Separation, Preconcentration and Spectrophotometry in Inorganic Analysis, Volume 10 (Analytical Spectroscopy Library)

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Analytical Spectroscopy Library 10

Separation, Preconcentration and Spectrophotometry in Inorganic Analysis

Analytical Spectroscopy Library A Series of Books Devoted to the Application of Spectroscopic Techniques to Chemical Analysis Volume 1 NMR for Liquid Fossil Fuels, by L. Petrakis and D. Allen Volume 2 Advances in Standards and Methodology in Spectrophotometry, edited by C. Burgess and K.D. Mielenz Volume 3 Introduction to Inductively Coupled Plasma Atomic Emission Spectrometry, by G.L. Moore Volume 4 Sample Introduction in Atomic Spectroscopy, edited by J. Sneddon Volume 5 Atomic Absorption Spectrometry. Theory, Design and Applications, edited by S.J. Haswell Volume 6 Spectrophotometry, Luminescence and Colour; Science and Compliance, edited by C. Burgess and D.G. Jones Volume 7 Applications of Synchrotron Radiation to Materials Analysis, edited by H. Saisho and Y. Gohshi Volume 8 Methods for Structure Elucidation by High-Resolution NMR, edited by Gy. Batta, K.E. K6ver and Cs. Sz~.ntay, Jr. Volume 9 Flow Analysis with Atomic Spectrometric Detectors, edited by A. Sanz-Medel Volume 10 Separation, Preconcentration and Spectrophotometry in Inorganic Analysis, by Z. Marczenko and M. Balcerzak

Analytical S p e c t r o s c o p y L i b r a r y -

10

Separation, Preconcentration and Spectrophotometry in Inorganic Analysis by Zygmunt Marczenko and Maria Balcerzak

Department of Analytical Chemistry, Warsaw University of Technology, Naokowskiego 3, 00-664 Warsaw, Poland Translated by Eugeniusz Kr

2000 ELSEVIER Amsterdam

- Lausanne - New York - Oxford - Shannon - Tokyo

This work is an enlarged translation of Spektrofotometryczne metody w analizie nieorganicznej by Zygmunt Marczenko and Maria Balcerzak 9 Wydawnictwo Naukowe PWN, SA, Warszawa, 1998 Published by arrangement with Polish Scientific Publishers PWN 9 2000 Elsevier Science B. V. for the English edition only ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

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First edition 2000 Library of Congress Cataloging in Publication Data A catalog record from the Library of Congress has been applied for.

ISBN: ISSN:

0 444 50524 5 0926 4345

The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). Printed in The Netherlands.

Contents Preface ...................................................................................................................................... Abbreviations ...........................................................................................................................

Part I. Chapter Chapter Chapter Chapter

3 4

General 1. 2. 3. 4.

Separation and preconcentration of elements ..................................................... 5 Principles of spectrophotometry ........................................................................ 26 Spectrophotometric methods ............................................................................. 39 Spectrophotometric reagents ............................................................................. 53

Part II.

Determination of Elements

Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter Chapter

Alkali metals ..................................................................................................... 77 Aluminium ........................................................................................................ 83 Antimony .......................................................................................................... 92 Arsenic .............................................................................................................. 99 Beryllium ........................................................................................................ 107 Bismuth ......................................................................................................... 113 Boron .............................................................................................................. 121 Bromine .......................................................................................................... 129 C a d m i u m ........................................................................................................ 133 Calcium .......................................................................................................... 140 Carbon ............................................................................................................ 147 Chlorine .......................................................................................................... 152 C h r o m i u m ....................................................................................................... 159 Cobalt ............................................................................................................. 167 Copper ............................................................................................................ 177 Fluorine .......................................................................................................... 189 Gallium ........................................................................................................... 198 G e r m a n i u m ..................................................................................................... 204 Gold ................................................................................................................ 210 Indium ............................................................................................................ 216 Iodine .............................................................................................................. 222 Iron ................................................................................................................. 226 Lead ................................................................................................................ 238 M a g n e s i u m ..................................................................................................... 247 Manganese ...................................................................................................... 253

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

Chapter 30. Mercury ..........................................................................................................262 Chapter 31. M o l y b d e n u m and tungsten ............................................................................. 270 Chapter 32. Nickel ............................................................................................................. 284 Chapter 33. N i ob i u m and tantalum .................................................................................... 293 Chapter 34. Nitrogen ..........................................................................................................304 Chapter 35. Oxygen ...........................................................................................................315 Chapter 36. Palladium ........................................................................................................318 Chapter 37. Phosphorus .....................................................................................................326 Chapter 38. Platinum ..........................................................................................................334 Chapter 39. Rare-earth elements ........................................................................................341 Chapter 40. R h e n i u m .........................................................................................................350 Chapter 41. R h o d i u m and iridium ......................................................................................357 Chapter 42. Ruthenium and o s m i u m ..................................................................................365 Chapter 43. Scandium ........................................................................................................375 Chapter 44. Selenium .........................................................................................................379 Chapter 45. Silicon .............................................................................................................385 Chapter 46. Silver ..............................................................................................................392 Chapter 47. Strontium and barium .....................................................................................399 Chapter 48. Sulphur ...........................................................................................................403 Chapter 49. Tellurium ........................................................................................................412 Chapter 50. Thallium .........................................................................................................418 Chapter 51. Thorium ..........................................................................................................424 Chapter 52. Tin ..................................................................................................................431 Chapter 53. Titanium .........................................................................................................438 Chapter 54. Uranium ..........................................................................................................446 Chapter 55. V a n a d i u m .......................................................................................................456 Chapter 56. Zinc .................................................................................................................466 Chapter 57. Zirconium and hafnium ..................................................................................474 A p p e n d i x ..............................................................................................................................483 Index .....................................................................................................................................514

Preface Spectrophotometry enables to determine, with a good precision and sensitivity, almost all the elements present in small and trace quantities in any materials. The method is particularly useful in the determination of non-metals. In the book, much attention has been paid to separation and preconcentration methods, since they play an essential role in increasing the selectivity and sensitivity of spectrophotometric methods. The separation and preconcentration methods have been utilised also in other determination techniques. Modern spectrophotometers, supplied with data-processing capabilities, enable the treatment of absorption spectra in the derivative spectrophotometry. The spectrophotometric methods can be easily automatized, e.g. in the flow injection analysis. Chapters 1-4 provide the characteristics of the separation and preconcentration methods: solvent extraction, flotation, coprecipitation with collectors, volatilization, ion exchange etc. These chapters deal also with the fundamentals of spectrophotometry, spectrophotometric methods of analysis, and most important chromogenic reagents. Chapters 5-57 have been devoted to individual elements or groups of related elements. In the monograph much attention has been paid to the application of the methods in analytical practice. The references, listed at the end of each chapter and critically selected, cover the works published until the end of 1999. The book has been designed for analytical chemists dealing with environment protection, geology, biology, many branches of industry, etc. It will also be a useful guide for students interested in becoming specialists in analytical chemistry. Zygmunt Marczenko and Maria Balcerzak

Abbreviations

BPHA 5-Br-PADAP CAS CP CTA DAM DAPM DDTC DIPE DMF ECR EDTA g

FIA HzDm H2Dz HTTA MIBK NTA oxine PAN phen ppb ppm REE SPADNS sp. abs., a TAN TAR TBP TEA TOA TOPO TPTZ

N-benzoyl-N-phenylhydroxylamine 2-(5-Br-2-pyridylazo)-5-diethylaminophenol Chrome Azurol S cetylpyridinium ion (or salt) cetyltrimethylammonium ion (or salt) diantipyrylmethane diantipyrylpropylmethane diethyldithiocarbamate di-isopropyl ether dimethylformamide Eriochrome Cyanine R ethylenediaminetetraacetic acid (or disodium salt) molar absorptivity flow-injection analysis dimethylglyoxime dithizone, diphenylthiocarbazone thenoyltrifluoroacetone wavelength methyl isobutyl ketone nitrilotriacetic acid 8-hydroxyquinoline, 8-quinolinol 1-(2-pyridylazo)resorcinol 1,10-phenanthroline parts per billion parts per million rare-earth elements 2-(4-sulphophenylazo)chromotropic acid specific absorptivity 1-(2- thiaz olyl azo)- 2-naphthol 4-(2-thiazolylazo)resorcinol tri-n-butyl phosphate triethanolamine tri-n-octylamine tri-n-octylphosphine oxide 2,4,6-tri(2'-pyridyl)-s-triazine

Chapter 1. Separation and preconcentration of elements The spectrophotometric determination of elements is usually preceded by their separation from major components (matrix) and from interfering elements, the effects of which cannot be eliminated by other methods such as masking or change of pH of the medium. In the trace analysis of high-purity materials, separation from the matrix involves simultaneous concentration of the trace components. General methods of preconcentrating and separating elements have been outlined in several monographs and reviews [1-4]. The present Section provides a discussion of the following separation and preconcentration methods: solvent extraction, precipitation and co-precipitation with collectors, volatilization, and methods based on the use of ion-exchangers and other sorbents. These methods are used not only with spectrophotometry, but also in conjunction with other methods of determination.

1.1. Solvent extraction 1.1.1. Introduction The extraction process and extractive methods for separation and preconcentration of elements are described in several monographs and reviews [5,6]. Solvent extraction separation is based on differences in the solubilities of elements and their compounds between two immiscible liquid phases. Usually, the initial phase is an aqueous solution and the second phase is an organic solvent, immiscible with water. Some properties of the more common organic solvents are listed in Table 1.1. The ion to be extracted into the non-aqueous phase should first be transformed into an uncharged species. Stripping ("re-extraction", "back-extraction", or "scrubbing") involves bringing the element from the organic extract back into the aqueous phase. The extraction efficiency, i.e., the degree of transfer of the species from the aqueous to the organic phase, is defined in terms of the distribution- (or extraction-) coefficient, (D). The quantity D is the ratio of total concentration (i.e., the concentration of all the existing forms) of the element in the organic phase (Ec0) to the total concentration in the aqueous phase (ECw) in the aqueous phase, at equilibrium

ZC o

The extraction efficiency (%E) is also expressed as the extraction percent %E =

100D

D+(Vw/Vo) where D is the distribution coefficient, and Vw and Vo are the volumes of the aqueous and the organic phases, respectively.

6

1. Separation and preconcentration of elements

When the distribution coefficient of a given element in a specified system is large (e.g., 1,000, i.e., log D = 3), a single extraction will suffice. In most extraction systems the partition coefficients change as the concentration of the substance extracted changes; in most cases they decrease with decreasing concentration [7]. T a b l e 1.1. Physical properties of s o m e organic s o l v e n t s Solvent Acetate, n-amyl n-butyl ethyl Acetone Alcohol, n-amyl n-butyl ethyl methyl Benzene Carbon tetrachloride Chloroform Cyclohexane o-Dichlorobenzene 1,2-Dichloroethane Dioxan Ethers, di(2-chloroethyl) Diethyl di-isopropyl (DIPE) Hexane Methyl isobutyl ketone (MIBK) Methylene chloride (dichloromethane) Mesityl oxide Nitrobenzene 1-Octanol Tetrachloroethylene Toluene Trichloroethylene misc. - completely miscible

Density 9/ml 0.87 0.88 0.90 0.89 0.81 0.81 0.79 0.80 0.89 1.59 1.50 0.78 1.30 1.26 1.03 1.22 0.72 0.73 0.66

Boiling point oC 149 126 77 57 138 118 78 65 80 77 61 81 180 83 101 178 35 68 69

Dielectric constant 4.8 5.0 6.0 20.7 13.8 17.1 24.3 32.6 2.3 2.2 4.8 2.0 9.9 10.4 2.2 23.0 4.3 3.9 1.9

Solubility in water % 0.2 0.5 8.6 misc. 2.2 7.9 misc. misc. 0.2 0.1 1.0 0.01 0.01 0.9 misc. 1.0 7.4 0.7 0.02

0.80

116

13.1

2.0

1.34 0.85 1.21 0.83 1.63 0.87 1.46

40 129 211 194 121 111 87

9.1 15.6 34.8 10.3 2.3 2.4 3.4

2.0 3.2 0.2 0.05 0.02 0.05 0.1

Shaking the phases in a separating funnel during the extraction or re-extraction must be continued until equilibrium is attained. The time required for the system to reach equilibrium varies from seconds to several minutes, depending on the kinetics of the process [8-10]. When the shaking time recommended is more than two minutes, it is advisable to use a mechanical shaker. Extraction is equally useful in the preconcentration and separation of small amounts of elements, and in the separation of macrocomponents from traces. Extraction methods generally require less time than precipitation methods. The former give also "purer" separation of elements owing to the small area of phase contact. Co-extraction occurring in some cases [ 11 ] has not been widely used in extraction separations.

1.1. Solvent extraction

7

1.1.2. Extraction systems Extraction systems may be divided into two classes: (1) uncharged covalent species (simple molecules and chelates), and (2) ion associates (ion pairs). Simple molecules (e.g., I2, HgC12, AsC13, BiI3, GeC14, OsO4) are extracted with nonpolar solvents such as benzene, CHC13, CC14. The extraction of this type of compound is comparatively selective and is widely applied in separation of some elements [5,12,13]. Inner chelates (uncharged chelates) are formed when metal ions react with bifunctional ligands, such as dithizone (formula 1.1), 8-hydroxyquinoline [14] (formula 1.2), dithiocarbamates (formulae 1.3 and 1.4), ethyl xanthates [15,16], cupferron (formula 1.5), BPHA (N-benzoyl-N-phenylhydroxylamine) [17,18], acetylacetone (formula 1.6) and thenoyltrifluoroacetone (HTTA) (formula 1.7) [19,20].

S=c/NH-NH-~

--N------NH~

Hs_C//

\"-J/

II

I

(1.1) CzH5

.,~S

CzH/

~S-Na §

\N__C// (].3)

(1.2)

O"NI'I~' N/~N~O

(1.5)

CzHs_ S "X~N--ICI- S-[NH2(CzHs)z]+ CzH/ (1.4)

H3C~ HC~,C---OH

iH

H3C/C~''~-'O (1.6)

(1.7)

Inner chelates are extracted with non-polar solvents (mostly with CHC13 and CC14). Synergism [21,22] is important in the extraction of some chelates. Some inner chelates were extracted into chloroform solutions of diantipyrylmethane (DAM) (formula 1.8) [23]. The effects of salting-out agent, solvent, and temperature on the

8


H3C----C~C--CH2-- C ~ C - - C H 3 I

.

I

t

H3C~N~N~C---'~ 0 0~----C~ 7N---'.-CH3

(1.8)

lC~Hs

i

C6Hs

extraction have been discussed [20]. Selectivity can be increased by using the exchange technique, in which a less-stable metal chelate is the source of the chelating agent [24]. The extraction of chelates is usually applied to preconcentration and separation of small amounts of metals. Owing to their low solubility in organic solvents, most chelates can not be used for the extraction of macrocomponents. Cupferronates and acetylacetonates are exceptions. Chelates of metal ions with alkyl- and arylphosphoric and thiophosphoric acids can be extracted into chloroform and other solvents [25,26]. Such systems enable one to separate, by means of extraction procedures, many metals from strongly acid solutions. Examples of such reagents are di-(2-ethylhexyl)phosphoric acid (HDEHP) and di-n-butyldithiophosphoric acid (formulae 1.9 and 1.10). HDEHP is a viscous liquid (density 0.98), slightly soluble in water, but readily soluble in benzene, hexane, and MIBK.

~zHs

CI.Hg~CH~CHz~O~

~,0

P, C~Hg~CH~CHz~O ~ ~OH ~zHs

(1.9)

(1.10)

Ion associates (ion-pairs) are formed by the electrostatic attraction of oppositelycharged ions which have, in general, high molecular weight. Ion-associates may be divided into several groups. Halometallic acids (e.g., HFeC14, HSbC16, HAuBr4, H2CdI4) are formed in reactions of multivalent metal ions with hydrohalic acids. These compounds are extractable from acid solutions containing high concentrations of halide ions by oxygen-containing solvents such as ethers, higher alcohols, ketones, and esters [27]. The extraction of halometallic acids is made possible by solvation of the protons by the solvent molecules, and often secondary solvation of the ion-pair formed. Since the solvent molecule co-ordinates through its oxygen atom, such systems are sometimes called oxonium extraction systems. Extractions from chloride- [28,29], bromide- [30], iodide- [29,31,32], fluoride- [33], and thiocyanate- [34] systems have been discussed.

Heteropoly acids [oxygen compounds of Mo(VI), W(VI), Si, P(V), As(V), Ge, and other elements] and their reduction products (molybdenum blues) are extracted into oxygencontaining solvents by a mechanism similar to that above. Another group of ion-association systems is represented by solvated salts (usually nitrates, but also halides and sulphates). Solutions (1-50%) of tri-n-butyl phosphate (TBP, formula 1.11) in hexane, CC14, and solutions (1-20 %) of tri-n-octylphosphine oxide (TOPO, formula 1.12) in cyclohexane are most often used as the extractants. Solvation with TBP or TOPO (through the strongly basic oxygen atom of the phosphoryl group) enables metal salts,

1.1. Solventextraction

9

such as UO2(NO3)2"2TBP, Th(NO3)4"2TBP, Zr(NO3)4"2TOPO, TiOSO4"2TOPO, to be extracted with non-polar solvents.

C~.H9C )

\

C4HgO-"-~P----~0

UO2C12-3TBP,

and

CaHr/

\

CeHI?----p ~----0

(1.11)

C4HgO/

(1.12)

CeH(

High molecular-weight amines are of particular importance in the extraction [35-39]. They form ion-associates with acids (e.g., HSCN, HReO4, HI), metal-complex acids [e.g., H2PtC16, HFeC14, H2UO2(SO4)212, and heteropoly acids. These complexes are extractable into non-polar solvents (e.g., C6H6, CHC13, CC14) and polar solvents (e.g., MIBK, amyl alcohol). Tertiary amines, such as tribenzylamine (TBA) and tri-n-octylamine (TOA) are most commonly used. Tertiary and secondary amines are used for extraction of anions from acid solutions, whereas quaternary ammonium salts enable also extraction from neutral and alkaline solutions. Secondary and tertiary amines and quaternary ammonium salts are applied as solutions in non-polar solvents. Diphenylguanidine (formula 1.13) is also frequently used in extraction.

~

N\

H

.C-m--NH

(1.13)

Some amines which extract anions from aqueous solutions as ion pairs are called liquid anion exchangers [40,41]. They are marketed commercially under such trade names as Aliquat-336 and Amberlite LA-1. They are mixtures of secondary or tertiary amines, or quaternary ammonium salts, with alkyl groups having 7-12 carbon atoms. Antipyrine and its derivatives [42,43] such as diantipyrylmethane (DAM, formula 1.8) are high molecular-weight amine extractants. In acid solutions, diantipyrylmethane is protonated, and the resulting cation combines with an anion to form an ion-associate (e.g., DAM.H+] [T1Br4-], [DAM.H+]z[CdI42-]. Ion associates formed by large cations and large anions need not be solvated and are extracted with inert solvents, such as CHC13, CC14, or toluene. Examples of large cations forming non-solvated salts are the tetraphenylarsonium ion (formula 1.14) and tetraphenylphosphonium ion. These cations form extractable ion-pairs with anions such as C104-, ReO4-, SbC16-, GaC14-, CdI42-, and SCN-.

(1.14)

Macrocyclic compounds have been proposed for selective extractive separation of metals, mostly alkali and alkaline earth metals [44-51 ]. An exhaustive review of applications of these compounds is given in articles [44,49]. Macrocyclic compounds form cationic

10


complexes with some metal ions, and when associated with suitable anions (e.g., picrate, perchlorate, tetraphenylborate) these can be extracted into non-polar solvents such as CH2C12, CHC13, C6H6.

(1.15) 0~6

~o 0

(1.16) H

An example of crown ether applied in selective separation of alkali metals is dibenzo18-crown-6 (formula 1.15). Extractive separations of metal ions are also performed with macrocyclic ligands containing nitrogen or oxygen atoms, as well as macrocycles with combinations of oxygen, nitrogen, and sulphur atoms (N-O, S-O, N-S) [45,48]. A macrocyclic compound with only nitrogen hetero-atoms (formula 1.16) is selective for copper. Cryptands (Greek kryptd = vault, crypt) are polycyclic compounds containing oxygen and nitrogen atoms, forming extractable cationic complexes (cryptates) with metal ions (in the presence of a suitable anion). Thiocrown ethers are also used for extraction separation of metal ions [52-57]. The cationic complexes formed by these ethers with Cu(I), Ag, Pd(II), Pt(II) are extractable from acidic media (e.g., by 1,2-dichloroethane) in the presence of, e.g., picric acid.

1.1.3. Isolation and separation of traces The elements which can be separated from each other and from the matrix in small and trace amounts by extraction are shown in Table 1.2. The symbols of elements are accompanied by the typical compounds used in the extractions. The extraction methods provide large possibilities in separation of traces. An important role in concentrating traces by extraction is played by organic reagents which form chelates with metal ions. Only the more important group reagents are mentioned in Table 1.2. Dithizones and dithiocarbamates, containing sulphur as the ligand atom, are particularly suitable for extraction of metals that form sparingly soluble sulphides. Ligands complexing through oxygen atoms, such as 8-hydroxyquinoline and cupferron, react preferentially with hydrolyzable metal ions. For the isolation and separation of some individual elements there exist highly selective, and even specific, extraction systems, such as nickel with dimethylglyoxime and cobalt with 1-nitroso-2-naphthol.

11

1.1. Solvent extraction

T a b l e 1.2. Separation of traces by solvent extraction

Be Bd Mg B Ca B Sr B Ba B

Ce bd

Th Bd

Sc bd Y bd La bd

Ti bd Zr bd Hf bd

V bcd Nb ebd Ta ed

Cr bdh Mo deb W ebg

Mn bc

Re eh

B e AI. bd Fe Co Ni Cu Zn Ga bcd ace acb acb acb bed Ru Rh Pd Ag Cd In he ech ace ac abc bae Os Ir Pt Au Hg TI he ce ace aec ace ace

Si

ge Ge eg Sn cde Pb acb

P

g As ceg Sb edc Bi ace

S

F Se Ecf Te Ce

Br F I F

U bd e

The traces are extracted as: a - dithizonates, b - 8-hydroxyquinolinates, c - dithiocarbamates, d - cupferronates, e - halogen complexes, f - elements, g - heteropoly acids, h - oxygen compounds

A group of the methods for pre-concentration of traces includes the extraction of metals by the resins modified with organic reagents, e.g., 8-hydroxyquinoline, DDTK [58], Bromopyrogallol Red [59], Zincon [60], and crown ethers [61]. Reviews of the methods have been presented [62,63]. In many cases the separation of mixtures must be preceded by conversion of the main component into a stable complex, in order to retain it in the solution and prevent its extraction along with the separated microcomponents. The masking agents most appropriate for individual elements are shown in Table 3.1.

1.1.4. Separation of macrocomponents The elements which can be extracted in larger quantities are shown in Table 1.3. The most numerous group comprises the halide and thiocyanate complexes, namely fluorides (Ta, Nb, Sn), chlorides [Fe(III), Sb, As, Ga, Ge, Au, Mo, T1], bromides (Au, In, T1, Ga), iodides (Bi, Sb, Cd, Hg, Sn), and thiocyanates [Zn, Co, Fe(III), Ti, Mo, U]. There are numerous examples of the extraction of macrocomponents as chloride complexes in the analysis of various materials. The extraction of iron(III) from hydrochloric acid medium, prior to determination of trace elements, has been thoroughly investigated [64]. Macroquantities of gallium were extracted from 6-7 M hydrochloric acid with di(2chloroethyl) ether [65,66], and gold(m) was extracted with isoamyl acetate [67]. As bromide complexes, indium has been extracted with diethyl ether [68] or diisopropyl ether, and gold(III) (from 3 M HBr), also with di(2-chloroethyl) ether [69].

12


T a b l e 1.3. Separation of the matrix by solvent extraction

Be

af

Sc

ba Y be La e

Ce e

Th bf

Ti ae Zr ef Hf ef

V aef Nb ae Ta ae

Cr df Mo ae W ae

Mn f

Re da

Fe aef Ru ad Os ad

Co Ae Rh A Ir A

Ni ae Pd af Pt a

Cu ae Ag a Au ab

Zn ae Cd a Hg ae

AI. fe Ga af In af TI af

Ge a Sn a Pb ae

As a Sb a Bi ae

S ca Se ca Te a

Br c I c

U bef

The macrocomponents are extracted as" a - halide compounds; b - nitrates; c - free elements" d - oxygen compounds; e - cupferronates; f - acetylacetonates.

Only those organic reagents, such as cupferron and acetylacetone, which form chelates highly soluble in non-polar organic solvents, can be used in the extraction of matrix elements. Larger amounts of some metals can be extracted as nitrates [U, Th, Ce(IV), Y, Sc] or oxides (Os, Ru). Matrix yttrium or scandium are extracted with TBP from 12-13 M HNO3 [70].

1.2. Precipitation Precipitation methods for the separation of elements are based on the differences in solubility of their compounds in aqueous solutions. Precipitation methods are used for separating trace elements alone, as well as for separating macrocomponents from the traces. Trace elements are separated quantitatively from the solution by using collectors (scavengers or carriers). When macrocomponents are precipitated, the aim is to prevent trace elements from co-precipitating with the large mass of the macrocomponent precipitate. This prerequisite restricts the application of the method to cases in which co-precipitation of trace elements with the macrocomponent precipitate is negligible.

1.2.1. Separation of traces with the use of carriers When a precipitant is added to a solution containing trace amounts of an ionic species (0.1100 ~tg in 100-250 ml, 10-8-10 .5 M), the ionic species may be only precipitated partly (or not at all), even though the solubility product has been exceeded. The formation of a precipitate, i.e., crystal growth, is a complex and slow process [71 ]. The process begins with nucleation. When nucleation and further growth of nuclei occur

1.2. Precipitation

13

slowly, a condition of permanent supersaturation exists in the solution. Nuclei are formed when the first portion of precipitant is added to the solution. If the ions to be precipitated occur in macro amounts in the solution, further addition of the precipitant causes rapid transformation of the nuclei into crystals by the formation of more of the sparingly soluble compound. If, however, the solution contains only trace amounts of the ions to be precipitated, the process is virtually complete after the nucleation stage. The rneehanisrn of the action of carriers depends on the nature of both the trace substance and the carrier involved. The co-precipitation consists in separation of ions coprecipitating from the solution with particles of the carrier formed in the solution. The coprecipitation may be either isomorphous (formation of solid solutions or mixed crystals) or based on adsorption phenomena. In most cases the co-precipitation of traces consists in the formation of solid solutions. The separation by co-precipitation is not restricted by the very low concentration of the trace species. Smaller amounts can be separated by co-precipitation than by solvent extraction, which is limited by the stability of the complex extracted. The formation of a solid solution, e.g., in the separation of Pb traces with lanthanum hydroxide, consists in the replacement of some La atoms in the crystal lattice by Pb atoms. If the trace element and the collector have opposite chemical properties (acidic, basic), the co-precipitation may be the result of formation of chemical compounds. For example, traces of germanium or vanadium form germanates or vanadates in co-precipitation with Fe(III)-, AI-, or La- hydroxides, while traces of tungsten or molybdenum, on co-precipitation with Fe(III) hydroxide, form the corresponding Fe(III) tungstate or molybdate. Elements which can be pre-concentrated by co-precipitation with carriers are collected in Table 1.4. The most important forms for the co-precipitation of particular elements are given. For most elements it is possible to find a suitable form that enables the separation of traces, along with an appropriate carrier. The carriers are normally selected among related elements, although this condition is not indispensable. The elements which are co-precipitated as hydroxides can also be separated by organic reagents of the R-OH type, such as 8-hydroxyquinoline, cupferron, or B-diketones. Metal ions giving sparingly soluble sulphides may be co-precipitated by organic reagents of the RSH type, e.g., dithiocarbamates. Enough collector should be added to the sample solution to ensure that the precipitation is rapid, and that sufficient precipitate is formed for easy filtration or centrifugation. At the same time the amount of the collector should be sufficiently small for adsorption of interfering ions to be negligible. The amount of collector used depends on the volume of precipitate formed. This amount may be smaller if the species is separated as the 8hydroxyquinolinate, than in cases of precipitation as hydroxide. In practice, 2-5 mg of collector are used per 50-200 ml of sample solution. Hydroxides are often used for precipitation of traces with collectors [72-74]. With Fe(III), A1, or La as collector, traces of most analytical group I-III metals are separated by addition of excess of ammonia. Metals forming ammine-complexes, e.g., Ag, Cu, Ni, Co, Zn, and Cd remain in solution. When excess of NaOH is used for precipitation, amphoteric metals such as A1, Pb, Zn, Sn, and Cr remain unprecipitated. In this case, Fe(III), Ti, Mg, or La may be used as the collector. Lanthanum is especially convenient, since it usually does not have to be determined in the trace concentrate. It has no chromophoric properties and it does not interfere in most spectrophotometric methods.

14

1. Separation and preconcentration of elements T a b l e 1.4. S e p a r a t i o n of t r a c e s by c o - p r e c i p i t a t i o n with carriers

Ce ae

Be A Mg ae Ca ec Sr ce Ba ce

Sc ae Y ae La ae

Th ae

U ae

AI. Ti

ae Zr ae Hf ae

V

ae Nb ae Ta ae

Cr ae Mo bea W ae

Mn ab

Re ba

Fe abe Ru db Os db

Co ab Rh db Ir db

Ni Ab Pd Db Pt Db

Cu abd Ag bda Au dab

Zn ab Cd ab Hg bd

ae Ga ae In abe TI abe

Si

a Ge ab Sn ab Pb abc

P

a As b Sb abd Bi abd

CI

Se da Te da

f Br f I f

The traces are precipitated as: a; hydroxides or acids; b - sulphides; c - sulphates; d elements after reduction" e - 8-hydroxyquinolinates; f - silver salts.

Traces of hydrolyzable metals [e.g., Sn, Sb, TI(III), Bi] are separated from acidic medium with MnO2aq as the collector [75,76]. Hydrogen sulphide or thioacetamide are used for the separation of traces of metals in the form of their sulphides. Copper, mercury, and other metals of the "hydrogen sulphide" group are used as collectors [77,78]. After the collector has been added and the pH adjusted, the solution is heated with thioacetamide or the hot solution is saturated with hydrogen sulphide. All the metals of analytical group I-III are precipitated when an ammoniacal solution (pH 89) is saturated with HzS. In the analysis of high purity metals, trace elements were pre-concentrated by partial dissolution of the matrix. The remaining small part of the matrix retains all trace elements that are electrochemically less noble than the matrix [79,80]. In this way the trace elements were pre-concentrated from silver-, cadmium-, gallium-, indium-, zinc-, lead-, manganese-, aluminium-, and lead-antimony alloys. Trace amounts of noble and semi-noble metals (e.g., Au, Ag, Hg, and Cu) are separated electrolytically from acid media on a small platinum or gold cathode [81]. In cementation methods (i.e., reduction to metal in situ by another metal) small amounts of semi-noble metals (Cu, Bi, Sb) are deposited on less noble metals such as tin, iron, or zinc. Trace elements can be separated from solutions of different metals by reducing a small amount of the matrix metal with sodium hydroborate (NaBH4) [82-85]. The metallic precipitate serves as a trace collector for all the elements that are electrochemically more noble than the matrix. The method has been used in the trace analysis of lead and its alloys [82-85]. Concentrates of trace amounts of many metals are precipitated with 8-hydroxyquinoline, thionalide, or dithiocarbamates [86,87]. In addition to group systems of separating elements, shown in Table 1.4, there are also specific methods for precipitation of particular elements. For example, Pd can be precipitated with Ni as collector (and vice versa) by means of dimethylglyoxime. The rare earths and

1.2. Precipitation

15

thorium can be separated as their sparingly soluble oxalates or fluorides. Molybdenum and tungsten are separated as benzoinoximates. Active carbon can serve as collector in the separation of some trace elements precipitated in various forms [88,89]. Silicic acid has been also used as collector in preconcentration of many elements [90]. Masking plays a substantial role in the separation of traces from macrocomponents by precipitation. The aim is to retain the macrocomponents in solution while the traces are coprecipitated with a collector. The masking agent selected must complex the sample matrix without interfering with the separation of the traces and the collector. Knowledge of the stability constants of complexes is not enough to predict their masking possibilities. The stability constants of complexes are apparent constants which vary with the pH and with the concentration of other species capable of complexation (competitive reactions).The effect of these factors is taken into account in the conditional stability constants of complexes [91].

1.2.2. Separation of traces by flotation Ions of precipitate particles are adsorbed or attached at the surface of bubbles rising through a liquid, and are thereby separated. A substance which is not surface-active itself can be made so through union with, or adherence to, a surface-active agent (surfactant). Froth flotation involves separation (pre-concentration) by frothing. If an insoluble product is formed in interaction between the ion to be separated and a surfactant, the process is called ion flotation. If the ion is first precipitated and the precipitate is then floated with or without the addition of a surfactant, the process is called precipitate flotation. Flotation is accomplished in a special cylindrical vessel provided with a sintered glass disk at the bottom to break the gas (nitrogen, air) stream into small bubbles [92]. Ion flotation has been used as a method for pre-concentration of heavy metal ions in water. Anionic complexes of these elements are formed by adding complexing agents, and then floated by using a cationic surfactant and nitrogen. The amount of the surfactant should be greater than the stoichiometric amount, but excessive concentration may decrease trace recoveries [93]. Precipitate flotation is applied in the analysis of natural waters. Trace ions in an aqueous solution are co-precipitated with colloidal metal hydroxide collectors and floated with the aid of a gas stream [94-96]. Tiny gas bubbles are trapped in the interstitial spaces and on the surfaces of the precipitates to give sufficient buoyancy. Surfactant ions having the charge opposite to the precipitate surfaces are used to make the surface hydrophobic. Another important role of the surfactants (e.g., sodium oleate, sodium dodecylsulphate) is to form a stable froth layer to support the precipitate on the solution surface, which is important for complete separation of the precipitate. Instead of passage of gas bubbles through the solution, the aqueous pseudo-solution can be shaken with non-polar solvent. In such cases, hydrophobic sparingly soluble compounds accumulate at the phase boundary or adhere to the wall of the separating funnel used. After careful removal of both liquid phases, and washing, the precipitate adhering to the wall can be dissolved in a polar solvent and the isolated trace elements can be determined. Numerous elements form multicharged anionic complexes which are able to associate with hydrophobic basic dyes. They include anions of the heteropoly acids of Si, Ge, P(V), and As(V), the bromide complexes of Bi and Te(IV), and the thiocyanate complexes of Mo and W. Flotation of sparingly soluble ion-associates formed between the multicharged anionic complexes (with halides, SnC13-, and thiocyanate) of the platinum-group metals and

16


gold with basic dyes (Rhodamine 6G, Crystal Violet, Methylene Blue, etc.) has been described [97,98]. The floating agents often used are DIPE, benzene, cyclohexane, and toluene. Flotation of these hydrophobic ion associates is carried out without the use of surfactants.

1.2.3. Separation of macrocomponents The precipitation of macrocomponents (matrix) to separate them from trace elements is relatively uncommon [99]. In the separation of macrocomponents their quantitative precipitation is not necessary. The largest group of elements comprises those isolated from solution in the elemental form as a result of reduction, usually electrochemical. In acid solution, the electrolytic deposition of metal on a solid cathode is limited to noble and semi-noble metals. Trace analysis of copper and its compounds may serve as an example [100]. An anodic dissolution technique may be applied for the isolation of macroscopic amounts of copper. A sample in the form of a bar, plate, or wire is the anode in the electrolytic system. When current is passed through the electrolyte (nitric acid + persulphate), Cu is deposited on the graphite cathode, while most trace elements accumulate in the solution. In the trace analysis of platinum, the matrix has been also separated on a cathode [101 ]. The use of a mercury cathode, which has a high overvoltage to hydrogen, enables a considerable number of metals to be isolated from dilute acid solutions. After electrolysis with the mercury cathode, the following metals remain in solution: A1, Ti, Zr, V, Nb, U, Th, Be, Mg, Ca, and rare-earth elements. A mercury cathode is used to separate Fe, Ni, Cr, Mo, and Mn when steel is analysed for certain elements (e.g., A1, V, Ti). Gold, silver, mercury, and platinum metals, as well as Se and Te, can be precipitated from acid solution in the elemental form by reduction with chemical reagents such as zinc, NHzOH, NzH4, SO2, or formic acid. In the trace analysis of high purity mercury the sample (about 100 g) is dissolved in HNO3 and the solution is warmed in the presence of formic acid. First of all, nitric acid, then mercury, is reduced. The mercury forms a separate liquid phase, and the impurities remain in the aqueous solution [ 102]. In the trace analysis of silver, the sample is dissolved in nitric acid, then formic acid and mercury are added. The silver liberated on reduction dissolves in the mercury to form an amalgam [ 102]. In the trace analysis of high-purity zinc, the sample is coated with a thin layer of mercury. After dissolution of the zinc in hydrochloric acid, a drop of mercury remains that contains amalgams of many trace metals from the zinc analysed. They can be determined after volatilization of mercury [ 103]. The belief that the isolation of macrocomponents from solution as sparingly soluble compounds is inadmissible in trace analysis, because of the considerable losses of traces caused by adsorption, is not necessarily true if the precipitation is done in acid medium. This has been confirmed in the following examples. In the trace analysis of lead, the matrix was precipitated from a nitric acid medium as lead sulphate. By using radioisotopes, it was found that none of the 24 elements investigated had co-precipitated with PbSO4 [104,105]. Most of the lead can also be separated as PbCI2 from nitric acid medium without perceptible co-precipitation of other components [ 104]. Before the determination of trace impurities in bismuth the latter is removed from nitric acid solution as the sparingly soluble iodide or the basic nitrate [106]. In the analysis for Cu, Zn, Cd, Ni, Pb, Mn, and Fe traces in silver, the matrix is precipitated as AgC1 from dilute nitric acid medium [107]. The trace elements do not collect

1.3. Volatility

17

with the precipitate. In the trace analysis of high-purity thallium, the matrix was separated as sparingly soluble TII [ 108]. Before the determination of impurities in metal chlorides (e.g., of sodium, potassium, calcium, aluminium), the matrix can be separated by saturating the solution with gaseous HC1.

1.3. Volatility The following group of methods for separating and preconcentrating is based on differences in the vapour pressures of individual elements and their compounds. Covalent compounds are generally fairly volatile whereas ionic compounds are not. Covalently bonded compounds are also more soluble in non-polar solvents. As both volatility and solubility in solvents depend on the strength of intermolecular attractions, there is a fair resemblance between the compounds which are volatile and those which are readily soluble in non-polar solvents. Examples are AsC13, GeC14, OsO4, and certain inner chelates [ 109].

1.3.1. Separation of traces Volatilization is usually utilized for separating individual trace elements from the sample before the determination. The methods based on volatilization are concerned mainly with non-metallic and amphoteric elements which have high vapour pressure in the elemental form (e.g., chlorine, bromine, sulphur), or in compounds with halogen, hydrogen, or oxygen. Other volatilization methods exist for the separation of certain elements, such as the distillation of boron as methyl borate. A method has been proposed for preconcentration of traces of the more volatile elements (e.g., Zn, Cd, T1, In, Pb) by heating samples in quartz tubes to about 1,000 ~ in a stream of hydrogen. The sublimed metals are collected on a cold-finger. Many metal halides have been volatilized from aluminium samples by heating to 990 ~ [110]. Volatile acetylacetonates and other g-diketonates have also been utilized for the separation of metals [lll]. The separation of traces is done in a closed system and involves absorbing the traces in a suitable sorbent, for example hydrogen sulphide in a zinc acetate solution, ammonia in dilute HC1, and methyl borate in dilute NaOH. In the Gutzeit method traces of arsenic, liberated in the form of ASH3, are absorbed by a strip of paper saturated with a reagent giving a colour effect with ASH3. In all such procedures a carrier gas, such as hydrogen, nitrogen, chlorine, or steam is indispensable The volatility of some elements can be reduced by binding them in complexes. The fact has been utilized for increasing the selectivity of separation of traces by distillation. For example, Ge and As can be separated from Sn by distillation as the chlorides, after Sn(IV) had been masked as the non-volatile phosphate complex.

1.3.2. Separation of macrocomponents If the macrocomponents can be removed by volatilization (distillation, sublimation) without the introduction of large amounts of reagents, the trace elements may be greatly concentrated. This favourable situation, in which relatively large samples can be used for trace analyses, arises when the sample is volatile, especially when it is a liquid such as water, an organic solvent, a volatile acid, or an ammonia solution.

18


During evaporation of the matrix, substances are sometimes added to reduce the volatility of the trace components which are to be retained in the vessel. A few drops of concentrated sulphuric acid are usually added, and the solution is evaporated until fuming [112]. In certain cases, the trace element is kept in solution as a less volatile complex during the distillation of the sample matrix. In the determination of boron traces in chlorosilanes, silicon is removed as the volatile fluoride complex. To prevent the formation of volatile BF3, mannitol is added to form a non-volatile complex with boron [ 113]. Chlorine, bromine, iodine, sulphur, and mercury are distilled as the elements. Examples of the distillation of major elements as the fluoride complexes can be found in the trace analysis of silicon [ 113]. Tin can be sublimed as the iodide [ 114]. In the trace analysis of titanium [115], and zirconium, volatile chlorides (TIC14 and ZrC14) are sublimed after heating the samples with chlorine. In the trace analysis of rubidium- and caesium arsenates, arsenic is distilled off as arsine [116]. In the trace analysis of high purity cadmium, the matrix can be separated by distillation at 630~ [ 1171. By heating aluminium with ethyl bromide the metal is converted into ethylaluminium bromide, a liquid which boils at 130~ under reduced pressure [ 118]. Mineralization of organic samples [119-121], which precedes the determination of inorganic components, is also an example of separation of major components by volatilization (principally as CO2 and H20). In the mineralization of organic substances by dry ashing, the mineral residue may be so small and light that considerable losses result from the formation of "volatile" aerosols. These losses are prevented by adding a mineral collector, e.g., by wetting the sample with a solution of Mg(NO3)2, K2SO4, or Na2CO3. The temperature should not exceed 400-500~ to prevent distillation of more volatile elements [122]. At 400~ As and Hg volatilize completely, and Ag, Au, Fe, Sb, Zn, and Pb partially volatilize. Mercury is lost during wet mineralization with acids. It volatilizes on heating a solution in sulphuric acid to fuming.

1.4. Ion-exchange and sorption 1.4.1. Introduction Organic ion-exchangers are used for the separation of ions and for the separation and preconcentration of traces from the sample macrocomponents (matrix). Ion-exchange processes are based on the differences in ionic charge, in stability of the complexes formed, and in the associated distribution coefficients [ 123,124]. The organic ion exchangers are obtained by co-polymerization. They contain ionizable functional groups. The functional groups in cation exchangers are: -SO3H,-COOH, and -OH; those in anion exchangers are:-NR3 +, -NR2,-NHR, and NH2. A strongly acidic cation exchanger is prepared by sulphonation of a co-polymer of styrene and divinylbenzene (DVB). Dowex 50 X8 is the ion exchanger Dowex 50 crosslinked with 8% DVB. The hydrogen ion of acid groups is exchanged for other cations in the exchange process. The cation exchanger may be used in the hydrogen (H+), sodium (Na+), or similar form, depending on the cation attached to its acid groups. A strongly acidic character is imparted to cation exchangers by sulphonic acid groups. In anion exchangers, a strongly basic character is imparted by quaternary ammonium groups. Anion exchangers are most often used in the hydroxide (OH-) or chloride (C1-) form.

1.4. Ion exchange and sorption

19

In separation methods, ion exchange is usually performed by the column technique. The wet swelled resin is placed in a glass, quartz, or plastic column, and the solution studied is run through the column. In simple ion exchange separations some kinds of ions are retained in the column while others are eluted. In ion exchange chromatography the ions retained by the ion exchanger are gradually eluted by appropriate, selective eluents, and are collected in different portions of the eluate. The size of the column (resin bed) is selected according to the quantity of ions to be retained in the column. The depth of the resin bed in the column should be 10-20 times its diameter. Columns 8-10 mm in inner diameter are often used in laboratories. When the quantities of ions retained are in the microgram or milligram range, columns 3-5 mm in inner diameter are sufficient. In columns of the dimensions given above, 30-50 mesh or 100-200 mesh resins may be used [ 125 The ion-exchange capacity of the column depends on the quantity of resin in the column and on the ion-exchange capacity of the resin used. The latter is expressed in meq of ionic species per gram of dry resin. For strongly basic anion-exchangers it amounts to 4-5 meq per gram. In practice, it is not the total capacity of the ion-exchange column which is important, but the breakthrough capacity, which is lower and depends on the column shape, resin particle size, elution rate, and other experimental conditions. The distribution coefficient (D) for a particular element, on a given ion exchanger, and in the given medium, is the ratio of the element concentration in the resin bed phase, Cr (in millimoles per gram of dry resin) to the element concentration in the solution in equilibrium with the ion exchanger, c~ (in millimoles per ml of solution). D=

Cr Cs

The ability of an ion exchanger to retain an element from a solution of a particular complexing agent is shown by a graph of log D v s . complexing-agent concentration. The concentrations of complexing agents modify the affinity of the ions to the ion exchangers. The ion exchange methods were applied also in mixed aqueous-organic media [126]. The addition of organic solvents to the aqueous system modifies the affinity of the ions for ion exchangers. The fact may be explained by change in the solvation of the ions, reduction in the dielectric constant of the medium, and stabilization of complexes which are weak in aqueous solution. Most ion exchangers have a microporous structure, in which the pore size does not exceed 4 nm. Separations in mixed aqueous-organic media or in organic solutions are usually performed with the use of macroporous ion-exchange resins with mean pore size ranging from 130 nm. They are characterized by lesser swelling ability, more rapid exchange processes, and higher chemical resistance. The application of ion-exchange chromatography in inorganic analysis has been discussed widely [127-132]. The development of methods involving chelating ionexchangers and other sorbents will be discussed in further paragraphs. The use of molecular spectroscopy as a detection method in chromatography has also been discussed [133].

1.4.2. Separation with the use of cation exchangers Strongly acidic cation exchangers, such as Dowex 50 and Amberlite IR-120, are most often used in ion-exchange separations.

20


The behaviour of elements on strongly acidic cation-exchangers in hydrohalic acid solutions has been extensively investigated. Distribution curves have been reported for individual elements separated on cation exchanger in 0.5-12 M HC1 [134], and separations in mixed media HCl-water-organic solvent were studied [135,136]. Studies of ion exchange on macroporous ion-exchangers were also carried out in the media of HCl-water-acetone [137] and HCl-acetic acid-water [138]. The behaviour of metal ions on a strongly acidic cation-exchanger in the mixed medium HBr-water-organic solvents was studied [139,140]. The behaviour of metal ions in the presence of thiourea has been described [141]. In the separation of elements on strongly acidic cation-exchangers, use was also made of fluorides [142], thiocyanates in aqueousorganic media [143], perchlorates [144], oxalates [145], and tartrates [146]. An interesting application of a cation exchanger has been given in relation to trace analysis of sodium-, potassium-, barium-, and strontium chlorides [147]. Metal cations are retained by the cation exchanger. Concentrated HC1 run onto the column precipitates NaC1, KC1, BaC12, and SrCI2, whereas trace metals present are eluted.

1.4.3. Separations with use of anion exchange resins Strongly basic anion-exchange resins, such as Dowex 1 and Amberlite IRA-400, are mostly used in the separation and preconcentrations to be discussed. The behaviour of elements on strongly basic anion-exchangers in hydrochloric acid medium has been studied [148,149]. The behaviour of some elements in mixed media HC1water-organic solvents has been a subject of some work [ 150,151 ]. Detailed data have been published on the behaviour of some elements on anion exchangers in the media of fluoride [152], nitric acid [153], sulphuric acid [154,155], and in the mixed medium HCl-acetic acid-water [138]. Studies were also carried out on the behaviour of elements on strongly basic anion exchangers in the media of HBr [156,157], phosphoric acid [158], oxalic acid [159], thiocyanate [160], tartrate [161], and thiosulphate [162]. Schemes have been given for the separation of iron and alloy components, and admixtures in steel of A1, Bi, Co, Cr, Mn, Mo, Nb, Ni, Sn, Ta, Ti, V, W, and Zr. Columns with strongly basic anion-exchangers and with strongly acidic cation exchangers were used, and various media (HF, HC1, and H2804 were applied) [163]. Weakly acidic anion exchangers were used in the separation of metals in chloride [ 164] and thiocyanate [165] media.

1.4.4. Chelating resins and other sorbents Chelating resins play a substantial role in the preconcentration and separation of trace elements 166-168]. These sorbents are especially useful in trace analysis of natural waters. Chelating resins are characterized by high selectivity, which depends mainly on the chelating groups involved. Various matrices are used in the synthesis of chelating resins: copolymers of styrene and divinylbenzene, polystyrene, polymethacrylate, and fibrous materials such as cellulose. Also used are modified anion exchangers, charcoal, and other materials impregnated (loaded) with chelating agents. Chelex-100 is a well-known resin based on styrene-divinylbenzene copolymer with iminodiacetate groups. It has found many applications in preconcentration of traces of heavy metals such as Cu, Cd, Zn, Ni, Co, Mn, Fe, Pb, and Hg [169-172].

References

21

From among numerous chelating agents used in chelating resins can be mentioned: oxine [173,174], dithiocarbamates [175-178], dithizone [179], nitrosonaphthols [180], thiazolothiol [181], Arsenazo I [182], and formazans [183]. Anion exchangers have been modified with chelating agents, such as dithizone, Arsenazo III, Sulphonazo III [184], Xylenol Orange [185], and BPHA [186]. The sorption of metal traces on resins modified with azo reagents was also studied [187,188]. Cellulose is a natural hydrophilic porous polymer. Owing to the presence of hydroxyl groups in cellulose it is possible to introduce chelating groups into it. Cellulose sorbents are characterized by good kinetic properties and high distribution coefficients [189]. Cellulose sorbents with functional groups derived from dithiocarbamates [190] or 1-(2hydroxyphenylazo)-2-naphthol [191] are known. Fibrillar chelating sorbents based on synthetic organic products have been proposed [192]. Chelating sorbents based on silica gel have been proposed. Among organic reagents supported on silica gel are: oxine [193], rhodanine [194], and other reagents containing thiol groups [195]. The use of macrocyclic ligands (bonded with silica gel) for the separation of metal ions has been investigated [ 196,197].

Inorganic ion exchangers have not been applied widely, but in some cases they can be useful in separation and preconcentration of trace elements. They include hydrous oxides and acid salts of multivalent metals, heteropoly acids (e.g., phosphomolybdenic acid), sparingly soluble ferrocyanides, and synthetic aluminosilicates (zeolites) [198,199].

References 1. Zolotov Yu.A., Pure Appl. Chem., 50, 129 (1978). 2. Minczewski J., Chwastowska J., Dybczyfiski R., Separation and Preconcentration Methods in Inorganic Trace Analysis, Horwood, Chichester 1982. 3. Mizuike A., Enrichment Techniques for Inorganic Trace Analysis. Chemical Laboratory Practice, Springer, Berlin 1983. 4. Zolotov Yu. A., Kuz'min N.M., Preconcentration of Trace Elements, Elsevier, Amsterdam, 1990. 5. Marcus Y., Talanta, 23, 203 (1976). 6. Rydberg J., Musikas C., Choppin G.R., Principles and Practices of Solvent Extraction, Dekker, New York 1992. 7. Bock R., Freitag K.D., Z. Anal. Chem., 254, 104 (1971). 8. Alimarin I.P., Zolotov Yu.A., Bodnya V.A., Pure Appl. Chem., 25, 667 (1971). 9. Danesi P.R., Chiarizia R., Crit. Rev. Anal. Chem., 10, 1 (1980). 10. Akiba K., Freiser H., Anal. Chim. Acta, 136, 329 (1982). 11. Bagreev V.V., Pomandopulo Yu.I., Zh. Anal. Khim., 35, 358 (1980). 12. Studlar K., Coil. Czech. Chem. Comm., 31, 1999 (1966). 13. Byrne A.R., Gorenc D., Anal. Chim. Acta, 59, 81 (1972). 14. Blokhin A.A. et al., Zavod. Lab., 45, 693 (1979). 15. Donaldson E.M., Talanta, 23, 411,417 (1976). 16. Donaldson E.M., Mark E., Talanta, 29, 663 (1982). 17. Riedel A., J. Radioanal. Chem., 13, 125 (1973). 18. Chwastowska J., Lissowska K., Sterlifiska E., Chem. Anal. (Warsaw), 19, 671 (1974). 19. Onishi H., Sekine K., Talanta, 19, 473 (1972). 20. Ionov V.P., Chicherina N.Yu., Zh. Anal. Khim., 36, 236 (1981); 39, 1581 (1984). 21. Zolotov Yu.A., Gavrilova L.G., Zh. Anal. Khim., 25, 813, 1054 (1970).

22

1. Separationand preconcentrationof elements

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118. Neeb K.H., Z. Anal. Chem., 221, 200 (1966). 119. Adrian W.J.,Analyst, 98, 213 (1973). 120. Schachter M.M., Boyer K.W., Anal. Chem., 52, 360 (1980). 121. Blanu~a M., Bregki D., Talanta, 28, 681 (1981). 122. Strohal P., Luli6 S., Jelisav6i6_ O., Analyst, 94, 678 (1969). 123. Marhol M., Ion Exchangers in Analytical Chemistry. Their Properties and Use in Inorganic Chemistry, Academia, Prague, 1982. 124. Harland C.E., Ion Exchange: Theory and Practice, Royal Soc. Chem., Cambridge, 1994. 125. Aubouin G., Laverlochbre J., J. Radioanal. Chem., 1, 123 (1968). 126. Moody G.J., Thomas J.D., Analyst, 93, 557 (1968). 127. Yi Z., Zhuang G.S., Brown P.R., J. Liq. Chromatogr., 16, 3133 (1993). 128. Xianren Q., Chong-Yu X., Baeyens W., J. Chromatogr., 640, 3 (1993). 129. Timberbaev A.R., Bonn G.K., J. Chromatogr. A., 640, 195 (1993). 130. Nesterenko P.N., Tarasenko D.A., Shpigun O.A., Zh. Anal. Khim., 49, 244 (1994). 131. Hu W. et al., Anal. Chem., 67, 3713 (1995). 132. Lucy C.A., J. Chromatogr. A., 739, 3 (1996). 133. Korte E.H., Fresenius'J. Anal. Chem., 333, 753 (1989). 134. Mann C.K., Swanson C.L., Anal. Chem., 33, 459 (1961). 135. Strelow F.W., Victor A.H., Van Zyl C.R., Eloff C., Anal. Chem., 43, 870 (1971). 136. Strelow F.W., Victor A.H., Talanta, 19, 1019 (1972). 137. Strelow F.W., Talanta, 35, 385 (1988). 138. Kasiura K., Chem. Anal. (Warsaw), 42, 809 (1997). 139. Korkisch J., Klakl E., Talanta, 16, 377 (1969). 140. Strelow F.W., Hanekom M.D., Victor A.H., Eloff C., Anal. Chim. Acta, 76, 377 (1975). 141. Weinert C.H., Strelow F.W., Talanta, 30, 755 (1983). 142. Korkisch J., Huber A., Talanta, 15, 119 (1968). 143. Pietrzyk D.J., Kiser D.L., Anal. Chem., 37, 233 (1965). 144. Strelow F.W., Sondorp H., Talanta, 19, 1113 (1972). 145. Nozaki T. et al., Bull. Chem. Soc. Jpn., 42, 245 (1969). 146. Strelow F.W., Van der Walt T.N., Anal. Chem., 54, 457 (1982). 147. Tera F., Ruch R.R., Morrison G.H., Anal. Chem., 37, 358 (1965). 148. Yoshimura J., Waki H., Bull. Chem. Soc.Jpn., 35, 416 (1962). 149. Seymour M.D., Fritz J.S.,Anal. Chem., 45, 1394 (1973). 150. Fritz J.S., Gillette M.L., Talanta, 15, 287 (1968). 151. Koch W., Korkisch J., Mikrochim. Acta, 1972, 687; 1973, 245,263,877. 152. Ferraro T.A., Talanta, 16, 669 (1969). 153. Walter C.W., Korkisch J., Mikrochim. Acta, 1971, 81,137, 158, 181,194. 154. Strelow F.W., Bothma C.J., Anal. Chem., 39, 595 (1967). 155. Lavrukhina A.K., Akol'zina L.D., Zh. Anal. Khim., 22, 542 (1967). 156. Strelow F.W.,Anal. Chem., 50, 1359 (1978). 157. W6dkiewicz L., Dybczyfiski R., Chem. Anal. (Warsaw), 26, 419 (1981). 158. Polkowska-Motrenko H., Dybczyfiski R., J. Chromatogr., 88, 387 (1974); Chem. Anal. (Warsaw), 22, 1021 (1977). 159. Strelow F.W., Weinert C.H., EloffC.,Anal. Chem., 44, 2352 (1972). 160. Kiriyama T., Kuroda R., Anal. Chim. Acta, 101,207 (1978). 161. Pitstick G.F., Sweet T.R., Morie G.P.,Anal. Chem., 35, 995 (1963). 162. Eusebius L.C., Ghose A.K., Mahan A., Dey A.K., Analyst, 105, 52 (1980).

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Chapter 2. Principles of Spectrophotometry 2.1. Introduction Spectrophotometric methods are among the oldest methods of analytical chemistry. The absorption of visible light by certain chemical substances has long been used for visual determination of their concentration. As early as in the middle of the XIX century methods were known for the determination of bromide in natural waters by oxidation and extraction of the resulting bromine into ether, of ammonia by Nessler's method, of titanium by the peroxide method, and of molybdenum by the thiocyanate method. The term "colorimetry" was used for those analytical methods, in which chemical elements were determined by comparing the colour of unknown samples with appropriate standards, either in graduated cylinders or in visual comparators. The use of photoelectric instruments has given rise to measurement of the absorption of radiation as it passes through the analysed samples and has enabled us to extend the useful radiation range outside the visible region. Nowadays, spectrophotometry is regarded as an instrumental technique, based on the measurement of the absorption of electromagnetic radiation in the ultraviolet (UV, 200-380 nm), visible (VIS, 380-780 nm), and near infrared region. Inorganic analysis uses UV-VIS spectrophotometry. The UV region is used mostly in the analysis of organic compounds. Irrespective of their usefulness in quantitative analysis, spectrophotometric methods have also been utilized in fundamental studies. They are applied, for example, in the determination of the composition of chemical compounds, dissociation constants of acids and bases, or stability constants of complex compounds. Spectrophotometry as a measuring technique has developed enormously as a consequence of the progress in technology, and in the development of new materials and of methods of data processing. The development of specialized optics, and of spectrophotometers coupled with microprocessors controlling their operation, has extended considerably the possibilities of using these instruments, the recording of absorption spectra, and the treatment of the data collected. Spectrophotometric methods have proved to be particularly suitable for automation, both in analytical procedures and in the treatment of data. They belong to the detection techniques most frequently used in automatic flow injection analysis (FIA).

2.2. Absorption and molecular structure Spectrophotometric methods of identification and determination of substances are based on the existence of relationships between the position and intensity of absorption bands of electromagnetic radiation, on the one hand, and molecular structure on the other. Electronic spectra result from changes in the energy states of electrons [o, x, and free electron pairs (n)] in a molecule as a result of absorption in the UV-VIS region. The changes depend on the probability of electronic transitions between the individual energy states of the molecule. The number of absorption bands, and their positions, intensities and shapes are the spectral parameters utilized in qualitative and quantitative chemical analyses [1-3]. The positions of individual absorption bands recorded in the spectra depend on the energy of the absorbed radiation. Radiation from the near-infrared region gives rise to changes in the rotational and oscillation energy states in a molecule. Narrow bands that are due to small

2.2. Absorption and molecular structure

27

changes in wave number are connected with characteristic groups of atoms (functional groups) and are used for identification of such groups. The UV-VIS radiation gives rise to changes in the energy of electronic states of a molecule. The probability of electronic transitions in a molecule depends on the presence of multiple bonds in the molecule and on the kind, number and positions of the substituent groups. Determination of the kind of transitions corresponding to the observed bands of absorption spectra enables one to determine the structure of the molecule. Spectral transitions of electrons associated with absorption of radiation correspond to transitions from binding orbitals ((y, x, n) to anti-bonding orbitals of higher energy state (o*, ~*). The energy of the respective transitions decreases in the following order: o--~ o * > n --~ o * > 7t--~7t*> n ~

m*

The o---> cy* transitions may take place in the far ultraviolet, which is generally not recorded in spectrophotometers. Other transitions occur in the near ultraviolet and visible regions. The n ~ ~* transitions are characterized by high intensity which varies depending on the number and kind of multiple bonds in the molecule. An increase in the number of conjugated bonds results in a reduction of the distance between the ~ and x* levels, an increase in the probability of transition, and increase of intensity of the spectrum recorded. The considerable bandwidth and the high intensity are characteristic for absorption bands due to intermolecular charge transfer without ionization. They appear when an electron is transferred, under the effect of radiation energy, from a donor molecule to a free orbital of another molecule (acceptor). The charge transfer bands occur in the near ultraviolet and are broad in general. The visible and the near UV regions are characterized by absorption bands owing to intraatomic d - d transitions. This kind of transition is specific for ions of transition metals with an incomplete d shell. Splitting of the d sub-levels in the ligand field creates a possibility of transitions between the levels due to absorption of appropriate radiation quanta. The colour of solutions of transition metal ions is connected with the intra-atomic d - d transitions. The transition metals are capable of forming coloured complexes even with colourless reagents, which do not contain chromophoric groups. Changes in the energy of electrons within the ligands, charge-transfer bands, and bands owing to intra-atomic transitions determine finally the shape of the absorption spectrum of the given compound. The position and the intensity of electronic spectra may change when substituents are introduced into the molecule or when its environment is changed. The colour of a molecule is an effect of the presence of ehromophorie groups. A chromophore may be a group of atoms containing easily excitable ~ electrons (formula 2.1), including the most important groups for the visible region: the azo group - N = N - and the pquinonoid system.

,---N---~'O

~

.--C~N

/N~--~.S

(2.1)

The changes in the energy levels of these electrons owing to the absorption of radiation quanta give rise to characteristic bands in the absorption spectrum. The more easily the bond electrons in a molecule are excited, the more intense is the colour of the compound. The shape of the spectrum and the intensity of the absorption band depend on the position of the

28

2. Principles of spectrophotometry

chromophores in the molecule. In isolated systems, where multiple bonds are separated from each other by at least two single bonds, the spectra contain absorption bands characteristic for individual chromophores. Where double bonds are present in conjugated systems, intense absorption bands, shifted towards longer wavelengths, appear in most cases. The features of the absorption spectra change if the so-called auxoehromes (e.g., -NH2, -NR2, -SH, -OH, -OR) are introduced into the molecules. The presence of free electron pairs in the auxochromic group, that interact with Ic- electrons of the chromophoric group (e.g., the free electron pair at nitrogen in t h e - N H 2 group) leads to a state of conjugation which may result in formation of a new absorption band in the spectrum. An action of a substituent or a solvent may give rise to a shift of absorption band towards longer wavelengths - the bathochromic effect, or towards shorter wavelengths - the hypsochromic effect. An increase or a decrease of band intensity is referred to as the hyperchromic or the hypsochromic effect, respectively. The shape of a spectrum and its intensity depend on the positions of substituents in the molecule and on their inter-relationships, as in cis- and trans- isomers. In general, higher band intensities, and shifts to the higher wavelength part of the spectrum, are characteristics of the trans isomers, in which the coupling of ~- electrons is stronger for spatial reasons. The intensities and the positions of absorption bands may also be influenced by solvent molecules. Electrostatic dipole interactions, and specific interactions that lead to formation of complexes based on hydrogen bonds and of charge-transfer type complexes, result in changes in the spectra of chemical compounds. As the pH is increased, spectrophotometric reagents ionize and their electronic structure becomes deformed, which often leads to a bathochromic shift of the absorption maximum. Ionization causes polarization of the chromophoric system. The formation of a chelate complex disturbs the electronic state of the organic molecule to produce, as a rule, a bathochromic shift.

2.3.

Absorption laws

Spectrophotometric measurements are generally made on solutions, either in water or in organic solvents, contained in a measuring cell which is placed in the path of a beam of monochromatic radiation of chosen wavelength. From the total radiation of intensity Io that impinges upon a layer of solution, one fraction of the beam Ia is absorbed on passing through the solution, another fraction/t is transmitted, and still another fraction/r is reflected by the cell walls and scattered: l o = Ia + l, + lr

As absorption measurements are always made by comparison with a standard solution, and both the sample and the standard are placed in identical cells, the part of radiation denoted as/r is constant and may be neglected. The amount of radiation absorbed depends on the thickness of the absorbing layer and on the concentration of the solution [4,5]. In 1729 Bouguer established the relationship between the amount of absorption (the absorbance) and the thickness of the absorbing layer. A mathematical formulation of this relationship was given by Lambert in 1769. In 1852, Beer settled a relationship between the absorbance and the concentration of coloured solutions. In the formula derived (the Bouguer-Lambert-Beer law) both the solution concentration and the layer thickness are taken into account.

2.3. Absorption laws When a parallel beam of monochromatic radiation of intensity Io impinges upon solution of thickness dl, a part of the radiant energy is absorbed. If the layer changes, the absorption changes proportionally. The fraction absorbed exponentially with linear increase of the layer thickness: dI -kdl I where k is a constant, and the minus sign denotes that the intensity of the transmitted decreases as the thickness of the layer increases. Integration of the above equation gives the following expression:

29 a layer of thickness increases

=

radiation

In ~I~= kl I where Io denotes the initial beam intensity (for 1 = 0). By conversion of natural logarithms to decimal ones the above equation assumes the following form: log Io/'It - 0.434 In Idlt = 0.434 K1 - A where K is a new constant, and A is the absorbance. The absorbanee is a logarithm of the ratio of incident beam intensity, Io' to the intensity of the beam transmitted,/t. If the concentration, c, of the absorbing species is doubled and the absorbing layer thickness is reduced by a factor of two, then the total number of absorbing molecules remains the same, hence the absorbance A will also remain the same. Therefore the absorbance is a function of the number of absorbing centres in the light-beam, i.e., of the product cl, and the above equation can be given the form: A = log Io/It = ,F_cl where e is a new constant called the molar absorptivity (or absorption coefficient), c is the concentration of absorbing species (M, in moles per litre), and l is the layer thickness (in

cm). The equation is a mathematical expression of a fundamental law of spectrophotometry, the B o u g u e r - L a m b e r t - B e e r law, which states that absorption of radiation depends on the total number of absorbing centres, i.e., on the product of concentration and layer thickness of the solution. In spectrophotometric measurements the thickness of the sample layer is usually identical to that of the reference solution, and only Beer's law, which relates the absorbance with the concentration of the sample solution, is of practical significance. If a solution contains more than one absorbing species and there is no interaction between the components, the total absorbance of the solution is equal to the sum of all the component absorbances. The law of additivity of absorbance (providing the optical path length is constant) is expressed by the formula: A = (elCl + e 2 C 2 + . . . + enCn)l The additivity of absorbance constitutes the basis for studies of multicomponent systems. If a coloured solution obeys Beer's law, the graph of A = f(c) is a straight line passing through the origin. From a practical point of view it is desirable that the solution should follow Beer's law for the concentration range corresponding to absorbances not exceeding 1 (unity). Deviations from Beer's law may result from either chemical reasons connected with the sample, or physical ones connected with the instruments involved [6-8]. In the former case

30


the deviations are due to changes in the form of the determined component as a consequence of chemical reactions (e.g., hydrolysis, solvation, association, polymerization) associated with changes in the analyte concentration. Any change in the form of the substance being determined gives rise, as a rule, to changes in its optical properties. Thus, for example, dichromate ions (orange) are transformed, on dilution, into yellow chromate ions: Cr2072- + H20 ----~2CrO42- +2H +. In the case of weak complexes, a dilution of the solution leads to a dissociation and decomposition of the complex thus giving rise to deviations from Beer's law. Beer's law is not obeyed in systems where complexes are formed in a stepwise manner. The reaction constants and the quantitative ratios of individual complexes depend on the concentration ratio of the reactants and the pH of the reaction medium. The optical medium must be homogeneous. Turbid solutions give deviations from Beer's law. Such deviations occur in two-phase systems insufficiently homogenized by protective colloids. Deviations from Beer's law may also arise from insufficient quality of measuring instruments, mainly from the use of non-ideal monochromatic light, improper width of the spectral band, or scattering of radiation. The detector signal should be proportional, over a wide range, to the intensity of the radiation recorded. Despite the many possibilities of deviation from Beer's law, in the absorbance range of practical interest for analytical purposes, colour systems not conforming to Beer's law are fairly rare.

2.4. Spectrophotometric apparatus The quality of the measuring instrument has a strong influence on the reliability of the results obtained. The standard spectrophotometric apparatus used in modern analytical laboratories is very different from the equipment used in the initial period of application of this technique. The progress in the development of spectrophotometric apparatus up to the middle of the 1980s has been described [9-11]. The trends in the development of UV-VIS spectrophotometry, with special consideration to improved detection, and modern methods of data treatment, have been discussed [12]. The following discussion will be devoted to apparatus installed in most analytical laboratories. The set of components that enable us to record a radiation absorption spectrum consists of: radiation source, monochromator, cuvette, and detector with the data treatment system (Fig. 2.1). Radiation source

hydrogen, xenon, tungsten, or halogen lamp

~

Monochromator

Cuvette

prism or diffraction grating

sample

solution

I-t

Detector

~-~ systemMeasuring

photocell, photomultiplier, photoresistor, or photodiode

galvanometer or

microprocessor

Fig. 2.1. Block diagram of spectrophotometer The intensity of light emitted by the source, the effectiveness of its monochromatization, and the sensitivity of the detector are decisive for the quality of the spectrophotometer.

2.4. Spectrophotometric apparatus

31

2.4.1. Radiation sources and monochromators In most cases spectrophotometers are equipped with two independent radiation sources: UV and VIS. The UV source is usually a deuterium- or xenon lamp that emits radiation in the range of 180-400 nm or 190-750 nm, respectively. The development of the UV radiation sources has been reviewed [13]. The sources emitting visible light are tungsten- and halogen lamps. A feature of the halogen lamps is their wider spectral range, higher radiation intensity, and longer lifetime. In modem spectrophotometers the exchange between the UV and VIS proceeds automatically. The increasing use of lasers as high intensity sources of monochromatic radiation is observed [ 14]. The principal element of the spectrophotometer is the monochromator which serves for dispersion of the radiation emitted by the source and isolation of a beam of monochromatic radiation of definite wavelength. The monochromator comprises a system of slits, a collimator, a light-dispersing element, and lenses or mirrors to focus the dispersed radiation. The dispersing system is the essential part of the monochromator. The degree of monochromatization is an important feature of the dispersing element. Beams of monochromatic radiation or radiation of wavelength comprised within a specified narrow range are isolated by means of filters, prisms, or diffraction gratings. The beams of radiation of a limited range of wavelength are separated from the continuous spectra by means of properly selected colour filters. Modem spectrophotometers are equipped with diffraction gratings, whose dispersion is independent of the kind of material used and the wavelength of radiation applied. Gratings of 1,800 and 2,400 grooves/mm are used for the UV region, and those with 600 and 1,200 grooves/mm are applicable for the visible light. The separation of the grooves, denoted as the grating constant, is the parameter characteristic for the given grating. The high precision of forming the grooves and the regularity of their separations are characteristic for holographic diffraction gratings having up to 6,000 grooves/mm. The substitution of diffraction gratings for prisms enabled researchers to increase the spectral resolution and to extend the measuring range from 1 to 4 in the absorbance scale. To record a diffraction spectrum in a required wavelength range it is necessary to change the position of the grating to isolate the beam of a given wavelength. The manual method of changing the position, used in former instruments, has been replaced by mechanical systems. Quick and precise changes in position of the diffraction grating may be obtained by means of a laser beam.

2.4.2. Measuring cuvettes Measuring cuvettes, in which sample solutions are placed, are made of various materials depending on the range of radiation used in the measurement. Measurements in the UV are performed with the use of quartz cuvettes. Synthetic quartz, which is less contaminated with traces of metals, has better optical properties. Measurements in the VIS range are made using quartz, glass, or plastic cuvettes. The cuvette should provide maximum transmission of radiation and definite, precisely known thickness of the light-absorbing layer. Cuvettes of different thicknesses within the range 5 ~tm - 10 cm are produced. Small cuvettes capable of accepting samples of volumes down to 100 ~tl are also available. Small volume cuvettes that enable multiple passage of the beam of radiation are of special interest [ 15].

32


The cuvette material should be resistant to the action of chemicals. The cuvettes are placed in measuring chambers in special holders that provide accurate and reproducible location of the sample in the path of the radiation beam. Cuvettes of special design are used for measurements over wide ranges of temperature and pressure or under conditions of permanent flow. An automatic method for cleaning the cuvettes has been proposed [16].

2.4.3. Detectors After traversing the measuring cuvette the radiation impinges on the detector. The role of the detector is to convert the energy of the incoming electromagnetic radiation into electrical energy. The signal transformation should be linear, which means that the electrical signal generated should be proportional to the optical signal received. This condition is successfully fulfilled by photocells, photomultipliers, photoresistors and photodiodes. A comparison of various detectors used in UV and VIS spectrophotometry has been given [ 17]. The operation of photocells and photomultipliers is based on the external photoelectric effect. Photons impinging on the surface of a photosensitive cathode (photocathode) knock out electrons which are then accelerated in the electrical field between the cathode and the anode and give rise to electric current in the outer circuit. The spectral sensitivity of a photocell depends on the material of the photocathode. The photocathode usually consists of three layers: a conductive layer (made, e.g., of silver), a semiconductive layer (bimetallic or oxide layer) and a thin absorptive surface layer (a metal from the alkali metal group, usually Cs). A photocathode of the composition, Ag, Cs-Sb alloy, Cs (blue photocell), is photosensitive in the wavelength range above 650 nm; for longer wavelengths the red photocell with Ag, Cs-O-Cs, Cs is used. The response time of the photocell (the time constant) is of the order of 10-s s. Photomultipliers are equipped with several supplementary diodes (dynodes) to which the electrons emitted from the photocathode are directed. The electrons impinging on the dynodes give rise to the emission of secondary electrons from the successive dynodes and they thus amplify the signal generated by a factor of up to 108. In the photoresistors and photodiodes use is made of the internal photoelectric phenomenon and of specific properties of semiconducting materials. Photons impinging on the photosensitive element generate an electrical current, which flows through the photoconductor and is amplified by the effect of a small applied voltage. The increase of the current intensity is proportional to the intensity of photons that strike the photosensitive element. The microcrystalline layer of lead(H) sulphide deposited on a dielectric (glass or quartz) plate may serve as an example of a photoresistor applied in the wavelength range above 700 nm. Photodiodes are made of two or three layers of semiconducting materials containing suitable admixtures. Silicon photodiodes are used in the UV-VIS range Modern spectrophotometers are equipped with multichannel detecting devices that contain a large number of photodiodes (a photodiode array) and enable simultaneous detection over the whole range of the spectrum. Details of the design and the advantages of using such detectors in spectrophotometric measurements have been presented [17-20].

2.4.4. Data recording and processing The application of microprocessors and the rapid development of computer techniques has made it possible to automate the analytical operations from the step of sampling up to

2.5. Spectrophotometric techniques

33

full processing of the data obtained. In modern spectrophotometers, microprocessors are applied to control many operations that were formerly operated manually. The functions now realized by microprocessors include the control of the optical system (lamp and analytical wavelength selection), selection of the kind of data collected (e.g., absorbance, concentration), zero-adjustment, autocalibration and control of measurement parameters [21]. The microprocessor determines the equation of the regression curve and provides statistical processing of the results. It can also be programmed to measure the absorbance, the % transmittance at a selected wavelength, or the concentration based on the relationship (linear or non-linear) established between the measured absorbance and the concentration. The advanced spectrophotometers are coupled with computers that facilitate the recording of results and the processing of the data obtained. Appropriate software enables the presentation of results on the display, smoothing of the obtained spectrum, calculation of peak heights with respect to the base-line, and mathematical processing of the results that provides the possibility of, e.g., resolving signals owing to individual components of the sample analysed (see "Derivative Spectrophotometry" in Section 2.5). The development of the computer techniques has facilitated the identification of the structures of chemical compounds by enabling quick and easy access to catalogues of UV-VIS spectra. The data recorded and the results obtained can be stored in the computer memory. This gives the possibility of comparing the obtained results and evaluating their quality by rapid comparison with greater numbers of data. A critical evaluation of the obtained results always remains the task of the analyst.

2.5. Spectrophotometric techniques If the value of the molar absorptivity, e, for the wavelength used in measurement of absorbance of the given system is known, it is possible to determine directly the concentration of the analyte by means of an equation based on Beer's law. The value of e is determined from the measurement of absorbance of several solutions containing precisely known amounts of the analyte under conditions identical to those used in the measurement of the sample solution. In analytical practice, the concentration of the given analyte is, in most cases, determined by the standard curve technique. The technique is based on the determination of the relationship between the absorbance and the analyte concentration under the measuring conditions. The relationship is given in terms of the regression equation, or graphically in the form of a standard curve. For systems that obey Beer's law this curve is a straight line. The determination error is smaller if the absorption of radiation is a consequence of the nature of the analyte itself, as with the coloured ions of transition metals. Conversion of the analyte into a form capable of absorbing radiation in proportion to its concentration requires some additional procedures (such as the use of a chromogenic reagent, pH adjustment, or addition of masking agents), that must be identical in the treatment of standard solutions and of the sample solution. The absorbance measurements must be carried out after the equilibrium has been settled in the system. If the absorbance varies with time, the time of measurement should be strictly specified. In analytical practice, use is sometime made of standard curves in which the changes in absorbance are inversely proportional to changes in the analyte concentration. The analyte concentration is found from the reduction of absorbance of the system, which is proportional to the amount of the analyte. The accuracy and the precision of determination depend on the

34


precise knowledge of the initial reagent concentration and on the reproducibility of the reaction conditions for different concentrations of the analyte. Differential spectrophotometry [22,23] consists in the measurement of the absorbance of a solution of the given element, not with reference to the solvent used, but with reference to a solution of this element (in the form of a coloured complex) of known concentration, slightly lower than the concentration of the solution studied. In this technique, the measuring error is in proportion to the difference of concentrations (and not to the concentration of the analyte in the solution under test), which enables one to reduce the relative error. Grey filters of appropriate absorbance have been proposed as references [24]. The theoretical bases of differential spectrophotometry have been presented [22]. The relative error of absorbance measurement is 0.2-0.5%, and is less in differential spectrophotometry [25-28] than in the regular method. Hence, the precision of differential spectrophotometry is comparable with that of gravimetric and titrimetric methods. This fact enables the technique to be applied in the determination of higher contents of the analytes. In definite coloured systems the concentrations of reference solutions are selected with a view to obtaining maximum precision of the measurements. Particular attention is required in the preparation of standard solutions. In cases where temperature variations may influence the absorbance measurements, thermostating of the system is required.

Spectrophotometric titration [29-32] consists in repeated measurement of an absorbance which changes in the course of titration of the sample solution. The use of this method depends on the existence of a linear relationship between the absorbance measured and the concentration of the absorbing substance in the solution being titrated. The course of the titration is represented graphically by two intersecting straight lines. To find the titration end-point it is necessary to determine the absorbance at two points before and two points behind it. The graphs are drawn in the system of A (absorbance) v e r s u s v (volume of titrant solution). To increase the accuracy of determination, corrections are made for the dilution caused by the addition of the titrant solution. The sample solution is titrated, at a definite wavelength, in a titration vessel placed inside the spectrophotometer. For this reason the spectrophotometers commonly applied require some adaptations that enable one to place a suitable titration vessel, the tip of the burette, and a mixer inside it. In this technique, the parameter of primary importance is not the absolute value of the absorbance measured but its changes during the course of titration. To reduce the effect of dilution on the absorbance one is recommended to use concentrated titrant solutions and micrometric syringes. Spectrophotometric titrations are used in cases where it is difficult to determine the endpoint visually as, for example, when there is a permanent change in the colour of the system. Good results are obtained in titrations of rather dilute solutions, of the order of 10.5 M. Spectrophotometric titrations are often performed in automatic systems. Dual-wavelength spectrophotometry [33-36] is applied in systems where the difference in absorbance of two absorbing components at definite wavelengths may be used for determination of their concentrations in the given solution. The choice of the wavelengths depends on the system studied. Usually one wavelength corresponds to the maximum absorption of the analyte, and the other may correspond to an absorption maximum of the reagent or of an interfering species. The highest sensitivity is obtained when the absorption is measured at )Lmaxof the analyte. A necessary condition is that the

2.5. Spectrophotometrictechniques

35

individual components obey Beer's law. The errors associated with the use of the dualwavelength technique have been estimated [34-36].

Derivative spectrophotometry [37-44] is an analytical technique that uses the 1 st to 5 th (I-V) -order derivatives of absorption spectra in the VIS and UV ranges. The recorded curve of the derivative of the spectrum (the derivative spectrum) represents the values of absorbance differentials as a function of wavelength (wave-number) according to the following equation: d"A ~="Dxx d,t

= f (,t)

where n denotes the order of derivative and "Dx,,~ is the value of the n-th derivative of absorption spectrum of the substance X at the given wavelength, 2. The height of the signal of the respective derivative is proportional to the analyte concentration: d"e "Dx,,t - d2, "1. c and is additive in cases where the system contains more than one component absorbing in the radiation range studied. The shape of the derivative spectrum depends on the shape of the zero-order spectrum. The width of the half-height band (L- band-width in the middle of its height) is an important parameter characterizing the system in derivative spectrophotometry. The derivative spectrophotometry methods provide higher selectivity and higher sensitivity than do the methods based on normal (zero-order) absorption spectra. The increase in selectivity (with reduction or elimination of the effect of the spectrum of one substance on the spectrum of another one) results from reducing the band-width in the derivative spectra. An appropriate order of derivative spectrum may give complete separation of the spectra owing to the corresponding components of the system). The increase of selectivity in the derivative spectrophotometry methods results from the fact that the values of derivatives increase, in the case of basic spectra characterized by sharp peaks, and decrease in cases of broad-band zero-order spectra (Fig. 2.2). The sharp-peak spectra enable one to make determinations of analytes in the presence of considerable excess of elements having flat spectra. An example may be the direct determination of traces of manganese (as MnO4-) in nickel salts, based on the fourth-order derivative spectrum [45]. An increase of selectivity may also be obtained by proper selection of the instrument setting parameters in recording the derivative spectra. The derivative spectra are obtained in spectrophotometers fitted with microprocessors which enable digital processing of the spectra recorded [10,44-46]. Derivative spectra may also be obtained using spectrophotometers coupled with analogue differentiating systems. The instrument parameters affecting the shape of the obtained spectra are: scanning rate, integration time, distance between measurement points, and degree of amplification. The Savitzky-Golay algorithm [47,48] is that used most frequently in the treatment of basic spectra aiming at obtaining suitable derivatives. The analytical value of the derivative is determined mostly by the zero-crossing method (determination of the derivative at the zero point of the derivative for the interfering component), by the peak-to-peak method (determination of the amplitude of the derivative spectrum in a point corresponding to the maximum difference between the derivatives of the

36


analyte and the interfering component), and by the baseline-to-peak method (determination of the derivative of the analyte spectrum at its maximum). The sources of errors in determinations by derivative spectrophotometry have been discussed [48]. o)

b)

c)

d)

~e.eeejpme~ .....

e)

. ,

. . . . .

wavelength, nm

Fig. 2.2. Zero-order absorption spectra (a) and their derivatives: 1st order (b), 2nd order (c), 3rd order (d), and 4th order (e). Broken lines - substance 1 and substance 2; continuous line - mixture of substances 1 and 2.

Derivative spectrophotometry is applied more and more widely in the determination of inorganic and organic substances without preliminary separation. It is also used for the identification of organic substances. The dependence of the recorded signal upon the instrument parameters is a disadvantage of this technique. Reproducible results are obtained on using one M and the same M type of spectrophotometer and identical conditions of spectra recording, or by adaptation of a definite method to the available apparatus. Flow injection analysis (FIA) is an automated method which consists in the injection of the sample solution to a continuous stream of an inactive carrier (e.g., a pH buffer or water) [49-51 ]. The diluted analyte is transported to a reaction spiral where a chromogenic reagent is added to the mixture. The dimensions of the spiral, the volume of the sample injected, and

References

37

the flow rate are optimized to provide proper reaction conditions, sensitivity, and selectivity. The liquid zone formed in the spiral and carrying the analyte is transported, by means of a pump, to the detector. The signals from the detector, which are proportional to the analyte concentration in the sample injected into the carrier stream, are recorded continuously. Spectrophotometry is a technique most frequently applied in flow injection analysis, mainly owing to the easy coupling of the two methods, and good reproducibility of the measuring conditions. Practically all rapid reactions suitable for use in spectrophotometric determinations may be utilized under conditions of flow injection analysis. FIA coupled with spectrophotometry is a rapid (several dozens of determinations per hour) and sufficiently precise analytical technique [50-54]. The method uses small sample volume (10-100 ~tl), which is the basis of high sensitivity and is particularly useful in cases where small amounts of sample material are available (e.g., physiological fluids). The automation includes often also the sample injection. The spectrophotometry coupled with FIA has found numerous applications in determinations of chemical elements in environmental and clinical samples, especially in laboratories involved in rapid serial analyses.

Turbidimetry is a technique based on measurement of the absorbance by suspensions of sparingly soluble compounds. The media determined should be turbid systems, and not colloidal solutions stabilized by protective colloids. Turbidimetric determinations are often realized by visual methods or by comparison with standards in measuring cylinders, e.g., the determination of sulphate as BaSO4, or chloride as AgC1. The absorption of light by a suspension depends on the dispersion of the suspended solid which depends, in turn, on the concentration of the ions determined, the rate of adding the reagents, the temperature, and the ionic strength of the solution, and the presence of organic solvents miscible with water. Turbid solutions used in this technique do not generally obey Beer's law and the precision of determinations is rather low, since it is difficult to keep experimental conditions strictly reproducible. Higher sensitivity is attained in the case of coloured suspensions, such as metal sulphides.

References 1. Sommer L., Analytical Absorption Spectrophotometry in the Visible and Ultraviolet. The Principles, Elsevier, Amsterdam 1989. 2. K~cki Z., Podstawy spektroskopii molekularnej, PWN, Warszawa 1992. 3. Cygafiski A., Metody spektroskopowe w chemii analitycznej, WNT, Warszawa 1997. 4. Lothian G. F.,Analyst, 88, 678 (1963) 5. Buijs K., Maurice M. J., Anal. Chim. Acta, 47, 469 (1969). 6. Agterdenbos J., Vlogtman J., van Breekhoven L., Talanta, 21,225 (1974). 7. Agterdenbos J., Vlogtman J., Talanta, 21, 231 (1974). 8. Youmans H. L., Brown H.,Anal. Chem., 48, 1152 (1976). 9. Altemose I. R., J. Chem. Educ., 63, A 216 (1986). 10. Altemose I. R., De Long L. E., Locke L. E., J. Chem. Educ., 63, A262 (1986). 11. Nowicka-Jankowska T., Wieteska E., Gorczyfiska K., Michalik A., Spektrofotometria UV/VIS w analizie chemicznej, PWN, Warszawa 1988. 12. Lobifiski R., Marczenko Z., Crit. Rev. Anal. Chem., 23, 55 (1992). 13. Jones K. P., Trends Anal. Chem., 9, 195 (1990). 14. Imasaka T., Shibashi N.,Anal. Chem., 62, 363A (1990). 15. Dagsupta P. K.,Anal. Chem., 56, 1401 (1984).

38


16. Bautz D. E., Ingle J. D. Jr., Anal. Chem., 59, 2534 (1987) 17. Grossman W. E. J., J. Chem. Educ., 66, 697 (1989). 18. Borman S. A., Anal. Chem., 55, 836A (1983). 19. Jones D. Anal. Chem., 1057A, 1207A (1985). 20. Dose E. V., Guiochon G.,Anal. Chem. 61, 2571 (1989). 21. George W. O., Willis H. A., Computer Methods in UV, VIS and IR Spectroscopy, Royal Society of Chemistry, Cambridge, 1990. 22. Barkovski V. F., Ganopolski V. I., Spektrofotometryczna analiza r62nicowa, WNT, Warszawa 1971. 23. Grossmann O., Z. Anal. Chem., 321,442 (1985). 24. Marczenko Z., Ramsza A., Chem. Anal. (Warsaw), 21, 805 (1976). 25. Ingle J. D., Anal. Chem., 45, 861 (1973). 26. Blank A. B., Zh. Anal. Khim., 28, 1435 (1973). 27. Kotar' N. P., Samoilov V. P., Zh. Anal. Khim., 30, 465 (1975). 28. Grossmann O., Z. Anal. Chem., 320, 112, 223,229 (1985). 29. Ringbom A., Skrifvars B., Still E.,Anal. Chem., 39, 1217 (1967). 30. Galik A., Talanta, 13, 109 (1966); 15, 771 (1968); 17, 115 (1970). 31. Sato H.,Anal. Chim. Acta, 96, 215 (1978). 32. Johns P., Price W. J., Analyst, 95,138 (1970). 33. Shibata S., Furukawa M., Honkawa T., Anal. Chim. Acta, 78, 487 (1975). 34. Ratzlaff K. L., Natusch D. F., Anal. Chem., 49, 2170 (1977). 35. Ratzlaff K. L., Daraus H. B., Anal. Chem., 51, 256 (1979). 36. Ratzlaff K. L., Natusch D. F.,Anal. Chem., 51, 1209 (1997). 37. O'Haver T. C., Green G. L., 48, 312 (1976). 38. Ishii H., Z. Anal. Chem., 319, 23 (1984). 39. Perfil'ev V. A., Mishchenko V. T., Poluektov N. S., Zh. Anal. Khim., 49, 1349 (1985). 40. Levillain P., Fompeydie D. Analusis, 14, 1 (1986). 41. Dubrovkin I. M., Zh. Anal. Khim., 43, 968 (1988). 42. Talsky G., Derivative Spectrophotometry, CCH, Weinheim 1994. 43. Bosh Ojeda C., Sanchez Rojas F., Cano Pavon J. M., Talanta, 36, 549 (1980). 44. Ku~ M., Marczenko Z., Obarski N., Chem. Anal. (Warsaw), 41,899 (1996). 45. Ku~ M., Marczenko Z., Talanta, 36, 1139 (1996). 46. Majer J. R., Azzouz A. S., Talanta, 27, 549 (1980). 47. Savitzky A., Golay M. J., Anal. Chem., 36, 1627 (1964). 48. Steinier J., Termonia Y., Dektour J.,Anal. Chem.. 44, 1906 (1972). 49. Ru~i6ka J., Hansen E.,Anal. Chim. Acta, 179, 1 (1986). 50. Trojanowicz M., Automatyzacja w analizie chemicznej, WNT, Warszawa 1992. 51. Karlberg B., Pacey G. E., Wstrzykowa analiza przeptywowa dla praktyk6w, WNT, Warszawa 1994. 52. Ru~i6ka J., Hansen E.,Anal. Chim. Acta, 78, 145 (1975); 99, 37 (1978). 53. Ranger C. B., Anal. Chem., 53, 21A (1981). 54. Stewart K. K., Talanta, 28, 789 (1981).

Chapter 3. Spectrophotometric methods 3.1. Introduction The spectrophotometric methods to be discussed (methods of molecular absorption spectrometry) are based on the measurement of absorption of radiation, in the visible and near ultraviolet regions, owing to coloured compounds formed, before the determination, by the elements to be determined. Only seldom is use made of the intrinsic colour of the element itself, in its ionic form. In cases where an element neither forms coloured compounds nor occurs in a coloured form, indirect spectrophotometric methods are applied. Spectrophotometric methods are characterized by high versatility, sensitivity, and precision. They may be used for the determination of almost all chemical elements over a wide range of concentrations, from macroquantities (by means of differential spectrophotometry) to traces ranging from 10-6-10 -8 % (after suitable preconcentration). Spectrophotometric methods are among the most precise instrumental methods of chemical analysis. The advantages mentioned of the spectrophotometric methods are made greater by their availability. A spectrophotometer, which is the basic instrument in this field, is cheaper than most other fundamental instruments used in chemical analysis. Spectrophotometric methods are extensively discussed in the literature [ 1-4]. Spectrophotometric methods were preceded by colorimetric methods. The colorimetric determinations were first performed in cylinders, then visual colorimeters with filters came into use. The first photoelectric colorimeters were introduced into laboratory practice in the 19308, and they were next replaced by spectrophotometers. Spectrophotometers do not measure or compare the colour, but measure the absorbance of solutions. The application of organic reagents, the development of the knowledge of complexes, then the use of spectrophotometers equipped with microprocessors that enable rapid processing of absorption spectra [5-9] have led to a very rapid development of spectrophotometric methods [ 10-24]. A paper has been published on the Recommendations of the Analytical Chemistry Section of the International Union of Pure and Applied Chemistry (IUPAC) concerning the nomenclature, symbols and units applied in molecular absorption spectrometry [25].

3.2. Sensitivity The sensitivity of an analytical method means a minimum concentration, minimum amount, or a minimum difference in concentrations, of an element that can be determined by this method. Later on, use will be made of the first meaning of the term. The numerical value of the sensitivity of spectrophotometric methods is usually determined in terms of the molar absorption coefficient (molar absorbance, ~, a coefficient) measured at the wavelength at which the absorbance is being measured: A ~=

cl

40

3. Spectrophotometric methods

where A is the absorbance, c is the concentration (in mole per litre) and l is the layer thickness (in cm). The molar absorption coefficient (e) is expressed in 1.mole-l.cm-1. Conforming to the principles of metrology, the value of the coefficient should be given with consideration to the meaningful number of digits or the precision of measurements. Therefore it is rational to give, e . g . , ~ - 4.2.104 or 4.20.104, not ~ = 42,000. In the former cases the number of significant digits is two or three, whereas in the latter case it is five. In sensitive spectrophotometric methods the values of the molar absorption coefficient are usually greater then 2.104 , whereas values below 1.103 correspond to methods of low sensitivity. As a result of the underlying quantum theory, the value of e cannot be greater than 1.5-105. Higher values are possible in some indirect methods, e . g . , in spectrophotometric amplification methods. The knowledge of e enables one to compare the sensitivities of the methods of determination of one and the same element or of elements having similar atomic masses. The sensitivity of spectrophotometric methods can be expressed conveniently and compared (especially in the case of elements differing considerably in atomic mass) in terms of the specific absorbanee (a) [26] which is obtained by dividing ~ by the molar mass of the element and by 1,000: a--

c mol.m. . 1000

The value of a corresponds to the absorbance of a solution of a given element at concentration 1 lag/ml (= 1 ppm)in a cuvette of layer thickness 1 cm. The sensitivity of the dithizone method for determination of copper (Xmax= 550, molar mass of Cu = 63.54 g/mole) may be expressed as follows: molar absorptivity (coefficient e) specific absorptivity, a

4.52-104 1 .mole-l.cm-1 0.71 ml.~g-l.cm -1.

The value of e is most readily determined under conditions where the spectrophotometric reagent has zero absorbance at ~max of the complex, and only one stable coloured complex is formed in the system. In cases where the complex is formed stepwise [as in the system Fe(III) - SCN-] the value of e depends on the excess of the reagent. It is more difficult to determine the value of e in cases where the reagent itself absorbs the radiation at ~maxof the complex. If only one coloured complex is formed in the system, the absorbance can be measured with respect to a reference solution in which the reagent's concentration is identical with the concentration of uncombined reagent in the measured solution. If, however, more than one coloured complex is formed in presence of a coloured reagent, absorbing partly in the region of ~max of the complex, it is difficult to find a concentration range in which Beer's law is obeyed. The values of e then have only a relatively low significance, as they depend on the contributions of individual complexes in the mixture, and hence on conditions such as the excess of reagent or the pH value. In some indirect methods, the element being determined causes bleaching or changes the colour of the system. In such cases, the value of e is calculated from the change in absorption caused by a definite amount of the analyte. The determination of the coefficient e is simple in the extractive spectrophotometric methods. In many cases the extraction is associated with transformation of dichromatic solution into a monochromatic one. Several complexes may coexist in an aqueous solution,

3.2. Sensitivity

41

and the extract contains usually one complex of definite composition. Extraction usually leads to an increase in the sensitivity of the method. In cases where the element to be determined is not completely extracted into the organic phase, because its partition coefficient is too low, and the percent of extraction varies depending on the phase-volume ratio, the value of e should be specified along with the extraction conditions (phase-volume ratio, multiplicity of extractions). Despite the fact that a fraction of the analyte remains in the aqueous phase, the whole quantity of the element present in the initial solution is taken into account. Some ambiguities occurring in the calculation of the coefficient e may arise from the definition of the mole. For example, 127 g of iodine is 1 mole of I or 0.5 mole of I2; hence it is necessary to give always the chemical form of the substance for which the value of e has been determined. In some methods, the sensitivity depends on the quality of reagent used to produce the colour reaction. This is of particular importance in the case of reagents of natural origin, although some differences have been noticed also in synthetic organic reagents. The differences in sensitivity are a result of the presence of some admixtures that have a bearing on the reactions of the elements being determined, or that give rise to competitive reactions. The minimum concentration of an element (in moles per litre) that can be determined by spectrophotometric methods may be calculated from the expression: A = ecl. If one assumes that the minimum measurable absorbance of a solution is A = 0.02, with a cuvette width 1 - 2 cm, and for a moderately sensitive spectrophotometric method, e = 1.104, the respective concentration will be: c - 0.02/2.104 = 10-6 M. If we assume a mean molecular mass of the element as equal to 100 (in this case the specific absorptivity a -- 104/102.103 = 0.1) the minimum measurable concentration of the element will be: 106.102/103 = 10 -7 g/ml - 0.1 ~tg/ml In order to determine the absorbance of a solution in a cuvette of thickness 2 cm one should have 6-7 ml of coloured solution and a volumetric flask of capacity 10 ml, which corresponds to 1 gg of the element. If one assumes that saturated solutions of soluble salts have concentrations about 10 percent, then 10 ml of such a solution corresponds to 1 g of the sample. So, if 1 gg of the analyte is contained in 1 g of the sample, its concentration in the sample is 10 -4 %. In more sensitive methods, where e is of the order of 105 and the specific absorptivity is about 1.0, the limit shifts to about 10-5 %. Trace concentrations below 10 -4 % are below the sensitivity range of many methods. To have a possibility of determining them by spectrophotometric methods a preliminary concentration of trace components usually becomes necessary (see Chapter 1 on methods of preconcentration and separation of elements). Depending on the kind of sample and its weight (e.g., 10 g, 100 g, or more) such an operation can increase the sensitivity (shift the limit of determination) by 1-2 or more orders of magnitude. In such a way, the sensitivity of spectrophotometric methods can be increased to 10-6-10 -7 %. An important role in trace analysis is played by the blank test, particularly in the determination of the more common elements such as Fe, Zn, Ca, A1, Si. It happens sometimes that the content of an element in the blank exceeds its content in the sample. In such cases the blank test determines the limiting concentration of the element in the given

42

3. Spectrophotometricmethods

material. Thus, it is possible to increase the sensitivity by reducing the value of the blank test (e.g., by purification of reagents, or the use of quartz or polyethylene vessels instead of glass).

3.3. Precision and accuracy In analytical chemistry the term "precision" denotes the reproducibility of results, their scatter and consistency. The term, "accuracy" denotes the degree of proximity of the obtained results to the real (true) value. The precision of spectrophotometric methods [27-29] depends on the range of the values determined. Under optimum conditions, it ranges usually from 0.5-2%. In the differential technique, precisions ranging from 0.2 to 0.5% are attained. A matter of considerable importance for the precision of a method is the measuring error. When very low concentrations are determined the relative error is large since the (absolute) measuring error is high as compared with the value measured. When the absorbance of a strongly coloured solution is being measured, only a small part of the incident radiation is transmitted through the solution. The divisions on the logarithmic absorbance scale are so small that considerable reading errors are made. For example, absorbance 2 corresponds to 1% transmission and absorbance 3 to 0.1% transmission. Hence 0.9% of the whole scale corresponds to a 50% change in the concentration. The theoretical value of absorbance measured with maximum precision may be found in the following way: c = a log x

dc-

dc c

m

(a log e)dx

(a log e)dx axlogx

0.434dx xlogx

where c is a concentration, x is a ratio of lo/It, and a is a constant. Differentiation of the above equation, and putting the second derivative equal to zero shows that the error is minimum when log x (=A) is 0.43. In modern absorptiometric instruments, with digital reading of absorbance, the precision depends on the noise of photomultiplier used as a detector. The signal is subjected to electronic processing. The respective curve of precision error has a broad minimum at A equal to about 0.9. Such spectrophotometers can record absorbance with good precision up to values of about 2. The errors involved in the absorbance measurements are usually smaller than those associated with chemical operations. In some methods the colour reaction is not reproducible. In other methods, the colour varies with time, and the absorbance should be measured after a strictly determined lapse of time. In some systems even small variations of temperature (e.g., 3-5 ~ result in changes in colour. Some reactions are sensitive to changes of pH. A small change in pH, e.g., through 0.1 unit, may cause a 5% error. Other possible errors are caused by competitive reactions occurring in the system, or by changes in the ionic strength. The total error in an analysis of a definite material is the sum of errors made in individual stages of the analytical procedure, e.g., in sampling, sample dissolution,

3.4. Selectivity

43

preconcentration and separation of the elements, and measurement of absorbance. In the course of these operations some elements may get to the sample from the environment or may be partly lost. A matter of great importance is the determination of a blank (in trace analysis) and taking it into account in calculation of the final result. The effect of the blank test on the precision of analysis increases as smaller and smaller trace amounts of elements are determined. In the determination of concentrations ranging from 10-3-10-4% the error is usually within +10%, whereas in determinations of trace amounts in the range 10-6-10-7%, with preconcentration, the error may be as high as +30%. The size of the total error may be evaluated by comparing the obtained results of analysis with values considered to be true. Where standard samples of known composition are not available, the accuracy of determination may be evaluated if known amounts of the elements to be determined are added to the weighed sample at the beginning of the analytical procedure. The amounts added should be similar to the amounts present originally in the sample. The chemical form of the additions should be selected so that in the chemical operations they behave similarly to the original sample components. A model procedure recommended for the determination of the precision (and of the sensitivity and detection limit) of spectrophotometric methods, based on experimental data and IUPAC recommendations has been developed [30], and the analysis of errors occurring in determinations of mixture components by means of modem computerized techniques, has been published [31,32].

3.4. Selectivity Conforming to a decision of IUPAC, a selective reagent is one that reacts with only a small number of elements, and a specific reagent is one that under definite conditions reacts with only one element. The selectivity of colour reactions and of the corresponding determination methods depends on the nature of the reagent used, the degree of oxidation of the elements determined, the pH of analytical medium, and of complexing agents that mask the interfering ions. If, despite the above agents, some ions still interfere in the determination, the analyte should be isolated from the interfering elements (or conversely). Separation and isolation methods are discussed in Chapter 1. There are very few specific reagents and reactions. Among the scarce examples are cuproine for Cu(I) and bathophenthroline for Fe(II). A change of valence of some ions effectively prevents their reaction with certain reagents. As an example, Nb can be determined by the thiocyanate method in presence of Fe if this had been reduced to Fe(II). The selectivity of most methods can be increased by proper selection of the pH of the analytical medium. For reagents of the R-OH type there exists a relationship between colour reactions and hydrolytic reactions of certain elements. In strongly acid solutions, colour reactions proceed with those elements that have easily hydrolyzable cations, such as Zr, Hf, Th, U(IV) and Ti. In moderately acidic solutions the reactions also proceed with Fe(III), A1, and U(IV); in weakly acid and neutral solutions, with rare-earth elements, Fe(II), Cu, Mn, and in alkaline solutions, with Ca, Sr, and Mg. Easily hydrolyzable species react in more acidic solutions, whereas other ions react only in less acidic ones. Reagents of the R-SH type react in acid solutions with cations of the second analytical group that form the most stable sulphides. In weakly acidic and neutral solutions they react with metal cations of the third analytical group.

44

3. Spectrophotometric methods

The acidity of the medium has an important influence on the form of certain ions in the solution (e.g., Zr, Ti, V) and this form is decisive for the kind of reaction (in the case of Zr: Zr 4+, ZrO 2+, polymerized forms). In the reactions of chemical species with some reagents, the optimum pH range may be narrow, either because of the maximum sensitivity of the colour reaction or the effect of other ions. The pH of coloured solutions is often stabilized by means of suitable buffers. Tartrate, citrate, phosphate and phthalate solutions are usually avoided since they can bind many metals in stable complexes. Acetate solutions are often used, although they also can form complexes with some metals (e.g., A1, Be). Acetate buffers may be replaced by hexamethylenetetramine buffers. An increase of selectivity is usually achieved by masking the interfering ions [33-36]. The masking consists in conversion of the interfering ion into a stable complex formed with the complexing agent added. Owing to this, the ion cannot participate in the colour reaction with the spectrophotometric reagent. Some most important masking reagents are presented in Table 3.1. High selectivity of spectrophotometric methods is achieved by choosing proper masking reagents and appropriate pH values. The stability of the complexes is not constant, but it varies depending on pH and other parameters, such as the concentration of masking agent and the presence of other complexing compounds. Table 3.1. The most common masking agents

Ba

Masking agent CN-, 82032, I, CI, NH3 F, acetate, tartrate, EDTA, OHS2-, OH CN-, CI, Br-, $2032 F, hydroxy acids EDTA, citrate, SO42

Be

F-, citrate, tartrate

Bi

citrate, EDTA, I EDTA, citrate, tartrate EDTA, CN, I-, tartrate F-, EDTA, citrate NH3, SCN, EDTA EDTA, tartrate, acetate NH3, ON, S2032, EDTA, citrate H3BO3,AI, Be, Ti F, PO43, EDTA, tartrate CN, $2032, phenantr. EDTA, tartrate, citrate oxalate, FI, CN, ClEDTA, CI-, citrate CI, SCN, NH3 EDTA, oxalate, tartrate

Element Ag AI As Au B

Ca Cd Ce Co Cr(lll) Cu F Fe(lll) Fe(ll) Ga Ge Hg In Ir Mg

,,

Element Mn Mo Nb Ni Os Pb Pd

Pt Rh Ru Sb Sc Sn Sr Ta Th Ti TI U V W Zn Zr

Masking agent oxalate, EDTA, citrate F, H202, citrate, EDTA F-, tartrate, H202 CN, EDTA, NH3 CN, SCN, Clacetate, I-, citrate, tartrate, EDTA, SO42CN-, I-, citrate, tartrate, EDTA, SO42I-, ON-, NO2-, NH3 CI-, citrate, tartrate CN, Cl tartrate, I-, S2, OH-, F Citrate, EDTA tart., OH-, S2, FSO42-, EDTA, citrate F-, citrate, tartrate F-, EDTA, citrate, acetate H202, F-, citrate CI, EDTA, citrate, tartrate F, CO32, H202, citrate H202, EDTA, FF, H202, tartrate, citrate CN, EDTA, OH-, NH3 F, oxalate, citrate, H202, PO43

Although the number of known specific reagents is rather small, there exist some methods based on the use of group reagents (that react with many elements), which in the presence of suitable masking agents and at proper pH have a specific action. Examples of such systems may be the determination of Zn with dithizone at pH 4-5 in the presence of

3.5. Colour systems

45

thiosulphate, or the determination of A1 with 8-hydroxyquinoline at pH 9 in the presence of EDTA, cyanide and H202 as masking agents. The selectivity of spectrophotometric methods has been greatly increased by the development of derivative spectrophotometry (see Chapter 1.5). Derivative spectrophotometry enables one to single out, by means of various mathematical algorithms of data processing, a separate signal due to a selected component, from the sum of absorbances of the analysed mixture. This technique was successfully applied in determinations of a number of elements in mixtures such as Pd, Pt and Au [37], Pd and Pt in iodide solutions [38], Au, Pd and Pt in bromide solutions [39], Ru(III) and Rh(III) in the form of octadecyldithiocarbamate complexes [40], Ru and Os in chloride solutions [41 ], Cu, Hg and Pb as dithizonates [42], complexes of various metals with 4-(2-pyridylazo)resorcinol [43], Fe(III) with EDTA in the presence of Cr(III), A1 and Mn [44], Cr(III) and Cu(II) with EDTA [45], and Cu and Co in a flow system [46]. Derivative spectrophotometry was also used in the study of Sr- complexing reactions with various crown ethers [47].

3.5. Colour systems Most of the colour systems, used as a basis of spectrophotometric methods are formed as a result of complexing reactions. The most important spectrophotometric reagents are presented in Chapter 4. A large group of the methods is based on difunctional organic reagents that with metal ions form inner chelates, soluble in non-polar solvents. Such reagents are generally used in the extraction-spectrophotometric methods involving, e.g., dithizone, dithiocarbamates, 8-hydroxyquinoline, 1-nitroso-2-naphthol, dioximes and PAN [1-(2pyridylazo)-2-naphthol]. Another group of methods is based on organic reagents giving electrically charged, water-soluble complexes. Such reagents comprise hydrophilic, mostly sulphonic groups. Such types of complexes are formed by arsenazo III, PAR [4-(2-pyridylazo)resorcinol], 5Br-PADAP [2-(5-Br-2-pyridylazo)-5-diethylaminophenol], Eriochrome Black T, Chrome Azurol S, and Bromopyrogallol Red. Cationic complexes of metals are formed in reactions with 1,10-phenanthroline or cuproin. A large group of sensitive methods is based on ternary systems[ 10,48]. Reactions of easily hydrolyzable metal ions [e.g., Be, A1, In, Sc, Ti, Th, Fe(III), Zr] with some chelating chromogenic reagents (e.g., CAS, ECR, phenylfluoron) give strongly coloured ternary complexes (with a large bathochromic shift of the order of 100-200 nm) in the presence of surfaetants [49-61]. Among the most often used cationic surfactants are bromide or chloride salts of cetyltrimethylammonium (CTA, formula 3.1) cetylpyridinium (CP, formula 3.2) and tetradecyldimethylbenzylammonium (Zephiramine, formula 3.3). CH3 C16H33--N--CH3

I

CH3

C.H3 (3.1)

C,6H33--

(3.2)

Clt.H29m

--CPI 2 CH3

An example of a non-ionic surfactant is Triton X-100 (octylphenyl polyethyleneglycol ether). Basic dyes (triphenylmethane, xanthene, azine, etc., dyes) form ion-association compounds (ion-pairs) with anionic halide complexes of metals and non-metals (e.g., SbC16-, AuBr4-, TaF6-, BF4-). The resulting compounds, that may be extracted into non-polar organic solvents, may serve as a basis for sensitive spectrophotometric methods [62-65].

46


Extractable ionic associates are also formed in reactions of heteropoly acids with basic dyes [62,66,67]. In some systems, more complex ionic associates are formed. These compounds are not extractable, but may form a basis for very sensitive flotationspectrophotometric methods [68,69]. Another group of methods is based on ionic associates formed by acid dyes (e.g., eosine, Bengal Rose, Methyl Orange) with hydrophobic cationic complexes of some metals [e.g., Fe(II), Ag(I), Zr, Cd] with 1,10-phenanthroline and other organic bases [10,62]. With metal ions, some organic reagents form coloured compounds that are sparingly soluble both in water and in organic solvents. They are either polynuclear complexes, such as formed by phenylfluoron with Sn(IV) or with Ge(IV), or adsorption-type compounds such as those formed by titanium yellow with Mg. In such cases, the absorbance is measured for suspensions of coloured pseudo-solutions stabilized with protective colloids [e.g., gum arabic, gelatine, poly(vinyl alcohol)]. Complexes of metal ions with inorganic reagents are also applied in spectrophotometric methods, e.g., thiocyanate [Fe(III), Co, Nb, Mo, Re, W, U, Ti], iodide (Bi, Sb, Pd), or hydrogen peroxide (Ti, V, U). Determinations based on such complexes are carried out in aqueous solutions or after extraction with oxygen-containing organic solvents. A group of elements (P, As, Si, Ge, V, W, Mo etc.) forms yellow heteropoly acids which, on being reduced, give intensely coloured blue compounds. Both the heteropoly acids and their reduced forms are used for determination of the elements. All these forms are extracted with oxygen-containing solvents. It should also be mentioned that the heteropoly acid anions form extractable ionic associates with basic dyes. Colour systems suitable for use in the spectrophotometric method may also be formed in redox reactions. Some examples of such reactions are: the oxidation of Mn(II) to MnO4or Cr(III) to CrO42-, oxidation of dimethylnaphthidine with vanadium(V) or chromium(VI), oxidation of o-tolidine with cerium(IV) or with chlorine. Examples of oxidation reactions are also the iodide methods, in which iodide ions are oxidized with bromine to give iodate ions which, in turn, react with the excess of iodide anions to form free iodine (see Chapter 25). A colour effect of reduction also occurs, for example, in determinations of Se and Te in the form of coloured sols produced in the reduction of Se(IV) or Te(IV) to their elementary forms. In some methods the colour system is a product of a synthesis reaction. Examples of such methods are: formation of an indophenol dye in a method for determination of ammonia, the synthesis of azo dyes in the determination of nitrite, the formation of Methylene Blue in the determination of sulphur, the pararosaniline method for determination of sulphite, and the benzidine-pyridine method for determination of cyanide. There are also indirect speetrophotometrie methods, in which the element determined provokes a change in colour. This group comprises most of the methods for the determination of fluoride. Being capable of forming stable complexes with some metals, fluoride anions can decompose colour complexes of those metals. Thus, in the method involving a sulphosalicylate complex of Fe(III) the solution is discoloured by F- and a change of colour is observed in the method based on the use of the Zr complex with ECR. Still another example is the determination of phosphate with the use of lanthanum chloranilate. Phosphate anions react to form the less soluble LaPO4 and release coloured chloranilate ions. Amplification methods [70-73], owing to their high sensitivity, are used in the determination of trace amounts of certain elements, including halides and some metals (e.g.,

Mo, Cr, Bi).

3.5. Colour systems

47

The development of a new spectrophotometric method is usually preceded by studies of the colour system (complex composition and stability), which is the basis of the method. Fundamental physicochemical studies of the colour complexes existing in the solution enable one to establish the optimum parameters of the method [74-76]. The composition of the complex (molar ratio of metal to ligand) in the solution is determined by Job's continuous variation method, the Bent and French method of equilibrium shift, the method of mole ratio proposed by Yoe and Jones, and the method based on the ratio of slopes and isosbestic points. Studies of complex compounds in solutions are described in many works, especially [77-80]. The role of mixed media (water and water-miscible organic solvents) in colour reactions which are useful in spectrophotometric methods has also been discussed [81 ]. Problems occurring in the development and publishing of new methods have also been considered [82.

3.6. Analytical procedure Spectrophotometric methods based on the use of calibration curves are comparative methods. Their accuracy depends to a considerable extent on the proper preparation of the standard solutions of the elements to be determined, which are used for the preparation of calibration graphs. Standard solutions are divided into stock and working solutions. Sufficiently concentrated stock solutions may usually be stored for a long time. More dilute working solutions, obtained by suitable dilution of stock solutions cannot, in general, be stored for a long time. Stock solutions, usually containing 1 mg of element (or ion) per ml less often, 10 mg/ml - - are obtained by dissolving sufficiently pure salts of definite composition. Water is used as a solvent, or dilute acids if the metal ions would hydrolyse or precipitate as carbonates with atmospheric carbon dioxide. The concentrations of the working solutions are related to the sensitivity of the spectrophotometric method: 0.1 rag, 10 ~tg, or 1 ~tg of element per ml. Solutions of concentration 10 ~tg/ml, and especially 1 ~tg/ml, are prepared freshly on the day of use. The instability with time, observed in more dilute working solutions, is mainly due to sorption of ions on the surface of the vessel [83,84]. When preparing standard working solutions it is best to conform with the rule of tenfold dilution and using adequately accurate pipettes and standard flasks (e.g., a 10 ml pipette and a 100 ml flask. From a solution of concentration 1 mg/ml one obtains a solution of 0.1 mg/ml and, in turn, 10 ~tg/ml. If there is no available salt of constant composition, such as copper(II) sulphate or iron(III), aluminium or chromium alums, it is advisable to prepare first a stock solution of an approximate concentration, slightly higher than required and to determine the concentration by a gravimetric or volumetric method. After suitable calculations the solution is diluted with pure solvent to obtain a solution containing exactly, e.g., 1 mg/ml of the given element. In some cases, standard solutions are obtained by dissolving a precisely weighed amount of the element in its pure form. Sometimes it is necessary for the standard solution to contain no complexing anions, and therefore a perchlorate or a nitrate is used. These can be obtained by starting with any other salt available (e.g., chloride, sulphate). The salt is dissolved in water or dilute acid, the metal is precipitated as hydroxide by means of ammonia or NaOH solution, and the

48


precipitate is filtered off, dissolved in perchloric or nitric acid, and diluted with water to a definite volume. In order to select the most suitable wavelength for the spectrophotometric measurements it is necessary to know the absorption spectrum of the complex, as well as that of the reagent if it is coloured. The absorption spectrum (absorption curve) is a plot of absorbance v s . wavelength. It is obtained with a recording spectrophotometer or plotted manually from measurements made on a non-recording instrument at 10-20 nm intervals, and at 2-5 nm intervals near the absorption maximum. The solvent is used as reference. The absorption spectra are drawn in the system: wavelength (~, in nm) as abscissa, and absorbance (A) or coefficient, e, as ordinate. When the analytical wavelength has been selected, on the basis of absorption spectra of the complex and the reagent, it is normal to prepare the standard curve. The absorbance measurements are usually made at ~max of the coloured compound. It can happen, when the complex and the reagent absorb in the same wavelength range, that the absorbance is measured not at the ~ma• but at a wavelength corresponding to a maximum difference between the absorbances of the complex and the reagent. The s t a n d a r d curve (calibration graph, analytical curve, calibration curve), relating the absorbance to the analyte concentration, is prepared by applying the colour reaction procedure to a set of standard solutions, regularly spaced over a concentration range that will give a maximum absorbance of 0.8-1.0. The conditions specified in the analytic procedure must be strictly adhered to. The curve is plotted with the scale arranged so that the curve is at about 45 o to the abscissa, as shown in Fig. 3.1.

0,8 0,6 O,t, 0,2

0

0,5

1,0

1,5 2.0 concentration

Fig. 3.1. Standard curve

The absorbances of standards and samples are measured in cuvettes of the same pathlengths, usually 1 or 2 cm, although 0.1-, 0.5-, and 4 cm cells are sometimes used. The choice of cuvette depends on the sensitivity of the method, the quantity of the analyte and the behaviour of the solution in relation to Beer's law. If, at low values of absorbance, deviations from Beer's law are observed, then the use of a longer pathway cuvette is recommended so that higher absorbance values are obtained for more dilute solutions. The reference (comparison) solution in one-colour spectrophotometric measurements is usually the solvent (if the reagent is colourless), and in two-colour methods a reagent solution is used. As well as the spectrophotometric reagent the reference should contain other reagents and have pH identical to the analyte solution. It happens that at ~max the absorbance of the colour reagent is zero and pure solvent may be applied as the reference.

49

References

3.7. Trace analysis Sensitive spectrophotometric methods are used in trace analysis either directly or after a preconcentration of the trace elements to be determined [ 10,85-88]. The value of the blank test plays an essential role in spectrophotometric analysis, especially when common elements are determined. The blank can serve as a reference in measurements of the absorbance. It also eliminates the effect of analyte admixtures introduced with the reagents, vessels or environment [89-94]. Blank determinations are made in parallel with measurements of analyte samples, and the results are obtained as the difference. Reducing the size of the blank permits the determination of smaller quantities of traces in the samples, especially for the more common elements such as Fe, Zn, Ca, Mg, A1, Si. The size of the blank can be reduced in various ways. The equipment used for sampling and for commination of the sample should be made, if possible, from material which does not contain the element to be determined. In some cases the admixtures can be removed by the use of an appropriate solvent, which does not react with the sample but dissolves the impurities (e.g., the use of hydrochloric acid to remove iron from a sample of silicon comminuted in a steel mortar). Quartz, polyethylene or Teflon vessels are used instead of glassware. To prevent contact of the sample with laboratory air, certain chemical operations may be conducted in closed chambers (dry boxes) flushed with purified air or inert gas [95, 96]. The reagents are the source of the major contaminants in the blank test. The water used should be distilled in a quartz apparatus after demineralisation with ion-exchangers [97]. Acids (H2SO4, HNO3, HC104, HC1, HBr) can be purified by slow distillation in quartz vessels. Hydrofluoric acid is purified by distillation in platinum or palladium vessels. Ammonia of high purity is obtained by saturation of distilled water with gaseous NH3. Reagents thus purified are kept in polyethylene bottles. Solutions of other reagents are usually purified by solvent extraction and coprecipitation with collectors. Some volatile reagents (hydrochloric acid, hydrobromic acid, ammonia) can be brought to a very high degree of purity by isothermal (isopiestic) distillation [98,99]. Iron(III) traces can be removed from concentrated hydrochloric and hydrobromic acids by passing the acids through a strongly basic anion-exchanger. For control of the accuracy in determination of traces of elements, standard reference materials are of great importance [100-106].

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50


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49. Tikhonov V. N., Zh. Anal. Khim., 32, 1435 (1977). 50. Chernova R.K., Zh. Anal. Khim., 32, 1477 (1977). 51. Chernova R.K. et al., Zh. Anal. Khim., 33, 858 (1978). 52. Savvin S.B., Chernova R.K. et al., Zh. Anal. Khim., 33, 1473 (1978). 53. Nemodruk A.A. et al., Zh. Anal. Khim., 34, 1266 (1979). 54. Tikhonov V.N., Aleksandrova N.L., Zh. Anal. Khim., 36, 242 (1981). 55. Tikhonov V.N., Pavlova O.K., Zh. Anal. Khim., 37, 1809 (1982). 56. Tikhonov V.N. et al., Zh. Anal. Khim., 38, 216 (1983). 57. Chernova R.K. et al., Zh. Anal. Khim., 39, 1019 (1984). 58. Tananayko M.M. et al., Zh. Anal. Khim., 39, 1034 (1984). 59. Callahan J.H., Cook K.D.,Anal. Chem., 56, 1632 (1984). 60. Antonovich V.P., Novoselova M.M., Nazarenko V.A., Zh. Anal. Khim., 39, 1157 (1984). 61. Jarosz M., Marczenko Z., Chem. Anal. (Warsaw), 37, 63 (1992). 62. Marczenko Z., Mikrochim. Acta, 1977 II, 651. 63. Motomizu S., Fujiwara S., T6ei K., Anal. Chim. Acta, 128, 185 (1981). 64. T6ei K., Anal. Sci., 3, 479 (1987). 65. Balog I.S., Kish P.P., Bagreev V.V., Zh. Anal. Khim., 43, 1750 (1988). 66. Alimarin I.P. et al., Zh. Anal. Khim., 39, 965 (1984). 67. Lychnikov D.S., Dorokhova E.N., Gracheva N.A., Zh. Anal. Khim., 43, 802 (1988). 68. Marczenko Z., Pure Appl. Chem., 57, 849 (1985). 69. Marczenko Z., Kalinowski K., Chem. Anal. (Warsaw); 32, 451 (1987). 70. Belcher R., Talanta, 15, 357 (1968); 24, 533 (1977). 71. Weisz H., Fritsche U., Mikrochim. Acta, 19"/3, 361" 1974, 701. 72. Flaschka H.A., Hornstein J.V., Microchem. J., 23, 488 (1978). 73. Burns D.T., Townshend A., Talanta, 39, 715 (1992). 74. McBryde W.A., Talanta, 21,979 (1974). 75. Sommer L., Kuba_ V., Langov~ M., Z. Anal. Chem., 310, 51 (1982). 76. Sommer L., Langova M., Crit. Rev. Anal. Chem., 19, 225 (1988). 77. Klausen K.S., Langmyhr F.J.,Anal. Chim. Acta, 40, 167 (1968). 78. Klausen K.S., Anal. Chim. Acta, 44, 377 (1969). 79. Likussar W., Boltz D.F., Anal. Chem., 43, 1265 (1971). 80. Ringbom A., Harju L., Anal. Chim. Acta, 59, 33, 49 (1972). 81. Petrova T.V., Savvin S.B., Zh. Anal. Khim., 42, 1925 (1987). 82. Kirkbright G.F., Pure Appl. Chem., 50, 237 (1978). 83. Struempler A.W.,Anal. Chem., 45, 2251 (1973). 84. A1-Sibaai A.A., Fogg A.G.,Analyst, 98, 732 (1973). 85. Nazarenko V.A., Flyantikova G.V., Zh. Anal. Khim., 32, 1217 (1977). 86. Sandell E.B., Onishi H., Photometric Determination of Traces of Metals. General Aspects (Chapters 1--4), Wiley, New York 1978. 87. Zolotov Yu.A., Kuz'min N.M., Preconcentration of Trace Elements, Elsevier, Amsterdam 1990. 88. Lobifiski R., Marczenko Z., Spectrochemical Trace Analysis for Metals and Metalloids, Elsevier, Amsterdam 1996. 89. Robertson D.E., Anal. Chem., 40, 1067 (1968). 90. Kloster M.B., Hach C.C., Anal. Chem., 44, 1061 (1972). 91. Mizuike A., Pinta M., Pure Appl. Chem., 50, 1519 (1978). 92. T61g G., Pure Appl. Chem., 50, 1075 (1978). 93. T61g G., Z. Anal. Chem., 294, 1 (1979).

52


94. Kosta L., Talanta, 29, 985 (1982). 95. Zief M., Mitchell J.W., Contamination Control in Trace Element Analysis, Wiley, New York 1976. 96. Van Grieken R., Van de Velde R., Robberecht H.,Anal. Chim. Acta, 118, 137 (1980). 97. Petrick H.J., Schulze F.W., Cammenga H.K., Mikrochim. Acta, 1981 II, 277. 98. Kuehner E.C., Alvarez R., Paulsen P.J., Murphy T.J., Anal. Chem., 44, 2050 (1972). 99. Rovinskii F.Ya., Gasilina N.K., Zh. Anal. Khim., 33, 160 (1978). 100. Knowles A., Burgess C., Standards in Absorption Spectrophotometry, Chapman-Hall, London 1981. 101. Cali J.P.,Anal. Chem., 48, 802A (1976); Z. Anal. Chem., 297, 1 (1979). 102. Koch O.G., Pure Appl. Chem., 50, 1531, 1951 (1978). 103. Quevauviller E.A., Maier E.A., Griepink B., Anal. Chim. Acta, 283, 583 (1993). 104. Michalke B., Fresenius'J. Anal. Chem., 350, 2 (1994). 105. Caroli S.,Anal. Chim. Acta, 283, 573 (1993). 106. Kuselman I., Talanta, 40, 1 (1993).

Chapter 4. Spectrophotometric reagents Reagents which give the colour reactions, upon which spectrophotometric methods are based, are called spectrophotometric reagents. Most spectrophotometric methods are based on organic reagents [1-4]. The theory of organic reagents, the mechanism of their reactions with metal ions, the structure of the complexes formed, and the effect of the dissociation constants and of the reagents and of hydrolysis constants of metal ions upon the formation of complexes have been presented in publications [5,6]. A historical survey of the organic reagents has been given [7]. A review of reagents used for determination of inorganic species has also been published [8,9]. This Section deals with the more important groups of spectrophotometric reagents. Some important reagents are presented, with formulas, in chapters devoted to individual elements.

4.1. Azo reagents The azo reagents form a large group of spectrophotometric organic reagents. This group comprises such reagents as 1-(2-pyridylazo)-2-naphthol (PAN), 4-(2-pyridylazo)resorcinol, (PAR), 5-Br-PADAP, Arsenazo III and Chlorophosphonazo III. Methods using azo dyes are sensitive.

4.1.1. N-Heterocyclic azo compounds Pyridylazo- and thiazolylazo reagents have become of great importance in the methods of determination of metals [ 10,11 ]. 1-(2-pyridylazo)-2-naphthol (PAN) (formula 4.1) can exist in solutions in three forms, depending on the pH. Acid solutions (pH < 2) contain the water-soluble yellow-green protonated HzR+ ion. Between pH 3 and 11, PAN occurs as the neutral HR molecule, soluble in organic solvents to give a yellow colour (colloidal suspensions can be formed in aqueous systems). In alkaline solutions of pH > 11 PAN exists as the water-soluble red R ~anion.

OH PAN is normally used in methanol or ethanol solutions. It acts as a terdentate ligand, complexing with metals through the hydroxyl oxygen atom, pyridine nitrogen atom, and one of the azo group nitrogen atoms. Metal complexes with PAN are sparingly soluble in water. Metal ions form neutral complexes extractable with inert solvents, such as chloroform or benzene. This permits the extractive-spectrophotometric determination of Mn, Zn, Cd, Cu, Ni, Co, In, U, Ga and Pd [12-14]. The molar absorptivities of the PAN complexes lie within

54

4. Spectrophotometric reagents

the range (2-6). 104. During extraction with an organic solvent, the metal complex and the uncombined PAN pass to the extract. The absorption maxima ~max of the complexes are usually very different from that of the reagent. The selectivity of the methods using PAN is enhanced by suitable selection of pH and masking agents. Iron, cobalt, and nickel, for example, react with PAN at pH = 4. At such a low pH the reagent does not form complexes with Mn, Zn and Cd. Cyanide enables manganese to be determined in the presence of Ni, Zn, Cd, Co, and Cu, which form stable cyanide complexes. Zinc and cadmium can be demasked from their cyanide complexes with formaldehyde. Unlike PAN, 4-(2-pyridylazo)resoreinol (PAR) (formula 4.2) is water-soluble and forms water-soluble complexes with metal ions [15,16]. In the range from 90% H2SO4 to pH 2 the following protonated forms are present: HsR 3+, H4R 2+ and H3R+. The neutral PAR molecule exists between pH 2.1 and 4.2 (~,max 385 nm). The anion HR- (~max 413 nm) occurs over the pH range 4.2-7. In alkaline solutions (pH 11-13) both hydroxyl groups are dissociated. The R 2- form has ~max 490 nm.

(4.2) OH

PAR gives coloured complexes with metal ions. The determination of metals with PAR is performed in aqueous solutions. In 0.5-0.05 M H2804 the reagent reacts with Cu, Bi, Ti, Zr, Pd, and TI(III) ions. In acetate medium (pH 3-6) PAR gives colour reactions with Zn, Cd, Co, Ni, Hg, U, Pb and Ga. Solutions of PAR complexes have a red or violet colour. Complexes of PAR with Co, Ni, Cu, Zn, and Cd can be extracted into chloroform in the presence of diphenylguanidine [ 17]. The bromo- and ehloro-derivatives pyridylazo derivatives [18-21] are very valuable, because they form the basis of highly sensitive methods. Their complexes with metal ions have molar absorptivities e about 105. The most widely applied 2-(5-bromo-2-pyridylazo)-5diethylaminophenol (5-Br-PADAP) (formula 4.3) has become the basis for very sensitive methods of determination of Cd, Co, Ni and other metals.

N.~-----.

(4.3)

NICzI..Is) 2

/

HO

CI,,

(4.4) HO

(4.5) HzN

Other examples of reagents of this group are: 2-(5-chloro-2-pyridylazo)-5diethylaminophenol (5-C1-PADAP) (formula 4.4), 4- (5-bromo-2-pyridylazo)- 1,3-

4.1. Azo reagents

55

diaminobenzene (5-Br-PADAB) (formula 4.5) and 2-(3,5-dibromo-2-pyridylazo)-5diethylaminophenol (3,5-diBr-PADAP) used in determinations of U and Zr. Thiazole azo compounds have similar properties to PAN and PAR as spectrophotometric reagents. Their reactions with metals are more selective, principally as a consequence of the lower stability of their complexes. The following reagents are examples: 1-(2-thiazolylazo)-2-naphthol (TAN) [22] (formula 4.6) and 4-thiazolylazo)resorcinol (TAR) [23, 24] (formula 4.7).

s

I

s

o.

(4.6)

(4.7)

A comprehensive review on applications of thiazolylazo reagents in spectrophotometric methods has been given [25].

4.1.2. Arsonic azo compounds The first reagents of this group were Arsenazo I (formula 4.8) and Thoron I (formula 4.9). This group of reagents is characterized by the presence of an arsonic acid group ortho to the azo group. The hydroxyl group is usually also ortho to the azo group. AsO3Hz

(4.8)

HO

S03H

(4.9)

The presence of the arsonic acid group, AsO3H2, causes the formation of stable complexes of some metals even in fairly strong acid solutions. The presence of the azo group ensures the colour reaction, whereas the hydroxyl group enables a second ring to be formed with the metal. This not only stabilizes the complex but also causes a considerable deepening of the colour obtained. The sulphonic acid groups render these reagents and their complexes watersoluble. Until Arsenazo I and related reagents were introduced into spectrophotometric analysis, there were no sensitive reagents for such elements as Th, Zr, Hf, U, and rare earths. Arsenazo I and a number of its analogues are derivatives of chromotropic acid (1,8dihydroxynaphthalene-3,6-disulphonic acid). Arsenazo I is applied to the determination of rare earths, Th, U, Zr, Ti, Nb, and other metals. Thoron I is less sensitive but more selective than Arsenazo I. It is most commonly applied to the determination of thorium and also Li, U, Zr and Be.

56


Arsenazo III (formula 4.10) is a very useful bis-azo dye based on chromotropic acid and o-aminophenylarsonic acid. It is moderately soluble in neutral and acid solutions, and readily soluble in slightly alkaline solutions. Strong oxidizing (H202, C12, Br2) and strong reducing agents (e.g., TIC13) cause decomposition. In acid solutions (from 10 M hydrochloric acid to pH 4) Arsenazo III has a purplish-red colour, while at higher pH values it is blueviolet.

~~__ s03Hz

H0

OH

HzO~As

(4.10) In strongly acid solutions (1-10 M HC1) Arsenazo III reacts only with Th, Zr, Hf and U(IV). The molar absorptivities, e, of the complexes with these metals are about 105. At pH 1-4 Arsenazo HI reacts with U(VI), Sc, Fe(III), Bi, and rare earths. The sensitivity of the colour reactions is lower in this case (e -5-104). The use of Arsenazo III in strongly acid medium overcomes difficulties connected with the hydrolysis of some multivalent metals (e.g., Zr, Th, U). In the determination of these metals the high acidity enhances the selectivity of the reagent. The absorbance of free Arsenazo III (~max 520-530 nm) at the absorption maxima of the metal complexes (~ 655-665 nm) is very slight. The large difference (A~) between the wavelengths of the absorption maximums of the complexes and the free reagent is important. In the case of Th and U the spectrophotometric method with Arsenazo HI is specific owing to the use of masking agents (oxalic acid, HF) and an appropriate acidity of the medium. Only one side of the symmetrical molecule Arsenazo III participates in the formation of complexes with metal ions. The metal ion bonds to the nitrogen atom of the azo group, the oxygen atom of the arsonic acid group, and the oxygen atom of the hydroxyl group. The distortion of the symmetry of the reagent molecule gives rise to two neighbouring absorption maxima in the visible spectra of the Arsenazo III metal complexes. It is possible to determine Arsenazo III by measuring its absorbance in concentrated sulphuric acid medium at 675 nm. Arsenazo I does not absorb under these conditions. The synthesis conditions and properties of high-purity Arsenazo III have been discussed [28-31 ]. A large number of other spectrophotometric reagents containing arsonic acid groups have been suggested. The reagents and their applications are critically reviewed [27,32,33]. The solubility and mechanism of reactions of these reagents have been discussed [34]. A related group of reagents comprises azo dyes containing phosphonic acid groups, for example, the Chlorophosphonazo HI (formula 4.11). This reagent is recommended for determinations of Zr, Ti, U, Sc, Ca, and Sr, among others [27].

P03Hz

H0

OH

Hz03 P~

(4.11) HO3S'"

v

~ f

"SO3H

4.1. Azo reagents

57

4.1.3. Other Azo Reagents Many azo reagents of other classes, such as o,o'-dihydroxyarylazo compounds have been employed in spectrophotometric determinations of various metals An example of the latter group of reagents is Sulphochlorophenol S (formula 4.12), which is recommended for determining such elements as Nb, Zr, Mo, Cu, V, and A1 in acid media [27].

HO3S~ - H -O ~OH HO \ /S03H N~N ' N=N--~ CI/ ....

H03S / ' w ~ / * ~ V J ~ ' ~ S

03H

(4.12)

\CI

An analogue of Sulphochlorophenol S is Sulphonitrophenol M, with nitro groups substituted for the two chlorine atoms of the former compound. This reagent has been applied for determination of Pb, Nb, A1, Ga, Zr, and V. Other examples of o,o'dihydroxyarylazo reagents are: Picramine-epsilon (formula 4.13) and the known reagents for determination of magnesium, namely Eriochrome Black T and Calmagite. OzN

OH

HO

S03H

(4.~3)

OzN

H03S

Examples of o-hydroxyarylazo compounds are: Chromotrope 2B (formula 4.14), a reagent for thorium and rare-earth elements, and 2-(4-sulphophenylazo)chromotropic acid (SPADNS) (formula 4.15) used in determinations of A1, Zn, Th, U and other metals [35]. In both these groups of azo reagents the oxygen atom of the o-hydroxyl group and the nitrogen atom of the azo group participate in complex formation with metal ions.

HO

OH

(4.14)

HO

OH

(4.15)

The presence of the sulphonic acid group in the azo reagents makes these and their complexes soluble in aqueous solutions. These complexes can be extracted in the presence of Aliquat 336 or tetrahexylammonium iodide in chloroform [36]. The sensitivity of reactions with azo reagents increases in the presence of organic solvents, such as acetone, propanol, or acetic acid [37]. 8-Hydroxyquinoline azo derivatives [38], pyrocatechol azo derivatives [39], and othiazo derivatives of p-cresol and 2-naphthol [ 4 0 ] have been reviewed.

58

4. Spectrophotometricreagents

4.2. Triphenylmethane reagents Owing to the presence of p-quinonoid tings triphenylmethane reagents are intensely coloured and provide the basis of spectrophotometric methods for the determination of a number of metals. Pyrocatechol Violet (formula 4.16) is a frequently used chelating reagent, readily soluble in water and in aqueous ethanol. An aqueous solution of the reagent is yellow (pH 18), and the colour of the solution changes to violet with increasing pH as a result of proton dissociation from the hydroxyl groups. Pyrocatechol Violet forms coloured (most often blue) chelates with many metals (e.g., Be, A1, Bi, Co, Cu, Fe, Ga, In, Mn, Pb, V, Zn) in weakly acidic and weakly basic solutions. OH

OH

(4.16) SO3'H

Eriochrome Cyanine R (ECR) (formula 4.17) and Chrome Azurol S (CAS) (formula 4.18) have carboxylate groups besides hydroxyl and sulphonate groups [41,42]. These reagents can be purified by various methods [43,44]. ECR and CAS react with many metals in weakly acidic or neutral solutions [45-47]. The yellow-orange colour of the reagents changes to blue or violet solutions of the metal chelates. C.H3

HOOC"" v

C,H3

"C~

+

~

C'H3

"COOH

HO0

SO~H

C'H3

C

I

(4.17)

OOH

(4.18)

Xylenol Orange (formula 4.19) [48,49] and its relative Methylthymol Blue [48,50] are characterized by the presence of the iminodiacetic acid groups, which occur in complexones such as EDTA. CH3

CH3

HOOCHzC HOOCI.IzC/'''~zu

CHzCOOH

S03H

~CHzCOOH

(4.19)

These reagents can be purified by various methods [51-53]. They are usually contaminated with unchanged starting materials and with products containing only one

4.3. Xanthene reagents

59

iminodiacetic group (e.g., Semi-Xylenol Orange). This group comprises also the Metalphthalein, used in determinations of Sr and Ba. Surface-active substances, cationic (CTA, CP, Zephiramine) (formulae 3.1, 3.2, 3.3) and non-ionic (e.g., Triton X-100) surfactants make it easier to dissociate the protons of chelating triphenylmethane reagents and, in consequence, facilitate the reactions of these chromogenic reagents (R) with easily hydrolysable metals [Be, A1, Ga, In, Fe(III), Sc, Ti, Zr] [54-61]. Ternary complexes are formed, with hyperchromic (increase of absorbance) and pronounced bathochromic (shift of ~max towards longer wavelengths) effects. In these complexes the ratio R:M. is higher than in binary systems (without surfactants). The mechanism of colour reactions involving surfactants was an object of studies that have led to fairly differing hypotheses. An important feature is the formation of surfactant micelles at considerable concentrations and appropriate pH values. In the determination of metals, values exceeding 105 are attained.

4.3. Xanthene reagents Among most frequently used chelating xanthene reagents are: Gallein (formula 4.20) [62], Bremepyregallel Red (formula 4.21) [63], and Pyregallel Red [63,64]. Xanthene reagents form chelates with many metals, such as Bi, In, Mo, Sn, Sb, Th, Ti, and Zr. The sensitivity of the determination methods increases in the presence of cationic surfactants [65]. OH

v

OH

OH

"C~" ~

B(

(4.20)

v

OH

-C=,-

v

"Br

(4.21)

Caleein (Fluorexone, Fluorescein Complexone) (formula 4.22), used as reagent for the determination of calcium, contains iminodiacetic acid groups in positions 2 and 7, and only one hydroxyl group in position 3. In the case of Calcein (like Xylenol Orange), the nitrogen atom of the iminodiacetic acid group participates in bonding the metal ion.

HO.~

0

~

~

O

HOOCHtC ~ N H z C ~ C ~ C I . . ~ N HOOCHzC/ COOH

CH2COOH / ~CHzCOOH

(4.22)

60


(4.23)

Another group of chelating xanthene reagents comprises 2,3,7-trihydroxy-6-fluorones, called simply fluorones [66]. The most often used reagents of this group are phenylfluorone (reagent for Ge and Sn) (formula 4.23) and methylfluorone, used for determination of Sb. Among more commonly known fluorones are salicylfluorone (formula 4.24 and disulfophenylfluorone (formula 4.25).

S03H

(4.24)

(4.25)

The sensitivity and the selectivity of the methods involving 2,3,7-trihydroxy-6-fluorones increase markedly in the presence of surfactants [67].

4.4. Non-chelating organic reagents 4.4.1. Basic dyes A number of elements give anionic complexes with halide and other ligands. These complexes can form ion-associates (ion pairs) with basic dyes, and are extractable into nonpolar solvents. The extracts form the basis of sensitive extraction-spectrophotometric methods [68,69]. The basic dyes are generally used as their halide salts. The form in which the dye can form the ion-association compound is its singly-charged cation. The solvents most often used are benzene, toluene, chloroform, and 1,2-dichloroethane. Sometimes a small amount of a donor-active oxygen-containing solvent (e.g., MIBK, butanol, di-isopropyl ether) is added to enhance the extraction. The acidity of the aqueous phase can vary over a wide range, from moderately concentrated mineral acid solutions (HC1, H2SO4) to pH 3-5, depending on the basic dye and the extraction solvent used.

4.4. Non-chelatingorganic reagents

61

Triaryhnethane (most often triphenylmethane) dyes constitute the most numerous group. The main representatives are Malachite Green, Brilliant Green (C2H5 instead of CH3 group) (formula 4.26) Crystal Violet (formula 4.27) and Methyl Violet [NHCH3 instead of N(CH3)2 on one benzene ring]. This group of dyes also includes Fuchsine (formula 27.1).

(CH3)zN~ .~NICH3)z -C~

(CzHs]zN~ ~N(CzHsIz

+

4-

V

- C ~'-

v

v

N(CH3] 2 (4.26)

(4.27)

The diphenylnaphthylmethane dyes comprise Victoria Blue 4R (formula 4.28) and Victoria Blue B (H instead of CH3 on the naphthyl group). Figure 4.1 shows the absorption spectra of some triarylmethane basic dyes.

(CH3)zN,~ ,~N(CH3lz +

C (4.28)

H3C//'N~csHs 2

500

600 . 6&O,

wavelength, nm

700

Fig. 4.1. Absorption spectra of Brilliant Green (1), Crystal Violet (2) and Victoria Blue 4R (3)

The theory of extraction of the ion-associates of basic triarylmethane dyes with complex anions has been discussed [70,71 ].

62


Rhodamine B (formula 4.29) is a xanthene basic dye [72]. Other reagents in this class are Rhodamine 3B (Ethyl Rhodamine B) (the ethyl ester of Rhodamine B) [73], Butyl Rhodamine B (the butyl ester) and Rhodamine 6G (formula 4.30).

CzHsNH~0~IHCzHs

4-

H3C" ~

(4.29)

~ "13

+

CH3 C00CzHs

(4.30)

The existence of several protonated forms of rhodamine dyes in acid solutions has been established [74]. Figure 4.2 shows the absorption spectra of xanthene basic dyes. The azine basic dyes are often applied in the spectrophotometric methods. Methylene Blue (formula 48.1) and Methylene Green are thiazine dyes.

2 rJ3

., 1

~00

530

S~176

nm6~176

Fig. 4.2. Absorption spectra of Rhodamine B (1) and Rhodamine 6G (2) (CzHs)zN-~~o~ICH3)z 4"

,,..u3P-"

v

~ 1 ~-

(4.31)

v

(4.32)

(4.33)

Among oxazine dyes are: Capri Blue (formula 4.31), Nile Blue A (formula 4.32) and Mendola's Blue. Safranine T [75] represents a phenazine dye (formula 4.33). The absorption spectra of the three azine dyes are presented in Fig. 4.3.

63

4.4. Non-chelating organic reagents

!

I

600

500

wavelength, nm

7O0

Fig. 4.3. Absorption spectra of Methylene Blue (1), Capri Blue (2) and Safranine T (3) Other types of basic dyes proposed for spectrophotometric determination of elements include the antipyrine dyes (Chrompyrazoles) [76,77], azo dyes [78], and the indamine dye Bindschedler' s Green (formula 30.1).

4.4.2. Acid dyes Acid dyes are used less frequently in spectrophotometric methods than the basic ones. Acid dyes form extractable ion-associates with hydrophobic cationic metal complexes. The dyes used are generally acid-base indicators. The acid xanthene dyes comprise Eosin (tetrabromofluorescein, formula 4.34) [79,80], Erythrosin (tetraiodofluorescein) and Rose Bengal B (formula 4.35) [79]. All these reagents are available as sodium salts. Br

Br

I

Cl"

(4.34)

I

y CI

"CI

(4.35) Br

Br

(4.36) S03H

64

4. Spectrophotometricreagents

Of the azo dyes, Methyl Orange has been successfully applied. The formula (4.36) shows the acid triphenylmethane dye, Bromophenol Blue (tetrabromophenolsulphophthalein) [81,82]. The cationic complexes which form ion-association compounds with acid nonchelating dyes are most commonly those of Zn, Cd, Pd, Fe(II), Cu with 1,10-phenanthroline. The e values often exceed 5.104. Chloroform and toluene are the commonly used extractants. As in the case of basic dyes, simple salts of acid dyes (e.g., sodium salts) are practically not extracted.

4.5. Dithizone Dithizone (HzDz, diphenylthiocarbazone, 3-mercapto-l,5-diphenylformazane) is one of the foremost organic spectrophotometric reagents [83-85]. It provides the basis of sensitive methods for the determination of Pb, Zn, Cd, Ag, Pd, Hg, Cu, Bi, and other metals. It has often been used in the extractive separation of traces of metals before their determination. Dithizone is insoluble in aqueous solutions at pH 12 is at 410 nm, whereas a colloidal solution of the adsorption compound formed with Mg(OH)2 and Titan Yellow is pink with )Lmax at 545 nm. At that wavelength, the absorbance of the free reagent is negligible. S03H

SOzH

(28.1)

The molar absorptivity of the pseudo-solution of the magnesium compound with Titan Yellow is 3.6.104 at 545 (a = 1.5). The intensity and reproducibility of the colour obtained are affected by the method of pH adjustment, the excess of Titan Yellow, the protective colloid used, the temperature of the solution, and the time of standing. Immediately after the start of the colour reaction, an increase in absorbance is noticed, but after 10-30 min the colour of the solution remains almost constant. After this it weakens progressively. Hydroxylamine is reported to stabilize the colour [ 12]. Solutions containing more than 2 ~tg of Mg per ml are unstable and soon become turbid as Mg(OH)2 coagulates. Protective colloids, such as poly(vinyl alcohol) prevent the coagulation. Gelatine, gum arabic, and starch are also used. Temperature greatly affects the reproducibility of the results. Heating a coloured solution from 20 to 30~ increases the absorbance by -~20%. A synergistic effect on the determination of magnesium is exerted by calcium. Although calcium itself gives no colour reaction with Titan Yellow, its presence with the magnesium causes increased absorbance. Since no further increase occurs above a certain concentration of calcium, the increased absorbance is exploited by adding excess of calcium to the sample and the standard solutions. Species which decrease the quantity of Mg(OH)2 precipitated upon the addition of NaOH, such as ammonium salts, and anions which precipitate Mg (e.g., phosphate), interfere in the Titan Yellow method. A number of metal cations decrease or increase the colour intensity. Decreased absorbance in the presence of A1, Zn, or Sn(IV) is believed result from adsorption of aluminate, zincate or stannate ions on the Mg(OH)2, which reduces the amount of dye adsorbed. A high concentration of salts in the sample solution interferes by increasing the solubility of Mg(OH)2.

Reagents Titan Yellow, 0.01% solution. Dissolve 10 mg of the reagent in 100 ml of water. The solution is stable for at least one week. Standard magnesium solution: 1 mg/ml. Dissolve 10.1350 g of MgSO4.7H20 (or 4.9500

28.2. Methods of determination

249

g of the salt ignited at 400-500 o C) in water containing 1 ml of conc. H2SO4, and dilute the solution with water to volume in a 1-1itre standard flask. Calcium chloride, 2% solution of CaC12.2H20, free of Mg. Add sodium hydroxide to this solution until its concentration is -~1 M . After 1 h, filter off the precipitated Mg(OH)2 and slightly acidify the solution with hydrochloric acid.

Procedure To the sample solution, in a 25-ml standard flask, containing not more than 12 gg of Mg, add 1 ml of the calcium chloride solution, 2.5 ml of the Titan Yellow solution, 3 ml of 1% poly(vinyl alcohol) solution, and water to -17 ml. While swirling the solution vigorously, add 1 M NaOH dropwise from a burette until the solution changes colour. Then add 2.5 ml more NaOH solution with continued mixing, and dilute the solution with water to the mark. After 15 min, measure the absorbance of the solution at 545 nm against a blank solution containing the same amount of calcium.

28.2.2. Eriochrome Black T method The reaction of a blue Eriochrome Black T (Erio T, formula 28.2) solution in an alkaline medium (pH 7.5-11.5) with magnesium ions, to form a pink complex, has been employed in the determination of magnesium [ 1,6]. OH OH N~-'N

S03H

(28.2)

N0x

Since slight alteration of the pH changes the blue colour of the Eriochrome Black T, the pH of the sample and the standard solution must be controlled carefully. The sensitivity of the method depends on the quality and excess of the Erio T used. The sensitivity decreases rapidly with increasing ionic strength. The molar absorptivity of the complex at pH 9.6 at )Lmax 520 nm is 1.8.104 (a - 0.76). Although the method has greatest sensitivity at pH 10.4 (e = 2.3.104), a pH of 9.6 is more convenient since calcium (in four-fold amount relative to magnesium) does not interfere at the lower pH value. Larger quantities of calcium must be separated, e.g., by precipitation as the oxalate or sulphate (in 90% methanol), before the determination of magnesium. The Analytical Group I-III metals must also be separated first. Small amounts of Fe, Cu, Zn, and Ni may be masked with cyanide. Phosphate should be removed, e.g., by ion-exchange separation. One version of the method proposed includes extraction of the Mg-Eriochrome Black T complex into butanol [16]. Extractable ion-associates of the anionic Mg-Erio T complex with TOA (CHC13, ~ = 5.5.104 at 555 nm) [17] have also been used for determining Mg. The FIA technique has also been used for the determination of Mg [ 18].

Reagents Eriochrome Black T (Erio T), 0.2% solution in methanol; this solution is stable for about one

250

28. Magnesium

week. Standard magnesium solution: 1 mg/m. Preparation as in Section 28.2.1. Buffer solution: pH 9.6. Dissolve 60 g of NH4C1 in water, add 120 ml of conc. ammonia solution, and dilute the solution with water to 1 litre.

Procedure To a slightly alkaline sample solution (pH 8-9), containing not more than 25 ~tg of Mg and free from the Group I-III metals add, in a 25-ml volumetric flask, 10 mg of ascorbic acid and 10 mg of potassium cyanide (to mask possible traces of Fe, Ni, Cu, Zn, and other metals). After 5 min, add 1 ml of buffer solution and 25 ml of the Eriochrome Black T reagent. Dilute the solution with water to volume in a 25-ml standard flask and measure the absorbance at 520 nm against a reagent blank.

Note. Under these conditions a fourfold amount of calcium relative to magnesium does not interfere. 28.2.3. Other methods Besides Titan Yellow, several other organic dyes have been recommended which react with Mg(OH)2 to form adsorption compounds in alkaline media, e.g., Magneson II, Phenazo [11 ], and polymethine dyes [ 19-21 ]. Most of the other methods are based, as with Erio T, on reactions of magnesium ions to form coloured chelates with azo reagents in alkaline media. Among these reagents are: Calmagite (formula 28.3) (e = 2.0.104 at 540 nm) [9,22,23], PAR [24-26], and Arsenazo III [27]. High sensitivity, as in the Calmagite method, has been obtained with the use of Xylidyl Blue I (Magon Sulphonate) and Xylidyl Blue II (Magon) [28-31]. OH

HO S-) which form coloured, water-soluble complexes with H2Dm interfere. However, the complexes of these metals are decomposed by EDTA, and a preliminary extraction as Ni(Hl)m)2 allows nickel to be isolated from Cu, Co, Fe, Cr, AI, and Mn. The presence of hydroxylamine ensures the quantitative extraction of nickel and prevents interference from Cu and Mn. In the presence of large quantities of Co and Fe the use of triethanolamine as masking agent is recommended [ 13]. When the chloroform extract is shaken with 0.5 M HC 1, Ni(HDm)2 is decomposed and the nickel is stripped into the aqueous phase. After the CHC13 has been removed, this aqueous solution is treated with H2Dm, oxidant, and ammonia (or NaOH solution) to forln Ni(Dm)32- . The dimethylglyoxime is added as an alcoholic solution or as a solution in 0.2 M NaOH.

286

32. Nickel

r,.)

9 r~ ,.cb c~

360

/400 436 /./,5

500

wavelength, nm

600

Fig. 32.1. Absorption spectra of nickel(ll) with dimethylglyoxime (H2Dm) in CHCla (1), nickel(IV) with H2Dm in alkaline solution (2), and nickel(ll) with ct-furyldioxime in CHCla (3).

The anionic nickel complex with H2Dm, formed in NaOH medium in the presence of an oxidant can be extracted into organic solvents in the presence of diphenylguanidine [14]. Nickel has been determined by the dimethylglyoxime method in the presence of iodine, by the flow injection technique (FIA) [15].

Reagents Dimethylglyoxime (H2Dm), 1% solution in ethanol. Standard nickel solution: 1 mg/ml. Dissolve 6.7300 g of (NH4)2Ni(SO4)2.6H20 in water containing 2 ml of conc. H2SO4, and dilute the solution with water to 1 litre. It is also possible to prepare a more concentrated solution of a nickel salt, determine the concentration of nickel gravimetrically, and then dilute the solution with water until it contains exactly 1 mg of Ni per ml. Bromine water, saturated aqueous solution. Potassium persulphate, 4% solution, freshly prepared.

Procedure Extractive separation of Ni. To the solution containing not more than 50 gg of Ni, add 1-3 ml of 20% sodium potassium tartrate solution, 1 ml of 10% NH2OH.HC1 solution, 2 ml of the H2Dm solution, and ammonia to pH 9-10. Shake the solution for about half a minute in a separating funnel with two portions of CHC13. Wash the combined extracts by shaking with dilute ammonia solution (1+50), then strip the nickel from the organic phase by shaking for 1 min with 0.5 M HC1. Discard the chloroform layer. Determination of Ni. Quantitatively transfer the solution obtained into a 25 ml standard flask and add, successively, 1 ml of the HzDm solution, 1 ml of bromine water (or persulphate solution), and 2.5 ml of conc. NH3 solution. Dilute the solution to the mark with water, and mix well. After 10 min, measure the absorbance at 445 nm against water.

32.2.2. tx-Furildioxime method ct-Furildioxime sometimes called Neonickelone, formula 32.2) reacts with nickel ions in a similar way to dimethylglyoxime and other dioximes, forming a chelate which is sparingly


287

water-soluble, but is extractable into chloroform and similar solvents. The yellow colour of the organic extract provides the basis of a specific spectrophotometric method for determining nickel [ 16]. This method is more convenient than the method discussed above with HzDm and oxidant. The sensitivities of the dimethylglyoxime (+ oxidant) and the ~-furildioxime methods are similar, but the latter method is simpler since it enables one to extract Ni from the aqueous phase to a smaller volume of an organic solvent. The nickel c~-furildioxime complex is formed and extracted quantitatively in the rather narrow pH range from 7.5 to 9.0. The pH can be adjusted most conveniently by adding small portions of sodium bicarbonate to the slightly acidic sample solution. Alternatively, a drop of 1% solution of phenolphthalein can be added to the solution, and ammonia added carefully until the indicator just turns pink.

(32.2)

Chloroform, CC14,o-dichlorobenzene, and ethyl acetate have been used as solvents. The solubility of the complex in CC14 is rather low and hence this solvent can be used only for extracting limited amounts of nickel (5 M HC1 solutions. Niobium(V) can be reduced to coloured species of Nb(III) and Nb(IV). In an acid medium, zinc metal reduces Nb(V), but not Ta(V). Tantalum (Ta, at. mass 180.95) is, in its chemical properties, similar to Nb. The Ta(V) complexes are less stable than the corresponding Nb complexes. Tantalum(V) is harder to reduce to the lower, coloured, oxidation states, than is Nb(V).

33.1. Methods of separation and preconcentration 33.1.1. Precipitation On being heated in acid solutions (or pseudo-solutions), niobium and tantalum hydrolyse and coagulate to form hydrous oxides. The following compounds may be used as collectors for traces of niobium or tantalum: Zr(OH)4 and MnOzaq. in acid solutions, and Fe(OH)3 and Mg(OH)2 in alkaline solutions. When an alkaline melt (NazCO3, NaOH) is leached, Nb and Ta remain in the solid phase, while W, Mo, V, and Re pass into the aqueous solution [1 ]. From solutions containing not too much oxalic or tartaric acid, or EDTA, Nb and Ta may be precipitated by cupferron [2,3] or phenylarsonic acid [4]; zirconium is often used as collector. When boric acid is added to a solution of Nb-, Ta-, W-, Mo-, or Zr fluoride complexes, the boron displaces the fluoride (formation of BF4-) from the Nb, Ta, and W complexes, causing the precipitation of these metals, while Zr and Mo remain in solution, since their fluoride complexes are more stable [5].

33.1.2. Extraction Niobium and tantalum form stable fluoride complexes which are extractable with oxygencontaining solvents and can thus be separated from many metals, such as Ti, Zr, Sn, Mo, W, U, and Fe [6,7]. By suitable choice of concentrations of HF, HC1, and H2SO4, tantalum can be separated from niobium and v i c e v e r s a [6]. MIBK is most often used as the solvent. Niobium and Ta can be stripped from the organic phase with a H202 solution. Niobium can be separated from Ta and other metals in HF or HC1 media by extracting into CHC13, CC14, cyclohexane, 1,2-dichloroethane, or xylene the ion-associates, which are formed by Nb (and Ta) complexes with tetraphenylarsonium cation, TOA [8,9], DAM [10], or Aliquat 336 [11-13]. The thiocyanate complexes of Nb with triphenylphosphonium cation [14] or with amides [ 15] have also been extracted. Highly selective separation of Nb has been based on extraction from 5 M HC1 by means of a-benzoinoxime in CHC13 [ 16,17].

294

33. Niobium and tantalum

Ion-associates of niobium anionic complexes with pyrocatechol [18], or oxalate [19], and quaternary ammonium ions have been used for separating Nb from other refractory metals (Ti, W, Mo, V). Niobium has been extracted from solutions containing Fe(III), Ti(IV), and V(IV) with the use of 4,5-dibromopyrocatechol [20]. The crown-ether dibenzo18-crown-6 has also been used for the extraction of Nb [21]. Tantalum has been separated from V (in 6 M HC1) by extraction into triphenylphosphine oxide in toluene [22].

33.1.3. Ion exchange and sorption Many methods for isolating and separating Nb and Ta are based on their fluoride complexes [23,24]. The complexes of Nb, Ta, and some other metals are retained in a polyethylene column filled with an anion exchanger, from which they are then washed out with suitable eluents. If a 6 M HC1 and 1 M HF medium is applied, Nb is retained in the column and Zr passes into the eluate [25]. Nb and Ta have been separated on the anion-exchanger Dowex 1 from a medium containing HC1 and H2C204 [26]. Either Ta is first eluted with a mixture of 0.1 M H2C204 and 2 M HC1, or Nb is eluted first with 0.5 M H2C204 and 1 M HC1. The procedure can be used for determination of Nb in tantalum and v i c e v e r s a . Niobium and tantalum have been also sorbed on the anion-exchanger Dowex 1 from mixtures of H F H2SO4 and HF-HC1 [27,28]. Cation exchangers were also used for separation of Nb, Ta, and other metals, using H202 mixtures with HC1, H2SO4, or HNO3 [29]. The use of liquid ion exchangers for the separation of Nb and Ta (and other metals) has been reviewed [30]. Tantalum has been separated from niobium with the aid of chelating ion-exchangers [31,32]. Polyurethane foam saturated with DAM, TBP, or MIBK has also been used in the separations [33].

33.2. Determination of niobium A description is given below of the classical thiocyanate method and of the more sensitive method based on Bromopyrogallol Red.

33.2.1. Thiocyanate method Niobium(V) reacts with thiocyanate in HC1 solutions to form a yellow complex, which has been a basis of determining Nb. The niobium is determined spectrophotometrically either after extraction of the complex [34-36] or in an aqueous acetone medium. The sensitivities in both cases are similar, but the extraction method is less subject to interference by other metals. Diethyl ether is commonly used as the solvent, but ketones, esters, and higher alcohols are also suitable. The absorption maximum of an ethereal solution of the Nb thiocyanate complex is at 385 nm. The molar absorptivity of the complex is 3.5.104 (a = 0.38). Niobium is extracted in the presence of SnCI2 which reduces Fe(III) and other interfering oxidants. The concentrations of SnCI2, HC1, and thiocyanate greatly affect the intensity of the colour and the reproducibility of the results. These concentrations should not be lower than 4% SnCI2, 2 M HC1, and 10% KSCN. Since some thiocyanic acid is also extracted by ether, one is recommended to use ether saturated with thiocyanic acid. A mixture of diethyl ether and CC14 (1+1) makes a useful extractant since it is denser than water.

33.3. Determinationof niobium

295

Niobium is kept in solution by the addition of tartaric acid, whose presence prevents the hydrolysis of tantalum and the consequent occlusion of niobium in the precipitate. Tungsten, molybdenum, and vanadium interfere in the determination of niobium. In contrast to the corresponding tungsten complex, the niobium-thiocyanate complex is decomposed by oxalic acid. Fe([II), U, Ti, and Ta do not interfere if they are present in no greater than hundred-fold amounts relative to niobium. Phosphate and fluoride interfere, but the latter can be masked with aluminium ions [37]. The thiocyanate complex of niobium may be extracted in the presence of ethylenebis(triphenylphosphonium) cation [38], promazine [39], triphenylguanidine [40], and dibenzo- 18-crown-6 ether [41 ]. Chloroform, trichloroethylene, toluene and benzene are used as the solvents.

Reagents Potassium thiocyanate, 30% solution. Standard niobium solution: 1 mg/ml. Fuse 0.1430 g of Nb205 with 4 g of K2S207 in a quartz or platinum crucible. Dissolve the melt in a hot 5% tartaric acid solution, allow to cool, and dilute with the tartaric acid solution to 100 ml in a volumetric flask. Working solutions are obtained by suitable dilution of the stock solution with 2% tartaric acid. Stannous chloride: 20% solution in 2 M HC1.

Procedure To the sample solution containing not more than 50 ~tg of Nb ( complexed with tartrate), add concentrated HC1 and the SnCI2 and thiocyanate solutions until the solution is --3 M in HC1, 5% in SnCI2, and 12% in KSCN. After 5 min, extract the niobium thiocyanate complex with 2 portions of diethyl ether. Transfer the extracts to a 25-ml standard flask, make up to the mark with ether, and measure the absorbance at 385 nm against a reagent blank solution.

Note. The ratio of the volumes of extractant to the volume of the aqueous phase must be identical for both the sample and the standard solutions. 33.2.2. Bromopyrogallol Red method In a mixed EDTA-tartrate medium at pH --6, Bromopyrogallol Red (BPR) (formula 4.21) reacts with niobium to form a blue complex used as a basis for Nb determination [42,43]. At 610 nm, the molar absorptivity is 4.75-104 (specific absorptivity 0.51). The absorption maximum of the reagent is at 560 nm. Since the niobium-BPR complex is insoluble in water, gelatine is added to give a stable colloidal dispersion. The colour reaction proceeds so slowly that maximum absorbance is attained only after 90 min, after which it remains constant. The greatest sensitivity is obtained in solutions buffered between pH 5.8 and 6.6 with ammonium acetate. The Nb complex with BPR is very stable. Its formation is not affected by 1,000-fold excess of oxalate, fluoride, or phosphate. Most interfering cations are masked with EDTA. Tartrate masks milligram quantities of Ta, Ti, W, Mo, Sb(V), and Sn(IV); U(VI) and Zr can be masked with phosphate; fluoride masks A1 and Th; cyanide masks any silver present. The interfering effect of Ce(IV) and V(V) can be eliminated by addition of ascorbic acid which reduces them to Ce(III) and V(IV), respectively. Thus, the BPR method can be regarded as specific, provided that

296


suitable masking agents are used. If the sample solution contains Nb separated from other metals, no masking agents, including EDTA and tartrate, are necessary. In this case the sensitivity of the Nb reaction with BPR is higher (~ = 6.0-104) [42]. Niobium has also been determined in 1 M HC1 in the presence of CP (15% DMF) [44].

Reagents Bromopyrogallol Red (BPR), 0.02% solution. Dissolve 20 mg of the reagent in 50 ml of ethanol, and dilute the solution to volume with water in a 100-ml standard flask. The solution should not be used if over a week old. Standard niobium solution: 1 mg/ml. Preparation as in Section 33.2.1. Acetate buffer, pH 6.0. Dissolve 80 g of ammonium acetate in water, add 6 ml of glacial acetic acid, and dilute the solution with water to 1 litre.

Procedure Add the sample solution (--5 ml) containing not more than 30 pg of Nb (complexed with oxalate or tartrate) to a 25-ml standard flask. Neutralise to pH -~6, and add 3 ml of 20% potassium sodium tartrate solution, 3 ml of 5 % EDTA solution, 2 ml of the BPR solution, 3 ml of the acetate buffer, and 0.5 ml of the gelatine solution. Mix the solution and set aside for 90 min. Then dilute with water to the mark, and measure the absorbance at 610 nm against a reagent blank solution.

Note. If A1 or Th is present, add NaF; if U or Zr is present, add Na3PO4; if Ag is present, add KCN (Caution!). 33.2.3. O t h e r m e t h o d s Several other azo compounds have been employed as spectrophotometric reagents for niobium. The reactions are carried out in the presence of complexants (tartrate, oxalate, H202). The coloured species produced are generally ternary niobium complexes. PAR (e = 3.6.104) [45-53] and Sulphochlorophenol S ( e - 3.3.104) [54-61] are often employed. Higher sensitivity is obtained in reactions of Nb with Sulphonitrophenol M (formula 33.1) (~ - 5.3-104) [55,62,63]. Other reagents recommended for Nb determination include Sulphonitrazo E [64], TAR [21,65], arsonophenylazochromotropic acid [66], an azo derivative of 8-hydroxyquinoline [67], and 2-(5-chloro-2-pyridylazo)-5-dimethylaminophenol (MIBK) [68]. HO3S,

0 2N

OH

HO

HO3s"

g

OH

v

; 03H

"SO3H

(33.1)

Among 2,3,7-trihydroxy-6-fluorones, the best results in the determination of Nb were obtained with phenylfluorone and o-nitrophenylfluorone (~ - 1.3.105 - 1.7-105) [69-71 ]. A very sensitive method has been based on a mixed complex of Nb with o-nitrophenylfluorone and DAM (~ = 1.9-105) [72]. Even higher sensitivity (~ = 2.1.105) has been obtained with the

33.3. Determination of tantalum

297

use of salicylfluorone and surfactants [73,74]. A high sensitivity (e = 2.1.105) is also characteristic for the flotation method, based on an ion-associate formed by the anionic oxalate complex of Nb(V) with 3,5dinitropyrocatechol and Rhodamine B [16]. Other organic spectrophotometric reagents for Nb include 8-hydroxyquinoline-5sulphonic acid [75], 5,7-dichloro-8-hydroxyquinoline (e =1.3.104 at 400 nm) [76], 5-chloro7-iodo-8-hydroxyquinoline [77], Lumogallion [78], and thioglycolic acid [79]. Nb has also been determined after extraction of its complex with 3-hydroxyflavone [80] and N-mphenylstyrylacrylhydroxamic acids [81 ]. The yellow niobium peroxide complex gives a less sensitive method (e = 1.0.103) suitable for determining larger quantities of Nb [82]. The method is suitable for determination of Nb in the presence of Ta, Ti, and other metals. Nb has been determined in the presence of Ti by derivative spectrophotometry, as a complex with H202 and 5-BrPADAP [83]. The ion-associate of the anionic Nb complex with tetrabromopyrocatechol and Brilliant Green has been used in the extractive method (toluene) [84,85]. Niobium has been determined in the presence of Zr with the use of Xylenol Orange [86].

33.3. Determination of tantalum The pyrogallol method for determining Ta is quite selective but rather insensitive. For traces, the extractive spectrophotometric methods using Methyl Violet or other basic dyes are recommended.

33.3.1. Pyrogallol method In an acid medium (HC1, H2SO4), pyrogallol reacts with Ta(V) to form a yellow complex, whose absorption maximum occurs in the near-ultraviolet. This reaction has long been the basis of a simple and selective method for determining Ta [87,88]. The absorption spectrum of the Ta complex and the interferences owing to other elements (particularly Nb and Ti) vary according to the reaction conditions. A solution of 4 M HC1 and 0.02 M (NH4)2C204 is a suitable medium, since the colour from pyrogallol complexes with niobium and titanium is insignificant. In this medium, a ternary tantalum complex with pyrogallol and oxalic acid, and the colourless Nb oxalate complex are formed. To reduce the interference by Nb, tartrate is sometimes added. The molar absorptivity of the tantalum-pyrogallol complex in 4 M HC1 and 0.02 M (NH4)2C204 solution is 2.4.103 (a = 0.013) at ~max = 335 nm. It is advisable, however, to measure the absorbance at longer wavelengths to avoid interference by the excess of pyrogallol, whose absorption maximum occurs at 315 nm. The colour reaction of Ta should be carried out in a reducing medium, as pyrogallol is readily oxidized in contact with air to give dark-coloured products. The excess of pyrogallol influences the intensity of the colour obtained. Mo, W, Sb, U, and fluoride interfere seriously in the determination of Ta with pyrogallol. Fluoride can be masked with boric acid. The tantalum-pyrogallol complex may be extracted in the presence of quaternary ammonium bases [88,89].

298


Reagents Pyrogallol, 20% solution. Dissolve 20 g of resublimed pyrogallol in water, add 10 ml of conc. HC1 and 2 g of SnClz.2H20 (dissolved in 5 ml of conc. HC1), and dilute the solution with 0.1 M HC1 to 100 ml. Standard tantalum solution: 1 mg/ml. Fuse 0.1220 g of Ta2Os, with 4 g of K2S207 in a silica or platinum crucible. Dissolve the melt in 4% (NH4)2C204 solution and dilute to the mark with the same reagent in a 100-ml standard flask. Working solutions are obtained by suitable dilution of the stock solution with 2% solution of ammonium oxalate. Ammonium oxalate and hydrochloric acid solution containing 15 g of (NH4)2C204 and 760 ml of conc. HC1 per litre.

Procedure Place the sample solution (7-8 ml) containing not more than 1.5 mg of Ta in a 25-ml standard flask. Add 10 ml of the (NH4)2C204 and HC1 solution, and 5 ml of the pyrogallol solution. Dilute the solution with water to the mark. After 30 min, measure the absorbance of the solution at 350 nm against a reagent blank.

33.3.2. Methyl Violet method In dilute HF medium, tantalum forms a complex, TaF6-, which combines with the basic dye Methyl Violet to form an ion-pair, which can be extracted with benzene. The coloured extract has been the basis for a sensitive method of determining Ta [90]. Maximum absorbance of the coloured extract is obtained when the pH of the reaction medium is 2.1-2.3, and the HF concentration is 0.2-0.3 M. When the volumes of the aqueous phase and benzene are 30 ml and 10 ml, respectively, 80% of the tantalum is extracted. Extraction of free Methyl Violet is only slight. The molar absorptivity of the benzene extract obtained under the conditions specified in the procedure below is 7.5.104 (specific absorptivity 0.42) at ~max605 nm. Low concentrations of HC1 and H2804 do not interfere with the extraction of Ta. In the presence of HNO3, however, more free Methyl Violet is extracted. Concentrations of niobium up to 0.2 mg/ml can be tolerated in this method. Rhenium at concentrations >5 ~tg/ml causes high results for tantalum. High concentrations of Mo and A1 cause low results since they mask the hydrofluoric acid as stable fluoride complexes. Moderate amounts of Zr, Ti, W, Fe, and Cu do not interfere.

Reagents Methyl Violet, 0.1% solution in 0.2 M HF. Store the solution in a polyethylene bottle. Standard tantalum solution: 1 mg/ml. Dissolve 0.1000 g of tantalum in 5 ml of conc. HF and a few drops of conc. HNO3. Evaporate the solution (in a platinum or Teflon vessel) to dryness, add a few drops of conc. HC1 and 2 ml of conc. HF, and evaporate to dryness again. Dissolve the residue in 1 ml of conc. HF and dilute the solution with water (with stirring) to 100 ml. Store the solution in a polyethylene bottle. Working solutions are obtained by suitable dilution of this stock solution with 0.2 M HF.

33.3. Determination of tantalum

299

Procedure To the sample solution (in a polyethylene separating funnel) containing not more than 20 ~tg of Ta, add 2 ml of the Methyl Violet solution and sufficient hydrofluoric acid to make its concentration 0.2-0.3 M in a volume of 30 ml (pH 2.1-2.3). Add 10 ml of benzene and shake for 1 min. Measure the absorbance of the benzene extract at 605 nm against benzene.

Note. When there are small amounts of tantalum in the sample a reagent blank should be used as reference because of the small solubility of Methyl Violet in benzene. 33.3.3. Other methods Many other basic dyes besides Methyl Violet have been used in sensitive extractionspectrophotometric methods for the determination of Ta as the anionic complex TaF6- [92]. Mention may be made of Crystal Violet (formula 4.27) (e = 8.5.10 4) [91-93], Brilliant Green (~ = 1.2.105) [94,95], Malachite Green [96,97], Methyl Green (~ = 1.2.105) [98], Rhodamine 6G and butylrhodamine B [99], Methylene Blue (~ = 9.1.104) [98], Nile Blue A [100], Capri Blue (~ = 1.1.105) [101], and Victoria Blue B [102]. Ion-associates with these dyes are extractable from acid solutions into benzene, toluene, CHC13, xylene, or dichloroethane. Some 2,3,7-trihydroxy-6-fluorones are sensitive reagents for Ta, e.g., phenylfluorone [103], salicylfluorone [104,105], and 9-(2'-hydroxyphenyl)-2,3,7-trihydroxy-6-fluorone (~ -2.1.105 at 505 nm) [ 106]. High sensitivity (~ = 1.6-105-1.8 9105) has been obtained in the presence of surfactants [ 105]. This method has been used for the determination of tantalum in niobium metal. Tantalum has been determined with the use of 4,5-dibromo-onitrophenylfluorone [ 107]. From the group of azo reagents the following have been recommended for determination of Ta: PAR [22,108-110], Arsenazo I [111], and 2-(2-thiazolylazo)-5dimethylaminophenol (TAM)(~ = 4.1-104) [112]. Other organic reagents for determination of Ta include pyrogallolsulphonic acid [ 113] and dibromogallic acid [ 114]. Tantalum has also been determined as tantalomolybdenum blue [115], and after the extraction (CHC13) of its thiocyanate complex, associated with triphenylguanidine [ 116]. Tantalum has been determined in the presence of Nb by derivative spectrophotometry with the use of Picramine-epsilon [ 117].

33.4. Analytical applications 33.4.1. Separation and determination of Nb The thiocyanate method has been used for determining niobium in steels [35,118], tantalum and its compounds [6,7,119], cobalt alloys [37], uranium [120], rocks and minerals [1,121], sodium metal [122], and thin Nb-Ti films [123]. Niobium has been determined in various metals and alloys with the use of Bromopyrogallol Red [ 124]. PAR has been applied in determinations of niobium in silicate rocks [29], steels [48,49,125], magnetic and electric alloys [22,50], zirconium and titanium alloys [51], copper alloys [52], thin Nb-Ge films [53], and high-phosphorus optical materials [ 126]. Niobium has been determined with the use of Sulphochlorophenol S in rocks and minerals [60,127-129], steels [57,58], tungsten, uranium, and beryllium [59], and titanium

300


dioxide and ilmenite [61]. Sulphonitrophenol M was applied in determinations of niobium in minerals [63]. Trace amounts of niobium (--1-10-4 %) in geological materials were determined by the flotation method with the use of Rhodamine B [ 16]. Other methods mentioned above were used in determining Nb standard samples of alloys and steels [81 ], in lithium fluoride [78], and volcano dusts [32].

33.4.2. Separation and determination of Ta The pyrogallol method has been used for determining tantalum in ores and minerals [130], steels [ 131 ], niobium [ 132], zirconium alloys [ 133], and beryllium and its oxide [ 134]. Tantalum has been determined using Methyl Violet in concentrates [135]. Larger amounts of tantalum were determined by differential spectrophotometry [ 136]. Other basic dyes were used in determination of tantalum in ores [94], niobium and its compounds [97], and uranium and zirconium [96]. Tantalum was determined with the use of PAR in ores [110], steels [108], alloys [22], and compounds of rare earth elements [ 109].

References 1. Grimaldi F.S., Anal. Chem., 32, 119 (1960). 2. Hibbits J.O. et al., Talanta, 8, 209 (1961). 3. Geissler M., Lorenz G., Z. Anal. Chem., 244, 235 (1969). 4. Patrovsk37 V., Coll. Czech. Chem. Comm., 30, 1727 (1965). 5. Ponomarev A.I., Bykovskaya Yu.I., Zh. Anal. Khim., 21, 1427 (1966). 6. Luke C.L., Anal. Chim. Acta, 34, 165 (1966). 7. R62ycki C., Boguszewska Z., Samsel T., Zh. Anal. Khim., 33, 2148 (1978). 8. Pal'shin E.S., Ivanova L.A., Zh. Anal. Khim., 28, 1741 (1973). 9. Das N.R., Lahiri S., Anal. Sci., 8, 317 (1992). 10. Caletka R., Krivan V.,Anal. Chim. Acta, 162, 67 (1984). 11. Das N.R., Chattopadhyay P., J. Radioanal. Nucl. Chem., 109, 45 (1987). 12. Das N.R., Chattopadhyay P., Bull. Chem. Soc. Jpn., 61, 4423 (1988). 13. Karve M.A., Khopkar S.M., Talanta, 40, 913 (1993). 14. Burns D.T., Chimpalee D.,Anal. Chim. Acta, 256, 307 (1992). 15. Patel K.S., Das M.,Anal. Lett., 24, 1273 (1991). 16. Lobifiski R., Marczenko Z., Anal. Chim. Acta, 226, 281 (1989). 17. Jarosz M., Oszwaldowski S., Anal. Sci., 9, 285 (1993). 18. Nardillo A.M., Catoggio J.A., Anal. Chim. Acta, 66, 359 (1973). 19. Taiurski V.A., Shastina Z.N., Pronin V.A., Zh. Anal. Khim., 33, 192 (1978). 20. Simonenko V.I., Tyltin A.K., Lysak N.B., Sukhan V.V., Ukr. Khim. Zh., 56, 614 (1990). 21. Deorkar N.V., Khopkar S.M.,Analyst, 116, 961 (1991). 22. Kakade S.M., Shinde V.M., Bull. Chem. Soc. Jpn., 67, 1306 (1994). 23. Headridge J.B., Dixon E.J., Analyst, 87, 32 (1962). 24. Dixon E.J., Headridge J.B., Analyst, 89, 185 (1964). 25. Holloway J.H., Nelson F., J. Chromatogr., 14, 255 (1964). 26. Speecke A., Hoste J., Talanta, 2, 332 (1959). 27. Caletka R., Hausbeck R., Krivan V., J. Radioanal. Nucl. Chem., 131, 343 (1989). 28. Schumann D. et al., J. Radioanal. Nucl. Chem., 187, 9 (1994).

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29. Mazzucotelli A. et al., Analyst, 102, 825 (1977). 30. Das N.R., Lahiri S., Solvent Extr. Ion Exch., 9, 337 (1991). 31. Savvin S.B., Myasoedov B.F., Eliseeva O.P., Zh. Anal. Khim., 24, 1023 (1969). 32. Pobi M., Das J., Anal. Lett. 26, 793 (1993). 33. Caletka R., Krivan V., Z. Anal. Chem., 321, 61 (1985). 34. Mari E.A., Anal. Chim. Acta, 29, 303, 312 (1963). 35. Minczewski J., R62ycki C., Chem. Anal. (Warsaw), 9, 601 (1964); 10, 463,701 (1965). 36. Djordjevid C., Tamhina B.,Anal. Chem., 40, 1512 (1968). 37. Canada D.C., Anal. Chem., 39, 381 (1967). 38. Burns D.T., Chimpalee D., Anal. Chim. Acta, 204, 365 (1988). 39. Tarasiewicz M. et al., Microchem. J., 37, 36 (1988). 40. Verdizade N.A., Amrakhov T.J., Zh. Anal. Khim., 44, 671 (1989). 41. Blanco Gomis D., Arribas Jimeno S., Sanz-Mendel A., Talanm, 29, 761 (1982). 42. Belcher R., Ramakrishna T.V., West T.S., Talanta, 12, 681 (1965). 43. West T.S., Zh. Anal. Khim., 21, 913 (1966). 44. Diaz Garcia M.E., Sanz-Medel A., Talanm, 32, 189 (1985). 45. Belcher R., Ramakrishna T.V., West T.S., Talanta, 9, 943 (1962); 10, 1013 (1963). 46. ~,iroki M., Djordjevid C., Anal. Chem., 43, 1375 (1971). 47. Siroki M. et al., Anal. Chem., 48, 55 (1976). 48. Pakalns P., Anal. Chim. Acta, 41, 283 (1968). 49. Gagliardi E., H611inger W., Mikrochim. Acre, 1972, 136. 50. Bagdasarov K.N., Osmanov Kh.A., Zavod. Lab., 34, 1044 (1968). 51. Wood D.F., Jones J.T., Analyst, 93, 131 (1968). 52. Elinson S.V., Piatiletova N.M., Novikova I.S., Zavod. Lab., 36, 659 (1970). 53. Bodkin J.B., Rogowski D.A.,Analyst, 102, 110 (1977). 54. Alimarin I.P., Savvin S.B., Talanta, 13, 689 (1966). 55. Alimarin I.P., Savvin S.B., Okhanova L.A., Talanta, 15, 601 (1968). 56. Savvin S.B. et al., Zh. Anal. Khim., 23, 1117 (1968). 57. (~i~ek Z., Dole~al J., Anal. Chim. Acta, 109, 381 (1979). 58. Ci~ek Z., Studlarova V.,Analyst, 108, 524 (1983). 59. Elinson S.V., Savvin S.B., Mirzoyan N.A., Zavod. Lab., 34, 136 (1968). 60. Childress A.E., Greenfield L.P., Anal. Chim. Acta, 116, 185 (1980). 61. Ilsemann K., Bock R., Z Anal. Chem., 274, 185 (1975). 62. Savvin S.B., Propistsova R.F., Okhanova L.A., Zh. Anal. Khim., 24, 1634 (1969). 63. Gerkhardt L.I. et al., Zavod. Lab., 39, 769 (1973) 64. Elinson S.V., Dedkov Yu.M., Tsvetkova V.T., Zh. Anal. Khim., 31, 1372 (1976). 65. Pilipenko A.T. et al., Zavod. Lab., 46, 389 (1980). 66. Sanz-Medel A. et al., Anal. Chem., 52, 1035 (1980). 67. Camara-Rica C., Gallego-Andreu R., Sanz-Medel A., Microchem. J., 28, 203 (1983). 68. Anton R.I., Marini H.J., Olsina R.A., Anal. Lett., 28, 663 (1995). 69. Nazarenko V.A., Yagnyatinskaya G.Ya., Zavod. Lab., 38, 1427 (1972); 40, 21 (1974). 70. Nazarenko V.A., Yagnyatinskaya G.Ya., Antonovich V.P., Zh. Anal. Khim., 29, 1977 (1974). 71. Nukatsuka I., Miura T., Ohzeki K., Ishida R., Anal. Chim. Acta, 248, 529 (1991). 72. Ganago L.I., Bukhteeva L.N., Zh. Anal. Khim., 34, 2186 (1979). 73. Chernysheva M.A. et al., Zh. Anal. Khim., 42, 1963 (1987). 74. Wang Z., Li J., Shen H.,Anal. Chim. Acta, 212, 145 (1988). 75. Garcia Alonso J.I., Diaz Garcia M.E., Sanz-Medel A., Talanta, 31, 361 (1984). 76. Sanz Medel A., Diaz Garcia M.E.,Analyst, 106, 1268 (1981).

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77. Sharma Y., Mikrochim. Acta, 1982 II, 297. 78. Egorova L.A. et al., Zavod. Lab., 59, No 6, 13 (1993). 79. Dutta R.K., Banerjee S., Talanta, 21, 1091 (1974). 80. Chhakkar A.K., Kakkar L.R., Mikrochim. Acta, 117, 137 (1995). 81. Patel K., Menon S.K., Agrawal Y.K., Microchem. J., 53, 158 (1996). 82. Partashnikova M.Z., Oboznenko V.A., Khrepinyuk T.E., Zh. Anal. Khim., 31, 187 (1976). 83. Jarosz M., Oszwa~dowski S., Ku~ S., Analusis, 22, 141 (1994). 84. Antonovich V.P. et al., Zh. Anal. Khim., 44, 80 (1989). 85. Stoyanova I.V. et al., Zavod. Lab., 58, No 8, 13 (1992). 86. Gromova T.A., Elinson S.V., Astafuvov V.I., Zavod. Lab., 58, No 8, 9 (1992). 87. Sarry B., Lange A., Z. Anal. Chem., 241, 186 (1968). 88. Catoggio J.A., Rogers L.B., Talanta, 9, 387 (1962). 89. Scott B.B., Analyst, 91, 506 (1966). 90. Rutkowski W., W~sowicz S., Chem. Anal. (Warsaw), 11, 971 (1966); 91. Dobkina B.M. et al., Zavod. Lab., 39, 671 (1973). 92. Nevzorov A.N., Mordvinova S.N., Kapitsa A.V., Zh. Anal. Khim., 29, 2281 (1974) 93. Malyutina T.M. et al., Zh. Anal. Khim., 29, 1429 (1974). 94. Donaldson E.M., Talanta, 30, 497 (1983). 95. Danilin E.S., Malyutina T.M., Mirontseva S.V., Zavod. Lab., 53, No 1, 11 (1987). 96. Eberle A.R., Lerner W.M., Anal. Chem., 39, 662 (1967). 97. Grossmann O., Z. Anal. Chem., 245, 135 (1969). 98. Tarayan V.M., Ovsepyan E.N., Barkhudaryan S.R., Zh. Anal. Khim., 27, 19 (1972). 99. Makarova C.B., Alimarin I.P., Zh. Anal. Khim., 19, 564, 847 (1964). 100. Gagliardi E., Wolf E., Mikrochim. Acta, 1969, 888. 101. Elinson S.V. et al., Zh. Anal. Khim., 29, 1234 (1974). 102. Kirkbright G.F., Mayhew M.D., West T.S., Anal. Chem., 40, 2210 (1968). 103. Bingham C.D., Maseda M.S., Johnson B.G., Anal. Chem., 41, 1144 (1969). 104. Amelina V.G., Chernova R.K., Zh. Anal. Khim., 38, 1446 (1983). 105. Antonovich V.P. et al., Zh. Anal. Khim., 39, 480 (1984). 106. Tsykhanskii V.D., Konusova V.V., Nazarenko V.A., Zh. Anal. Khim., 30, 2390 (1975). 107. Wu Z., Hu Z., Jia X., Anal. Chim. Acta, 231, 101 (1990). 108. Elinson S.V., Pobedina L.I., Rezova A.T., Zavod. Lab., 37, 521 (1971). 109. Piryutko M.M., Benediktova N.V., Zh. Anal. Khim., 30, 1556 (1975). 110. Aznarez J., Vidal J.C., Vaquero C., Coll. Czech. Chem. Comm., 55, 1686 (1990). 111. Sizonenko N.T., Egorova L.A., Timchenko A.K., Zh. Anal. Khim., 34, 2182 (1979). 112. Tsurumi C., Furuya K., Kamada H., Analyst, 106, 944 (1981). 113. Horak J., Oka6 A., Coll. Czech. Chem. Comm., 28, 2563 (1963). 114. Ackermann G., Koch S., Talanta, 16, 95,284, 288 (1969); 17, 757 (1970). 115. Guyon J.C.,Anal. Chim. Acta, 30, 395 (1964). 116. Verdizade N.A., Amrakhov T.I., Zh. Anal. Khim., 44, 671 (1989). 117. Kvaratskheli Yu.K. et al., Zh. Anal. Khim., 45, 67 (1990). 118. Iyer C.S., Kamath V.A., Talanta, 27, 537 (1980). 119. Bergstresser K.S.,Anal. Chem., 31, 1812 (1959). 120. Shrimal S.K., Varde M.S.,Anal. Chim. Acta, 33, 683 (1965). 121. Esson J., Analyst, 90, 488 (1965). 122. Sattler P.F., Schreinlechner I.E., Anal. Chem., 49, 80 (1977). 123. Godovskaya K.I., Babenko A.S., Alferov E.A., Zavod. Lab., 39, 1169 (1973). 124. Williams A.I., Analyst, 92, 43 (1967).

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125. Kavipurapu C.S., Srivastava S.C., Gupta K.K., Pandey L.P., Mikrochim. Acta, 111, 127 (1993). 126. Ptushkina M.N., Baranova I.O., Zavod. Lab., 57, No 3, 14 (1991). 127. Basargin N.N. et al., Zavod. Lab., 55, No 5, 12 (1989). 128. Brykina G.D., Lebedeva G.G., Shpigun O.A., Zavod. Lab., 59, No 1, 4 (1993). 129. Koz'menko O.A., Mustafaev R.F., Lazutkin A.M., Zavod. Lab., 57, No 6, 7 (1991). 130. Webb H.W., Ashworth V., Hills J.M., Analyst, 88, 142 (1963). 131. Tietze B., Mikrochim. Acta, 1972, 658. 132. Theodore M.L.,Anal. Chem., 30, 465 (1958). 133. Wood D.F., Scholes I.R.,Anal. Chim. Acta, 21, 121 (1959). 134. Hibbits J.O. et al., Talanta, 8, 209 (1961). 135. Kuchminstaya G.I., Nadezhdina G.B., Dobkina B.M., Zavod. Lab., 36, 275 (1970). 136. Dobkina B.M. et al., Zh. Anal. Khim., 22, 1510 (1967).

Chapter 34. Nitrogen Nitrogen (N, at. mass 14.01) is a chemically inert gas (N2) at room temperature. It occurs in its compounds in the following oxidation states: -III in ammonia, -II in hydrazine, -I in hydroxylamine, III in nitrite, and V in nitrate. From other elements it is usually separated as ammonia.

34.1. Separation of nitrogen as ammonia The separation of small amounts of nitrogen as ammonia consists in distillation from alkaline medium [1-4]. Ammonia escapes quantitatively when a strong base is added in excess and the solution is subsequently heated. The ammonia and steam driven off are condensed and collected in dilute acid. To prevent the liquid from being superheated, porous porcelain chunks are added to the still. Ammonia has also been separated by vacuum distillation and by isothermal diffusion. Nitrate and nitrite nitrogen are reduced to ammonia which is then distilled off [5]. Powdered Devarda's alloy (50% Cu, 45% A1, 5% Zn) in a cold alkaline medium is used as the reducing agent. The reaction is carried out in the same flask from which ammonia is afterwards distilled. The flask is cooled with water if the reduction is too rapid. In Kjeldahl's method, organic nitrogen is converted into ammonia which is separated by distillation. The method entails boiling a sample with concentrated H2804 in the presence of a catalyst (Hg, Cu, or Se) [1,6]. If the nitrogen is present in a group which contains oxygen, a reduction step is also necessary. Amine- and imine nitrogen forms are converted into ammonia, which is retained in the solution as the ammonium ion and can be titrated with hydrochloric acid. The method is used for determination of nitrogen in organic compounds [7,8]. Nitrogen has also been determined in an automatic system [9]. The results of determining nitrogen in water by the FIA technique and by the classic method have been compared [10]. Optimum conditions for sample decomposition by microwaves, before the separation of nitrogen by the Kjeldahl method, have been studied [ 11 ].

34.2. Methods for ammonia determination Two methods of similar sensitivity are commonly used for determining ammonia. These are the classical Nessler method and the indophenol method. The indophenol method is more convenient since the blue reaction product, which is the basis for the spectrophotometric measurement, is soluble in water.

34.2.1. Indophenol method The reaction of ammonia with hypochlorite and phenol in an alkaline medium (Berthelot's reaction) yields a blue product which is the basis of a sensitive and specific spectrophotometric method for determining nitrogen as ammonia [12-19]. The probable mechanism of the reaction is:

305

34.3. Methods for nitrite determination

'••

CIO-+NHa

OH ~

0

N--CI

0,...._2_

+

@OH

.-~

(34.1)

H§

Chloramine, which is formed as the first step of the reaction, reacts with phenol to yield quinonechloramine. This reacts with another phenol molecule to give indophenol. The blue colour is due to the indophenol anion, formed in alkaline medium. The intensity of the blue colour is greatly increased by adding a little acetone (-0.2 ml of acetone per 25 ml of solution). The molar absorptivity a t )~max "- 625 nm is 4.5.103 (a = 0.32). The indophenol can be extracted with isobutyl- or isoamyl alcohol after adding a considerable amount of sodium chloride to the aqueous solution as a salting-out agent. The organic extract is, however, less intensely coloured than a corresponding solution in aqueous acetone [20]. The reaction is similar if thymol is used instead of phenol [21,22]. Ammonia is usually determined after being separated from a strongly alkaline medium and then absorbed in dilute H2SO4 or HC1. It is sometimes possible to carry out the indophenol reaction without separating the ammonia (e.g., in natural waters). In the presence of EDTA, moderate quantities (0.1-0.5 mg) of Ca, Mg, and A1 do not interfere. The addition of tartrate prevents the precipitation of hydrolysable metals. Phosphate interferes in the colour reaction [23]. Since the sensitivity of the method is often limited by the high blank values caused by the presence of traces of ammonia in the reagents, purification of the reagents by distilling ammonia from their alkaline solutions may be necessary.

Reagents Phenol-acetone solution. Dissolve 70 g of phenol in 15 ml of ethanol, add 20 ml of acetone, and dilute the solution with ethanol to 100 ml. Sodium phenolate solution. Immediately before use, mix 10 ml of the phenol-acetone solution with 10 ml of 30% aqueous NaOH solution, and dilute with water to 50 ml. Sodium hypochlorite, 2% solution. (Check the content of C10- in the solution iodometrically). Standard ammonia solution: 1 mg NH3/ml. Dissolve 3.1410 g of ammonium chloride (previously dried at --100~ in water, and dilute the solution with water to 1 litre in a volumetric flask. Standard ammonia solution: 1 mg N/ml. Prepare as above but with 3.8190 g of dried NH4C1. Sodium hydroxide: 30% aqueous solution. Boil the solution for 10-15 min in an open vessel to remove traces of ammonia.

Procedure

Distillation of NH3. Place the sample solution containing ammonia in a 70-150 ml still. Immerse the condenser outlet in a receiver containing 5 ml of water and 5 drops of 0.1 M

306

34. Nitrogen

H2SO4. Pour 10-20 ml of 30% NaOH into the still, and dilute with water to 40-80 ml. Add a few fragments of porous porcelain to promote regular ebullition, and distil a quarter of the liquid volume from the still into the receiver. Determination of NH3. Place all or part of the distillate, containing not more than 40 gg of NH3, in a 25-ml standard flask. Add 2.5 ml of the sodium phenolate solution and 1 ml of the sodium hypochlorite solution. Dilute the solution to the mark with water, and mix thoroughly. After 30 min, measure the absorbance at 625 nm against water (or a reagent blank solution when traces of ammonia are being determined).

Note. For determination of traces of ammonia, the sample solution should be made slightly acidic with 0.5 ml of 0.1 M H2S04, and concentrated by evaporation, before the distillation. 34.2.2. Nessler's method In 1856, Nessler recommended an alkaline solution of mercury(II) iodide and potassium iodide as a reagent for the determination of ammonia [24]. Nessler's reagent reacts with ammonia in an alkaline medium, to give a brown-orange, sparingly soluble product, according to the following equation: 2HgI42- + NH3 + 3OH- --->NH2Hg2IO$ + 7I- + 2H20 Since the product forms a stable dispersion only at very low concentrations, protective colloids such as gum arabic, gelatine, or poly(vinyl alcohol) are added. The molar absorptivity at e~max = 370 nm is 6.8.103; at 400 nm, it is 5.1.103 (specific absorptivity 0.36). The determination of ammonia is usually preceded by a distillation from strongly alkaline solution. Nessler's method is commonly used for determining ammonia in natural waters. Since Ca and Mg present in water interfere, they are masked with tartrate.

Reagents Nessler's reagent. Dissolve 2.5 g of KI in l0 ml of water. Add saturated HgC12 solution until a permanent precipitate forms. Add 25 ml of 30% NaOH solution, dilute to 200 ml with water, and mix well. Decant the clear solution from the precipitate, and store the solution in an amber-glass bottle. Standard ammonia solution. Preparation as in Section 34.2.1. Potassium sodium tartrate (Seignette salt), 20% solution. Remove traces of ammonia by making the solution alkaline with sodium hydroxide, and boiling for 10-15 min.

Procedure Place the clear, colourless, neutral solution, containing not more than 50 ~tg of NH3, in a 25ml standard flask, add 1 ml of the tartrate solution, 1 ml of 1% gum arabic solution, and 1 ml of Nessler's reagent, and dilute the solution to the mark with water. After 10 min, measure the absorbance of the pseudo-solution at 400 nm, using a reagent blank solution as reference.

307

34.3. Methods for nitrite determination

34.2.3. Other methods There are methods, similar to the indophenol method, in which coloured products of ammonia reaction are formed in alkaline media with the use of salicylic and dichlorocyanuric acids [25], phenol, hypochlorite and nitroprusside [26], or salicylic acid, hypochlorite, and nitroprusside (e = 1.5.104 at 698 nm) [27,28]. The last method has been used for determining ammonia in soils [29]. Ammonia may also be oxidized to nitrite, which is then determined by a suitable method

[30].

34.2.4. Analytical applications The indophenol method has been applied for determination of nitrogen (as ammonia) in biological materials [31,32], plant materials [33,34], foods [1,2,35], air [36], boiler water [37], and other waters [38-40], organic substances [17,41], refractory alloys [42], tantalum alloys [43], vanadium, titanium, and uranium [21], alkali and alkaline earth metals [22]. The indophenol method has been applied also in a fully automated version (including distillation of ammonia) [2,3,13,44] and with the FIA technique [45,46]. The two versions were applied for determining nitrogen in soil extracts [13,47], plants [45], natural waters [3,44,46,48], and crude oil [49]. Nessler's method has been used for the determination of nitrogen (as ammonia) in biological materials [32], plant materials [50], air [51], waters [39,52,53], fodder [54], and tungsten [55]. Nitrogen has been determined in soils and plants by the FIA technique with the use of salicylic acid, nitroprusside, and dichloroisocyanurate [56]. Nessler's method has been applied in the FIA technique [50,53,57].

34.3. Methods for nitrite determination The classical Griess method for nitrite determination is very well known. However, some of the more recent methods which are also based upon the formation of azo dyes may be better. A critical review of the nitrite determination methods has been published [58].

34.3.1. Modified Griess method In an acid medium, nitrite reacts with primary aromatic amines to form a diazonium salt. The salt is then coupled with a suitable aromatic compound containing an-NH2 o r - O H group to yield an azo dye which is the basis of the spectrophotometric method [59,60]. In the Griess method (1879), nitrite, sulphanilic acid, and 1-naphthylamine are reacted as follows: HO3S--~NHz

+

NOz

+

~

2H+

NHz

~

"•+ _•

HO~S

HO3S

N~N

~

N"'~--~N

+

2Hz0

NHz

+

H+(34"2)

308

34. Nitrogen

The Griess method is highly sensitive. The molar absorptivity at )~max= 520 nm is 4.0-104, specific absorptivity 2.8). The method is specific, but only of moderate precision. The solution in which nitrite is determined must not contain oxidants, reductants, or coloured substances. Neither should urea or aliphatic amines be present since they may react with nitrite to liberate free nitrogen. Copper(II) ions catalyse the decomposition of the diazonium salt, thereby causing low results. Since 1-naphthylamine is a well-known carcinogen [61], it is recommended to replace it by 1-naphthylamine-7-sulphonic acid. This reagent is used in the procedure given below.

Reagents Sulphanilic acid solution. Dissolve 0.50 g of sulphanilic acid in 120 ml of water and 30 ml of glacial acetic acid. Store in a brown bottle. 1-Naphthylamine-7-sulphonic acid solution. Dissolve 0.50 g of the reagent in 120 ml of hot water. Filter, cool, and add 30 ml of glacial acetic acid. Store in a brown bottle. Standard nitrite solution: 1 mg NOz/ml. Dissolve 1.5000 g of anhydrous NaNO2 in water, add 1 ml of CHC13 to stabilize the solution, and 0.2 g of NaOH, and dilute with water to 1 litre in a volumetric flask.

Procedure To the neutral solution (-~15 ml), containing not more than 20 btg of nitrite (NO2-), add 1.0 ml of the sulphanilic acid solution, mix, and allow to stand for 10 min. Add 1.0 ml of the 1naphthylamine-7-sulphonic acid solution, dilute to the mark in a 25-ml standard flask, and mix. After 20 min, measure the absorbance at 520 nm, using a reagent blank solution as reference.

34.3.2. Other methods A number of other organic compounds is suitable for the diazotization and coupling reactions [62-69]. For example, the reaction with o-nitroaniline and N-(1naphthyl)ethylenediamine leads to a method with e = 6.0.104 at 545 nm [68]. In some cases, 8-hydroxyquinoline is used for the coupling [70,71]. Many azo dyes formed by the use of various reagents are extractable [72-74]. In one of the methods [74], with hexanol as extractant, e = 5.2.104 at 610 nm. Some authors [75-77] have proposed using flow-injection analysis for determining nitrite, through the formation of azo dyes. Other reagents used for determining nitrites have been reviewed [78-80].

34.3.3. Analytical applications The Griess method has been applied widely for determining nitrite in foods and waters. The method has also been used for automatic determination of nitrite (and NO3- after reduction to NO2-) in waters and in soil extracts [81], in living cell fluids [82], and in blood [83]. The methods based on azo dyes have found application in determinations of nitrite in soil extracts [67,68], blood [73,84], waters [63,65,77], waste-water effluents [84a], and fruits [84b].

34.4. Methods for nitrate determination

309

34.4. Methods for nitrate determination Most spectrophotometric methods for determining nitrate are based either on: 1), nitration or oxidation of appropriate organic reagents to form coloured compounds or on, 2), reduction of NO3- to NO2- or NH3 with subsequent determination of this species. Prevalent among the methods belonging to the first group are those using phenoldisulphonic acid, and xylenols.

34.4.1. Phenoldisulphonic acid method Reaction between 1-phenol-2,4-disulphonic acid (1-hydroxybenzene-2,4-disulphonic acid) and HNO3 occurs when a dry sample (or dry residue from the evaporation of the sample solution) containing nitrate is mixed with a solution of the reagent in concentrated ti2SO4. The reaction product, nitrophenoldisulphonic acid, is pale yellow, but when the solution is made alkaline, the intensely coloured anion which is the basis of this spectrophotometric method is formed [85]. The reaction is specific for nitrate. The absorption maximum of the nitrophenoldisulphonic acid is at 410 nm. The molar absorptivity is 9.4.103 (a = 0~ Neither the nature nor the excess of the reagent used to raise the pH (NH3, NaOH, KOH) affects the colour. Chloride causes low results owing to the reaction between HCI and HNO3 when the phenoldisulphonic acid in concentrated sulphuric acid is added to the sample (3C1-~ NO3 ~+ 4H+ --~ C12 + NOCI + 2H20). This effect is, however, negligible if the amount of chloride present is less than twice that of nitrate. Larger amounts of chloride should be separated beforehand by precipitation as AgCI. Since the presence of silver ions in the solution after removal of AgC1 is detrimental, the excess silver is precipitated with sodium phosphate. If the concentration of chloride in the solution is known exactly, it is better to use a stoichiometric (or slightly lesser) quantity of silver sulphate as precipitant. Nitrite interferes, since it may be partially converted into nitrate under the conditions of the determination. When the concentration of nitrite is not higher than that of nitrate, the effect is negligible. Large quantities of nitrite must be removed, e.g., by reduction with sodium azide, urea, or hydrazine. Ammonium ions cause low results when nitrate is determined. Preliminary expulsion of ammonia by heating the solution after it has been made alkaline with sodium hydroxide is recommended. EDTA is added before the evaporation when larger quantities of Ca and Mg are present, thus preventing precipitation when the solution is finally made alkaline. To prevent loss of HNO3 during evaporation to dryness, the solution is neutralized with NaOH or CaCOs.

Reagents Phenoldisulphonic acid, solution in conc. H2SO4. Dissolve 12.5 g of phenol in 75 ml of conc. H2SO4, add 37.5 ml of 13% oleum, and stir well. Heat the solution in a 250-ml conical flask for 2 hr on a boiling water-bath with occasional stirring. Standard nitrate solution: 1 mg NO3-/ml. Dissolve in water 1.6310 g of KNO3 (previously dried at 110~ and dilute the solution with water to 1 litre in a volumetric flask. Silver sulphate solution. Dissolve in water 1.10 mg of Ag2SO4 and dilute the solution with water to 250 ml in a volumetric flask. One ml of solution is equivalent to 1 mg of CI-. Calcium carbonate suspension. Mix 1 g of CaCO3 with 100 ml of water.

310

34. Nitrogen

Procedure Neutralize the solution (containing not more than 100 gg of NO3-) in a small evaporating dish, add 1 ml of the CaCO3 suspension, and evaporate to dryness on a water-bath. Treat the cooled residue with 1 ml of phenoldisulphonic acid reagent and stir well. After 5 min, dilute with 10 ml of water, and transfer the solution quantitatively to a 25-ml standard flask. Add conc. ammonia solution until the solution becomes intensely yellow, then add 5 ml more, and dilute the solution with water to the mark. Measure the absorbance of the yellow solution at 410 nm against water or a reagent blank solution.

Notes. 1) If, after being made alkaline, the solution becomes turbid or a precipitate is formed, the solution should be filtered before the absorbance is measured. 2) If the amount of C1- present exceeds twice that of NO3-, the former should be separated by adding a slightly less than stoichiometric quantity of AgzSO4 to the acid solution. After 30 rain, the precipitated AgC1 is filtered off, and the filter paper is washed with a small volume of dilute NazSO4 solution. The combined filtrate and washings are neutralized, and analysed for nitrate as described above. 34.4.2. Other methods The following dimethylphenols yield coloured nitration products in sulphuric acid media: 2,6-xylenol (e = 7.9.103 at 330 nm) [86], 2,4-xylenol [87,88], and 3,4-xylenol [89,90]. Other organic reagents nitrated include: phenol [91,92], 2-butylphenol [93], 4,5-dihydroxycoumarin [94], 2,7-diaminofluorene [95], and resorcinol [96]. Some sensitive spectrophotometric methods for determining nitrate utilize extractable ion-associates of the nitrate ion with the basic dyes: Crystal Violet (chlorobenzene, pH 6) [97], Nile Blue A [98], and Methylene Blue (1,2-dichloroethane) [99]. Nitrogen has been determined also by the FIA technique with the use of Malachite Green [ 100]. Nitrate is often determined, after reduction to nitrite by cadmium [64,76,101-103], hydrazine [104], or titanium trichloride [105] as an azo dye. The flow-injection technique has been often applied [75,76,104,106,107]. Nitrate can also be determined after reduction to ammonia [5].

34.4.3. Analytical applications Nitrate has been determined, after the reduction to nitrite, in soil extracts [81,103], waters [48,64,81,101,103-105,107], and plants [106]. Nitrite has been determined in soil extracts also after the reduction to ammonia [28]. The reactions of nitration have been applied in the determination of nitrate in waters, plants, vegetables, and soil extracts [89,91 ]. Nitrate has been determined by other methods in waters [86,90] and vegetables [93].

34.5. Determination of other nitrogen compounds Hydrazine (NH2.NH2) can be determined with the use of 2-hydroxy-l-naphthaldehyde (e = 2.7-104 at 412 nm) [108], and pyridylpyridinium chloride [109]. A yellow Schiff's base is obtained in condensation of hydrazine with vanillin in acid media (e = 5.3-10 4 at 400 nm) [110]. The reduction of Ag + by hydrazine yields a red sol of silver (~ = 4.2.10 4 at 415 nm)

34.5. Determinationof other nitrogen compound

311

[111]. In an indirect method hydrazine is determined with the use of Fe(III) and ferrozine [112]. Hydroxylamine (NH2OH) can be determined by its colour reaction with di(2-pyridyl) ketone guanylhydrazone [113]. It can also be determined in an indirect method by its reaction with biacetyl to give dimethylglyoxime, which then reacts with Ni(II) [114]. The FIA technique has been applied in the determination of NH2OH with the use of ferrozine [115]. A review has been given of methods used for the determination of hydrazine [116]. Nitrogen dioxide absorbed in alkali can react to form azo dyes [117-120]. These diazotization reactions are suitable for continuous automatic monitoring of atmospheric NO 2 [117]. Oxides of nitrogen in cigarette smoke have been determined with brucine [121]. The amount of NO in liquid N204 (0~ can be determined from the green colour of the N203 [122]. NO2 in the atmospheric air has been absorbed in Cu(DDTC)2 in toluene and determined from the decrease in the initial absorbance at 437 nm [123]. In a proposed indirect method, NO reduces Cu(II) to Cu(I), which is then determined with the use of cuproine [ 124]. Azide (N3-) forms a coloured complex with Cu(II), which has ~maxat 375 nm [125]. Azide reduces Ce(1V) to Ce(III) which gives a coloured complex with Arsenazo III [126]. Azide can also by determined from the decrease of yellow colour of Ce(IV) which is reduced by azide [127].

References 1. Thaler H., Sturm W., Z. Anal. Chem., 244, 379; 246, 315 (1969), 250, 120; 251, 30 (1970). 2. Kaul A.K., Sharma T.R., Z. Anal. Chem., 280, 133 (1976). 3. Crowther J., Evans J., Analyst, 105, 84 l, 849 (1980). 4. Kawakubo S., Fukasawa T., Anal. Chim. Acta, 149, 363 (1983). 5. Bremner J.M., Keeney D.R., Anal. Chim. Acta, 32, 485 (1965). 6. McKenzie H.A., Trends Anal. Chem., 13, 138 (1994). 7. Shirai M., Kawashima T., Bull. Chem. Soc. Jpn., 66, 2541 (1993). 8. Yoshikuni N., Talanta, 41, 89 (1994). 9. Davidson J., Mathieson J., Boyne A.W., Analyst, 95, 181 (1970). 10. Kroon H.,Anal. Chim. Acta, 276, 287 (1993). I 1. Feinberg M.H., Ireland-Ripert J., Mourel R.M., Anal. Chim. Acta, 272, 83 (1993). 12. Oliver R.,Analusis, 6, 126 (1978). 13. Keay J., Menage P.M., Analyst, 94, 895 (1969); 95, 379 (1970). 14. Glebko L.I., Ulkina Zh.I., Lognenko E.M., Mikrochim. Acta, 1975 II, 641. 15. Patton C.J., Crouch S.R., Anal. Chem., 49, 464 (1977). 16. Searle P.L., Analyst, 109, 549 (1984). 17. Reay P.F.,Anal. Chim. Acta, 176, 275 (1985). 18. Harfman R.G., Crouch S.R., Talanta, 36, 261 (1989). 19. Kempers A.J., Kok C.J., Anal. Chim. Acta, 221, 147 (1989). 20. Namiki M., Kakita Y., Goto H., Talanta, 11, 813 (1964). 21. Hashitani H., Yoshida H., Adachi T.,Anal. Chim. Acta, 76, 85 (1975). 22. Malikova E.D., Kunin L.L., Uritskaya T.P., Zh. Anal. Khim., 32, 88 (1977). 23. Ngo T.T., Phan A.P., Yam C.F., LenhoffH.M.,Anal. Chem., 54, 46 (1982). 24. M611er G., Z. Anal. Chem., 245, 155 (1969). 25. Krom M.D., Analyst, 105, 305 (1980). 26. Guebel D.V., Nudel B.C., Giulietti A.M., Biotechnol. Tech., 5, 427 (1991).

312

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27. Qiu X.C., Zhu Y.Q., Analusis, 15, 254 (1987). 28. Sims G.K., Ellsworth T.R., Mulvaney R.L., Commun. Soil Sci. Plant Anal., 26, 303 (1995). 29. Qiu X.C., Liu G.P., Zhu Y.Q.,Analyst, 112, 909 (1987). 30. Matsunaga K., Nishimura M., Anal. Chim. Acta, 73, 204 (1974). 31. Mann L.T., Anal. Chem., 35, 2179 (1963). 32. Jacobs S., Crit. Rev. Anal. Chem., 7, 297 (1978). 33. Reis B.F. et al., Anal. Chim. Acta, 119, 305 (1980). 34. Mroczkowski W., Cygafiski A., Chem. Anal. (Warsaw), 28, 319 (1983). 35. Fogg A.G., Summan A.M.,Analyst, 108, 691 (1983). 36. Leithe W., Petschl G., Z. Anal. Chem., 230, 344 (1967). 37. Tetlow J.A., Wilson A.L., Analyst, 89, 453 (1964). 38. Boo H.K., Ma T.S., Mikrochim. Acta, 1976 II, 515. 39. Blauchard P., Madec C., Courtot-Coupez J.,Analusis, 10, 155 (1982). 40. Analytical Quality Control Committee, Analyst, 107, 680 (1982). 41. Strukova M.P., Veslova G.I., Zh. Anal. Khim., 28, 1025 (1973). 42. Kallmann S., Hobart E.W., Oberthin H.K., Brienza W.C., Anal. Chem., 40, 332 (1968). 43. Davis W.F., Graab J.W., Merkle E.J., Talanta, 18, 263 (1971). 44. Crowther J., Wright B., Wright W., Anal. Chim. Acta, 119, 313 (1980). 45. Stewart J.W., Ru2i6ka J., Filho H.B., Zagatto E.A., Anal. Chim. Acta, 81, 371; 82, 137 (1976). 46. Krug F.J. et al., Anal. Chim. Acta, 151, 39 (1983). 47. Selmer-Olsen A.R., Analyst, 96, 565 (1971). 48. Takeda K., Fujiwara K., Water Res., 30, 323 (1996). 49. Heistand R.N., Anal. Chem., 42, 903 (1970). 50. Van Ginkel J.H., Sinnaeve J., Analyst, 105, 1199 (1980). 51. Buck M., Stratmann H,, Z. Anal. Chem., 213, 241 (1965). 52. Crosby N.T., Analyst, 93, 406 (1968). 53. Bergamin H. et al.,Anal. Chim. Acta, 117, 81 (1980). 54. Williams P.C., Analyst, 89, 276 (1964). 55. Awasthi S.P., Sahasranaman S., Sundaresan M., Analyst, 92, 650 (1967). 56. McLeod S., Anal. Chim. Acta, 266, 113 (1992). 57. Krug F.J., Ru2i6ka J., Hansen E.H., Analyst, 104, 47 (1979). 58. Fox J.B., Crit. Rev. Anal. Chem., 15, 283 (1985). 59. Fox J.B.,Anal. Chem., 51, 1493 (1979). 60. Norwitz G., Keliher P.N.,Analyst, 110, 689 (1985); 111, 1033 (1986). 61. Bunton N.G., Crosby N.T., Patterson S.J.,Analyst, 94, 585 (1969). 62. R6hl K., Sch6nherr S., Talanta, 25, 535 (1978). 63. Chao K.C., Higuchi T., Sternson L.A., Anal. Chem., 50, 1670 (1978). 64. Okada M., Miyata H., T6ei K.,Analyst, 104, 1195 (1979). 65. Flamerz S., Bashir W.A.,Analyst, 106, 243 (1981). 66. Tsao F.P., Underwood A.L., Anal. Chim. Acta, 136, 129 (1982). 67. Wu Q.F., Liu P.F., Talanta, 30, 374 (1983). 68. Chanbe A., Baveja A.K., Gupta V.K., Talanta, 31, 391 (1984). 69. Norwitz G., Keliher P.N., Analyst, 109, 1281 (1984). 70. Bashir W.A., Flamerz S., Talanta, 28, 697 (1981). 71. Chanbe A., Baveja A.K., Gupta V.K., Anal. Chim. Acta, 143, 273 (1982). 72. T6ei K., Kiyose T., Anal. Chim. Acta, 88, 125 (1977). 73. Renner G., Hopfer C., Z. Anal. Chem., 304, 405 (1980).

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74. Baveja A.K., Nair J., Gupta V.K., Analyst, 106, 955 (1981). 75. Anderson L., Anal. Chim. Acta, 110, 123 (1979). 76. Gin6 M.F. et al.,Anal. Chim. Acta, 114, 191 (1980). 77. Nakashima S. et al., Anal. Chim. Acta, 155, 263 (1983). 78. Raman V., Dabras M.S., Microchem. J., 40, 242 (1989). 79. Nikonorov V.V., Moskvin L.N., Zh. Anal. Khim., 51, 737 (1996). 80. Kawakami T., Igarashi S., Anal. Chim. Acta, 354, 159 (1997). 81. Henriksen A., Selmer-Olsen A.R., Analyst, 95, 514 (1970). 82. Boudard F., Vallot N., Cabaner C., Bastide M., J. Immunol. Methods, 174, 259 (1994). 83. Litchfield M.H., Analyst, 92, 132 (1967). 84. Shechter H., Gruener N., Shuval H.J., Anal. Chim. Acta, 60, 93 (1972). 84a. Staden F.J., Merwe T.A., Microchim. Acta, 129, 35 (1998). 84b. Wang G.F., Satake M., Horita K., Talanta, 46, 671 (1998). 85. Mubarak A., Howald R.A., WoodriffR.,Anal. Chem., 49, 857 (1977). 86. Andrews D.W., Analyst, 89, 730 (1964). 87. Norwitz G., Keliher P.N., Anal. Chim. Acta, 98, 323 (1978); 109, 373 (1979). 88. Norwitz G., Farino J., Keliher P.N., Anal. Chim. Acta, 105, 335 (1979). 89. Elton-Bott R.R., Anal. Chim. Acta, 90, 215 (1977). 90. Osibanjo O., Ajayi S.O., Analyst, 105, 908 (1980). 91. Elton-Bott R.R.,Anal. Chim. Acta, 108, 285 (1979). 92. Velghe N., Claeys A.,Analyst, 108, 1018 (1983). 93. Tanaka A., Nose N., Iwasaki H., Analyst, 107, 190 (1982). 94. Nakamura M., Analyst, 106, 483 (1981); Mikrochim. Acta, 1983 II, 69. 95. Hainberger L., Nozaki J., Mikrochim. Acta, 1979 I, 75; 1979 II, 187. 96. Velgh N., Claeys A.,Analyst, 110, 313 (1985). 97. Baca P., Freiser H., Anal. Chem., 49, 2249 (1977). 98. Pokorny G., Likussar W., Anal. Chim. Acta, 42, 253 (1968). 99. Ciesielski H., Soignet G. et al., Analusis, 6, 38 (1978). 100. Aoyagi M., Yasumasa Y., Nishida A.,Anal. Sci., 5, 235 (1988). 101. Davison W., Woof C.,Analyst, 104, 385 (1979). 102. Elliott R.J., Porter A.G.,Analyst, 96, 522 (1971). 103. Pandurangappa M., Balasubramanian N., Mikrochim. Acta, 124, 137 (1996). 104. Madsen B.C.,AnaL Chim. Acta, 124, 437 (1981). 105. A1-Wehaid A., Townshend A.,Anal. Chim. Acta, 186, 289 (1986). 106. Gin6 M.F. et al., Anal. Chim. Acta, 155, 131 (1983). 107. Nakashima S. et al., Z. Anal. Chem., 319, 506 (1984). 108. Mafies J., Campillos P., Font G., Martre H., Prognon P., Analyst, 112, 1183 (1987). 109. Asmus E., Ganzke J., Schwarz W., Z. Anal. Chem., 253, 102 (1971). 110. Amlathe S., Gupta V.K., Analyst, 113, 1481 (1988). 111. Pal T., Maity D.S., Ganguly A., Analyst, 111, 1413 (1986). 112. Dias F., Olojola A.S., Jaselskis B., Talanta, 26, 47 (1979). 113. Kavlentis E., Microchem. J., 37, 22 (1988). 114. Pittwell L.R., Mikrochim. Acta, 1975 II, 425. 115. Bourke G.C., Stedman G., Wade A.P.,Anal. Chim. Acta, 153, 277 (1983). 116. Kolasa T., Wardencki W., Talanta, 21, 845 (1974). 117. H~intzsch S., Nietruch F., Prescher K.E., Mikrochim. Acta, 1969, 550. 118. Fisher G.E., Becknell D.E. Anal. Chem., 44, 863 (1972). 119. Bultez A.,Analusis, 2, 190 (1973). 120. Ishii K., Aoki K., Anal. Chem., 55, 604 (1983).

314

34. Nitrogen

121. Smith G.A., Sullivan P.J., Irvine W.J., Analyst, 92, 456 (1967). 122. Wright C.M., Orr A.A., Bailing W.J.,Anal. Chem., 40, 29 (1968). 123. Zhelyazkova B.G., Yardev P.B., Yordanov N.D., Talanta, 30, 185 (1983). 124. Kinoshita S., Wakita H., Masuda I., Anal. Chim. Acta, 169, 373 (1985). 125. Neves E.A., Oliveira E., Sant'Agostino L., Anal. Chim. Acta, 87, 243 (1976). 126. Kubaszewski E., Kurzawa Z., Chem. Anal. (Warsaw), 30, 609 (1985). 127. Terpinski E.A., Analyst, 110, 1403 (1985).

Chapter 35. O x y g e n Oxygen (O, at. mass 16.00) is a gas, 0 2 ( 0 3 in ozone). It occurs in most compounds in the -II oxidation state, and in the -I state in peroxides. With other elements, it forms numerous 9 222+ 2+ oxide complexes such as C r O 4 , M o O 4 , V O , U O 2 , 8 0 4 2 - , NO3-. Volatile oxides include OsO4 and CO2. Oxygen compounds of great importance in analysis are hydrogen peroxide and the peroxide complexes of a number of metals, e.g., Ti, V, Nb, U, and Zr.

35.1. Determination of oxygen The well-known titrimetric method for determining oxygen in water is based on oxidizing the Mn(II) in Mn(OH)2 with oxygen. After the addition of KI and acidification of the solution with H2SO4, an equivalent amount of iodine is liberated. In the spectrophotometric modification of the method the iodine is determined either as a blue complex with starch, or after the iodine has been extracted into chloroform. Oxygen impurities (0.1-0.0001%) in various gases can be determined by the colour reaction with anthraquinone-2-sulphonate in alkaline solution. The red solution of the reagent (reduced with zinc amalgam) is decolorized when oxidized by oxygen. This reaction is suitable for the continuous spectrophotometric determination of oxygen [ 1]. The colour reaction with Indigo Carmine is useful for determining oxygen dissolved in water [2-4]. The yellow reduced form of the reagent (leucobase) turns red 0~max = 555 nm) under the influence of oxygen. Leuco forms of other dyes have also been proposed for determining oxygen. These include Methylene Blue [4-7], Berbelin Blue (formula 35.1) [8], Methyl Red [9], and Safranine T [4].

~

eHs

~6Hs I

,

if" "}'Oz H3C"

~

"N-

I

v

-CH z

+

=

4" OHH~C"

v

-'Nr,-"

~-"

(35.1)

"C 3

H

In other methods for determining oxygen in water, 3,3-dimethylnaphthidine [10], DCTA [+ Mn(II)] [11,12], and EDTA [+ Mn(II)] [13] have been used. The determination of oxygen in water has also been based on the reduction of absorbance by a sol of gold in the presence of cyanide (formation of a colourless complex Au(CN)2- ) [14] and a silver cyanide complex [15]. The oxidation of Fe(II) to Fe(III) [determination of Fe(II) with 1,10phenanthroline] has been used for determining oxygen in semiconducting oxides [16]. The colour reduction of the blue Fe(II) complex with Cacotheline Blue has also been used for the determination of oxygen dissolved in water [17].

35.2. Determination of ozone Since ozone is a stronger oxidant than oxygen, it is determined on the basis of colour redox reactions with reagents which are not oxidized by oxygen.

316

35. Oxygen

The iodine liberated by ozone from slightly acidic iodide solutions can be determined by the iodine-starch method [18]. This method has been used for determining ozone in ozonised air [18a]. Organic reagents which have been used for spectrophotometric determination of ozone include 1,1-diphenylethylene [19,20], 1,2-di-(4-pyridyl)ethylene [21], indigo disulphonate (c = 2.2.104) [22], and indigo trisulphonate [23]. Determination of ozone in air is based on its reaction with bis(terpyridyl)iron(II) [24]. In another method ozone oxidizes Mn(II) in phosphoric acid medium to Mn(III), which is then made to react with o-tolidine [25]~ The data on ozone determination methods and continuous automatic determination of Os have been collected [26].

35.3. Deternfination of hydrogen peroxide Spectrophotometric methods for determining hydrogen peroxide are based on its capacity to form stable peroxide complexes as well as on its oxidizing and reducing properties. A widely known, but relatively insensitive, method is the titanium method [27-29], based on the orange-yellow titanium peroxide complex formed in acid ([-12SO4) medium. The titanium peroxide- 8-hydroxyquinolinate method is more sensitive (c = 3~ :~ at 45(1i nm) [30]. Mixed complexes of Ti with H202 and oxalate [31], Xylenol Orange, or Chrome Azurol S [32] also provide a basis for determining hydrogen peroxide. The Xylenol ()range complex with V(V) is discoloured in the presence of HeO2 [33,134]. Some sensitive methods for determining H202 in aqueous media are based on the reactions of H202 with 4-aminoantipyrine and phenol, or with N-ethyl-N-(sulpho~ propyl)aniline and 4-aminoantipyrene [35]. The FIA technique has also been applied [35]. Hydrogen peroxide can reduce Cu(II) in the presence of Neocuproin in excess. The resulting coioured complex of Neocuproin with Cu(I) is equivalent to the amount of tteO~ present [36]~ From an iodide solution, containing some Mo(VI) as a catalyst, He0::, iil~m'ates an equivalent amount ot iodine, which is then determined [37]. A sensitive method for determining hydrogen peroxide depends on the oxidation of the colourless leucobase of phenolphthalein in alkaline medium containing copper(II), to form the familiar red colour [37,38]. The alkaline phenolphthalein solution is converted into the leucobase by heating with zinc dust. Oxo-peroxo-pyridine-2,6-dicarboxylato-vanadate(V) complex is used for detmmining hydrogen peroxide in rainwater [39]. Hydrogen peroxide can also be determined [40] from the decrease in absorbance (al 418 nm) of an alkaline i>rricyanide solution as a result of the reaction; 2Fe(CN)e, > + |t202 4- 2OH- ~ 2Fe(CN)64- + 2H20 + ()2 In a similar method, [-1202 decolorizes (reduces) a green alkaline solution of manganate (mnO4 2- ) [41 ].

References 1. Waclawik J., Waszak S., Chem. Anal. (Warsaw), 4, 343 (1959); 8, 633 (1963). 2. Meyling A.[t., Frank G.H., Analyst, 87, 60 (1962). 3. John P.A., Winefordner J.D., Silver W.S., Anal. Chim. Acta, 30, 49 (1964). 4. Hamlin P.A., Lambert J.L., Anal. Chem., 43, 618 (1971). 5. Goodfellow G.|., Webber H.M., Analyst, 104, I 105, 1119 (1979).

References

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317

Chapter 36. Palladium Palladium (Pd, at. mass 106.42) is a platinum-group metal, and occurs in the II and IV oxidation states. Palladium(II) compounds are the more stable. Unlike the other platinum metals, palladium is soluble in conc. HNO3. Brown-red Pd(OH)2 precipitates at pH ~-4, but dissolves in excess of an alkali-metal hydroxide. Palladium(U) gives stable nitrite, ammine, cyanide, chloride, bromide, and iodide complexes. Palladium(II)- and (IV) are reduced to the metal by SO2, Fe(II), and ethanol.

36.1. Methods of separation and preconcentration Methods for separating the platinum metals (including palladium) are discussed in the Chapter 38 on platinum.

36.1.1. Extraction Palladium(II) dioximates are specifically extracted from dilute acid with chloroform [1,2]. Dithizone can be used to separate Pd from Pt [3,4]. Extraction of the iodide [5,6]-, bromide [7,8]-, chloride [6,8-10]-, and thiocyanate [11] complexes of palladium gives convenient separations from a number of metals. Chloropyridine [5], di-n-octyl sulphide [6], TBP [7], and TOPO [8] have been used in these extractions.

36.1.2. Precipitation The precipitation of palladium dimethylglyoximate from an acid medium is an excellent separation method. In the separation of microgram quantities of Pd, nickel has been used as a collector [12]. The optimum pH for the precipitation is 6.5 (acetate medium). If Au(III) is separated beforehand by reduction with oxalic acid, and if copper is masked with EDTA, the separation of traces of palladium (with Ni as collector) is specific. Palladium metal can be co-precipitated with tellurium (SnCI2 is used as the reducing agent) [ 13]. Traces of palladium can also be precipitated as the sulphide with Pb as collector, or as the hydroxide with Fe(III) as collector. Traces of palladium have been separated with AgCN as collector [ 14,15].

36.1.3. Other methods Ion-exchange separation methods are based on the retention of the palladium chloride complex on strongly basic anion-exchangers, and of the cationic ammine complex on cation-exchangers [ 16]. A cellulose anion-exchanger has been used to separate Pd from Pt and Ir [17], and from Ir [16]. Traces of Pd have been preconcentrated from water with silica gel impregnated with thionalide [ 18], PAN [ 19,20], or 2- [2-(triethoxysilyl)ethylthio] aniline [21 ]. Like other platinum metals, Pd is separated from ores and concentrates by fire assay and eupellation methods with lead, tin, copper; iron, nickel, and copper alloys [22,23], or


319

nickel- or cuprous sulphides [24] as collectors. The cupellation methods are combined with ion-exchange [22,23].

36.2. M e t h o d s of determination A dithizone method, a very sensitive thio-Michler's ketone method, and a less sensitive iodide method are discussed below. A simple modification of the iodide method gives a very sensitive method involving the formation of the blue starch-iodine complex.

36.2.1. Iodide methods In an acid medium (HC1, H2SO4) containing excess of iodide, palladium forms a brown-red complex, PdI42-, which provides the basis for a moderately sensitive spectrophotometric method of determining Pd [5,14,25,26]. The concentration of HC1 or H2SO4 (up to 10 M) does not affect the colour. A reductant (e.g., ascorbic acid) is added to reduce the iodine liberated by atmospheric oxygen. The molar absorptivity of the complex at )~max= 410 nm is 1.02.104 (a - 0.10). The PdI42- complex may be extracted as the ion-association complex with DAM [27]. In the presence of a small excess of iodide, Pd gives PdI2 which is sparingly soluble in acidic media. When shaken with benzene or DIPE, PdI2 passes into the organic phase; it has been the basis for a sensitive indirect determination method. The suspension of PdI2 in the organic solvent is stripped by dilute ammonia. The aqueous solution is acidified, then the iodide is oxidized by bromine to IO3-. This iodate reacts with added I- to liberate iodine, which is determined as its coloured complex with starch (see Section 25.2.1). One Pd atom in PdI2 is equivalent to twelve atoms of iodine. In the above-given method, e - 2.2.105 (spec. Abs. 2.1) at 590 nm as ~,max of the blue starch--iodine complex. The combination of the iodide method with the extractive separation of Pd as its dimethylglyoximate makes the determination specific for palladium.

Reagents Potassium iodide, 20% and 0.01% solution (iodine-free), freshly prepared. Standard palladium solution: 1 mg/ml. Dissolve 0.1000 g of metallic palladium in aqua regia. Evaporate the solution nearly to dryness, add 3 ml of conc. HC1, and evaporate to half volume. Dilute the solution with water to volume in a 100-ml standard flask. Dimethylglyoxime (H2Dm), 1% solution in ethanol. Starch, 1% solution. Preparation as in p.25.2.1.

Procedure

Extractive separation of Pd. Acidify the sample solution with hydrochloric acid (to --0.2 M in HC1), add 2 ml of the HzDm solution and 5 ml of 0.1 M EDTA, mix well, and allow to stand for 10 min. Extract the Pd(HDm)2 with two portions of CHC13, shaking for 1 min. Wash the combined chloroform extracts with two portions of 0.2 M HC1, and evaporate the organic phase to dryness on a water-bath. Mineralize the residue by heating with a few drops of conc. H2SO4 and conc. HNO3. Expel the nitric acid, allow the residual solution to cool, dilute it with water, and heat it until it clears.

320

36. Palladium

Determination of Pd (as PdI42). To the sample solution containing not more than 0.2 mg of Pd, add 2 ml of HC1 (1 + 1), 5 ml of 20% KI solution, and 1 ml of 1% ascorbic acid solution. Dilute the solution to the mark with water in a 25-ml standard flask, and measure the absorbance at 410 nm against water. Determination of Pd (by the indirect starch-iodine method). To the sample solution containing not more than 10 gg of Pd, add 1 ml of 0.01% KI solution and 2 ml of H2SO4 (1+3), and dilute with water to --15 ml. Shake the solution with two 10-ml portions of benzene for 1 min. Wash the benzene phase with two portions of 1 M H2SO4. Strip the palladium iodide from the benzene phase by shaking for 15 s with 10 ml of ammonia solution (1+9). Place the ammoniacal solution in a 25-ml standard flask, acidify with dil. H2SO4, add 2 drops of bromine water, mix, and after 1 min add 2 drops of 1% phenol solution in glacial acetic acid. After 1 min, add 1 ml of 0.5% KI solution and 1 ml of the starch. Dilute the solution with water to the mark, and measure the absorbance at 590 nm against a reagent blank solution or water.

36.2.2. Dithizone method When an acidic solution of Pd(II) is shaken with an excess of dithizone (H2Dz) (formula 1.1) in CC14 the grey-green dithizonate, Pd(HDz)2, soluble in CC14and CHC13, is formed. With a deficiency of dithizone, the red dithizonate PdDz, readily soluble in CHC13, is formed [28]. The grey-green Pd(HDz)2 is useful for spectrophotometric determination of palladium [3,4,12,28]. It is resistant both to acids (e.g., 3 M H2SO4) and ammonia solutions (up to 3 M NH3). This enables free dithizone to be stripped from the CC14phase with dilute ammonia. Figure 38.1 shows the absorption spectrum of Pd(HDz)2 in CC14. The compound has two absorption maxima in the visible spectrum. The molar absorptivity is 3.55.10 4 at ~max 635 nm (a = 0.33). Unlike other metal-dithizone systems, for palladium there is only a slight difference between the colours of the complex and the free reagent. Pd(HDz)2 is formed rather slowly. The extraction rate increases with increasing excess of dithizone and in the presence of SnC12 (in small amounts) or I-. In acid medium, other noble metals [Au, Pt(II), Hg, Ag] and Cu also react with dithizone. Palladium and platinum can be determined successively with dithizone in SnC12 [3] or iodide [4] media. Silver is masked by chloride. Iodide masks interfering Au and Hg. Traces of palladium may be isolated, before determination with dithizone, by coprecipitation with Ni(HDm)2 from an acetate medium (pH --6.5) [ 12]. Copper can be masked with EDTA, and Au(III) is removed after reduction to the element with oxalic acid. The separated Pd(HDm)2 precipitate dissolves slowly in 6 M HC1. However, when a suspension of Pd(HDm)2 in 2 M HC1 is shaken with a solution of dithizone in CC14, Pd(HDz)2 is formed. For determination of traces of Pd in platinum, it is advisable to use an introductory isolation of Pd as PdI2 floated with benzene [4]. Simultaneous determination of Pd and Pt as dithizonates can be made with the use of 5 th order derivative spectrophotometry [29] -

"

Reagents Dithizone, 0.01% solution in CC14. Preparation as in Section 46.2.1. Standard palladium solution: 1 mg/ml. Preparation as in Section 36.2.1. Dimethylglyoxime (H2Dm), 1% solution in ethanol. Nickel solution: -- 1 mg/ml. Dissolve 0.67 g of (NH4)2Ni(SO4)2.6H20 in 100 ml of water


321

Procedure Separation of Pd with a collector. To the sample solution (-~100 ml) in 0.1 M HC1, containing not more than 50 gg of Pd and heated to --80~ add a macerated filter paper and 1 ml of 5% oxalic acid solution. Keep the solution at -~80~ for 1 h, then allow it to cool. Filter off the precipitate of elemental gold and silver chloride together with the paper. To the filtrate add successively 2 mg of nickel (as its sulphate solution), 2 ml of 20% potassium sodium tartrate solution [to mask Fe(III), A1, Ti, etc.], 2 g of sodium acetate, 1 ml of 0.1 M EDTA, and 2 ml of the H2Dm solution (pH -~6.5). After 30 min, filter off the precipitate of nickel- and palladium dimethylglyoximates. Wash the precipitate from the filter paper into a beaker, add 1 ml of conc. HC1, and evaporate to 5-10 ml, depending on the quantity of Pd in the solution. Determination of Pd. Shake the solution containing the Pd(HDm)2 suspension for 5 min with 5 ml of 0.01% dithizone solution (1 ml of this solution corresponds to 21 ~tg of Pd). Wash the organic extract by shaking with two portions of 2 M HC1, and strip the residual dithizone with dilute ammonia solution (1+50). Transfer the organic phase to a standard flask of suitable capacity, dilute to the mark with CC14, and measure the absorbance at 450 nm, using CC14 as the reference.

36.2.3. Thio-Michler's ketone method Palladium and thio-Michler's ketone (TMK) (formula 46.2) form red mixed ligand complexes Pd(TMK)2X2 (X = CI-, Br-, I-, or SCN-) on extraction with CHC13, in the presence of suitable halide ions in weakly acidic aqueous solution. The complexes [Pd(TMK)4(Sol)2] 2+ are formed when the extraction is done with mixtures of CHC13 with a polar solvent (Sol) such as ethanol or DMF, or with only a polar solvent, e.g., amyl alcohol. Similar complexes (Pd:TMK = 1:4) form in mixed organic-aqueous solutions containing 30-40% of DMF or 40-50% of ethanol. The thio-Michler's ketone has been proposed for a very sensitive spectrophotometric determination of palladium [30,31]. Depending on the conditions, the value of e for the method is 1.105 - 3-105. The use of the mixed medium (water-DMF) is recommended because the molar absorptivity is 3.0-105 (a = 2.8) at 530 nm. TMK in DMF solution has its maximum absorbance at 450 nm. The optimal acidity corresponds to pH 3.0_+0.2. Only Au, Hg, and Pt interfere in determination of palladium. The reaction of TMK with Hg(II) is slower (about 2 h). Oxidants also interfere; the reagent behaves as a reductor. Chloride masks silver ions. EDTA can be used to prevent hydrolysis of some metal ions. Palladium can be separated, before its determination with TMK, by extraction of its bromocomplex with TOA in toluene [32].

Reagents Thio-Michler's ketone (TMK), 0.001 M solution in DMF (28.5 mg of TMK in 100 ml). The solution is not very stable and should be kept in darkness. Standard palladium solution: 1 mg/ml. Preparation as in Section 36.2.1. Acetate buffer, pH 3.0. Mix 50 g of CH3COONa.4H20 in 100 ml of water with 350 ml of glacial acetic acid, adjust to pH 3.0, and dilute to 500 ml with water.

322

36. Palladium

Procedure To a weakly acidic solution containing in l0 ml not more than 8 pg of Pd, add 2 ml of acetate buffer, 7 ml of DMF and 2.5 ml of TMK solution. Dilute to the mark with water in a 25-ml standard flask, and mix thoroughly. After 10 min, measure the absorbance at 530 nm against a reagent blank.

36.2.4. Other methods Many organic spectrophotometric reagents for Pd incorporate sulphur as a ligand atom. Apart from dithizone and thio-Michler's ketone, which have been discussed above, examples include the thiourea derivatives [33-36]: p-dimethylaminobenzylidene-rhodanine (rhodanine) (e = 4.9.104) [37] and its derivatives [38], thiodibenzoylmethane [39,40], and thiosemicarbazone derivatives [41-47]. Oxime reagents give highly selective extraction methods for determining palladium. The commonest are: ct-furildioxime (e = 2.25.104 at 380 nm, in CHC13) [1], dimethylglyoxime (HzDm) (e = 1.7.104 at 380 nm) [26], t~-benziloxime [48], and heptanone oxime [49,50]. Certain nitroso compounds are sensitive spectrophotometric reagents for Pd. Of particular value is p-nitrosodimethylaniline (formula 38.1), which reacts with palladium in the cold at pH 2-2.5 to form a red complex (e = 8.6.104 at 535 nm). The corresponding platinum complex is not formed unless the solution is heated. Further reagents of this group are p-nitrosodiphenylamine [13], nitrosodibenzylaniline [51], and 2-nitroso-5-diethylaminophenol [52]. Many azo compounds have been suggested as reagents for Pd, e.g., PAR (e = 1.8.104 at 510 nm) [53-58], PAR in the presence of Zephiramine [59] and in the presence of diphenylguanidine [60], p-Cl-phenylazo-R-acid [61], thiazolylazo derivatives [62,63], 5-BrPADAP (e = 8.4.104) [64,65], other bromo- and chloro-pyridylazo derivatives [66-68], Sulphonitrophenol M (formula 33.1) [69,70], 5-phenylazo-8-aminoquinoline (extraction with MIBK, e = 7.9.104 at 620 nm) [71], sulphochlorophenol-azorhodanine (e = 1.2.105) [72,73], Arsenazo III [74,75], and Palladiazo (formula 36.1) (e 5.7.104 at 640 nm) [76-78]. =

HO

OH

(36.1)

A considerable increase in the sensitivity of methods for palladium determination has been found with triphenylmethane reagents in ternary systems that include cationic surfactants, e.g., Eriochrome Cyanine R with CP [79], Chromal Blue G with CTA (e = 1.0-105) [80], Chrome Azurol S-CTA ( o r - C P ) [81], Eriochrome Azurol B-CTA (e = 1.15.105) [82], and Eriochrome Azurol G-CTA [83]. Methods for Pd determination based on ion-associates with basic dyes are often very sensitive. In extraction-spectrophotometric methods, thiocyanate [84-89], chloride, and bromide [84] anionic complexes of palladium are associated with Brilliant Green [84], Malachite Green [85,88], Rhodamine B (e = 9.0.104) (86), Rhodamine 6G [89], and Methylene Blue [87]. In flotation-spectrophotometric methods, ion-associates formed by the


323

bromide complex of palladium and Rhodamine 6G (e = 3.0.105) [90], the thiocyanate complex and Methylene Blue [91], and Pd complex with SnC13- and Rhodamine 6G (e = 2.8.105 ) [92] have been described. The solvents used in the flotation methods include benzene (DMF for dissolving the compound), benzene and acetone [91 ], and DIPE [92]. Methods based on coloured Pd complexes formed in acid-chloride (~ =2.8-103) [93-97] or -bromide [93,98] media, often in the presence of Sn(II), are not very sensitive. These complexes may be extracted in the presence of TOA (benzene) [ 1], tetraphenylarsonium ion (CHC13) [94], DAM (CHC13, ~ = 1.5.104) [98], triphenylphosphine (~; = 1.5.104 at 346 nm) [951. Derivative spectrophotometry has been used for the determination of Pd in the presence of Pt and Au in bromide solutions [ 100], and in the presence of Pt in iodide solutions [ 101 ].

36.3. Analytical applications The methods described in detail in Section 36.2, or only mentioned, have been used as follows for spectrophotometric determination of palladium: the thio-Michler's ketone - - in silver, copper, and anodic slime [32], in catalysts [31]; with thiosemicarbazide derivatives m in water [44] and alloys [46]; with palladium-carbon powder with o~-benzilmonoxime [48]; with PAR in catalysts and ores [58]; with thiazolylazo derivatives m in Ni-AI catalysts [63]; with 5-Br-PADAP m in titanium alloys; with pyridylazo derivatives - in nickel alloys [68]; with sulphonitrophenol- in silver alloys [70]; with Arsenazo Ill ~ in iron and meteorites; and with Palladiazo in catalysts, minerals, silica gel, and calcium carbonate [78].

References 1. Marczenko Z., Krasiejko M., Chem. Anal. (Warsaw), 15, 1233 (1970). 2. Holland W.J., Walker R.J., Mikrochim. Acta, 1973, 193. 3. Marczenko Z., Ku~ S., Mojski M., Talanta, 31,959 (1984). 4. Marczenko Z., Ku~ S.,Analyst, 110, 1005 (1985). 5. Egli R.A., Z Anal Chem., 194, 401 (1963). 6. Mojski M., Talanta, 25, 163 (1978). 7. Patil S.P., Shinde V.M., Mikrochim. Acta, 1974, 853. 8. Mojski M., J. Radioanal. Chem., 46, 239 (1978). 9. Khopkar S.M., Anal. Chem., 38, 360 (1966). 10. Masana A. et al., Anal. Sci., 5, 201 (1989). 11. A1-Bazi S.J., Chow A., Talanta, 29, 507 (1982); 30, 487 (1983). 12. Marczenko Z., Krasiejko M., Chem. Anal. (Warsaw), 9, 291 (1964). 13. Marhenke E.R., Sandell E.B., Anal. Chim. Acta, 28, 259 (1963). 14. Jackwerth E., Graffmann G., Lohmar J., Z. Anal. Chem., 247, 149 (1969). 15. Jackwerth E., Z. Anal. Chem., 251, 353 (1970). 16. Brajter K., Stonawska K., Mikrochim. Acta, 1989 I, 137. 17. Brajter K., Slonawska K., Talanta, 30, 471 (1983). 18. Terada K., Matsumoto K., Taniguchi Y.,Anal. Chim. Acta, 147, 411 (1983). 19. Liu F. et al., Microchem. J., 52, 274 (1995). 20. Ivanov V.M., Beketova N.A., Basova E.M., Bol'shova A.T., Zh. Anal. Khim., 46, 1512 (1991). 21. Seshadri T., Haupt H.J., Anal. Chem., 60, 47 (1988).

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22. Faye G.H., Inman W.R., Anal. Chem., 33, 278 (1961). 23. Banbury L.M., Beamish F.E., Z. Anal. Chem., 211, 178 (1965). 24. Kallmann S., Talanta, 33, 75 (1986). 25. Morrow J.J., Markham J.J.,Anal. Chem., 36, 1159 (1964). 26. Marczenko Z., Ramsza A., Chem. Anal. (Warsaw), 23, 23 (1978). 27. Pilipenko A.T., Ol'khovich P.F., Ukr. Khim. Zh., 34, 83,286 (1968). 28. Minczewski J., Krasiejko M., Marczenko Z., Chem. Anal. (Warsaw), 15, 43 (1970). 29. Ku~ S., Marczenko Z., Analyst, 112, 1503 (1987). 30. Cheng K.L., Goydish B.L., Microchem. J., 10, 158 (1966). 31. Pilipenko A.T., Ryabushko O.P., Matsibuta G.S., Zh. Anal. Khim., 35, 1545 (1980). 32. Tsukahara I., Bunseki Kagaku, 28, 253 (1979). 33. Yoda R., Mikrochim. Acta, 1982 II, 271. 34. Yoda R., Yamamoto Y., Murakami Y., Mikrochim. Acta, 1983 II, 75. 35. Degtev M.I., Toropov L.I., Zavod. Lab., 52, No 4, 3 (1986). 36. Vest P., Schuster M., Koenig K.H., Z. Anal. Chem., 335, 759 (1989). 37. Borrisova R., Koeva M., Topalova E., Talanta, 22, 791 (1975). 38. E1-Sayed A.A., Mikrochim. Acta, 117, 161 (1995). 39. Uhlemann E., Hoppe J., Waltz D.,Anal. Chim. Acta, 83, 195 (1976). 40. Prabhu B.N., Khopkar S.M., Bull. Chem Soc. Jpn., 49, 2139 (1976). 41. Mas'ko L.I., Kerentseva V.P., Lipanova M.D., Zh. Anal. Khim., 30, 315 (1975). 42. Yamaguchi S., Uesugi K., Analyst, 110, 1241 (1985). 43. Shravah K., Sinha P.P., Sindhwani S.K., Analyst, 11, 1339 (1986). 44. Ghazy S.E., Kabil M.A., E1-Asmy A.A., Sherief Y.A., Anal. Lett., 29, 1215 (1996). 45. Uesugi K. et al., Microchem. J., 50, 88 (1994). 46. Shetty A.N., Gadag R.V., Bull. Chem. Soc. Jpn., 66, 2536 (1993). 47. Murthy G.V., Reddy T.S., Talanta, 39, 697 (1992). 48. Ensafi A.A., Eskandari H., Microchem.J. 63, 266 (1999). 49. Dunn P.J., Holland W.J., Walker R.J., Mikrochim. Acta, 1975 II, 129. 50. Dunn P.J., Holland W.J., Mikrochim. Acta, 1977 I, 363. 51. Kothny E.L., Mikrochim. Acta, 1978 I, 425. 52. T6ei K., Motomizu S., Hamada S.,Anal. Chim. Acta, 101, 169 (1978). 53. Flaschka H., Hicks J., Microchem. J., 11, 517 (1966). 54. Mizuno K., Miyatani G., Bull. Chem. Soc. Jpn., 49, 2479 (1976). 55. Ivanov V.M., Figurovskaya V.N., Troyanovskii I.V., Zh. Anal. Khim., 39, 60 (1984). 56. Langade A.D., Shinde V.M., Analyst, 107, 708 (1982). 57. Chhakkar A.K., Kakkar L.R., Fresenius'J. Anal. Chem., 340, 19 (1991). 58. Sundaramurthi N.M., Shinda V.M., Bull. Chem. Soc. Jpn., 63, 1508 (1990). 59. Yotsuyanagi T., Hoshino H., Aomura K., Anal. Chim. Acta, 71, 349 (1974). 60. Confalova O. et al., Microchem. J., 32, 24 (1985). 61. Hanna W.G., Talanta, 50, 809 (1999). 62. Sutthivaiyakit P., Kettrup A., Z. Anal. Chem., 320, 769 (1985). 63. Liu P., Liu H., Wu C., Talanta, 38, 1143 (1991). 64. Chang Y.P., Zhou N., Talanta, 33, 939 (1983). 65. Michaylova V., Evtimova B., Nonova D.,Anal. Chim. Acta, 207, 373 (1988). 66. Shibata S., Ishiguro Y., Nakashima R., Anal. Chim. Acta, 64, 305 (1973). 67. Fontan C.A., Malone C.B., Oloina R., Bull. Chem. Soc. Jpn., 61, 4113, 4125 (1988). 68. Chen X.G., Li G.B., Hu D.Z., Mikrochim. Acta, 122, 143 (1996). 69. Savvin S.B., Propistsova P.F., Okhanova L.A., Zh. Anal. Khim., 24, 1634 (1969). 70. Gur'eva R.F., Savvin S.B., et al., Zh. Anal. Khim., 32, 1009 (1977).

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71. Blanco M., Maspoch S., Mikrochim. Acta, 1983 IIl, 11. 72. Gur'eva R.F., Savvin S.B., Zh. Anal. Khim., 43, 476 (1988). 73. Shiundu P.M., Wentzell P.D., Wade A.P., Talanta, 37, 329 (1990). 74. Sen Gupta J.G.,Anal. Chem., 39, 18 (1967); Anal. Chim. Acta, 42, 481 (1968). 75. Rizvi G.H., Natarajan P.R., Fresenius' J. Anal. Chem., 336, 498 (1990). 76. Perez-Bustamante J.A., Burriel-Marti F., Anal Chim. Acta, 37, 49, 62 (1967); 51, 277 (1970). 77. Perez-Bustamante J.A., Burriel-Marti F., Talanta, 18, 183,717 (1971). 78. Bocanegra Sierra L., Perez-Bustamante J.A., Burriel-Marti F., Anal. Chim. Acta, 59, 231, 249 (1972). 79. Duchkova H., Malat M., (~ermakova L, Anal. Lett., 9, 487 (1976). 80. Uesugi K., Shigematsu T., Anal. Chim. Acta, 84, 377 (1976). 81. Kant R., Srivastava R., Prakash O., Croat. Chem. Acta, 54, 465 (1981). 82. Gregorowicz Z. et al., Mikrochim. Acta, 1983 II, 181. 83. Uesugi K., Miyawaki M., Microchem. J., 41, 78 (1990). 84. Khvatkova Z.M., Schpeg M.A., Golovina V.V., Zh. Anal. Khim., 36, 2367 (1981). 85. Pilipenko A.T., Khvatkova Z.M., Zh. Anal. Khim., 41, 2045 (1986). 86. Lopez Garcia I., Martinez Avilles, Cordoba M.M., Talanta, 33, 411 (1986). 87. Mafiko R.W., Chem. Anal. (Warsaw), 36, 763 (1991). 88. Mafiko R.W., Chem. Anal. (Warsaw), 36, 17 (1991). 89. Ramalingom P.A. et al., Chem. Anal. (Warsaw), 42, 75 (1997). 90. Marczenko Z., Jarosz M., Talanta, 28, 561 (1981). 91. Marczenko Z., Jarosz M.,Analyst, 106, 751 (1981). 92. Kalinowski K., Marczenko Z., Anal. Chim. Acta, 186, 331 (1986). 93. Dalziel J.A., Donaldson J.D., Woodget B.W., Talanta, 16, 1477 (1969). 94. Nasouri F.G., Witwit A.S., Talanta, 16, 1492 (1969). 95. Mojski M., Plesifiska M., Microchem. J., 24, 117 (1979). 96. Khalonin A.S. et al., Zavod. Lab., 52, 7 (1986). 97. Koch K.R., Auer D., Talanta, 40, 1975 (1993). 98. Danilova V.N., Lisitchenok S.L., Zh. Anal. Khim., 26, 1157 (1971). 99. Kuroda R., Hayashibe Y., Yoshitsuka K., Fresenius'J. Anal. Chem., 336, 494 (1990). 100. Obarski N., Marczenko Z., Chem. Anal. (Warsaw), 40, 385 (1995). 101. Marczenko Z., Obarski N., Chem. Anal. (Warsaw), 39, 347 (1994).

Chapter 37. Phosphorus Phosphorus (P, at. mass 30.97) occurs mainly in the V oxidation state as phosphate (derived from orthophosphoric acid) and the condensed forms, pyro-, meta-, and polyphosphate. Phosphoric acid gives stable heteropoly acids with Mo(VI), W(VI), V(V) etc. Phosphorus occurs also in the III, I and -HI oxidation states in phosphite, hypophosphite and phosphine (PH3), respectively.

37.1. Methods of separating phosphate When a sample is dissolved, the phosphorus usually passes into solution as P(V). Rather than isolate the phosphate, it is often better to isolate the interfering elements, leaving the phosphate to be determined in the mother liquor. Examples of such separations include distillation of Si, As, and Ge as volatile halides [1] or of boron as trimethyl borate [2], precipitation of heavy metals as sulphides from an acid medium, retention of cations on a strongly acidic cation exchanger, and electrolytic separation of metals. The separation of P(V) from other elements, in particular from Si, is often achieved by extracting phosphorus as a heteropoly acid from a slightly acidic solution of pH --1.4. Higher alcohols, esters, and ethers are suitable extractants [3-5]. By extraction with a mixture of butanol and chloroform, molybdophosphoric acid can be separated from molybdoarsenic acid [6]. Isobutyl acetate extracts molybdophosphoric acid, but not molybdosilicic acid, from a solution at pH 0.3-1.0. In the determination of traces of P in silicon tetrachloride, shaking the sample with concentrated sulphuric acid causes the phosphorus to pass into the acid layer [7]. Trace amounts of phosphate can be co-precipitated with AI(OH)3, Fe(OH)3, or Be(OH)2 as collector [8-10]. Traces of phosphate, co-precipitated with AI(OH)3 at pH 8.5, have been floated with Na oleate, by passing a stream of nitrogen [ 11 ]. Traces of phosphate were concentrated on a cation-exchanger impregnated with Fe(III) [12,13] or with barium chloranilate [14]. Mixtures of P(V), As(V), and Si(IV) were separated on an anion-exchange column [15].

37.2. Methods of determining phosphate Microgram quantities of phosphorus are conventionally determined by the phosphomolybdenum blue method. The molybdovanadophosphoric acid method is suitable for determining relatively large quantities of P(V). Sensitive spectrophotometric methods based on ion-associates with basic dyes deserve attention.

37.2.1. Phosphomolybdenum blue method In an acid medium containing excess of molybdate, orthophosphate forms pale yellow molybdophosphoric acid ("Mo-P"). This reaction is useful for determining phosphate at fairly high concentrations, but the sensitivity can be increased considerably by the use of aqueous acetone [ 16-18]. A sensitive spectrophotometric method for determining phosphorus is based on the

37.2. Methods of determining phosphate

327

reduction of molybdophosphoric acid to phosphomolybdenum blue ("P-Mo") under mild conditions to prevent reduction of the free molybdic acid [19-24]. Reductants employed include hydrazine, SnC12, ascorbic acid, sulphite, and other reducing agents [25,26]. In the preparation of "P-Mo" blue, a single reagent consisting of ammonium molybdate, hydrazine sulphate, and H2SO4 is convenient (see procedure below). Molybdophosphoric acid is reduced either in aqueous medium (-~0.5 M H2SO4) or in the organic phase (usually n-butanol) after molybdophosphoric acid has been extracted. Alternatively, the phosphomolybdenum blue may be formed in the aqueous phase, then extracted into n-butanol [27]. Aqueous acetone medium is also used [28]. The absorbance of the phosphomolybdenum blue depends on the medium, its acidity, and the kind of the reducing agent used. The molar absorptivity of the blue solution in butanol after hydrazine reduction is 2.5.104 (a = 0.81) at ~max 780 nm. Extraction of phosphomolybdenum blue displaces the absorption maximum slightly towards shorter wavelengths. Interfering species in the determination of phosphorus by the phosphomolybdenum blue method are As(V), Si, and Ge, which also react with molybdate to form the corresponding acids which are reduced to the respective heteropoly blues. Arsenic(V) does not interfere when reduced to As(III) using sulphite or thiourea. In the presence of vanadium(V), molybdovanadophosphoric acid is produced. Large amounts of vanadium(V) are reduced with Mohr's salt to V(IV) before the molybdate is added. The difference in the rates of formation of the phosphomolybdenum- and silicomolybdenum- blues has been utilized for the determination of phosphorus in the presence of silicon [29]. The interference of silicon can be prevented by the use of a sufficiently acidic medium [30]. Oxalic-, tartaric-, and citric acids, and EDTA affect the completeness of reduction of "Mo-P" acid [31 ]. Before the determination of phosphate, any nitrate must first be reduced to ammonia, which is then distilled from the alkaline medium [32]. Phosphomolybdenum blue may be extracted with CHC13 in the presence of dioctylamine, trioctylamine, or propylene carbonate [33]. The FIA technique has also been used in the determination of phosphorus(V) [15,34].

Reagents Molybdenum reagent. Solution (1). Dissolve 1.0 g of ammonium molybdate in 100 ml of 2 M H2804. Solution (2). Dissolve 0.10 g of hydrazine sulphate in 100 ml of water. Immediately before use, mix 10 ml of solution (1) with 10 ml of solution (2), and dilute to 100 ml with water. Solutions (1) and (2) should not be stored longer than 4 days. Standard phosphorus(V) solution: 1 mg/ml. Dissolve in water 4.3900 g of potassium dihydrogen phosphate (KH2PO4) dried at l l0~ add 1 ml of CHC13 (to prevent the formation of mould), and dilute with water to 1 litre. Ammonium molybdate, 10% solution adjusted with ammonia to pH 7.4_+0.2.

Procedure

Extractive separation of P. Evaporate the sample solution freed from As (e.g., by extraction as AsC13, cf. Section 8.1.1) nearly to dryness, dilute with 20 ml of water, add 2 ml of the ammonium molybdate solution and adjust the pH to 1.4_+0.1 with 0.5 M H2SO4. After 5 min, transfer the solution to a separating funnel and extract the "Mo-P" acid with two 10-ml portions of butanol. Wash the alcoholic extract with 0.05 M H2SO4. Determination of P. Evaporate an aliquot of the extract (or an aqueous sample solution

328

37. Phosphorus

freed from As, Ge, and Si), containing not more than 30 ~tg of P, to dryness in a beaker with nitric acid. Add 12 ml of the molybdenum reagent, and place the beaker on a boiling waterbath for 10 min. Transfer the cooled solution to a separating funnel, and extract the phosphomolybdenum blue with two portions of n-butanol. Dilute the extract to the mark with the solvent in a 25-ml standard flask, and measure the absorbance at 720 nm, using the solvent as reference. Notes. 1. The absorbance of the P-Mo blue may be measured in the aqueous solution. In this case, the coloured aqueous solution is diluted to the mark in the volumetric flask with the molybdenum reagent. 2. Molybdophosphoric acid may be extracted, then stripped with dilute ammonia solution (1 +50), the solution acidified with nitric acid and evaporated, and the complex reduced to the heteropoly blue.

37.2.2. Molybdovanadophosphoric acid method Addition of molybdate to an acidic solution containing orthophosphate and vanadate, results in the formation of the yellow-orange molybdovanadophosphoric acid having the Mo:V:P ratio of 11:1:1 [35,36]. The absorption maximum of the compound is at 314 nm (e = 2.0-104). At 400 nm, e = 2.5.103 (specific absorptivity 0.08). In the molybdovanadophosphoric acid method, the absorbance is measured either at 315 nm (sensitivity as high as that in the "P-Mo" blue method), or at 400-470 nm (much lower sensitivity). The colour depends on the acidity of the solution and on the concentrations of the reagents used. The optimum acid concentration is 0.5-1.0 M HNO3 (H2SO4, HC104, or HC1). In insufficiently acid solutions, the yellow colour is produced even in the absence of orthophosphate; in excessively acid solutions, the formation of molybdovanadophosphoric acid proceeds too slowly. The concentration of V(V) and Mo(VI) in the final solution should be --0.002M and --0.01M, respectively. Since the reagents also produce a slight colour in the absence of phosphate, the absorbance must be measured against a reagent blank solution. In --0.8 M HNO3 silicon does not interfere provided it is not present in greater amount than phosphorus. In more acidic media, even more silicon can be tolerated. At higher concentrations, silicic acid can be converted into the inert polymeric form by heating the sample solution to fumes with conc. HC104. Large amounts of Fe(III) interfere, but may be masked with fluoride, the excess of which is complexed with boric acid. Reductants and certain coloured metal ions [e.g., Cr(VI), Ni, Co, Cu, and U(VI)] also interfere. Molybdovanadophosphoric acid may be separated from many coloured ions by extraction with oxygen-containing organic solvents [35]. Reducing agents must be absent.

Reagents Ammonium metavanadate, 0.25% solution. Dissolve 1.25 g of NH4VO3 in 250 ml of hot water. Cool the solution and add 10 ml of conc. HNO3. Allow the solution to stand overnight, filter if necessary, and dilute with water to 500 ml. Store the solution in a polyethylene container. Ammonium molybdate, 5% solution. Dissolve 2.5 g of the reagent in 250 ml of water (at --50~ Allow the solution to stand overnight, filter if necessary, dilute with water to 500 ml, and store in a polyethylene container.

37.2. Methods of determining phosphate

329

Standard phosphorus(V) solution: 1 mg/ml. Preparation as in Section 37.2.1.

Procedure To the slightly acidic sample solution containing not more than 0.3 mg of P(V), add successively 2.5 ml of HNO3 (1+1), 2.5 ml of the vanadate solution, and 2.5 ml of the molybdate solution, mixing the solution after the addition of each reagent. Dilute the solution to volume with water in a 25-ml standard flask. After 30 min, measure the absorbance at 400 nm against a reagent blank solution.

37.2.3 Other methods With basic dyes, molybdophosphoric acid forms ion-associates which are the basis of sensitive extraction methods for determining P(V). Malachite Green [37--48], Crystal Violet [49-52], Brilliant Green [53,54], Ethyl Violet [55], Methylene Blue [56,57], Rhodamine B [58,59], and Rhodamine 6G [60,61] are among the most widely used. The ion-associates, which are sparingly soluble in water, can be extracted, and the absorbance of the extracts measured [39,40,55,56]. The associates may also be separated by flotation [44,49] or centrifugation [53,58] before being dissolved in polar solvents. Aqueous pseudo-solutions are often stabilized with, e.g., poly(vinyl alcohol) [38,41,50], Syntanol DS-10 [37], or Zephiramine [57]. The most sensitive methods include those utilizing Malachite Green (e = 3.3.105) [44] and Brilliant Green (e = 2.9.105) [53]. The derivative spectrophotometry technique has been applied in the determination of phosphate with Rhodamine 6G [61 ]. The molybdophosphate anion has been associated, in a HC1 medium, with a cationic complex of Co with 5-C1-PADAP. The sparingly soluble associate has been separated by flotation with butyl acetate and dissolved in methanol (~ - 3.4.105 at 560 nm) [62]. A method based on phosphoantimonylmolybdenum blue has also been proposed [62a]. Numerous indirect amplification methods have been devised for the determination of phosphate. The molybdenum in an extract of molybdophosphoric acid (Mo:P = 12:1) has been determined with thiocyanate [63], phenylfluorone [64], dithiol (e = 1.7-105), Sulphonitrophenol S (e - 4.6-105) [65], or 2,2'-diquinoxalyl [66]. The indirect method that involves the Fe(II)-ferrozine complex [67] is unusually sensitive (~ = 9.7.105). In another method involving the complex of Ce(III) and Arsenazo III, the phosphate gives a sparingly soluble CePO4 and liberates an equivalent quantity of Arsenazo III [68].

37.3. Analytical applications The phosphomolybdenum blue method has been used for determining phosphorus in biological materials [69-71], milk [72], vegetables [73], wine and blood serum [74], sewage [75,76], waters [5,11,77-87], soils and plants [88-94], rocks and minerals [95], geological deposits (sediments) [96], organic compounds [97], fertilisers [98], cast iron and steel [8,26,99,100], nickel and its alloys [26,100,101], copper alloys [101], aluminium alloys [102], platinum and gold [6], gallium and its compounds [103], iron ores [104], silicon [1], boron [2], tungsten materials [9], arsenic and its oxide [105], niobium- and tantalum oxides [106], neodymium and yttrium oxides [107], silicates [108-110], glass [111], coke [112], coal dust [91], silumins [ 10], and concentrated chloride solutions [ 113]. Optimum conditions for determining P in soils, waters, and plants were studied [81]. Phosphorus was determined by the FIA technique in soils [91] and waters [87]. The method has been applied for determination of phosphate in the presence of

330

37. Phosphorus

phosphate esters [114], pyrophosphate, and polyphosphate [115]. The condensed phosphates are converted into phosphates by boiling for 15 min in 2.5 M H2804 medium. When trace amounts of phosphates were determined in water, they were preconcentrated (as the P-Mo blue) in a column packed with silane-coated glass beads; DMF was used as the eluent [116]. The P-Mo blue may also be concentrated on a nitro- or acetylcellulose membrane in the presence of dodecyltrimethylammonium bromide [ 117]. The P-Mo blue method has been applied for automatic determination of P(V) in silicate rocks [118], soils [119], sea deposits [120], and water [121]. This method has been applied also in the FIA technique [122-126]. The method involving the M o - V - P acid has been used in determinations of phosphorus in biological tissues [127], plant material [128], fruits [129], fish products [130], foodstuffs [131], phosphate minerals [132], cast iron and steel [133,134], niobium, zirconium and its alloys, titanium and tungsten, aluminium, copper, and white metal [135], nickel alloys [134,135], metallurgy products [136], molybdenum concentrates [137], silicon tetrachloride [7], cement [138], and lubricants[139]. The flow injection technique has been applied for determining phosphate in minerals [140] and in plant materials [141 ]. The methods involving basic dyes have been used for determining phosphate in natural waters [38,39,41,43,48,55,59,142], biological materials [47,50,52], soils [45,51], uranium [37], iron [46,49], and nickel, cobalt, copper, and zinc [46]. The FIA technique has also been applied [40,43,48]. Various methods were applied for determining phosphorus in calcium chloride extracts [143]. The spectrophotometric methods for the determination (and speciation) of phosphorus in natural waters have been compared with other analytical techniques [144].

37.4. Determination of other phosphorus compounds Hypophosphite can be determined, in the presence of phosphate, from the colour it gives with ammonium molybdate in H2SO4 medium [145]. Hypophosphite may also be determined by its bleaching of the colour of the Fe(III)-SCN- complex [ 146 ]. Pyrophosphate is determined by using its effects on the colour reactions of Fe with SCN- or 1,10-phenanthroline [147]. A method for separation of hypophosphite, phosphite, phosphate, pyrophosphate, and triphosphate by ion exchange chromatography, with HC1 and KC1 solutions as eluents, has been proposed [148].

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Chapter 38. Platinum Platinum (Pt, at. mass 195.09) occurs in its compounds in the II and IV oxidation states, compounds of Pt(IV) being the more stable. The hydroxide Pt(OH)4 dissolves in excess of NaOH. Platinum(W) forms chloride, iodide, cyanide, and nitrite complexes. Platinum(H) and -(IV) are more difficult to reduce to the metal than is gold(HI). Zinc and aluminium in acid solution, and formaldehyde in an alkaline medium, are suitable reductants. Of the other platinum metals, palladium resembles platinum most closely, and osmium and ruthenium resemble it least.

38.1. Methods of separation and preconcentration of platinum metals 38.1.1. Fire assay and cupellation These methods, used to isolate small amounts of the noble metals (platinum metals, Au, Ag) from ores and concentrates, have been discussed in Section 23.1.1.

38.1.2. Extraction A comprehensive review of the solution chemistry and of solvent extraction and ionexchange methods for separating platinum metals has been published [1]. Thioeyanate [2] and chloride [3-8] complexes play an important role in the extractive separation of platinum metals. They are extracted in the presence of pyridine (MIBK), TBP, phosphonium salts (ethyl acetate), DAM, DAPM (CHC13), and TOPO (1,2-dichloroethane). Often, the extraction is carried out in hydrochloric acid media in the presence of SnCI2 (ligand SnC13-) which plays the role of labilizing agent [8-11]. Solvent extraction is often applied in separation of platinum from other metals [3,6-8]. Bromide complexes of Pt and other metals of this group are extracted from acid media with TBP, MIBK, amyl acetate [ 12] or TOPO [6]. Pt and Pd can be separated from Rh and Ir by extraction (with TBP) from iodide solution [13]. The same method has been used for separating platinum from copper [14]. Dithizone allows Pt and Pd to be separated from each other and from other metals [1518]. Pt has been separated from Pd in the presence of SnCI2 [ 16] or iodide [ 17]. Other organic reagents used for extractive separation and preconcentration of platinum group metals include thiourea derivatives [19] and organic sulphides [20]. Integrated schemes for extractive separation of all platinum metals and gold have been developed [21,22].

38.1.3. Other methods Platinum metals form stable anionic chloride complexes which allow their separation from a number of common metals on both anion- and cation-exchangers [1,23-25]. Platinum metals can also be separated as the bromide [26,27]. Columns with chelating resins have


335

also found use in the separation and preconcentration of platinum metals [28-32]. Platinum, as well as Pd, Au, and Rh, can be precipitated by the following reducing agents: formic acid, hydrazine, tin(II) chloride, calomel, and sodium tetrahydroborate [33]. Tellurium and selenium are used as collectors [34,35]. When an acid solution is boiled with powdered tellurium, Pt and Pd are precipitated, whereas Rh and Ir remain in the solution [34]. From mixtures of platinum metals, Ru and Os can be separated by distillation as volatile tetroxides, and a double precipitation of Pd, Rh, and Ir hydroxides (with Fe used as collector) at pH 8 enables them to be separated from platinum. Ion-flotation has been used to separate and preconcentrate platinum metals as chloride complexes in the presence of cationic surfactants [36].

38.2. Methods of determination Two methods for determining platinum are described here in detail: the well-known insensitive stannous chloride method, and a sensitive method based on an ion-associate of Pt(II) with SnC13- and Rhodamine 6G.

38.2.1. Tin(II) chloride method Stannous chloride reacts with Pt(IV) in dilute HC1 acid to yield a yellow-orange complex which is the basis of the spectrophotometric method for determining Pt [37]. The molar absorptivity is 1.3-104 a t ~max "- 403 nm (a --- 0.067). The most suitable concentration of HC1 is 0.8-1.5 M: the concentration of SnCI2 is not critical for the colour reaction. The coloured complex is extractable into esters and ethers, amyl acetate being the most recommended. The molar absorptivity in amyl acetate is similar to that in aqueous medium, but the absorption maximum is shifted slightly towards shorter wavelengths. The high distribution coefficient facilitates the concentration of platinum in a small volume of the organic solvent. Palladium and platinum react similarly with SnCI2. If, however, the sample solution containing palladium and platinum is first made alkaline with ammonia, then acidified with hydrochloric acid to -~1 M in HC1, and finally treated with SnCI2, only the platinum complex is extracted. Other platinum metals also form coloured complexes with SnCI2 [ 11 ]. Higher sensitivity is obtained by extracting the anionic Pt complex with chloroform or benzene in the presence of TOA or another long-chain high molecular-weight amine [9]. Platinum has been extracted with TOA before the determination in gold [38]. A related method for platinum is based on the orange-red complex obtained in acidic bromide solution in the presence of tin(II). The complex has been extracted with chloroform in the presence of DAM [39], or with benzene in the presence of DAPM [40].

Reagents Tin(II) chloride, 25% solution in HC1 (1+3). When covered with a layer of xylene, the solution is protected from atmospheric oxygen and may be stored for a long time. Standard platinum(IV) solution: 1 mg/ml. Dissolve 0.1000 g of platinum in 4 ml of aqua regia and evaporate the solution nearly to dryness. Add 5 ml of conc. HC1 and 0.1 g of NaC1, and evaporate the solution to dryness. Dissolve the solid in 20 ml of HC1 (1 + 1), and dilute

336

38. Platinum

the solution to volume with water in a 100-ml standard flask. Working solutions are obtained by appropriate dilutions of the stock solution with dilute HC1 (e.g., 0.2 M).

Procedure Place the sample solution, containing not more than 0.3 mg of Pt, in a 25-ml standard flask. Add 3 ml of conc. HC1 and 5 ml of the SnC12 solution, dilute the solution to the mark with 0.01 M HC1, mix well, and measure the absorbance at 403 nm against water. In the extraction modification of the method, add to the acidified sample solution (see above) 5 ml of 25% ammonium chloride solution, 5 ml of the SnCI2 solution, and 0.01 M HC1 to -20 ml. Extract the solution with two portions of amyl acetate (containing 1% of resorcinol). Dilute the combined extracts with the solvent to the mark in a standard flask of suitable capacity, and measure the absorbance at 400 nm against the solvent.

38.2.2. R h o d a m i n e 6G method This very sensitive flotation-spectrophotometric method is based on the ion associate formed by the anionic chlorostannate(II) complex of platinum(H) with the xanthene basic dye Rhodamine 6G (formula 4.30) [41]. When the aqueous phase is shaken with DIPE, the sparingly soluble compound collects on the wall of the separating funnel. The solid associate is washed, dissolved in acetone, and its absorbance is measured. Hydrochloric acid (-1 M) is a suitable medium for formation of the associate. The concentration of tin(II) should be 0.02-0.04 M, and Rhodamine 6G should be in -50-fold excess with respect to Pt. At higher SnCI2 and dye concentrations the absorbance of the reagent blank increases. The amount of DIPE used in flotation has no effect on the Pt separation efficiency. The time of shaking should not be shorter than 1 min. After the washing with 2 portions of 1 M HC1 the absorbance of the reagent blank is not higher than 0.05. Under the conditions of this procedure e = 2.8.105 (sp. abs. 1.4) at 530 nm. Beer's law is obeyed in the Pt concentration range 0.08-0.6 lag/ml. The high sensitivity of the method is accounted for by the fact that three cations of the dye are equivalent to one Pt atom [41 ]. Pd and Rh affect the determination of Pt. The smaller effect of other platinum group elements is due to the lower rate of formation of their complexes with SnC13-. A convenient method of separating Pt from Pd consists in extraction of Pt with triphenylphosphine oxide solution in 1,2-dichloroethane [7] (see the procedure below).

Reagents Rhodamine 6G, 0.001 M solution (about 0.05% ). Tin(II) chloride: 10% solution of SnC12.2H20 in 2 M HC1. Triphenylphosphine oxide (TPPO), 0.1 M solution in 1,2-dichloroethane. Standard platinum(IV) solution: 1 mg/ml. Preparation as in Section 38.2.1.

Procedure

Separation of Pt from Pd. Evaporate an acid sample solution (in HC1) to dryness on a water-bath. Dissolve the residue in 10 ml of 7-8 M HC1. Extract the platinum by shaking with three 5-ml portions of TPPO solution (each portion for 10 min). Wash the combined dichloroethane extracts by shaking with 10 ml of 7-8 M HC1 for 10 min.


337

Before determining Pt, evaporate the organic solvent and mineralize the residue with a few drops of conc. H2804 and conc. HNO3. Heat the residue (after evaporation) with some drops of aqua regia, and evaporate almost to dryness with several drops of conc. HC1 added. Determination of Pt. Evaporate to dryness the acidic (HC1) sample solution, containing not more than 15 gg of Pt, on a boiling water-bath. Add 2 ml of the SnCI2 solution. Transfer the solution to a separating funnel. Add appropriate amounts of conc. hydrochloric acid and water to obtain 20 ml of solution that is 0.8-1.2 M in HC1. Then add 1 ml of the Rhodamine 6G solution and 5 ml of DIPE, and shake for 1 min. Open the funnel at the top, allow the phases to separate, and slowly discard the aqueous layer. Wash the ether phase and the precipitate by shaking with two successive 20-ml portions of 1 M HC1 for 30 s each. Carefully remove the aqueous and organic layers and dissolve the isolated solid in acetone. Transfer the solution to a 25-ml standard flask and dilute to the mark with acetone. Measure the absorbance of the solution at 530 nm against a reagent blank prepared in the same way.

38.2.3. Other methods Besides the Rhodamine 6G-SnC12 flotation-spectrophotometric method described above, similar methods using other basle dyes, e.g., Victoria Blue B, Victoria Blue 4R, Capri Blue [41], Crystal Violet (~ = 2.1.105) [42], or Nile Blue A [43], have been proposed. An ionassociate of the chloride platinum complex with Methyl Green has been extracted with a mixture of 1,2-dichloroethane and CC14 (E = 1.45.105) [44]. The ion-associate of the Ptthiocyanate complex with Malachite Green has been extracted with benzene [45], and a thiocyanate- or iodide- Pt complex associated with Crystal Violet has been extracted into xylene or toluene [46]. The cationic hexa-ammine complex of Pt(IV) gives an ion-pair with the acid dye, ethyl eosin, and this is also used in determinations of platinum (e = 8.0.104) [47,48]. A sensitive but unselective method is based on the reaction with p-nitrosodimethylaniline (formula 38.1) or p-nitrosodiethylaniline [49].

'CH3'2N--N0

(38.~)

When a Pt(IV) solution (pH 2-3) is heated with excess of the reagent, an orange-red complex slowly forms (e = 5.7.104 at 525 nm). Because palladium reacts rapidly with the reagent, even in the cold, it should be separated before the determination of platinum. Alternatively, platinum can be determined from the difference between the absorbances before and after the solution is heated. The other platinum metals must be separated beforehand. Dithizone reacts with platinum(II) in an acid medium (1-4 M HC1 or H2804) containing tin(II), to form a brown-yellow dithizonate Pt(HDz)2, which is soluble in CC14 or CHC13.

338

38. Platinum

2 3

1

2

3

i

4,00

_/.90 z,,50 500

600 635

700

wavelength, nm

Fig. 38.1. Absorption spectra of dithizone (H2Dz) (1), Pd(HDz)2 (2), and Pt(HDz)2 in 0014

The molar absorptivity is 3.8.10 4 at 490 nm and 3.5.104 at 710 nm. The absorption spectrum is shown in Fig. 38.1. Pt and Pd can be separated and determined one after the other by using dithizone [ 16,17]. Traces of Pt in palladium were determined by the dithizone method after separation of the macro-amounts of Pd as the sparingly soluble Pd(NH3)I2 [50]. Simultaneous determination of Pt and Pd is possible in the 5th order derivative spectrophotometry technique [51 ]. Trace amounts of Pt in larger amounts of Pd have been determined by means of the 2nd order derivative of Pt(HDz)2 absorption curve [52]. Azo dyes have also been used in determining Pt, e.g., PAR [53,54], Sulphonitrophenol M [55], and azo compounds based on Rhodanine and Thiorhodanine, such as Tyrodine and Sulphochlorophenolrhodanine (e = 1.0.105) [55-59]. Derivative spectrophotometry has been applied for determining Pt in the presence of Au and Pd in chloride [60]-, bromide [61 ]-, and iodide [62] solutions.

38.3. Analytical applications The tin(II) method has been used for determining platinum in catalysts [63,64] and ores [65]. Differential spectrophotometry was used for determining platinum in its alloy with ruthenium [66] and in catalysts [67]. The contents of platinum in catalysts have also been determined by derivative spectrophotometry [68]. The Rhodamine 6G method has been applied for determining 5.10 -3 % concentrations of Pt in palladium [52] and in catalysts [69]. Platinum present in amounts 0.01-5% in catalysts has been determined as the ionassociate of the thiocyanate-Pt(IV) complex with Crystal Violet [46,70].

References 1. A1-Bazi S.J., Chow A., Talanta, 31, 815 (1984). 2. A1-Bazi S.J., Chow A.,Anal. Chem., 55, 1094 (1983). 3. Khopkar S.M., Anal. Chem., 38, 360 (1966). 4. Shendrikar A.D., Berg E.W., Anal. Chim. Acta, 47, 299 (1969). 5. Ionov V.P., Potapova S.A., Dubrovina Z.N., Zhavoronkov N.M., Zh. Anal. Khim., 30, 955 (1975). 6. Mojski M., J. Radioanal. Chem., 46, 239 (1978). 7. Mojski M., Kalinowski K., Microchem. J., 25, 507 (1980). 8. Koch K.R., Yates J.E., Anal. Chim. Acta, 147, 235 (1983). 9. Khattak M.A., Magee R.J., Talanta, 12, 733 (1965).

References 10. Ahmed N., Koch K.R.,Anal. Chim. Acta, 162, 347 (1984). 11. Balcerzak M., Analusis, 22, 353 (1994). 12. Berg E.W., Sanders J.R.,Anal. Chim. Acta, 38, 377 (1967). 13. Faye G.H., Inman W.R.,Anal. Chem., 35, 985 (1963). 14. Shkil' A.N., Zolotov Yu.A., Zh. Anal. Khim., 43, 1466 (1988). 15. Rakovskii E.E., Shkil' A.N., Zh. Anal. Khim., 34, 1795 (1979). 16. Marczenko Z., Ku~ S., Mojski M., Talanta, 31,959 (1984). 17. Marczenko Z., Kug S.,Analyst, 110, 1005 (1985). 18. Rakovskii E.E., Shvedova N.N., Shkil' A.N., Zh. Anal. Khim., 39, 1463 (1984). 19. Rakovskii E.E., Shvedova N.V., Berliner L.D., Zh. Anal. Khim., 30, 1775 (1975). 20. Rakovskii E.E., Starozhitskaya M.I., Zh. Anal. Khim., 29, 2094 (1974). 21. Diamantatos A., Verbeek A.A., Anal. Chim. Acta, 91, 287 (1977). 22. Diamantatos A., Anal. Chim. Acta, 94, 49 (1977). 23. Korkisch J., Klakl H., Talanta, 15, 339 (1968). 24. Rocklin R.D., Anal. Chem., 56, 1959 (1984). 25. Fujiwara M. et al.,Anal. Chim. Acta, 274, 293 (1993). 26. Dybczyfiski R., Maleszewska H., J. Radioanal. Chem., 21, 229 (1974). 27. Weinert C.H., Strelow F.W., Talanta, 30, 755 (1983). 28. Malofeeva G.I., Myasoedova G.V., Volynets M.P., Mikrochim. Acta, 1978 I, 391. 29. Koster G., Schmuckler G., Anal. Chim. Acta, 38, 179 (1967). 30. Siddhanta S., Das H.R., Talanta, 32, 457 (1985). 31. Liu F. et al., Microchem. J., 52, 274 (1995). 32. Kundu D., Roy K., Talanta, 39, 415 (1992). 33. Khain V.S., Volkov A.A., Zh. Anal. Khim., 39, 1945 (1984). 34. Fuller C.W., Himsworth G., Whitehead J., Analyst, 96, 177 (1971). 35. Beyermann K., Z. Anal. Chem., 200, 183 (1964). 36. Berg E.W., Downey D.M., Anal. Chim. Acta, 120, 237 (1980). 37. Elizarova G.L., Matvenko L.G., Zh. Anal. Khim., 25, 301 (1970). 38. Marczenko Z., Krasiejko M., Chem. Anal. (Warsaw), 15, 1233 (1970). 39. Danilova V.N., Lisichenok S.L., Zh. Anal. Khim., 24, 1061 (1969). 40. Akimov V.K., Emelyanova I.A., Busev A.I., Zh. Anal. Khim., 26, 2416 (1971). 41. Marczenko Z., Kalinowski K., Anal. Chim. Acta, 153, 219 (1983). 42. Marczenko Z., Maruszak J., Chem. Anal. (Warsaw), 24, 341 (1979). 43. Li Z.B., Wang J.L., Xu Q.H., Mikrochim. Acta, 118, 43 (1995). 44. Ovsepyan E.N., Zh. Anal. Khim., 38, 1277 (1983). 45. Pilipenko A.T., Khvatkova Z.M., Golovina V.V., Zh. Anal. Khim., 41, 2045 (1986). 46. Pilipenko A.T., Khvatkova Z.M., Golovina V.V., Zh. Anal. Khim., 43, 1664 (1988). 47. E1-Ghamry M.T., Frei R.W., Talanta, 16, 235 (1969). 48. Shkrobot E.P., Shebarshina N.I., Zh. Anal. Khim., 36, 1986 (1981). 49. Sen GuptaJ.G.,Anal. Chim. Acta, 42, 481 (1968). 50. Marczenko Z., Ku~ S., Anal. Chim. Acta, 196, 317 (1987). 51. Ku~ S., Marczenko Z., Analyst, 112, 1503 (1987). 52. Ku~ S., Marczenko Z.,Analyst, 114, 207 (1989). 53. Mizuno K., Miyatani G., Bull. Chem. Soc. Jpn., 49, 2479 (1976). 54. Ivanov V.M., Gorbunova G.N., Zh. Anal. Khim., 35, 2363 (1980). 55. Savvin S.B. et al., Zh. Anal. Khim., 37, 667 (1982). 56. Savvin S.B., Propistsova R.F., Rozovsky Yu.G., Zh. Anal. Khim., 27, 1554 (1972). 57. Savvin S.B. et al., Zh. Anal. Khim., 38, 132 (1983).

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58. Gur'eva R.F., Savvin S.B., Trutneva L.M., Chalisova N.N., Zh. Anal. Khim., 38, 881 (1983). 59. Gur'eva R.F., Savvin S.B., Zh. Anal. Khim., 43, 476 (1988). 60. Kuroda R., Hayashibe Y., Yoshitsuka K., Fresenius'J. Anal. Chem., 336, 494 (1990). 61. Obarski N., Marczenko Z., Chem. Anal. (Warsaw), 40, 385 (1995). 62. Marczenko Z., Obarski N., Chem. Anal. (Warsaw), 39, 347 (1994). 63. Rees T.D., Hill S.R., Talanta, 15, 1312 (1968). 64. R6Zycki C., Chem. Anal. (Warsaw), 25, 1075 (1980). 65. Coombes R.J., Chow A., Talanta, 26, 991 (1979). 66. Marczenko Z., Ramsza A.P., Chem. Anal. (Warsaw), 21, 845 (1976). 67. Kallmann S., Talanta, 23, 579 (1976). 68. Tummavuori J., Hankala S., Siltaloppi L., Fresenius'J. Anal. Chem., 339, 145 (1991). 69. Pillai A.R. et al., Chem. Anal. (Warsaw), 41,787 (1996). 70. Pilipenko A.T., Khvatkova Z.M., Golovina V.V., Zh. Anal. Khim., 43, 1664 (1988).

Chapter 39. Rare-earth elements The lanthanides are a group of elements with atomic numbers 57-71, which exhibit very similar chemical properties. They occur naturally in two slightly different groups - - the cerium group, (elements 57-63), and the yttrium group (elements 63-71). Yttrium has very similar chemical properties to the lanthanides in the second group and, although it is not itself a lanthanide (having no f- electrons), it is generally included with the lanthanides in the group of elements known as the rare-earth elements (REE). The cerium group includes lanthanum La, cerium Ce, praseodymium Pr, neodymium Nd, promethium Pm, samarium Sm, and europium Eu. The yttrium group consists of yttrium Y, gadolinium Gd, terbium Tb, dysprosium Dy, holmium Ho, erbium Er, thulium Tm, ytterbium Yb, and lutetium Lu. All the rare-earth elements occur in the III oxidation state in compounds, and can be separated and determined in this form to provide what is known as the total REE. Samarium, europium, and ytterbium also occur in the unstable II oxidation state, whereas cerium, praseodymium, and terbium can be found in the IV oxidation state. Rare-earth element hydroxides, M(OH)3, precipitate from nitrate solution at pH values above 6.3-7.8 and reveal no amphoteric properties. Like thorium, the rare-earth elements yield acid-insoluble fluorides and oxalates, and soluble EDTA-, tartrate-, and citratecomplexes. Cerium (Ce, at. mass 140.12), which is the most popular rare-earth element, occurs also in the IV oxidation state. In chemical properties, cerium(IV) resembles Th and U(IV). The yellow Ce(OH)4 precipitates at pH --1. Cerium(III) is oxidized to Ce(IV) in acidic media by bismuthate, silver(II) oxide, or persulphate (in the presence of Ag+).

39.1. Methods of separation and preconcentration 39.1.1. Ion-exchange. Chromatography The most important methods for separating the rare-earth elements are based on ionexchange. Rare-earth elements have been separated on strongly basic anion-exchangers by taking advantage of the small differences in the stabilities of the EDTA complexes [1]. Trace amounts of REE have been separated from uranium on Dowex 1 anion-exchanger, using mixtures of methanol with HC1 or acetic acid [2]. Anion-exchange separation of the rareearth metals with mixtures of HNO3 and water-miscible alcohols is also feasible [3,4]. EDTA has also found application in the cation-exchange separations. From an EDTA solution at pH 2.1, Dowex 50 cation-exchanger retains the REE elements, whereas Th is eluted as the more stable EDTA complex. The lighter lanthanides have been separated by eluting them from a cation-exchanger with EDTA or NTA solution. The REE have been separated on strongly acid cation-exchangers with citrate media [5,6]. REE have been separated from other elements on cation-exchangers using hydrochloric acid [7,8], HC1 + ethanol [9], and aq. HNO3 [10] media. The REE have been preconcentrated on a chelating ion-exchanger [ 11 ]. The chromatographic methods for separation and isolation of REE have been reviewed [12,13].

342

39. Rare-earth elements

39.1.2. Precipitation Rare-earth elements may be isolated by precipitation as their oxalates, fluorides, or hydroxides. When the oxalates are precipitated from a weakly acidic medium (pH 1-4), Ca is used as a collector [14,15]. If the medium is too acidic for precipitation of the oxalates, the REE can be separated as fluorides [16,17]. Thorium is co-precipitated, and calcium is a suitable collector. Both methods give good separation of REE from Fe, A1, Ti, Zr, U(VI), Nb, Ta, and certain other metals. In 0.2 M NazCO3, the light lanthanides (cerium group) are precipitated quantitatively, while the remaining lanthanides and scandium are only partly precipitated [18]. Separation of Ce(IV) as the hydroxide (pH --1) enables the separation of Ce from other REE. Ti, Zr, or Fe(III) can be used as carriers. Ce(IV) may also be precipitated as iodate.

39.1.3. Extraction Rare-earth elements can be separated from other metals by extraction methods using the complexes with TOPO (MIBK) [19], HTTA with TOPO (toluene) [20,21], HDEHP [22-24], DDTC [25], 8-hydroxyquinoline with phen [26] or with tetra-n-heptylammonium ion (CHC13) [27], high molecular-weight amine [28], EDTA in the presence of Capriquat [29], a-diketones [30], and 2-thenoyltrifluoroacetone [31-34]. Crown ethers have been used for extractive separation of lanthanides [35-40]. The effect of solvent on extraction of REE with 18-crown-6 ether (and trifluoroacetone) has been discussed [41 ]. Ce(IV) has been separated from other lanthanides in HNO3 by extraction with Aliquat 336 solution in xylene [42] or TOA in CHC13 [43]. The lanthanides were also extracted from thiocyanate media using TBP [44], or as ion-associates of complexes with 2,3naphthalenediol and surfactants [45]. Extraction of REE from multi-component mixtures by phosphoro-organic compounds and trialkylammonium nitrates has also been studied [46].

39.2. M e t h o d s of determination The Arsenazo III method presented below can be used to determine either the total REE or the individual lanthanides and yttrium. The 8-hydroxyquinoline method gives a possibility of determining cerium as Ce(IV) in the presence of all the remaining REE. The determination of lanthanides in geological samples has been reviewed [47].

39.2.1. Arsenazo III method In weakly acidic media the lanthanides and yttrium react with Arsenazo III (formula 4.10) to form coloured complexes which are the basis of the sensitive method for determining the total of REE or any element from this group [48-51 ]. In weakly acidic solution, the reagent is violet, and its complexes with the rare-earth elements are green. The maximum absorbance for Ce(III) is obtained at pH 2.3-2.7. The optimum pH values for the various REE differ slightly. The reagent does not absorb at the wavelength of the absorption maximum of the complex (650 nm). The molar absorptivity of the Arsenazo III complex with cerium is 5.6.104 (specific absorptivity 0.40). The other REE complexes have similar molar


343

absorptivities. Chloride, sulphate, and phosphate do not interfere in the determination of REE. Neither do small amounts (less than 1 mg per 50 ml of solution) of Ti, A1, Ca, and Fe [reduced to Fe(II) with ascorbic acid], but larger quantities of these metals and certain other metals (Th, Zr, U, Bi, Cu) should be removed. Many interfering metals can be masked with EDTA [5254]. The REE are separated from the calcium used as the collector in the precipitation of the oxalates or fluorides, by reprecipitation as the hydroxides with ammonia. In this instance, Fe(III) is used as the collector. In aqueous organic media, the sensitivity in determining REE with Arsenazo III is increased significantly [55]. The reaction with Arsenazo III has been used in the study of extraction equilibria in the systems of REE and phosphonic acid derivatives [56].

Reagents Arsenazo III, 0.05% solution. Dissolve 50 mg of the reagent in 100 ml of water. Standard lanthanum solution: 1 mg/ml. Dissolve 0.1170 g of La203, dehydrated by ignition, in 5 ml of hot HC1 (1+1). Dilute the solution to volume with water in a 100-ml standard flask. Standard cerium solution: 1 mg/ml. Dissolve 0.1228 g of cerium(W) oxide, CeO2 (dehydrated by ignition), in 5 ml of hot HC1 (1 + 1), and add 0.1 g of NHzOH.HC1. Dilute the solution to volume with water in a 100-ml standard flask. Standard yttrium solution: 1 mg/ml. Dissolve 0.1270 g of Y203 (dehydrated by ignition), in 5 ml of hot HC1 (1+1). Dilute the solution to volume with water in a 100-ml standard flask. Formate buffer, pH 3.5. Dissolve 60 ml of formic acid and 28 g of NaOH in water, and dilute the solution with water to 1 litre.

Procedure Separation of R E E as oxalates. To the acid solution (50-100 ml) add 5-10 ml of 8% oxalic acid solution. Adjust the pH of the solution to 2.0-2.5, heat to -~80~ and add 5 mg of calcium (as a chloride solution) dropwise with stirring. Heat the solution for 1 h, but do not boil. After several hours (or on the next day), filter off the precipitate and wash it thoroughly with 1% oxalic acid solution and water. Ignite the precipitate to the oxides, and dissolve it in a small amount of hot 4 M HC1. To separate the REE from calcium, dilute this solution with water to 10-15 ml, add 3 mg of A1 (as a salt solution), and co-precipitate REE hydroxides and aluminium hydroxide with ammonia (pH -~9). Dissolve the precipitate in a small amount of hot 2 M HC1. Determination of REE. To the acid solution (pH -~1) containing not more than 40 gg of REE, add 1 ml of 1% ascorbic acid solution. After several min, add 1 ml of the buffer and 2 ml of the Arsenazo III solution. Add water to 20 ml and mix well. Adjust pH to 2.6___0.1 with 0.1 M NaOH. Transfer the solution to a 25-ml standard flask, add water up to the mark, mix well and measure the absorbance at 650 nm, using the reagent blank solution as the reference.

344


39.2.2. 8-Hydroxyquinoline method 8-Hydroxyquinoline (oxine) (formula 4.42) reacts in ammoniacal media with cerium ions to give a sparingly soluble chelate, extractable into CHC13. The brown-red colour of the extract is used for determining cerium. It has been shown that the coloured extract contains cerium(IV) oxinate, which in the presence of reducing agents gives pale-yellow Ce(III) oxinate. In the absence of reductants the yellow Ce(III) oxinate is readily oxidized by atmospheric oxygen to the intensely coloured Ce(IV) oxinate. In CHC13 solutions ~maxof this compound is 495 nm, and e = 6.7.103 (sp. abs. 0.048). All the other REE and Th give, like Ce(III), pale yellow oxinates which exhibit a small absorption at 495 nm. Thus, in order to avoid the interference of REE and Th on the determination of cerium, one is recommended to measure the absorbance of Ce(IV) oxinate at 530 nm (although the sensitivity is slightly lower than at 495 nm). The extraction of Ce(IV) oxinate is complete when the pH of the ammoniacal solution is 9.9-10.6. The absorption spectra of Ce(IV) oxinate solutions in 1,2-dichloroethane, CC14, and benzene, differ slightly from that in the chloroform solutions. The presence of citrate in the aqueous solution interferes in the quantitative extraction of cerium oxinate. The solution should also not contain other Group 1-IV metals besides REE and Th. In the presence of EDTA, titanium is not extracted from solutions of pH 9.9-10.6 [57].

Reagents 8-Hydroxyquinoline (oxine), 1% solution in ethanol. Standard cerium solution: 1 mg/ml. Dissolve 3.1000 g of Ce(NO3)3.6H20 in water, add 2 ml of conc. HNO3, and dilute to 1 litre in a standard flask. Oxalic acid, 8% solution. Separation of Ce as oxalate. To the acid sample solution add 5-20 ml of oxalic acid solution and 3-5 mg of lanthanum (as a salt). Adjust the pH of the solution to 2-3, heat to 70-80~ and keep at this temperature for 1 h. After 2-3 h, filter off the precipitate and wash it thoroughly with 1% oxalic acid solution and water. Ignite the precipitate to the oxide, and dissolve it in a small volume of hot 4 M HC1. Determination of Ce. Dilute the obtained acid solution, containing not more than 0.3 mg of Ce, with water to 10 ml, add 1 ml of oxine solution, 1 drop of 1% phenolphthalein solution in ethanol, and ammonia (1+1) until the solution is rose coloured. Add 1 ml of ammonia (1+1) (the pH should be within 9.9-10.6) and transfer the solution to a separating funnel. Shake the solution with 2 portions of CHC13 (5 min shaking with each portion). Dilute the combined extracts with chloroform to 25 ml in a standard flask, and measure the absorbance at 530 nm, using the solvent as reference.

39.2.3. Other methods In addition to Arsenazo HI, the following azo dyes containing the arsonic or phosphonic acid group have also been recommended as spectrophotometric reagents for either individual lanthanides or the total REE: Arsenazo I [58,59], p-Acetylarsenazo [60], Carboxyarsenazo (e = 5.2.104 for La) [61], Dicarboxyarsenazo III [62], Chlorophosphonazo III [63,64], Chlorophosphonazo [65], Chlorophosphonazo DAL [66], p-Acetylchlorophosphonazo


345

[67,68], p-Nitrochlorophosphonazo [69]. In the extractive method for La with the use of Chlorophosphonazo III, diphenylguanidine and butanol, ~ = 1.6.105 [70]. Other azo reagents include PAR [71-75], TAR [76] and its derivatives [77], 5-BrPADAP [78-82], Carboxynitrazo (~ = 1.4.105) [83], Sulphonazo III [84], Calmagite [85], and Hydroxynaphthol Blue [86]. High sensitivity in determination of La and Ce was obtained in the presence of PAN [87]. Praseodymium and erbium were determined with PAN and its derivative in the presence of a non-ionic surfactant [88,89]. Neodymium was determined in glasses, based on the 4 th order derivative spectrum of its complex with PAN [90]. Xylenol Orange has been applied for determination of REE [91-95]. High sensitivity is obtained in the presence of CP (e = 9.2.104 at 625 nm for La) [96-99]. Other triphenylmethane reagents proposed are Methylthymol Blue [100], Chrome Azurol S [101-103], and Pyrocatechol Violet [104]. The sensitivity of these methods increases in the presence of CP [ 105-107]. From among the xanthene chelating reagents, Bromopyrogallol Red [108-110], and Gallein [111] have found application in the spectrophotometric determination of REE. Rhodamine B, and the anionic complexes of REE with oxine [112], salicylic acid derivatives [ll3,114], and 3,5-dinitrocatechol [115] have been the basis for sensitive methods of determining REE (e = 9.104 at 550 nm). Praseodymium has been determined by a sensitive flotation-spectrophotometric method (e = 1.8.105) with the use of 5,7-dichlorooxine and Rhodamine 6G [116]. REE ions with 1,10-phenanthroline form cationic complexes such as [La(phen)33+], which give ion-associates with xanthene acid dyes, eosin or Erythrosin [117,118]. Molar absorptivities are of the order of 1-105. Europium(II) has been determined by an indirect method, after reduction of Fe(III) and reaction of the resulting Fe(II) with 1,10phenanthroline [ 119]. Trivalent cations of REE in aqueous solutions, acidified with HC1, HNO3, or HC104, absorb in the UV or VIS. The absorption bands are narrow, with sharp, non-overlapping peaks, but the molar absorptivities are rather small (1-10), and individual species of REE can be determined at concentrations of the order of 1 mg/ml [120]. Higher sensitivities are obtained after the ions have been converted into EDTA complexes [121 ]. The determination can be made more selective and sensitive by the use of the derivative spectrophotometry techniques [ 122-124]. Neodymium and erbium have been determined in the mixtures of REE by the derivative spectrophotometry technique using ferron and diethylamine [ 125]. Europium(III) has been selectively reduced in the Jones column to Eu(II) which can reduce Methylene Blue to the colourless leuco- form [126]. Europium(II) also reduces phosphomolybdic acid to molybdenum blue [127] and gives a colour reaction in solutions containing 1,10-phenanthroline and Fe(III). 2,2'-Diquinoxalyl has also been used for determining Eu [ 128]. Cerium(IV) ions give a yellow colour in H 2 8 0 4 and HNO3 solutions, with maximum absorption in the UV region and e = 5.6.103 at 320 nm. Certain determination methods are based on coloured complexes of Ce(IV) with organic reagents, such as formaldoxime [129], N-p-tolylbenzohydroxamic acid [130], or 3-thianaphthenoyltrifluoroacetone (e = 5.5.103 at 424 nm) [ 131 ]. Some methods are based on the colour effects occurring in the oxidizing action of Ce(IV) on, e.g., o-tolidine and tetron (N,N'-tetramethyl-o-tolidine) (e = 2.5-104 at 470 nm) [132].

346


39.3. Analytical applications The Arsenazo III has been used for determining the total REE and individual metals of this group in natural waters [133], air [134], ores, rocks, and minerals [ 135-138], steels 19,139,140], chromium and its alloys [141 ], copper-nickel alloys [54], and fertilizers [ 142]. Continuous FIA has been applied [78,138]. The Arsenazo I method was applied for the determination of REE in steel [58] and in minerals [59]. The 8-hydroxyquinoline method was used for determining cerium in cast iron and steel [5]. Cerium was determined with PAR in rocks [8], with Xylenol Orange- in apatites [98], with p-Acetylchlorophosphonazo - in minerals [67], and with Chlorophosphonazo in plant materials [65].

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Chapter 40. Rhenium Rhenium (Re, at. mass 186.21) occurs mainly in oxidation states VII, V, and IV. Rhenium(VII) compounds are the most stable. The colourless perrhenate, ReO4- ion exhibits weak oxidizing properties. The chemical properties of rhenium resemble those of Mo and Mn. Rhenium(V) and (IV) form halide, oxalate, and tartrate complexes.

40.1. Methods of separation and preconcentration The main requirement in connection with the determination of small quantities of Re is its separation from Mo, which is usually the major interfering element. Small amounts of rhenium are normally found with Mo in natural materials, such as molybdenite.

40.1.1. Extraction Traces of rhenium can be separated from larger amounts of Mo by shaking a chloroform solution of tetraphenylarsonium chloride with an aqueous solution at pH 8-9 containing ReO4-, thereby forming the CHC13-soluble ion associate [(C6Hs)4As+][ReO4-]. Molybdate ions remain in the aqueous solution. The distribution coefficient for rhenium is -~1,000. At chloride concentrations greater than 0.1 M, the efficiency of Re extraction is worse. Tetraphenylarsonium perrhenate is extracted over a wide pH range, but a medium at pH 8-9 is most suitable for the separation from molybdenum. EDTA is added to prevent the precipitation of hydroxides of other metals [1,2]. AuC14-, MnO4-, C104-, and SCN- are extracted together with ReO4-. Tetraphenylphosphonium chloride may be substituted for tetraphenylarsonium chloride. From acid media, ReO4- can be extracted as its ion-pair with tribenzylammonium ion (dichloromethane) [3], tetraoctylammonium ion (CHC13) [4], tributylammonium ion [5], N-benzylaniline (CHC13) [5], or amiloride (4-methyl-2-pentanone) [6]. ReO4- can be extracted from 6 M NaOH with pyridine [7]. Order of citation changed: please confirm. Re(VII) has been separated by extraction with cyclohexanone from various inorganic acids [8]. Rhenium can be extracted from bromide media into CHC13 as ReOBr4 with dithiol [9]. Nitrotetrazolium chloride has been used for extraction (dichloroethane) of Re from mixed chloride and iodide media [10]. Rhenium has been extracted also with the crown ether, dibenzo-18-crown-6 (formula 1.11)[11]. Problems connected with the extractive separation of Re from Mo and other metals have been studied in detail [12-14].

40.1.2. Other methods Rhenium can be separated from many metals by precipitation with H2S in 5-6 M HC1 (or 2.5-3 M H2SO4). Arsenic(III) is a suitable collector since it does not interfere in the subsequent determination of rhenium by the thiocyanate method. At pH < 7.5 the MoO42- ion can be co-precipitated with Fe(III) used as collector, and ReO4-remains in the solution [15].


351

The sample containing Re is sintered at 600-700~ for 2-3 h with CaO and Ca(NO3)2, then the sinter is leached with hot dilute bromine water. Most metals (including molybdenum, which is then present as CaMoO4) remain in the solid residue while ReO4ions pass into solution [ 16]. Perrhenate is also leached with water from a sample melted with NazCO3 or NaOH. Rhenium heptoxide, Re207, is volatile and can be separated by distillation at 260280~ from a concentrated sulphuric acid medium [1,17,18]. It may be accompanied by As(HI), Hg, Se, and to a lesser degree by Sb, Te and Mo. In the presence of hydrochloric acid, rhenium distils as the oxychloride at a lower temperature, but As, Ge, Hg, Sn, Se, Mo, Te, and T1 are wholly or partly co-distilled. In view of the partial co-distillation of Mo, Re cannot be directly determined in the distillate. In the distillation of Re, carbon dioxide or air is used as the carrier gas. When a slightly acidic solution is run through a cation-exchange column, most metals (Fe, Cu, Ni, Mn, A1, etc.) are retained as cations, while rhenium is eluted as the ReO4- anion. Small amounts of ReO4- can be conveniently retained on an anionite column [19,20]. By using suitable eluents it is possible to separate Re from Mo and W [21 ].

40.2. Methods of determination A common spectrophotometric method for the determination of rhenium involves extraction of the thiocyanate complex into isoamyl alcohol. The ~-furildioxime method has similar sensitivity, the rhenium complex being measured either in the aqueous medium or in the CHC13 extract. Methods based on ion-pairs of ReO4- with basic dyes are the most sensitive.

40.2.1. Thiocyanate method The red-orange colour formed when SnC12 is added to an HC1 solution containing rhenium(VII) and thiocyanate is the basis of the spectrophotometric method for rhenium [5,22,23]. The reaction is complex, and the colour obtained depends on the concentration of the reducing agent and thiocyanate, and on the reaction time. It has been shown that two complexes can be formed: a greenish-yellow and a redorange complex with absorption maxima at 350 and 430 nm, respectively. The formula [ReOz(SCN)4] 3- has been proposed for the red-orange complex. Complexes of Re(IV) have also been reported [24]. Reproducible results in the determination of Re are obtained only when the reaction conditions are kept constant. Maximum colour intensity requires the presence of a small excess of SnCI2. The most suitable acid concentration is 2 M HC1, and the thiocyanate concentration should not be lower than 1%. Ascorbic acid has also been used instead of SnCI2 in Re determinations by the thiocyanate method [25,26]. Extraction of the thiocyanate complex with higher alcohols, diethyl ether, or DIPE enhances the sensitivity of the method, because the high distribution coefficients facilitate concentration of the complex in a small volume of organic extractant [27]. The molar absorptivity of the rhenium-thiocyanate complex in isoamyl alcohol is 3.8.104 (a = 0.21) at 430 nm. The absorption spectrum is shown in Fig. 40.1. The following species interfere in the determination of rhenium by the thiocyanate method; Mo, W, V, and Cr, as well as oxidizing and reducing agents. The thiocyanate complex of Re has been extracted with a thioacetanilide solution in benzene [28], DAPM [29], or hexamethylphosphoramide (or other amides) in CHC13 [30,31 ].

352

40. Rhenium

A mixed Re complex with thiocyanate and thiourea has been extracted into acetophenone [32]. The sensitivity of determining Re by the thiocyanate method increases in the presence of Fe(III) and NO3-[33]. 1

~oo

c~o '

2

soo "s3oL L" 6oo wavelengtla, nm

Fig. 40.1. Absorption spectra of the rhenium thiocyanate complex in isoamyl alcohol (1) and the rhenium a-furildioxime complex in aqueous acetone (2)

Reagents Potassium thiocyanate, 20% solution. Standard rhenium solution: 1 mg/ml. Dissolve 0.1550 g of KReO4 in water and dilute the solution to the mark with water in a 100-ml standard flask. Tin(II) chloride: fresh 0.1% solution prepared by dissolving 0.1 g of SnClz.2H20 in 100 ml of HC1 (1 +3). Tetraphenylarsonium chloride, 1% solution.

Procedure Extractive separation of Re. Adjust -~30 ml of the perchlorate-free sample solution to pH 8-8.5 with NaHCO3. If hydrolysable metals are present, add EDTA first. Transfer the solution to a separating funnel, add 1 ml of the tetraphenylarsonium chloride solution, and extract Re by shaking for 2 min with two 5-ml portions of CHC13. Before draining the extract, allow the phases to separate completely by letting them stand for 30 min. Wash the combined extracts with 1 ml of 1% NaHCO3 solution, and again allow the phases to stand for 30 min before separating them. Carefully separate the chloroform phase and evaporate it to dryness in a platinum crucible containing a little NazCO3 dissolved in a few drops of water. Fuse the residue with 0.5 g of NazCO3, dissolve the cooled melt in water, and neutralize the solution with 1 ml of H2S04 (1+3). Determination of Re. Acidify ~20 ml of solution containing not more than 100 gg of Re with 5 ml of conc. HC1, and add 3 ml of the thiocyanate solution. Heat the solution to 50~ in a beaker on a water-bath, add 0.5 ml of the SnC12 solution with vigorous stirring, and keep the solution at 50~ for 20 min on the water-bath. After cooling to room temperature, transfer the solution to a separating funnel and extract with two portions of isoamyl alcohol. Dilute the extracts to the mark with the solvent in a 25-ml standard flask, and measure the absorbance at 430 nm, using isoamyl alcohol or water as the reference.


353

40.2.2. ~-Furildioxime method In acid medium (HC1, H2804) containing SnC12 as a reducing agent, c~-furildioxime (formula 32.2) reacts with rhenium to form a red complex [34-36]. The coloured complex is sparingly soluble in aqueous medium, but dissolves in solutions containing 25% of acetone, which also accelerates the reaction. The reaction rate decreases, however, at acetone concentrations above 25%. Heating the solution (to not higher than 60~ since volatile acetone is present) also increases the reaction rate. The optimum acidity in the medium corresponds to 0.8-1 M HC1. The reaction with rhenium requires a considerable excess of ~-furildioxime, the quality of which affects the absorbance [34]. The minimum amount of stannous chloride necessary is 2.5 ml of 10% SnCI2 solution per 25 ml of the final solution. The molar absorptivity of the complex in aqueous acetone at ~max 530 nm is 4.1.10 4 (a = 0.22). The rhenium cz-furildioxime complex can be extracted into CHC13. The distribution coefficient is 150. Addition of isoamyl alcohol clears the chloroform extract. In CHC13 medium e = 3.7.104. Thiocyanate, nitrate, and fluoride interfere in this method for determining rhenium. Palladium and copper in acid media sparingly soluble ~-furildioximates. Normally, rhenium should be separated from Mo before the determination.

Reagents a-Furildioxime, 0.5% solution in acetone. If coloured, purify the solution by shaking it with active carbon. Standard rhenium solution: 1 mg/ml. Preparation as in Section 40.2.1. Tin(H) chloride: a freshly prepared 10% solution in HC1 (1 +9).

Procedure Place -~15 ml of the slightly acidic sample solution, containing not more than 100 ~tg of Re, in a 25-ml standard flask. Add 2 ml of conc. HC1 and 6 ml of the c~-furildioxime solution, and mix well. Add 2.5 ml of the SnCI2 solution and dilute to the mark. Heat the flask in a waterbath at 60~ for 20 min with occasional shaking. Cool the solution in a cold water-bath to room temperature, and measure the absorbance at 530 nm against a reagent blank or water. Extraction into chloroform. After heating at 60~ as above and cooling to room temperature, transfer the coloured solution to a separating funnel and extract the rhenium complex by shaking for 1 min with two portions of CHC13. Dilute the combined extracts in a standard flask with chloroform and isoamyl alcohol (4+1). Measure the absorbance of the extract at 530 nm, using the solvent as reference.

40.2.3. Other methods The most sensitive spectrophotometric methods for the determination of Re involve the extraction of ion-associates formed by ReO4- with basic dyes. From among the triarylmethane dyes, use has been made of Methyl Green (benzene, e - 1.2.105 at 640 nm) [37], fuchsine (formula 27.1) [1] Brilliant Green (benzene, e = 1.0.105) [38], Crystal Violet [39], Victoria Blue 4R (formula 4.28) [40], Rhodamine B [41], Safranine T [42], Nile Blue

354

40. Rhenium

(formula 4.32) (e = 7.2.104) [43], and Chrompyrazole II [44]. In another method [45] the associate of ReO4- with Brilliant Green is adsorbed on microcrystalline benzophenone which is then dissolved in benzene. A method based on the reaction of perrhenate with Nitrotetrazolium Blue and SnCI2 in CHC13 medium is very sensitive [46]. Trace amounts of Re were separated from large quantities of Mo by extraction into benzene solution of Brilliant Green [47]. In addition to a-furildioxime, other oximes are used as spectrophotometric reagents for rhenium, such as dimethylglyoxime [7,48,49], c~-pyridyldioxime [50], o~-benzoinoxime [51], methyl-2-pyridylketoxime [52], and phenyl-2-pyridylketoxime [53]. c~-Benzoyldioxime gives with Re, in thiocyanate medium, a complex ()~max= 430 nm) extractable into isoamyl acetate [54]. In each case the colour reaction occurs in the presence of SnCI2. In 3-5 M HC1 containing SnCI2, thiourea forms a yellow cationic rhenium complex which is soluble in water, thus affording the basis of a less sensitive method for determining Re (e -~ 6.103) [55-58]. The colour reaction is slow, and heating to 70~ is advantageous. Above 75~ browning of the solution can be observed as the thiourea complex decomposes. Several complexes can be formed in the rhenium-thiourea system. Which species predominates, depends on the amount of the excess of reagent and the acidity. The thiourea method is suitable for the differential spectrophotometric determination of larger quantities of rhenium [59]. Phenylthiourea (e - 9.5.103) [60] and 2-pyridylthiourea (e - 1.6.104) [61] have been proposed as spectrophotometric reagents for rhenium. Other thiourea derivatives recommended are 1-phenylthiosemicarbazide and 1,4-diphenylthiosemicarbazide (formula 42.1) [62]. Other organosulphur reagents for rhenium include dithiol [63], Bismuthiol II (formula 49.1) (~ = 2.2.104) [64], rubeanic acid [65], mercaptopropionic acid [66], o-hydroxythiobenzhydrazide [67], and thiobenzhydrazide [68]. A molybdo-rhenium heteropoly acid has also been applied for determination of rhenium [69,70].

40.3. Analytical applications The thiocyanate method has been used for determining rhenium in dusts and in molybdenite [51], copper ores and concentrates [28,71,72], molybdenum, tungsten, and vanadium [73], and coal [ 16]. o~-Furyldioxime has been applied in determinations of Re in copper concentrates [71], molybdenite [36], molybdenum concentrate [4], and molybdenum and tungsten alloys [74]. Dimethylglyoxime was used for determining Re in molybdenite, tungsten, and minerals [7]. Rhenium has been determined in copper concentrates and rhenium-tungsten alloys with the use of Nitrotetrazolium Blue [46]. The thiourea method has been applied for determining rhenium in used catalysts [75]. The results of Re determination in ores by the spectrophotometric method and by other methods have been compared [76].

References 1. Beyermann K., Z. Anal. Chem., 183, 91 (1961). 2. Thierig D., Umland F., Z. Anal. Chem., 240, 19 (1968). 3. Ziegler M., Schroeder M., Z. Anal. Chem., 212, 395 (1965).

References

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4. Budeginsky B.W., Analyst, 105, 278 (1980). 5. Khosla M.N., Rao S.P., Anal. Chim. Acta, 61, 239 (1972). 6. Burns D.T. et al., Anal. Chim. Acta, 322, 107 (1996). 7. D6ge H.G., Grosse-Ruyken H., Mikrochim. Acta, 1967, 98. 8. Jordanov N., Pavlova M., Bojkova D., Talanta, 23, 463 (1976). 9. Prasolova O.D., Borisova L.V., Zh. Anal. Khim., 44, 346 (1989). 10. Simeonova Z., Alexandrov A., Dzharkova M., Fresenius'J. Anal. Chem., 348, 329 (1994). 11. Koshima H., Onishi H., Anal. Chim. Acta, 232, 287 (1990). 12. Yatirajam V., Kakkar L.R.,Anal. Chim. Acta, 47, 568 (1969); 54, 152 (1971). 13. Yatirajam V., Kakkar L.R., Talanta, 17, 759 (1970). 14. Yatirajam V., Kakkar L.R., Mikrochim. Acta, 1970, 708; 1971, 479. 15. Novikov A.I., Zh. Anal. Khim., 16, 588 (1961). 16. Kuznetsova V.V., Zh. Anal. Khim., 16, 736 (1961). 17. Basifiska M., Rutkowski W., Chem. Anal. (Warsaw), 13, 799 (1968). 18. Solt M.W., Wahlberg J.S., Myers A.T., Talanta, 16, 37 (1969). 19. Hamaguchi H., Kawabuchi K., Kuroda R., Anal. Chem., 36, 1654 (1964). 20. Morgan J.W. Golightly D.W., Dorrzapt A.T. Jr., Talanta, 38, 259 (1991). 21. Korkisch J., Feik F., Anal. Chim. Acta, 37, 364 (1967). 22. Ko~licka M., W6jtowicz M., Adamiec J., Chem. Anal. (Warsaw), 15, 247 (1970). 23. Wahi A., Kakkar L.R., Fresenius' J. Anal. Chem., 338, 298 (1990). 24. Jordanov N., Pavlova M., Zh. Anal. Khim., 19, 221 (1964); 20, 591 (1965); 22, 212 (1967). 25. Wahi A., Kakkar L.R., Fresenius' J. Anal. Chem., 343, 904 (1992). 26. Wahi A., Kakkar L.R., Anal. Sci., 10, 509 (1994). 27. Skiba H., W6jtowicz M., Chem. Anal. (Warsaw), 10, 183 (1965). 28. Mishra N. et al., Anal. Sci., 6, 407 (1990). 29. Akimov V.K., Kliot L.Ya., Busev A.I., Zh. Anal. Khim., 28, 118 (1973). 30. Mitra M., Mitra B.K., Talanta, 25, 597 (1978). 31. Mishra N., Sinha S.K., Patel K.S., Mishra R.K., Bull. Chem. Soc. Jpn., 62, 3400 (1989). 32. Savariar C.P., Hariharan T.R., Mikrochim. Acta, 1975 I, 477. 33. Kuznetsova N.M., Monakhova N.G., Zh. Anal. Khim., 45, 683 (1990). 34. Fryer F.A., Galliford D.J., Yardley J.T., Analyst, 88, 188, 191 (1963). 35. Peshkova V.M., Ignat'eva N.G., Zh. Anal. Khim., 22, 757 (1967). 36. Dor2 D., Dobrowolski J., Chem. Anal. (Warsaw), 24, 465 (1979). 37. Taraian V.M., Mirzoian F.V., Sarkisian Zh.V., Zavod. Lab., 45, 409 (1979). 38. Fogg A.G., Burgess C., Burns D.T.,Analyst, 95, 1012 (1970). 39. Gajbakian A.G. et al., Zh. Anal. Khim., 42, 2093 (1987). 40. Pilipenko A.T., Kish P.P., Zheltvai I.I., Ukr. Khim. Zh., 37, 477 (1971). 41. Lebedeva S.P., Taraian V.M., Zh. Anal. Khim., 30, 1403 (1975). 42. Pilipenko A.T., Nguen Mong Shin', Ukr. Khim. Zh., 32, 1211 (1966). 43. Gagliardi E., Ftisselberger E., Mikrochim. Acta, 1972, 385. 44. Busev A.I. et al., Zh. Anal. Khim., 21,574 (1966); 22, 205 (1967). 45. Burns D.T., Tungkananuruk N.,Anal. Chim. Acta, 204, 359 (1988). 46. Plastinina E.I., Borisova L.V., Zh. Anal. Khim., 44, 2211 (1989). 47. Lyam Ngok Tkhu, Ukr. Khim. Zh., 57, 67 (1991). 48. Ko21icka M., W6jtowicz M., Adamiec J., Chem. Anal. (Warsaw), 15, 701 (1970). 49. Wahi A., Kakkar L.R.,Anal. Sci., 9, 409 (1993). 50. Trussell F., Thompson R.J., Anal. Chem., 36, 1870 (1964).

356

40. Rhenium

51. Khaira S., Kakkar L.R., Fresenius'Z. Anal. Chem., 335, 404 (1989). 52. Thompson R.J., Gore R.H., Anal. Chim. Acta, 31,590 (1964). 53. Guyon J., Murmann R.K.,Anal. Chem., 36, 1058 (1964). 54. Wahi A., Kakkar L.R., Fresenius'J. Anal. Chem., 352, 387 (1995). 55. Borisova L.V., Zh. Anal. Khim., 24, 1361 (1969). 56. Nemodruk A.A., Bezrogova E.V., Zh. Anal. Khim., 24, 1534 (1969). 57. Lazarev A.I., Lazareva V.I., Komarnaya V.D., Zavod. Lab., 42, 1304 (1976). 58. Bahr H., J~dras C., Chem. Anal. (Warsaw), 23, 1031 (1978). 59. Shanina T.M., Mikhailovskaya V.S., Gelman N.E., Zh. Anal. Khim., 29, 2059 (1974). 60. Pollock E.N., Anal. Chim. Acta, 32, 418 (1965); 47, 367 (1969). 61. Dutta G., Sur B., Mikrochim. Acta, 1986 I, 359. 62. Borisova L.V., Plastinina E.I., Ermakov A.N., Zh. Anal. Khim., 29, 743 (1974). 63. Koyama M. et al., Chem. Anal. (Warsaw), 17, 679 (1972). 64. Lazarev A.I., Lazareva V.I., Zh. Anal. Khim., 32, 751 (1977). 65. Bozhkov O.D., Jordanov N., Borisova L.V., Talanta, 35, 62 (1988). 66. Talipova L.L., Lapin S.B., Parpiev N.A., Zh. Anal. Khim., 31, 106 (1976). 67. Gangopadhyay P.K., Shome S.C., Anal. Chim. Acta, 75, 235 (1975). 68. Gangopadhyay S., Gangopadhyay P.K., Shome S.C., Anal. Chim. Acta, 83, 409 (1976). 69. Semenovskaya E.N., Basova E.M., Zh. Anal. Khim., 37, 2014 (1982). 70. Basova E.M., Semenovskaya E.N., Zh. Anal. Khim., 39, 1834 (1984). 71. Stefanov S., Jordanov N., Pavlova M., Mikrochim. Acta, 1976 II, 449. 72. Ghosh A., Patel K.S., Mishra R.K., Analyst, 115, 969 (1990). 73. Pavlova M., Kadieva S., Jordanov N., Z. Anal. Chem., 285, 271 (1977). 74. Cotton T.M., Woolf A.A., Anal. Chem., 36, 248 (1964). 75. Bochkova L.P., Borisova L.V., Korchemnaya E.K., Zh. Anal. Khim., 47, 809 (1992). 76. Kolosova L.P., Karchevskaya G.Ya. et al., Zavod. Lab., 56, No 12, 19 (1990).

Chapter 41. Rhodium and iridium Rhodium (Rh, at. mass 102.91) and iridium (Ir, at. mass 192.22) occur in the HI, IV and VI oxidation states, the compounds of rhodium(III) and iridium(IV) being the most stable. Powdered Rh and Ir, when fused with Na202, give RhO42- and IrO42-, respectively. Strong reducing agents (e.g., zinc, magnesium) reduce rhodium(III) (in solutions) to the metal. Iridium(III) and iridium(W) are not so readily reduced, although complete reduction may be achieved in the presence of Pd and Pt. Rhodium(III) and iridium(IV) give stable halide, cyanide, and ammine complexes.

41.1. Methods of separation and preconcentration 41.1.1. Extraction From 1-6 M HC1, containing SnC12, rhodium(III) can be separated from Ir, Pd, and Pt by extraction with benzene in the presence of DAPM [1], TOA [2,3], or 2-octylaminopyridine [4]. Rhodium is also extracted with isoamyl alcohol from an acid medium containing bromide and Sn(II) after heating at -~90~ [5]. From a 0.06 M HC1 medium hexachloroiridate(IV) can be extracted as an ion-pair with tetraphenylarsonium cation [6]. This method enables one to separate Ir from Rh and Pd, but not from Pt. Ir has been separated from Rh, also, by extraction with tetra-n-butylammonium ion [7]. Rh (also Ir, Pd, and Pt) can be extracted with bis-(2-ethylhexyl)dithiophosphoric acid [8]. Ir can be extracted with mercaptobenzothiazole [9] or thiobenzanilide [10]. Rhodium has been separated from Ir by the extraction (CHC13) of its complex with 2mercaptobenzimidazole [11 ]. HDEHP has been used for extraction separation of Rh from Ru and Ir [12]. Extraction of Rh (and other noble metals) by thiourea and its derivatives was studied [ 13].

41.1.2. Ion exchange and other methods Rhodium(HI) can exist as a cationic complex in hydrochloric acid medium, whereas Ir(IV), Pt(IV), and Pd(II) exist as anionic complexes. These properties enable their separation with the use of ion exchangers [ 14-16]. Chelate complexes of Rh with TAR [17,18], TAN [19], PAN [20-22], and 8hydroxyquinoline [23] and its derivatives [24] have been used for separation of Rh by the liquid chromatography. The sorption of Rh and Ir on polyurethane foam from thiocyanate medium has been applied for the isolation of Rh and Ir and for their separation [25-27]. Iridium can be separated from Rh and Pt owing to the fact that the anionic Ir chloride complex is precipitated with Ag + [7]. Precipitation methods have been proposed also for separation of Rh from Ir and Pt [28]. Rhodium(III) may be separated from iridium(W) by reduction to the metal [iridium being reduced only to Ir(III)] with Cu powder in 1 M HC1 or sodium borohydride (NaBH4)] [29,30]. Iridium can also be separated from Rh and Ru by flotation of the ion-associate formed by the chloride complex of Ir(IV) with a cationic

358

41. R h o d i u m and iridium

surfactant [31,32]. Iridium can be separated by distillation from boiling HC104 in a stream of chlorine. This method is specific for Ir in the absence of Ru and Os [33]. Fire-assay methods have been applied for separation of Rh and Ir [34-37]. These methods have been discussed in Section 23.1.1.

41.2. Methods of rhodium determination The determination of rhodium with the use of tin(II) in chloride or iodide medium, and a sensitive method based on the ion-associate of Rh-SnC13- complex with Rhodamine 6G, have been presented. 4 1 . 2 . 1 . T i n ( I I ) chloride (or iodide) method When SnC12 is added to a solution of rhodium(III) in HC1 and the system heated, the solution turns red. The red complex has been used for spectrophotometric determination of Rh [38,39]. The absorption maximum of the Rh complex with SnC13- is at 470 nm. The molar absorptivity, e = 4.2.103 (a = 0.041). If rhodium is present in solution as the sulphate complex, the solution should be heated for some time after the addition of HC1 to allow the rhodium sulphate complex to be converted into the chloride complex. The other platinum metals interfere in the determination. The SnC12 method has been used to determine rhodium in platinum concentrate [40]. A considerable increase in sensitivity is obtained when bromide [41] or iodide [42] is used instead of chloride. In the bromide method, a yellow-orange complex is obtained (~; = 2.9.104 at )~max 427 nm; a = 0.29). The rhodium-tin(II) iodide complex is red and has an absorption maximum at 460 nm (e = 3.9.104; a = 0.34). In the iodide method, the optimum concentration of HC1 is 1 M. The concentration of KI should not be lower than 4% in the final coloured solution. The quantity of SnC12 only slightly affects the absorbance. The iodine liberated by air in the initial stage of the procedure is reduced when the SnC12 is added.

Reagents Tin(II) chloride, 10% solution in 2 M HC1. Potassium iodide, 20% solution. Standard rhodium solution: 1 mg/ml. Fuse 0.1000 g of metallic rhodium powder with 2 g of potassium pyrosulphate in a silica crucible. Dissolve the melt in hot 1 M HC1 and make the solution up to volume with this acid in a 100-ml standard flask.

Procedure Tin(II) chloride method. To the sample solution containing not more than 0.4 mg of Rh, add 5 ml of the SnC12 solution. Heat the solution for 15 min nearly at boiling point. Cool the solution, dilute to the mark with 2 M HC1 in a 25-ml standard flask, and measure the absorbance at 470 nm against water as the reference. Tin(II) iodide method. To the sample solution containing not more than 50 ~tg of Rh, add 5 ml of the KI solution. Mix well, and heat for 15 min in a boiling water-bath. To the

41.2. Methods of rhodium determination

359

cooled solution, add 5 ml of the SnC12 solution. Dilute the solution to volume in a 25-ml standard flask with dilute HC1 so that the final HC1 concentration is 1 M. Place the unstoppered flask in the boiling water-bath for 2 min. Cool the solution and measure its absorbance at 460 nm against a reagent blank solution.

41.2.2. Rhodamine 6G method In -2 M HC1 medium rhodium forms anionic complexes with SnC13- and C1- ions. With a large excess of tin(H), and at room temperature, a stable red complex is obtained after 1 h. The same effect is reached with 10-15 min heating on a boiling water-bath. The complex gives with Rhodamine 6G (R6G) (formula 4.30) a sparingly soluble compound that collects on the inside surface of the separating funnel after shaking (for 45 s) of an aqueous phase with DIPE. The optimum SnC12 concentration during flotation is 0.01-0.02 M. At higher SnC12 concentration, the absorbance of the blank increases. The excess of basic dye should be at least 50-fold with respect to Rh. The precipitate in the separating funnel is washed with 2 M HC1, then dissolved in acetone. This solution is the basis of a very sensitive and precise method for spectrophotometric determination of rhodium [43]. The molar absorptivity is 4.0.105 (a = 3.9) at 530 nm. A 5-fold excess of Pd and Ir causes about 15% increase in the absorbance. The influence of Pt is more serious. These three metals should be separated from Rh before its determination. Ruthenium and osmium, when present, are readily volatilized as the tetroxides.

Reagents Rhodamine 6G (R6G), 1.10 -3 M (---0.05 %) solution. Tin(II) chloride, SnC12.2H20, 10% solution in 2 M HC1. Rhodium standard solution: mg/ml. Preparation as in Section 41.2.1. Rhodium(III) chloride can also be used to prepare a standard solution [43].

Procedure Evaporate to dryness the acid (HC1) sample solution (containing not more than 5 gg of Rh) on a boiling water-bath. Add 0.5 ml of SnCI2 solution and 2 M HC1 to about 15 ml. Heat the solution on a boiling water-bath for 15 min. Transfer the cooled solution to a separating funnel. Add 0.5 ml of R6G solution and 5 ml of DIPE, then shake for 40 s. Allow the phases to separate, and slowly discard the aqueous layer. Wash the ether phase and the precipitate by shaking with 10 ml of 2 M HC1 for 30 s. Remove the aqueous and the organic layers carefully, dissolve the precipitate in acetone, transfer the solution to a 25-ml standard flask and dilute to the mark with acetone. Measure the absorbance of the solution at 530 nm against a reagent blank.

41.2.3. Other methods The Malachite Green ion-associate with the Rh-SnC13- complex in HC1 medium has been used for flotation-spectrophotometric determination of rhodium [44]. Numerous azo reagents have been proposed for spectrophotometric determination of rhodium: PAN (e = 1.15.104) [20,45,46], PAR (in the presence of surfactant CP, e - 6.2.104)

360

41. Rhodium and iridium

[47], TAN [19], TAR [17,18], 4-(4-methyl-2-thiazolylazo)resorcinol in aqueous-organic medium [48] and other resorcinol derivatives [49], 2-(5-bromopyridylazo)-5-propyl-Nsulphopropylaminophenol [50], 5-(2-pyridylazo)-p-cresol (e = 8.5.104) [51 ], 2-thiazolylazop-cresol [52], and sulphochlorophenolazorhodanine [53]. Organic reagents for rhodium with sulphur as a ligand atom include sulphoallthiox [54,55]. Dithizone reacts with rhodium(III) at pH 8.5 on heating. The dithizonate formed ()~max= 560 nm) can be extracted, after cooling, into CHC13 [56]. Various other organic reagents have been proposed for determining Rh, viz. nitroso-R salt [57,58], 1,5-diphenylcarbazide [59], Chrome Azurol S (in the presence of CP) (e = 6.2-104) [60], and Chrompyrazole I in the presence of SnCI2 [61]. Rhodanine derivatives have been used for determination of Rh in the presence of Pd [62] and heavy metals [63]. Rhodium has been determined in the presence of Ru by the derivative spectrophotometry method as a complex with octadecyldithiocarbamate [64]. The 3rd order derivative spectrophotometry has been used for determining rhodium as a complex with ohydroxy-hydroquinonephthalein [65]. A number of spectrophotometric methods for determining Rh has been based on its catalytic effect on redox reactions, such as the oxidation of Methyl Red [66], Methyl Orange [67], or copper [68] with periodate. The catalytic reactions have been the basis for Rh determinations in technological samples [67] and in copper and nickel alloys [68].

41.3. Methods of iridium determination The old tin(II) bromide method and a new, very sensitive method involving the basic dye Rhodamine 6G are described. A critical review of Ir determination methods has been published [69].

41.3.1. Tin(II) bromide method When tin(II) in HBr solution is added to an acidic solution of the iridium bromide complex, a yellow complex is produced on heating. The absorbance of this complex is the basis of the spectrophotometric determination of iridium [70-72]. Maximum colour intensity is obtained by heating the solution for 1 min on a boiling water-bath. If the heating is continued for longer than 2 min the colour intensity decreases. The molar absorptivity of the complex a t )Lmax -- 402 nm is 5.10 4 (a = 0.26). Before its determination by this method, iridium must be separated from Rh, Pt, and Pd. Other metals (e.g., Co, Ni, Cu, Fe, Sb) interfere slightly. Low concentrations of HC1 (20 mg of A1, 20 mg of Fe, 2 mg of Zr, or 2 mg of Th) prevent the quantitative separation of scandium as the tartrate. Scandium is separated from yttrium by extraction as the thiocyanate complex.

376

43. Scandium

Trace amounts of scandium have been enriched from aqueous solutions by means of flotation with the use of Fe(OH)3, CTA, and a stream of nitrogen [17].

43.2. Methods of determination The Xylenol Orange method has been recommended for spectrophotometric determination of Sc. Numerous azo reagents have also been proposed for scandium. Some methods enable scandium to be determined in the presence of rare-earth elements.

43.2.1. Xylenol Orange method Xylenol Orange (XO) (formula 4.19) reacts in a slightly acidic medium with scandium to form a red-violet complex which is the basis of a sensitive spectrophotometric method for determining scandium [14,18-21]. The reagent is yellow-orange in acidic solution (pH 1-5), but turns red-violet above pH 5. Maximum absorbance of the scandium-XO complex is obtained between pH 2.5 and 2.7. As the acidity rises, the absorbance rapidly drops; with increase in pH, the absorbance slowly decreases. The absorption maximum of the complex occurs at 560 nm. At this wavelength the reagent absorbs imperceptibly. The molar absorptivity of the complex is 2.9.104 at ~max560 nm (a = 0.65). Solutions of the complex are stable with respect to time. At pH 2.6, Th, Zr, Ti, Fe(III), Bi, In, A1, and Y (but not the lanthanides) interfere. Reducing the pH minimizes these interferences and even masks yttrium completely. Iron(III) and cerium(IV) are masked by reduction with ascorbic acid. Oxalate, sulphate, fluoride, and phosphate interfere in the determination of scandium,

Reagents Xylenol Orange (XO), 0.05% aqueous solution. Standard scandium solution: 1 mg/ml. Dissolve 0.1530 g of Sc203 in l0 ml of hot 2 M HC1, and dilute the solution to volume with water in a 100-ml standard flask. Working solutions are obtained by suitable dilutions of the stock with 0.01 M HC1. Chloroacetate buffer, pH ~3. Dissolve in water 3 g of NaOH, add 15 ml of chloroacetic acid, and dilute the solution with water to 250 ml.

Procedure To the slightly acidic (pH "-1) sulphate-free sample solution containing not more than 30 gg of Sc, add 1 ml of 1% ascorbic acid solution, 1 ml of the chloroacetate buffer and 2.5 ml of the XO solution. Dilute the solution with water to --20 ml, and adjust its pH to 2.0__0.1). Transfer the solution to a 25-ml standard flask, and make up to the mark with water. After 10 min, measure the absorbance of the solution at 560 nm against a reagent blank solution.

Note. If yttrium is known to be present in the sample solution [introduced, for example, as a collector (2-5 mg) during the precipitation of Sc as tartrate], the same amount of yttrium should be added to the standard solutions used to prepare the calibration curve.


377

43.2.2. Other methods Many azo dyes have been recommended as spectrophotometric reagents for scandium. Arsenazo HI (formula 4.10) [22-24] gives similar sensitivity to Xylenol Orange. The optimum pH is between 1.5 and 3.0" the molar absorptivity is 2.9.10 4 a t 640 nm. Arsenazo I [8,10] is less sensitive and less selective. Other azo reagents used for determining scandium include Chlorophosphonazo HI [25], Chlorophosphonazo-p-C1 [26,27], pacetylchlorophosphonazo in the presence of Ce(III) [28], p-nitrochlorophosphonazo [29], PAR [30,31], and TAR (~ = 5.0.104 at 540 nm) [32]. The following triphenylmethane dyes have been employed for determination of Sc similarly to Xylenol Orange: Methylthymol Blue [33], Chrome Azurol S [34,35], Chromal Blue G [36], and Eriochrome Brilliant Violet B [37]. Much higher sensitivities have been obtained in the presence of some cationic surfactants [38-40]. In the method with Chrome Azurol S and Zephiramine, the e value is 1.5.105 at 610 nm, and in the method with Eriochrome Cyanine R and CP, e = 9.2.104 at 600 nm [40]. When o-hydroxyquinonephthalein and CP are used, the molar absorptivity is 1.1.105 at 555 nm [41]. Scandium has been determined with the use of Nile Blue in a poly(vinyl alcohol) medium [42]. Further organic reagents for scandium are Bromopyrogallol Red [43-45], o-chlorophenylfluorone and CTA (e = 1.31.105 at 569 nm) [46], indoferron [47], and 2-pyridylidene2-aminophenol [48].

43.3. Analytical applications Xylenol Orange has been applied for the determination of scandium in minerals and coal ashes [16], copper- and nickel alloys [49], and iron-rich materials [50]. The Arsenazo III method has been utilized for determining Sc in minerals [51]. Scandium in mixtures with rare earth elements was determined by derivative spectrophotometry with the use of Chlorophosphonazo-p-C1 [27]. p-Acetylchlorophosphonazo with Ce(III) has been used for determining Sc in copper, aluminium, manganese, and magnesium alloys [28]. Traces of scandium in silicate rocks and sediments were determined with the use of Bromopyrogallol Red [43].

References 1. Hamaguchi H., et al., J. Chromatogr., 16, 396 (1964); Anal. Chem., 36, 2305 (1964). 2. Hamaguchi H., et al., Talanta, 11,495 (1964). 3. Kuroda R., Hikawa I., J. Chromatogr., 25, 408 (1966). 4. Strelow F.W., Bothma C.J.,Anal. Chem., 36, 1217 (1964). 5. Hamaguchi H., et al., Talanta, 10, 153 (1963); J. Chromatogr., 22, 143 (1966). 6. Kalyanaraman S., Khopkar S.M., Anal. Chem., 49, 1192 (1977). 7. McDonald J.C., Yoe J.H., Anal. Chim. Acta, 28, 264 (1963). 8. Ohishi H., Banks C.V.,Anal. Chim. Acta, 29, 240 (1963). 9. Zolotov Yu.A., Shakhova N.V., Alimarin I.P., Zh. Anal. Khim., 23, 1321 (1968). 10. Langade A.D., Shinde V.M., Anal. Chem., 52, 2031 (1980). 11. Karve M.A., Khopkar S.M., Bull. Chem. Soc. Jpn., 64, 655 (1991). 12. Katsuta S., Imura H., Suzuki N., Anal. Sci., 7, 661 (1991). 13. Deorokar N.V., Khopkar S.M., Bull. Chem. Soc. Jpn., 64, 1962 (1991).

378

43. Scandium

14. Bykhovtsova T.T., Bykhovtsova I.V., Zh. Anal. Khim., 42, 1247 (1987). 15. Brudz' V.G. et al., Zh. Anal. Khim., 17, 568 (1962). 16. Belopol'skii M.P., Popov N.P., Zavod. Lab., 30, 1441 (1964). 17. Liang S.C., Zhong Y.L., Wang Z., Z. Anal. Chem., 318, 19 (1984). 18. Berman S.S., Duval G.R., Russell D.S.,Anal. Chem., 35, 1392 (1963). 19. Antonovich V.P., Nazarenko V.A., Zh. Anal. Khim., 23, 1143 (1968). 20. Romantseva T.I. et al., Zh. Anal. Khim., 36, 1529 (1981). 21. Shmanenkova G.I. et al., Zavod. Lab., 59, No. 1,1 (1993). 22. Nazarenko V.A., Antonovich V.P., Zh. Anal. Khim., 24, 1008 (1969). 23. Spitsyn P.K., Zavod. Lab., 46, 582 (1980). 24. Spitsyn P.K., Zavod. Lab., 49, No. 10, 22 (1983). 25. Bykhovtsova T.T., Tserkovnitskaya I.A., Zh. Anal. Khim., 37, 624 (1982). 26. Chen W.R. et al., Mikrochim. Acta, 1985 III, 417. 27. Hsu C., Wang X., Chen W., Pan J., Microchem. J., 40, 175 (1989). 28. Hsu C.G., Liu S.C., Pan J.M., Talanta, 42, 1905 (1995). 29. Hsu Ch.G., Xu Q., Pan J.M., Mikrochim. Acta, 126, 83 (1997). 30. Kui H.V., Petruhkin O.M., Zolotov Yu.A., Zh. Anal. Khim., 42, 649 (1987). 31. Kuznetsov V.V., Dong Z.D., Zh. Anal. Khim., g0, 136 (1995). 32. Shimizu T., Momo E., Anal. Chim. Acta, 52, 146 (1970). 33. Akhmedli M.K., Gambarov D.G., Zh. Anal. Khim., 22, 1183 (1967). 34. Ishida R., Hasegawa N., Bull. Chem. Soc. Jpn., 40, 1153 (1967). 35. Kwapulifiska G., Buhl F., Poledniok J., Chem. Anal. (Warsaw), 38, 201 (1993). 36. Uesugi K., Bull. Chem. Soc. Jpn., 42, 2051 (1969). 37. Uesugi K., Anal. Chim. Acta, 49, 597 (1970). 38. Verhande C., Munshi K.N., Microchem. J., 23, 28 (1978). 39. Jurkevi6iute J., Malat M., Coll. Czech. Chem. Comm., 44, 3236 (1979). 40. Jarosz M., Marczenko Z., Anal. Chim. Acta, 159, 309 (1984). 41. Mori I. et al., Bull. Chem. Soc. Jpn., 59, 1623 (1986). 42. Li Z.B., Wang J.L., Xu Q.H., Anal. Sci., 12, 259 (1996). 43. Shimizu T., Talanta, 14, 473 (1967); Bull. Chem. Soc. Jpn., 42, 1561 (1969). 44. Ganago L.I., Alinovskaya L.A., Zh. Anal. Khim., 35, 279 (1980). 45. Alinovskaya L.A., Zh. Anal. Khim., 44, 1040 (1989). 46. Luo Z., He W., Talanta, 37, 641 (1990). 47. Shimizu T., Ogami K., Talanta, 16, 1527 (1969). 48. Holzbecher Z., Huynh V.T., Coll. Czech. Chem. Comm.,41, 1506 (1976). 49. Postnikova I.S., Zavod. Lab., 36, 542 (1970). 50. Shmanenkova G.I. et al., Zh. Anal. Khim., 33, 699 (1978). 51. Basargin N.N. et al., Zavod. Lab., 61, No 1, 9 (1995).

Chapter 44. Selenium Selenium (Se, at. mass 78.96) forms selenide, selenite and selenate ions, in the oxidation states -II, IV, and VI, respectively. Selenium(IV) compounds are the most stable. Selenium dioxide sublimes readily (unlike TeO2). On dissolution in nitric acid, selenium is oxidized to Se(IV). Strong oxidizing agents (e.g., aqua regia) oxidize Se to Se(VI). Moderate reducing agents reduce Se(IV) and Se(VI) to the element. Selenium compounds are more easily reduced and less easily oxidized than the corresponding tellurium compounds.

44.1. Methods of separation and preconcentration 44.1.1. Distillation Selenium is usually separated by distillation as the volatile selenium bromide (SeBr4) or chloride (SeCI4) [1,2]. Selenium is distilled from concentrated HBr medium [in the presence of bromine to prevent the reduction of Se(IV)] and from concentrated HC1. Perchloric or sulphuric acid is added to the still, and distillation is continued until white fumes of H2SO4 or HC104 appear. Passage of nitrogen through the liquid promotes the distillation. During the distillation, Te remains quantitatively in the still, but As, Ge, and Sb are distilled with the selenium. Selenium may be separated from various non-volatile materials as the volatile SeO2, which forms when a stream of oxygen is passed over the sample in a tube heated to 1,000~ The SeO2 sublimed onto the cold part of the tube is dissolved and determined [3,4]. A special apparatus has been proposed for the separation of selenium as SeO2 [5]. The volatile hydrogen selenide, HzSe, is also used for separation of selenium [6].

44.1.2. Precipitation Selenium is readily separated by reduction to the element with SnC12, $02, hypophosphite, or hydrazine [7]. Arsenic [8,9] and tellurium [10] are suitable collectors for traces of Se. When selenium is precipitated from 1-8 M HC1 with SO2, the following are wholly or partly reduced to the element: Te, Au, Pt, Pd, Hg, Bi, Sb, Sn, and Cu. Traces of selenium can be co-precipitated with Fe(III), La, Mn, or aluminium hydroxide [11-13]. Se(VI) has been co-precipitated as PbSeO4 together with PbSO4 [14]. When samples mixed with NazCO3 and MgO are ignited at 800~ a sinter is formed, from which water leaches Se(VI), while sparingly soluble MgTeO4 remains in the solid residue [ 15].

44.1.3. Extraction and ion-exchange The selenium(W) chloride complex in 6-7 M HC1 reacts with methyl ethyl ketone to form a compound which can be extracted with chloroform. Such extraction separates Se from Te [16]. Complexes of selenium with xanthate [17], 4-nitro-o-phenylenediamine (toluene) [18], and DDTC [19,20] have also been used in extractive separation of Se from many elements.

380

44. Selenium

Selenium(IV) has been selectively retained on an anion-exchange column [21 ], and on a cation-exchanger modified with Bismuthiol II [22] or with Bismuthiol(II)-sulphonic acid [23]. Sorption of Se (along with Te) on a polyurethane foam has also been applied [24]. The ion-exchange methods for separating Se and Te are discussed in Chapter 49.

44.2. Methods of determination The sensitive method based on 3,3'-diaminobenzidine is widely used. Selenium is determined either in the aqueous medium or after extraction with toluene. The method based on the coloured sol of elemental selenium is much less sensitive. Spectrophotometric methods of determining selenium have been reviewed [25,26].

44.2.1.3,3'-Diaminobenzidine method Selenium(W) reacts with 3,3'-diaminobenzidine (DAB) in acid medium to form the yellow piazselenol, which is sparingly soluble in water and which is utilized for spectrophotometric determination of Se [27-29] (formula 44.1). HzN

NH~

H~N

~Se

The colour reaction is carried out in 0.1 M HC1, and the time necessary for colour development in the aqueous pseudo-solution is 50 min. In the extractive spectrophotometric method [36], the time for reaction at pH 2-3 (in the presence of formic acid) is 30 min, after which the solution is neutralized to pH 6-7, and the piazselenol is extracted into toluene. The colour reaction may be accelerated by heating the solution. Within the pH range 5-10, the distribution coefficient of piazselenol between toluene and water is high, and one portion of toluene extracts practically all the selenium complex into the organic phase. The free reagent (DAB) is also extracted. Related solvents such as benzene and xylene may be substituted for toluene. The two absorption maxima of piazselenol occur at 340 and 420 nm. Since DAB absorbs strongly at 340 nm but negligibly at 420 nm, absorbances are measured at 420 nm. The molar absorptivity of the toluene solution of piazselenol at 420 nm is 1.02.104 (sp. abs. 0.13). This method is specific for selenium. Tellurium does not react with DAB, but V(V) and Fe(III) oxidize DAB to give coloured oxidation products. Iron(III) can be masked with fluoride or phosphate. EDTA is used as masking agent to prevent the precipitation of metals in the neutral medium. Substances capable of reducing selenium to the element interfere in the determination of selenium by the 3,3'-diaminobenzidine method.

Reagents 3,3'-Diaminobenzidine hydrochloride (DAB), 0.5% solution in boiled and cooled water. The solution is stable for a few hours, then turns brown under the influence of atmospheric oxygen. Standard selenium solution: 1 mg/ml. Dissolve in water 1.4050 g of SeO2 (resublimed and stored over P205), and dilute the solution with water in a volumetric flask to 1 litre.


381

Procedure To the sample solution containing not more than 100 ~tg of Se in a volume o f - 2 0 ml, add 2 ml of 10% formic acid solution and 2.5 ml of DAB, and then adjust the pH to 2.0-2.5. Let the solution stand for 30 min, then neutralize with ammonia to pH 6-7. Extract the piazselenol by shaking for 1 min with two portions of toluene. Dilute the extract with toluene to the mark in a 25-ml standard flask, and measure the absorbance at 420 nm against water or a reagent blank solution.

Note. If metal ions are present (A1, Bi, Cu, Ni, etc.), add at the start of the procedure 1-5 ml of 5% EDTA solution as masking agent. To mask Fe(III), add also NaF (0.05-0.2 g, depending on the amount of Fe). 44.2.2. Selenium sol m e t h o d Reduction to a brown-yellow sol of elemental selenium in acid medium containing a protective colloid has been made the basis of a simple but rather insensitive method for determining selenium. Suitable protective colloids for preventing coagulation of the selenium sol are gum arabic, gelatine, and poly(vinyl alcohol). Tin(II) chloride, ascorbic acid, thiourea, or hydrazine are used to reduce selenium(W). In 3-4 M HC1 solutions, SnCI2 rapidly reduces Se(IV) in the cold. Depending on the reducing agent and acid strength, pseudo-solutions of different colour are obtained. The molar absorptivity of the selenium sol obtained with SnCI2 in 3 M HC1 containing poly(vinyl alcohol) is 1.7.103 at 400 nm (a = 0.022). Towards longer wavelengths, the absorptivity of the sol decreases; in the ultraviolet it increases. At 325 nm the absorptivity is twice as great, and at 450 nm half as great, as that at 400 nm. Interference in this method for determining selenium comes from Te, Hg, Au, and platinum metals, all of which are easily reduced to the element.

Reagents Tin(H) chloride, SnC12.2H20, 20% solution in 3 M HC1. Standard selenium solution: 1 mg/ml. Preparation as in Section 44.2.1. Poly(vinyl alcohol), 2% solution.

Procedure Distillation separation of Se. To the sample solution of selenium(iv) in a 50-ml still, add just enough conc. HC1 to bring its concentration to 7 M. Add 10 ml of H2SO4 (1+1) and a few fragments of porous porcelain, and connect the still to a condenser, the end of which is immersed in a small amount of 2 M HC1 in the receiver. Distil until white fumes of H2804 appear in the still. To the cooled still, add 5 ml of conc. HC1 and distil again until white fumes appear. The receiver should be cooled with ice-water. Determination of Se. To the sample solution (in -3 M HC1) containing not more than 0.5 mg of Se in a volume of--15 ml, add 3 ml of poly(vinyl alcohol) solution, and mix well. Add 1.0 ml of the SnCI2 solution with stirring. Measure the absorbance of the coloured pseudo-solution at 400 nm, vs. water as the reference.

382

44. Selenium

44.2.3. Other methods Similarly to 3,3'-diaminobenzidine, other aromatic o-diamines also react with selenium(IV) in HC1 medium. The o-phenylenediamine method [30,31] is more sensitive than the DAB method. The following reagents have been proposed for selenium: N-methyl-o-phenylenediamine (e = 1.9-104 at 346 rim) [32], 2-aminodiphenylenediamine [33], 4-nitro1,2-diaminobenzene [34], 4,5-diamino-2,6-dimercaptopyrimidine [35], 1,2-diamino-4chlorobenzene [36], 4,5,6-triaminopyrimidine [37], 3,4-diaminobenzoic acid [38], and 2,3diaminonaphthalene [39,40]. Among the organosulphur compounds proposed for the spectrophotometric determination of selenium, are: Bismuthiol II (known as a reagent for Te) [41], 1,4diphenylthiosemicarbazide [42], 2-mercaptoethanol [43], dithio-oxamide (rubeanic acid) [44], and DDTC [45]. Dithizone has been a basis for a sensitive method of determining Se (~ - 7.4.104) [46]. The mechanism of the reaction between Se(IV) and dithizone is still the subject of contradictory opinions [47,48]. In a sensitive, indirect method Se(IV, VI) is reduced by Cr(II) to hydrogen selenide which, on passing in a stream of nitrogen through an alkaline solution of Fe(CN)63-, reduces the latter to Fe(CN)64- [Se(-II) --+ Se(IV)]. The ferrocyanide formed reacts with the 1,10phenanthroline complex of Fe(III) to give an equivalent amount of the complex Fe(phen)32+. The absorbance of this complex is measured at 508 nm (e = 6.8.104) [49]. The molar absorptivity increases to ~ - 1.4.105 0Vmax= 535), if bathophenanthroline is used instead of 1,10-phenanthroline. In another indirect method Se(IV) oxidizes ferrocene to the ferricenium ion which is oxidized to Fe(III), then reduced to Fe(II), to be determined finally by the colour reaction with 1,10-phenanthroline (~ = 4.2-104) [50]. Selenium has been determined with 5,5-dimethyl-l,3-cyclohexanedione [51], 6-amino1-hydroxynaphthalenesulphonic acid [52,53], and 1-aminonaphthalene-7-sulphonic acid [54]. Determinations of Se involved also the following dyes: Rhodamine B [55], Methylene Blue [56], Xylenol Orange [57], and Rhodamine 6G (by the amplification method, in iodide medium, after oxidation of iodide to iodine, and reaction of the IO3- with the dye) [58]. Some spectrophotometric methods for Se determination are based on its catalytic effect on the redox reactions of various organic compounds [59-65].

44.3. Analytical applications The 3,3'-diaminobenzidine method has been applied for determination of Se in biological materials [28,66], soils [67], air [68], silicates [11], sulphide ores [1], copper [8,14,18], organic substances [69], lead [8,14], steel [29], antimony and bismuth tellurides [70], thin Cd-Se films [71], silver chloride and uranium oxide [12]. Aromatic diamines were used for determining selenium in biological materials [35,36], environmental samples [72], sewage [73], copper [38], non-ferrous metals alloys [31], semiconductors [35], sulphur [38], commercial sulphuric acid [30]. Selenium has been determined in tellurium with the aid of dithizone [46]. Selenium present in anodic slimes has been determined after the reaction with iodide and extraction of the liberated iodine [74]. In pharmaceutical products selenium was determined with the use of 1-aminonaphthalene-7-sulphonic acid [54].

References

References 1. Barcza L., Zsindely S., Z. Anal. Chem., 199, 10,117 (1964). 2. Rybakov A.A., Ostroumov E.A., Zh. Anal. Khim., 38, 446 (1983). 3. R62afiska B., R62owski J., Mikrochim. Acta, 1984 II, 481. 4. Meyer A., Hofer Ch., T61g G., Z. Anal. Chem., 290, 292 (1978). 5. Han H.B., Kaiser G., T61g G., Anal. Chim. Acta, 128, 9 (1981). 6. Remer D.C., Veillon C., Tokonsbalides P.T., Anal. Chem., 53, 245 (1981). 7. Bye R., Talanta, 30, 993 (1983). 8. Luke C.L., Anal. Chem., 31, 572 (1959). 9. Kujirai O. et al., Talanta, 30, 9 (1983). 10. Voronkova M.A., Sidorenko G.A., Zh. Anal. Khim., 22, 1085 (1967). 11. Chau Y.K., Riley J.P., Anal. Chim. Acta, 33, 36 (1965). 12. Russell B.G. et al., Talanta, 14, 957 (1967). 13. Reichel W., Bleakley B.G.,Anal. Chem., 46, 59 (1974). 14. Jackwerth E., Z. Anal. Chem., 235, 235 (1968). 15. Kniazeva R.N., Kleiman V.Ya., Zavod. Lab., 31,410 (1965). 16. Jordanov N., Futekov L., Talanta, 12, 371 (1965); 13, 163 (1966); 15, 850 (1968). 17. Donaldson E.M., Talanta, 24, 441 (1977). 18. Donaldson E.M., Talanta, 35, 633 (1988). 19. Desai G.R., Paul J., Microchem. J., 22, 176 (1977). 20. Lo J.M., Lin C.C., Yeh S.J.,Anal. Chim. Acta, 272, 169 (1993). 21. Nakayama M., Chikuma M., Tanaka H., Talanta, 30, 455 (1983). 22. Nakayama M. et al., Talanta, 31, 269 (1984). 23. Nakayama M. et al., Talanta, 34, 435 (1987). 24. Stewart I.I., Chow A., Talanta, 40, 1345 (1993). 25. Murashova V.I., Sushkova S.G., Zh. Anal. Khim., 24, 729 (1969). 26. Raptis S.E., Kaiser G., T61g G., Z. Anal. Chem., 316, 105 (1983). 27. Barcza L., Mikrochim. Acta, 1964, 967. 28. Cummins L.M., Martin J.L., Maag D.D., Anal. Chem., 37, 430 (1965). 29. Nivibre P., Chim. Anal., 47, 125 (1965). 30. T6ei K., Ito K., Talanta, 12, 773 (1965). 31. Shkrobot E.P., Shebarshina N.I., Zavod. Lab., 35, 417 (1969). 32. Kasterka B., Chem. Anal. (Warsaw), 25, 215 (1980). 33. Kasterka B., Mikrochim. Acta, 1989 I, 337. 34. Stibilj V., Dermelj M., Franko M., Byrne A.R., Anal. Sci., 10, 789 (1994). 35. Izquierdo A., Prat M.D., Aragones L., Analyst, 106, 720 (1981). 36. Nbve J., Hanocq M., Molle L., Mikrochim. Acta, 1980 I, 41,259. 37. Bodini M.E., Alzamora O.E., Talanta, 30, 409 (1983). 38. Kasterka B., Dobrowolski J., Chem. Anal. (Warsaw), 32, 749 (1987). 39. Huang X.R. et al., Fresenius'J. Anal. Chem., 354, 195 (1996). 40. Ramachandran K.N., Kumar G.S., Talanta, 43, 1711 (1996). 41. Navratil O., Sorfa J., Coil. Czech. Chem. Comm., 34, 975 (1969). 42. Sushkova S.G., Murashova V.I., Zh. Anal. Khim., 21, 1475 (1966). 43. Afsar H., Apak R., Tor I., Analyst, 114, 1319 (1989). 44. Lebed' N.B., Pantaler R.P., Zh. Anal. Khim., 41, 2224 (1986). 45. Warner D.A., Paul J., Microchem. J., 20, 292 (1975). 46. Kasterka B., Dobrowolski J., Chem. Anal. (Warsaw), 15, 303 (1970). 47. Stary J. et al., Anal. Chim. Acta, 57, 393 (1971).

383

384

44. Selenium

48. Campbell A.D., Yahaya A.H., Anal. Chim. Acta, 119, 171 (1980). 49. Bode H., Schulze K., Z. Anal. Chem., 327, 154 (1987). 50. Kamaya M., Murakami T., Ishii E., Talanta, 34, 664 (1987). 51. Bodini M.E. et al., Talanta, 37, 439 (1990). 52. Ramachandran K.N., Kaveeshwar R., Gupta V.K., Talanta, 40, 781 (1993). 53. Manish R., Ramachandran K.N., Gupta V.K., Talanta, 41, 1623 (1994). 54. Pyrzyfiska K., Anal. Sci., 13, 629 (1997). 55. Liu S., Zhou G., Huang Z., Talanta, 37, 749 (1990). 56. Bernal J.L. et al., Talanta, 37, 931 (1990). 57. Amin A.S., Zareh M.N., Anal. Lett., 29, 2177 (1996). 58. Ramesh A., Ramakrishna T.V., Subramanian M.S., Bull. Chem. Soc. Jpn., 67, 2121 (1994). 59. Ensafi A.A., Dehaghi G.B., Anal. Lett., 28, 335 (1995). 60. Ensafi A.A., Afldaami A., Massoumi A., Anal. Chim. Acta, 232, 351 (1990). 61. Safavi A., Afldaami A., Anal. Lett., 28, 1095 (1995). 61a. Afkhami A., Mosaed F., Zh. Anal. Khim., 54, 1271 (1999). 62. Shiundu P.M., Wade A.P., Anal. Chem., 63, 692 (1991). 63. Sanchez-Pedreno C., Albero M.I., Garcia M.S., Saez A., Talanta, 38, 677 (1991). 64. Parham H., Shamsipur M., Bull. Chem. Soc. Jpn., 64, 3067 (1991). 65. Gokmen I.G., Abdelqader E., Analyst, 119, 703 (1994). 66. Cummins L.M. et al., Anal. Chem., 36, 382 (1964). 67. Stanton R.E., McDonald A.J., Analyst, 90, 497 (1965). 68. West P.W., Cimeoman C., Anal. Chem., 36, 2013 (1964). 69. Domenech R., Chim. Anal., 51,440 (1969). 70. Cheng K.L., Goydish B.L.,Anal. Chem., 35, 1965 (1963). 71. Marczenko Z., Mojski M., Czarnecka I., Chem. Anal. (Warsaw), 18, 189 (1973). 72. Ramachandran K.N., Kumar G.S., Talanta, 43, 1711 (1996). 73. Kasterka B., Chem. Anal. (Warsaw), 37, 361 (1992). 74. Somer G., Ekmekci G., Anal. Sci., 13, 205 (1997).

Chapter 45. Silicon Silicon (Si, at. mass 28.09) occurs in its compounds in the IV oxidation state, e.g., in silica (SiO2) or in silicic acids. A characteristic feature of silicon(W) is its ability to form the tetrafluoride (SiF4) and heteropoly acids, e.g., with molybdic acid.

45.1. Methods of separation and preconcentration 45.1.1. Distillation. Extraction Volatile SiF4 distils from a hot HC104 or H2SO4 medium, containing excess of HF, in a closed apparatus made of platinum, silver, or Teflon [1]. The SiF4 liberated is absorbed in NaOH or H3BO3 solution. Separation of SiF4 by the Conway microdiffusion method is a slow process [2,3]. The sample to be analysed (e.g., copper, aluminium, iron alloys, dolomite, titanium dioxide) is decomposed with acids in a polystyrene Petri dish to which hydrofluoric acid is subsequently added. The SiF4 evolved is trapped in NaOH solution, e.g., in another Petri dish placed beside. Both vessels are placed inside another, tightly closed, polystyrene vessel. To achieve quantitative separation of silicon, the reaction is carried out at 70~ for-~ 18 h. Trace amounts of silicon can be evolved as SiF4 when a solid sample is heated with ammonium fluoride at 200-380~ [4]. The volatile SiF4 is carried by a nitrogen stream into an alkaline absorbing solution. Molybdosilicic acid can be extracted from a 1 M H2SO4 (or HNO3) with oxygencontaining organic solvents [5], silicon being determined directly as the yellow heteropoly molybdosilicic acid or as silicomolybdenum blue. The molybdosilicic acid must be formed at lower acidity ([H +] < 0.7 M) and the acidity raised just before extraction. In the presence of high molecular-weight amines, molybdosilicic acid or its reduced form (heteropoly blue) may be extracted with toluene, CHC13, or a mixture of CHC 13 and isoamyl alcohol.

45.1.3. Ion exchange. Precipitation Ion exchangers are applicable both for separating small amounts of silicon and for removing various anions and cations from silicon [6,7]. Silicon has been retained as SiF62- on a strongly basic anion-exchanger and then displaced from the fluoride complex (and the column) by elution with H3BO3 solution. Silicon traces have been also retained on strongly basic anion exchanger, and then eluted with dilute ammonia [8]. Weakly basic anionexchangers have been used to separate silicate from other anions [6]. Mixtures of silicon, P(V), and As(V) have been separated on anion-exchange columns [9]. Microgram amounts of silica can be co-precipitated with niobium, which precipitates as niobic acid. This method enables silicon to be quantitatively separated from major quantities of P(V), As(V), Fe(III), and A1 [10]. Trace amounts of silicon have been coprecipitated with molybdophosphoric acid from 8 M HNO3 [ 11 ]. Milligram amounts of silica can be precipitated without any collector by evaporation with HC104 to white fumes. When samples are fused with alkalies (NazCO3, NaOH) and the melt is leached with

386

45. Silicon

water, silicon passes into solution, while many elements (e.g., Fe, Ti, Cu, Ni, and Zr) remain in the solid residue.

45.2. Methods of determination Silicon is determined spectrophotometrically as the yellow heteropoly molybdosilicic acid (less sensitive method) or, after reduction, as silicomolybdenum blue. Very sensitive methods, based on ion-association complexes with basic dyes, are becoming increasingly important.

45.2.1. Molybdosilicic acid method In alkaline solutions, silica exists in the form of silicate ions (e.g., SIO32-).In dilute solutions (up to 0.1 mg of Si per ml) between pH 1 and 8, water-soluble monomeric silicic acid is the stable form. In more concentrated solutions of the same acidity, monosilicic acid condenses to disilicic acid and polysilicic acids which can be transformed into colloidal species. Soluble monosilicic acid reacts with molybdic acid at pH 1-2, in the presence of at least a 0.05 M excess of molybdenum, to form the yellow soluble 13-molybdosilicic acid. The yellow colour is the basis of a rather insensitive spectrophotometric method for silicon [1215]. The absorption maximum of the complex is in the ultraviolet. At 400 nm, the molar absorptivity is 2.2.103 (a = 0.08) (in the presence of acetone). At lower acidity, the reaction product is a-molybdosilicic acid, which is more stable than the 13-form but which absorbs only half as intensely at 400 nm [16,17]. Silicon is often determined from the colour of the a-form of molybdosilicic acid [18-20]. Besides acidity, the temperature, Si:Mo concentration ratio, and the degree of condensation of the molybdate ions influence the concentrations of the two heteropoly acid forms. At pH > 7, MoO42- ions are stable, but they condense when acidified. Precise results are obtained in this method only if the conditions are kept rigorously the same for both samples and standards. Silica is converted into monosilicic acid by fusing with sodium carbonate and acidifying the alkaline solution produced when the melt is dissolved in water. Alternatively, an acidic sample solution may be made alkaline with sodium hydroxide and heated to convert colloidal silica into silicate. Soluble monosilicic acid is formed after appropriate dilution and acidification. On heating with dilute hydrofluoric acid, silica is transformed into the soluble hexafluorosilicic acid, HzSiF6. Aluminium chloride or boric acid introduced subsequently mask the excess of HF and decompose HzSiF6 to monosilicic acid (the more stable A1F63- or BF4 complexes being produced). Phosphorus(V), Ge(IV), and As(V) which give yellow heteropoly acids, interfere. Before silicon is determined, Ge and As may be separated by volatilization or extraction of GeC14 and AsC13. Molybdophosphoric and molybdoarsenic acids are separated from molybdosilicic acid by extraction with butyl acetate at pH 0.3-1.0. The extractive separation of silicon from P(V) and As(V) has been discussed [5,21] . Molybdosilicic acid has been extracted with TOA in toluene [22]. Ion-associates with basic dyes, e.g., Chrompyrazole have been floated with a mixture of toluene and acetone [23]. Ferric ions interfere in the determination of silicon. Large quantities are separated by extraction, and smaller ones are masked with phosphoric acid.


387

Reagents Ammonium molybdate, 10% solution, at pH adjusted to 7.4_+0.2 by means of ammonia. Standard silicon solution: 0.1 mg/ml. Fuse 0.2140 g of ignited and comminuted silica, SiO2, with 2 g of NazCO3 in a platinum crucible. Dissolve the melt in water, dilute the solution with water to -~900 ml, acidify with 1 M H2SO4 to pH -1.5, and make up the solution to volume in a 1-1itre standard flask with water.

Procedure In a 25-ml standard flask place 5 ml of a 1:1 mixture of ammonium molybdate solution and 1 M sulphuric acid, and add 3 ml of acetone and an aliquot of sample solution (e.g., neutralized solution from sodium carbonate fusion) containing not more than 0.2 mg of Si. Dilute to the mark with water, mix well, let stand for 15 min, and measure the absorbance at 400 nm against a reagent blank.

45.2.2. Silicomolybdenum Blue method Molybdosilicic acid reacts with suitable reducing agents to yield the intensely coloured silicomolybdenum blue, upon which a sensitive method for determining silicon is based [2426]. The reaction conditions are adjusted so that only molybdosilicic acid, and not unreacted molybdic acid, is reduced. Tin(II) chloride or oxalate, Fe(II) (Mohr's salt), ascorbic acid, sodium sulphite, and other reagents have been used as reductants [27,28]. To prevent partial reduction of molybdic acid, molybdosilicic acid is reduced in sufficiently acidic medium. Molybdosilicic acid is produced in a slightly acidic medium but, once formed, it does not decompose if the acidity is strongly increased (up to 1.5 M H2SO4). The most suitable acidity for the reduction depends on the reducing agent used. The various forms of molybdosilicic acid (a, [~) and the various reductants yield products which differ in absorption spectra, absorption maxima, and stability [27]. Molybdosilicic acid can also be reduced after extraction into an oxygen-containing organic solvent (e.g., amyl alcohol) [29,30]. Alternatively, silicomolybdenum blue may be formed in the aqueous phase and then extracted. The heteropoly blue exhibits similar molar absorptivities in both the organic phase and in the aqueous solution, but the absorption maximum is shifted slightly towards shorter wavelengths in organic solvents. Silicomolybdenum blue produced by extraction of molybdosilicic acid into amyl alcohol and reduction with SnCI2 has its )~max at 750 nm. The molar absorptivity is 1.7.104, sp. abs. 0.60. P(V), Ge, and As(V), which form corresponding heteropoly acids and heteropoly blues, must be separated or masked before the determination of Si. It is possible to separate silicomolybdenum and phosphomolybdenum blues by extraction [31 ]. The silicon traces present in the reagents and water used interfere in the determination of microgram amounts of silicon. Analytical grade HC1, H2SO4, and HF, and distilled water contain 2.10 -5 %, 7.10 -5 %, 4.10 -2 %, and 2-10 -6 % of Si, respectively [ 10]. These reagents may be considerably purified by distillation in quartz or platinum apparatus. Platinum, Teflon, and polyethylene vessels should be used and the silicon in a reagent-blank solution should be taken into account when traces of silicon are determined. The interfering effect of various substances on the determination of Si as silicomolybdenum blue was studied after decomposition of the samples with HF [32].

388

45. Silicon

Reduced molybdosilicic acid may be extracted with CHC13 in the presence of 1,2propanediol carbonate [33] or with toluene in the presence of TOA [34]. Silicomolybdenum blue can be associated with dodecyltrimethylammonium ion and retained on a nitrocellulose membrane; the compound, along with the membrane, is then dissolved in DMF and its absorbance measured [35].

Reagents Ammonium molybdate, 10% solution adjusted with ammonia to pH 7.4_+0.2. Standard silicon solution: 0.1 mg/ml. Preparation as in Section 45.2.1. Tin(H) chloride, SnClz.2H20, 50% solution in HC1 (1 + 1). Niobium solution, --1 mg of Nb in 1 ml. Heat 0.145 g of Nb205 in a platinum crucible with 5 ml of conc. HF until the oxide dissolves. Evaporate the solution to --1 ml, add 2 ml of H2504 (1 + 1), and heat to white fumes. Let cool, rinse the walls of the crucible with water, and heat until white fumes appear. Let cool, rinse the walls of the crucible with water and heat to fumes again. Repeat the operation once more to remove HF completely. Pour the niobium solution in conc. H2504 into 30 ml of 5% aqueous ammonium oxalate solution, dilute the clear solution of niobium oxalate complex to 100 ml, with water, and mix well.

Procedure Separation of Si with a collector. To the sample solution, containing not more than 25 ~tg of Si, in a Teflon or platinum vessel, add 2 ml of the niobium solution and 5 ml of conc. HC104 and evaporate the solution to fumes, expelling most of the perchloric acid. Dilute the residue with 10-20 ml of HC104 (1+50), stir until the salts dissolve, add some macerated filter paper, filter off the precipitate, and wash it with very dilute perchloric acid. Ignite the filter paper and precipitate in a platinum crucible. Add 1 ml of 5% HF to the cooled crucible and heat in a sealed vessel in a water-bath at --70~ for 30 min. Transfer the solution from the crucible to a polyethylene beaker, dilute with water to --10 ml, and add 5 ml of 3 % boric acid solution. Determination o f Si. To the solution obtained as above, add 1 ml of the molybdate solution and adjust the pH to 1.4_+0.1 with 0.5 M H2SO4. After 5 min, transfer the solution to a separating funnel, add 5 ml of H2504 (1+4), and extract molybdosilicic acid with two 15ml portions of isoamyl alcohol. Wash the extract by shaking with 10 ml of 0.5 M H2SO4. Transfer the organic phase to a 25-ml standard flask, and add one drop of the SnC 12 solution, -~1 ml of diethyl ether (to clarify the solution), and isoamyl alcohol to the mark. Mix well, and after 5 min measure the absorbance of the blue solution at 750 nm, vs. a reagent blank as reference. Note. Because of the traces of Si present in the reagents the use of a reagent blank in absorbance measurement is recommended.

45.2.3. Other methods Sensitive methods for determining silicon are based on the formation of molybdosilicate ionassociates with certain basic dyes. The ion-associate with Crystal Violet (formula 4.26) can be extracted with a 3:2 mixture of cyclohexanol and isoamyl alcohol [36]. The sparingly soluble compound is centrifuged and then dissolved in acetone (e = 4.2-105 at 590 nm) [37]. The ion-associate with Rhodamine B is separated by flotation with DIPE and dissolved in ethanol (e= 5.0.105 at 555 nm) [38]. Other basic dyes have also been proposed, namely

45.3. Analytical applications

389

Chrompyrazole II (antipyrine dye) [23,39], Brilliant Green [40-42] , Safranine T (e = 1.5.105) [43], Methylene Blue, and Methylene Green [44]. In the case of Methylene Blue, the compound with the Mo-Si is centrifuged and dissolved in acetone (e = 4.3-105 at 660 nm). There exist several indirect methods for the spectrophotometric determination of silicon. After extraction of molybdosilicic acid, the Mo has been determined with phenylfluorone [45], or 2-amino-4-chlorobenzenethiol (e = 2.0.105) [46]. When silicic acid is added to a solution which contains hydrofluorotitanic acid and H202, a yellow titanium peroxide complex is formed. Chloranilic acid has also been used for determination of silicon [47].

45.3. Analytical applications Silicon has "been determined as molybdosilicic acid in sewage [48], organic compounds [49], rocks and minerals [19,50,51], bauxites [52], cast iron and steel [53], high purity copper [23], copper alloys [21], various metals [54], refractory materials [55], vanadium pentoxide [22], and semiconductors [56]. The method has been applied also in the differential spectrophotometry [57] and flow injection [58,59] techniques. The silicomolybdenum blue method has been used for determining silicon in biological materials [60], waters [25,61-64], air [65,66], soil extracts [67], industrial waste waters [27], organic materials [68,69], various inorganic reagents [10], phosphoric acid [3], cast iron and steel [70-74], copper and its alloys [3,75,76], nickel and its alloys [71], aluminium [3,71], uranium and its compounds [71], titanium and its compounds [3,71,77], platinum and gold [5], molybdenum and its compounds [78], tungsten trioxide [79], refractory metals [1], gallium arsenide [80], gallium phosphide [6], and ferrophosphorus [81]. The phosphomolybdenum blue has been applied in automatic determination of Si in natural waters [82], industrial solutions [83], as well as in the flow injection technique for determination of silicon in sea water [84], soil extracts [67,85], and glass [86]. The ion associates with Malachite Green have been used for determining Si in water [42] and glass [41 ].

References 1. Stobart J.A.,Analyst, 94, 1142 (1969). 2. Alon A., Bernas B., Frenkel M., Anal. Chim. Acta, 31, 279 (1964). 3. Householder R., Russell R.G., Anal. Chem., 36, 2279 (1964). 4. Szabo Z.G., Zapp E.E., Perczel S., Mikrochim. Acta, 1974, 167. 5. Marczenko Z., Lenarczyk L., Chem. Anal. (Warsaw), 19, 679 (1974). 6. Luke C.L., Anal. Chem., 36, 2036 (1964). 7. Duce F.A., Yamamura S.S., Talanta, 17, 143 (1970). 8. Bazzi A., Boltz D.F., Microchem. J., 20, 462 (1975). 9. Narusawa Y., Anal. Chim. Acta, 204, 53 (1988). 10. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 9, 321 (1964). 11. Barkovskii V.F., Radovska T.L., Zaporozhets A.S., Zh. Anal. Khim., 25, 1853 (1968). 12. Govett G.J., Anal. Chim. Acta, 25, 69 (1961). 13. Chalmers R.A., Sinclair A.G., Anal. Chim. Acta, 33, 384 (1965); 34, 412 (1966). 14. Truesdale V.W., Smith C.J., Smith P.J., Analyst, 102, 73 (1977). 15. Kircher C.C., Crouch S.R., Anal. Chem., 54, 2303 (1982). 16. Truesdale V.W., Smith C.J., Analyst, 100, 203,797 (1975).

390

45. Silicon

17. Truesdale V.W., Smith P.J., Smith C.J., Analyst, 104, 897 (1979). 18. Kato K., Anal. Chim. Acta, 82, 401 (1976). 19. Sarkar R.S., Das M.S., Anal. Chim. Acta, 134, 401 (1982). 20. Lamathe J., Hautbout R., Analusis, 13, 420 (1985). 21. Shkavavskii Yu.F., Lynchak K.A., Chernogorenko V.B., Zavod. Lab., 51, No 12, 5 (1985). 22. Dorokhova E.N. et al., Zh. Anal. Khim., 29, 2014 (1974). 23. Gurentsova O.I., Prokhorova G.V., Osipova E.A., Dorokhova E.N., Zh. Anal. Khim., 48, 332(1993). 24. Hargis L.G., Anal. Chem., 42, 1494, 1497 (1970). 25. Tarasova N.S., Dorokhova E.N., Alimarin I.P., Zh. Anal. Khim., 35, 1520 (1980). 26. Smith J.D., Milne P.J., Anal. Chim. Acta, 123, 263 (1981). 27. Morrison I.R., Wilson A.L., Analyst, 88, 88, 100 (1963). 28. Nag6rski B., Chem. Anal. (Warsaw), 20, 903 (1975). 29. Pakalns P., Flynn W.W., Anal. Chim. Acta, 38, 403 (1967). 30. Kakita Y., Goto H., Talanta, 14, 543 (1967). 31. Tarasova N.S., Dorokhova E.N., Alimarin I.P., Zh. Anal. Khim., 36, 459 (1981). 32. Sadiq M., A1-Muhanna H., Alam I., Commun. Soil Sci. Plant Anal., 19, 1693 (1988). 33. Trudell L.A., Boltz D.F., Anal. Chim. Acta, 52, 343 (1970). 34. Zhukova L.B., Dorokhova E.N., Tikhomirova T.I., Zh. Anal. Khim., 33, 710 (1978). 35. Kasahara I. et al., Anal. Chem., 59, 787 (1987). 36. Babko A.K., Shkaravskii Yu.F., Go~kowska A., Chem. Anal. (Warsaw), 11, 1091 (1966). 37. Mirzoyan F.V. et al., Zh. Anal. Khim., 34, 1515 (1979). 38. Go~kowska A., Pszonicki L., Talanta, 20, 749 (1973). 39. Dorokhova E.N., Gracheva N.A., Dracheva L.V., Zh. Anal. Khim., 43, 265 (1988). 40. Motomizu S., Oshima M., Ojima Y., Anal. Sci., 5, 85 (1989). 41. Motomizu S. et al., Analyst, 114, 1679 (1989). 42. Susanto J.P., Oshima M., Motomizu S., Analyst, 120, 2605 (1995). 43. Pilipenko A.T., Shkaravskii Yu.F., Ukr. Khim. Zh., 42, 1183 (1976). 44. Mirzoyan F.V., Tarayan V.M., Zh. Anal. Khim., 35, 1293 (1980). 45. Halasz A., Polyak K., Pungor E., Talanta, 18, 691 (1971). 46. Trudell L.A., Boltz D.F., Talanta, 19, 37 (1972). 47. Mazuranic K. et al., Microchem. J., 46, 374 (1992). 48. Sager M., Puxbaum H., Mikrochim. Acta, 1984 I, 361. 49. Debal E., Talanta, 19, 15 (1972). 50. Uchida T. et al., Anal. Sci., 4, 523 (1988). 51. Foner H.A., Gal I., Analyst, 106, 521 (1981). 52. Fresenius W., Schneider W., Z. Anal. Chem., 214, 341 (1965). 53. Macher F., Glasz M., Mikrochim. Acta, 1964, 104. 54. Bill J., Mikrochim. Acta, 1966, 1047. 55. Privalova M.M., Makhova G.P., Tulina M.D., Zh. Anal. Khim., 29, 279 (1974). 56. Kitazume E., Usami K., Mater. Trans., 30, 184 (1989). 57. Beshikdashyan M.T., Vasireva M.G., Zh. Anal. Khim., 36, 1082 (1981). 58. Yokoyama T. et al., Bull. Chem. Soc. Jpn., 55, 3477 (1982). 59. Kuroda R., Ida I., Kimura H., Talanta, 32, 353 (1985). 60. Tfima J., Mikrochim. Acta, 1962, 513. 61. Morrison I.R., Wilson A.L., Analyst, 94, 54 (1969). 62. Fanning K.A., Prison M.E., Anal. Chem., 45, 136 (1973).

References 63. 64. 65. 66. 67.

391

Ramachandran R., Gupta P.K., Anal. Chim. Acta, 172, 307 (1985). Gamo T. et al., Anal. Sci., 10, 843 (1994). Chu-Fang Wang et al., Anal. Chim. Acta, 342, 239 (1997). Wang C.F., Tu F.H., Jeng S.L., Anal. Chim. Acta, 342, 239 (1997). Raben-Lange B., Broe Bendtsen A., Jorgensen S.S., Commun. Soil Sci. Plant Anal., 25, 3241 (1994). 68. Christopher A.J., Fennell T.R., Webb J.R., Talanta, 11, 1323 (1964). 69. Bradley A., Altebrando D., Anal. Chem., 46, 2061 (1974). 70. Sauer K.H., Keller H.,Arch. Eisenhiittenw., 41, 961 (1970). 71. Pakalns P., Anal. Chim. Acta, 54, 281 (1971). 72. Tarasova N.S., Dorokhova E.N., Alimarin I.P., Zh. Anal. Khim., 38, 63 (1983). 73. Ptushkina M.N., Tikhonova L.I., Shelyapina T.D., Zavod. Lab., 57, No 1, 73 (1991). 74. Basak A.C. et al., Microchem. J., 54, 48 (1996). 75. Sturton J.M.,Anal. Chim. Acta, 32, 394 (1965). 76. Pakalns P., Anal. Chim. Acta, 40, 327 (1968). 77. Barkovskii V.F., Radovskaya T.L., Zavod. Lab., 35, 160 (1969). 78. Fukker K., Hegedfis A.J., Mikrochim. Acta, 1961, 227. 79. Chkanikova O.K., Dorokhova E.N., Zh. Anal. Khim., 34, 944 (1979). 80. Lin R.S., Yang M.H., Z. Anal. Chem., 325, 272 (1986). 81. Brooking K.A., Belcher C.B., Talanta, 22, 777 (1975). 82. Truesdale V.W., Smith C.J., Analyst, 101, 19 (1976). 83. Kowalski Z., Migdalski J., Kolder E., Chem. Anal. (Warsaw), 21, 655 (1976). 84. Thomsen J., Johnson K.S., Petty R.L., Anal. Chem., 55, 2378 (1983). 85. Borggaard O.K., JCrgensen S.S., Analyst, 110, 177 (1985). 86. Archer F.A., Street K.W. Jr., Anal. Chim. Acta, 262, 243 (1992).

Chapter 46. Silver Silver (Ag, at. mass 107.87) occurs in its compounds in the (I)- oxidation state. So far, silver(II) is only of limited value in spectrophotometry. Silver(I) -sulphide and-halides are sparingly soluble. Ammine, cyanide, and thiosulphate complexes of silver are formed. In the presence of excess of C1- or SCN-, traces of silver form soluble complexes.

46.1. Methods of separation and preconcentration 46.1.1. Extraction Silver is often separated from other metals by extraction with dithizone. The details of this separation are discussed below in the dithizone method for determining silver 9 Other methods for separating silver involve extraction of the diethyldithiocarbamate into chloroform [pH 4-11 (EDTA as masking agent)] [ 1]. The thiocyanate complex of silver can be extracted from 0.1-1 M solutions of H2SO4, HC1, or HC104 into TBP [2]. Selective separation and preconcentration of microgram quantities of silver are often based on the use of macrocyclic compounds. Thus, silver has been extracted from nitrate medium by means of a crown ether, dicyclohexyl-18-crown-6 [3], and macrocyclic compounds containing sulphur or nitrogen atoms, in 1,2-dichloroethane solutions [4-8]. Among several 15- and 19-membered macrocyclic compounds [9,10] containing nitrogen, oxygen, or sulphur as the electron-donor atoms, the most favourable properties have been found for 2,3:8,9-dibenzo-4,7,13-trithia- l, 10-diazacyclopentadecane (formula 46. l) used as a solution in 1,2-dichloroethane or chloroform, with dipicrylammate or picrate as the counterions [ 11 ].

(46.1)

46.1.2. Other methods 9Silver ions can be retained on a strongly basic anion-exchange column in the chloride form, and subsequently eluted with ammonia [12,13]. Silver may be separated from Ce, Zr, Th, Be, and Fe(III), on strongly acidic cationexchangers, by converting these metals into anionic complexes, or separated from Cu, U, A1, and Zn by selective elution with nitric acid [14]. After retention of Pb, Ag, and Hg on Dowex 50, lead is eluted first with 0.25 M ammonium acetate, then silver with 0.5 M ammonia solution 9 Silver has been separated on a cation-exchanger from Hg, Co, Ni, and Zn on the basis of the differing stabilities of their EDTA complexes at pH 4.6 [15]. Silver retained in a column with a macroporous cation-exchange resin bed has been eluted with 2 M HNO3 or 0.5 M HBr in aqueous acetone solution [ 16].


393

In convenient precipitation methods, traces of silver are separated with the use of collectors; tellurium has been recommended as a collector, and SnC12 as a reducing agent. Good separation of silver is also obtained by precipitation of the sulphide with Hg, Cu, or Pb as scavenger. It is possible to co-precipitate silver chloride or bromide with TI(I) as collector. Traces of silver have been collected on powdered dithizone [17,18], or rhodanine [19]. Selective sorption of Ag from solutions containing mixtures of various metals has been obtained with the use of a polyacrylamide sorbent, impregnated with a compound of a crown ether, dibenzo-18-crown-6 with molybdophosphoric acid [20]. Microgram amounts of silver are separated on shaking an aqueous solution (about 1 M in HNO3) with mercury (redox exchange). Less noble metals (e.g., Cu, Bi, Pb) remain in solution [21 ]. Silver can be isolated along with the platinum metals by fire assay, with copper(I) sulphide as collector [22]. Silver (and gold) present in the sample are collected in copper on melting at -~1,200~ After dissolving the copper alloy and diluting the obtained solution, silver (and gold) are reduced to the metals with formic acid or hydroquinone [23].

46.2. M e t h o d s of determination Among the many spectrophotometric methods for determining silver, the dithizone method is particularly important. Some newer methods, such as that using thio-Michler's ketone, and methods based on ion-associates with dyes, can be recommended.

46.2.1. Dithizone method Dithizone (formula 4.37) reacts with silver ions in acid medium (H2SO4, HNO3, HC104) to form the orange-yellow dithizonate, which is soluble in CC14 and other inert solvents. A solution of HzDz in carbon tetrachloride extracts silver rapidly, even from a 4 M H2804 medium. Silver dithizonate exhibits a molar absorptivity of 3.05-104 (a - 0.28), at ~max = 462 nm (the absorption spectrum of AgHDz is shown in Fig. 4.4) [24,25]. At higher pH values and in the presence of excess of Ag, the purple dithizonate AgzDz is formed, but this compound is readily transformed into AgHDz in acidic media in the presence of an excess of dithizone. AgHDz is stable even in 5% NaOH solutions. From a strongly acidic chloride-free medium, noble metals [Au, Pt(II), Pd, Hg] and Cu are extracted together with the silver. The presence of chloride in the acid solution prevents the formation of silver dithizonate [26]. Trace amounts (10-20 gg) of chloride do not interfere in the extraction. The AgHDz. can be decomposed by shaking the carbon tetrachloride extract with 1 M HC1, thereby separating the silver from the co-extracted metals. Similarly, an acidified thiocyanate solution strips silver from the extract containing Ag, Hg(II), and Cu(II) dithizonates. Dithizone extracts Ag from solutions containing chloride but at pH values of 2-5. In the presence of EDTA (pH 4-5) Ag (and also Hg and Au) can be extracted from a solution containing considerable amounts of Cu, Bi, Cd, Zn, Ni, and Pb. When a solution containing Ag, Au, and EDTA at pH 4.7 is heated to boiling, the gold is reduced by EDTA to the element. Mercury can be volatilized by igniting the sample before the determination of silver.

394

46. Silver

Since silver dithizonate is more stable than copper dithizonate, a solution of violet Cu(HDz)2 in CC14 may be used in the extractive titration technique instead of dithizone. The colour change from orange-yellow to violet is then more easily observed.

Reagents Dithizone (H2Dz), 0.01% solution in CC14. Dissolve enough reagent to provide 50 mg of "active" (non-oxidized) dithizone in 100 ml of CCla. Filter the solution through a filter paper into a 500-ml separating funnel. Shake the green solution with 100 ml of aqueous ammonia (1+50). Discard the brown CC14 layer containing oxidation products of HzDz. Acidify the orange ammoniacal dithizone solution with 1 M HC1, and shake it with 200 ml of CC14 to decolorization of the aqueous phase. Dilute the green CC14 solution of HzDz to 500 ml with the solvent, and keep in an amber glass bottle under a layer of 1 M H2SO4. Prepare working solutions (e.g., 0.001% HzDz) by suitable dilution of the stock solution (0.01%). Determine the concentration of dithizone in the CC14 solution either by measuring the absorbance of the green solution, or by extractive titration with a standard silver solution, as follows. Place in a separating funnel 100 ~tg of Ag (10 ml of a 0.01 mg/ml solution), add -~0.001% dithizone solution in portions from an amber-glass burette, and shake. Drain the resulting orange silver dithizonate solution from the separating funnel, and continue shaking with successive dithizone portions. Towards the end-point, add 0.5-0.2 ml portions of the H2Dz solution. The titration of 100 gg of Ag requires 23.75 ml of exactly 0.001% HzDz solution. Standard silver solution: 1 mg/ml. Dissolve 1.5750 g of dried (110~ silver nitrate in water containing 1 ml of conc. HNO3, and dilute the solution to the mark with water in a 1-1itre standard flask. Carbon tetrachloride. Regeneration of the solvent used or purification of the commercial product is carried out as follows. Shake a portion of the solvent (about 400 ml) in a 500-ml separating funnel successively with H2SO4 (1+2), 10% NaOH solution, water, 1% KMnOa solution in H2804 (1+9), and 5% Na2SO3 solution. Dehydrate the clear CC14 layer by mixing it in a conical flask with 50 g of anhydrous K2804. Pour the solvent into a still, add 1 g of Na2S203.5H20, and distil the CC14, collecting the fraction boiling at 7678~ It is often sufficient to purify commercial grade (or old) carbon tetrachloride by shaking with dilute (e.g., 0.1 M) Na2S203 and then with water.

Procedure Adjust the acidity of the chloride-free solution, containing not more than 50 ~tg of Ag, with H2804 or HNO3 until the concentration is 1-2 M, with respect to acid, and extract Ag (along with Au, Pd, and Pt) with small portions of dithizone in CC14 (1 ml of 0.001% HzDz solution corresponds to 4.2 gg of Ag). The last portion of dithizone added should not change from green to yellow, although it may turn violet [owing to Cu(HDz)2]. Shake the combined orange-yellow CC14 extracts for 20 s with 1 M HC1. Separate the aqueous layer (containing Ag), add a little EDTA, adjust to pH 4-5 with ammonia, and xtract Ag with portions of 0.001% dithizone solution in CC14. Remove free dithizone from the extract with dilute ammonia solution (2 drops of conc. NH3 solution in 25 ml of water). Dilute the AgHDz solution with carbon tetrachloride in a 25-ml standard flask, and measure the absorbance at 462 nm, using CC14 as reference.

References

395

46.2.2. Thio-Michler's ketone method Thio-Michler's ketone (TMK), 4,4'-bis(dimethylamino)thiobenzophenone has been proposed for a very sensitive spectrophotometric determination of silver (formula 46.2) [27]. The yellow reagent, soluble in many organic solvents, reacts with silver ions over a wide pH range (2-8) giving a red-violet complex, soluble in water-organic solvent media, and extractable into some organic solvents (e.g., butanol, or a mixture of butanol and CHC13). The complex is solvated by oxygen-containing solvents. The presence of anionic surfactants promotes the co-ordination of a greater number of the reagent molecules [28]. S

,,e/" ~

~

S-

"~c., ~

H3

H3

(46.2)

The molar absorptivity depends on the medium. It is 1.40.105 (a = 1.3) at 530 nm in DMF (40-50%) (pH -~3), 1.06.105 in ethanol (50-60%), 1.12.105 at 520 nm in a mixture CHC13-butanol, and 0.96.105 in butanol. Under the conditions suitable for determination of silver, TMK reacts with Au, Pd, Pt(II), Hg, and Cu. When they are present, a preliminary separation of Ag may be necessary. All halides, except fluoride, interfere in the determination of silver. An approximately equimolar amount of chloride can be tolerated. In certain cases, large amounts of chloride or bromide may be used to mask silver in the determination of other noble metals with TMK. Thiosulphate, thiocyanate, sulphide, phosphate, as well as reducing and oxidizing agents interfere in the silver determination. Tartaric acid and EDTA may be used for masking hydrolysable metal ions at pH about 3.

Reagents Thio-Michler's ketone (TMK), 0.001 M solution in dimethylformamide (DMF) (28.5 mg of TMK in 100 ml). The solution should be kept in darkness. Standard silver solution: 1 mg/ml. Preparation as in Section 46.2.1. Acetate buffer, pH 3.0. Mix 50 g of sodium acetate trihydrate in 100 ml of water with 350 ml of glacial acetic acid, adjust to pH 3.0, and dilute to 500 ml with water.

Procedure To an acid solution (pH -~3), containing acetate buffer, 6 ml of DMF and 2.5 ml with DMF in a 25-ml standard flask, absorbance at 530 nm vs. a reagent blank

in -10 ml not more than 20 gg of Ag, add 2 ml of of TMK solution. Make up the solution to volume and mix thoroughly. After 10 min, measure the solution as reference.

46.2.3. Other methods p-Dimethylaminobenzylidenerhodanine (rhodanine, formula 46.3) reacts with silver ions in an acid medium to form a compound which is sparingly soluble in water. The method for determining silver is based on the red pseudo-solution of the complex in the presence of an excess of yellow rhodanine [29,30]. The molar absorptivity is 2.0.104 at 450 nm.

396

46. Silver

HN--------C=O

.

S~C~s"

(46.3)

Protective colloids (gum arabic or gelatine) can be added as stabilizing agents. Au, Pt, Pd, and Hg react similarly to silver in an acid medium, but copper does not interfere. Anions that form sparingly soluble silver salts do interfere. The cationic complex of silver and 1,10-phenanthroline (phen) has been found to react with Bromopyrogallol Red to yield an ion-associate which can be extracted into nitrobenzene (~ = 3.2.10 4 at 590 nm) [31,32]. The cationic complex, Ag(phen)2 + gives ionassociates also with acid dyes, such as Rose Bengal (formula 4.35) and eosin (formula 4.34) (nitrobenzene) [33]. Extractable ion-associates of cationic silver complex with 1,4,8,11tetrathiacyclotetradecane (crown ether), and various chromogenic anions [34] should also be mentioned. Sensitive extraction-spectrophotometric methods are based on the extractable (into CHC13, 1,2-dichloroethane, benzene, or toluene) ion-associates of basic dyes and anionic Ag complexes with cyanide [35,36], iodide [37,38], and bromide [39]. In these methods, use has been made of such dyes as Crystal Violet [35,39], Brilliant Green [38,39], Malachite Green [39], Methylene Blue [36], and Nile Blue A [37]. In some of these methods the molar absorptivities are close to 1-105 [36,39]. A flotation method has been proposed, based on the addition compound [R6G+][Ag(SCN-)z]-[R6G+][SCN -] which is formed by silver ions (at pH 2-5) in the presence of thiocyanate and Rhodamine 6G (flotation with DIPE, the precipitated compound is washed and dissolved in acetone, e = 1.5-105) [40]. The complex Ag(CN)2-, associated with Crystal Violet, has been utilized in another flotation-spectrophotometric method of determining silver [41]. Silver has been determined also in a system comprising thiocyanate and Rhodamine B, as an aqueous pseudo-solution, in the presence of poly(vinyl alcohol) [42]. Other spectrophotometric organic reagents used for the determination of silver include 2-(3,5-dibromo-2-pyridylazo)-5-diethylaminophenol (3,5-diBr-PADAP) in the presence of the anionic surfactant lauryl sulphate (~ = 7.7.104) [43] and dodecyl sulphate (E = 6-104) [44], 4-(3,5-dibromo-2 pyridylazo)-N,N-diethylaniline in the presence of dodecyl sulphate [45], Cadion 2B in the presence of Triton X-100 (~ = 1.0-105) [46], 4-(2-quinolylazo)phenol [47], 4-(p-nitrophenylazo)-l-amino-3-pyridynol (~ = 1.07-105 at 605 rim) [48], and thyrodine (after extraction of silver with the use of macrocyclic compounds) [5,49]. In a rather insensitive method of determining silver (e -~1-104) use has been made of a coloured sol which is produced when silver ions are reduced to the element by means of ascorbic acid in the presence of gelatine (pH ~8) [50].

46.3. Analytical applications The dithizone method has been used for determining silver in sewage [12], aluminium and its compounds [51], uranium compounds [52], tin [26], lead [53], gold [54], metal tellurides [55,56], and glass and ceramic materials [57]. A related compound, di-2-naphthylthiocarbazone (dinaphthizone) has been used for determining Ag in selenium and tellurium (e = 4.7-104 at 505 rim) [58]. The thio-Michler's ketone was applied for the determination of silver in mineral waters [59,60], sewage [61], and lead [59,60].

References

397

Rhodanine and its derivatives have been used for separation and determination of silver in waters and in silicate rocks [62], ores [63,64], and copper, tin, and lead alloys [65]. Traces of silver in copper amalgam [32] and in tellurium [66] have been determined by the method involving 1,10-phenanthroline and Bromopyrogallol Red.

References 1. Zolotov Yu.A., Vanifatova N.G., Mikrochim. Acta, 1985 I, 281. 2. Kuznetsov R.A., Zh. Anal. Khim., 32, 2343 (1977). 3. Abashkin V.M., Yakshin V.V., Zh. Anal. Khim., 37, 1713 (1982). 4. Muroi M., Hamaguchi A., Sekido E.,Anal. Sci., 2, 351 (1986). 5. Morosanova E.I., Zolotov Yu.A., Kuz'min N.M., Sergeeva N.N., Zh. Anal. Khim., 42, 456 (1987). 6. Morosanova E.I., Kosyreva O.A., Kuz'min N.M., Zolotov Yu.A., Zh. Anal. Khim., 43, 1614(1988). 7. Suhkan V.V. Nazarenko A.Yu., Velidchenko E.D., Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tecknol., 32, 57 (1989). 8. Chayama K., et al.,Anal. Sci., 3, 535 (1987); 6, 883 (1990). 9. Morosanova WE.I., Zolotov Yu.A., Bodnya V.A., Formanovsky A.A., Mikrochim. Acta, 1989 III, 389. 10. Zolotov Yu.A., et al., Mikrochim. Acta, 1984 III, 399. 11. Poddubnik L.P., Zolotov Yu.A., Kuz'min N.M., Dmitrenko S.G., Zh. Anal. Khim., 43, 255 (1988). 12. Pierce T.B., Analyst, 85, 166 (1960). 13. Lukashenkova N.V., Tolmatseva N.S., Shkrobot E.L., Zavod. Lab., 39, 541 (1973). 14. Rangnekar A.V., Khopkar S.M., Mikrochim. Acta, 1965, 642. 15. Shrimal R.L., Talanta, 18, 1235 (1971). 16. Strelow F.W., Talanta, 32, 953 (I 985). 17. Fukuda K., Mizuike A., Anal. Chim. Acta, 51, 77 (1970). 18. Mizuike A., Hiraide M., Kawakubo S., Mikrochim. Acta, 1979 II, 487. 19. Mizuike A., Fukuda K.,Anal. Chim. Acre, 44, 193 (1969). 20. Der-Liang T., Jeng-Shang S., Yu-Chai Y.,Analyst, 112, 1413 (1987). 21. Mizuike A., Sakamoto T., Sugishima K., Mikrochim. Acre, 1973, 291. 22. Kallmann S., Talanta, 33, 75 (1986). 23. Diamantatos A., Talanm, 34, 736 (1987). 24. Dyer F.F., Schweitzer G.K., Anal. Chim. Acta, 23, 1 (1960). 25. Miller A.D., Grosse Yu.I., Zh. Anal. Khim., 30, 913 (1975). 26. Marczenko Z., Kasiura K., Chem. Anal. (Warsaw), 10, 449 (1965). 27. Cheng K.L., Mikrochim. Acta, 1967, 820. 28. Pilipenko A.T., Matsibura G.S., Terletskaya A.V., Zh. Anal. Khim., 41,829 (1986). 29. Stephen W.I., Townshend A., J. Chem. Soc., 1965, 3738. 30. Borisova R., Koeva M., Topalova E., Talanta, 22, 791 (1975). 31. Dagnall R.M., West T.S., Talanta, 8, 711 (1961); 11, 1533 (1964). 32. Yaroshenko O.P., Gavrilova V.N., Sumskaya N.R., Zavod. Lab., 57, No 9, 4 (1991). 33. E1-Ghamry M.T., Frei R.W., Anal. Chem., 40, 1986 (1968). 34. Saito K., Masuda Y., Sekido E., Bull. Chem. Soc. Jpn., 57, 189 (1984). 35. Markham J.J.,Anal. Chem., 39, 241 (1967). 36. Koh T., Katoh M.,Anal. Chim. Acta, 109, 107 (1979). 37. Likussar W., Raber H., Anal. Chim. Acta, 50, 173 (1970).

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38. Busev A.I., Shestidesyatnaya N.L., Zh. Anal. Khim., 29, 1138 (1974). 39. Shestidesyatnaya N.L., Kotelyanskaya L.I., Chuchulina I.A., Zh. Anal. Khim., 30, 1303 (1975). 40. Kalinowski K., Marczenko Z., Chem. Anal. (Warsaw), 32, 941 (1987). 41. Ishchenko N.N., Ganago L.I., Ivanova I.F., Zh. Anal. Khim., 52, 848 (1997). 42. Lopez Garcia I., Hernandez Cordoba M., Sanchez-Pedreno C., Analyst, 109, 1573 (1984). 43. Shui-Chien H., Chang-Ling Qu., Shiu-Sheng W., Talanta, 29, 85 (1982). 44. Jarosz M., Oszwa~dowski S., Marczenko Z., Chem. Anal. (Warsaw), 37, 335 (1992). 45. Ohohita K., Wada H., Nakagawa G., Anal. Chim. Acta, 182, 157 (1986). 46. Fu-sheng W., Fang Y., Talanta, 30, 190 (1983). 47. Barua S., Garg B.S., Singh R.P., Singh I.,Analyst, 105, 996 (1980). 48. Tarin P., Figuerola E., Blanco M., Mikrochim. Acta, 1986 I, 97. 49. Morosanova E.I., Kosyrieva O.A., Kuz'min N.M., Zolotov Yu.A., Zh. Anal. Khim., 43, 1614(1988). 50. Pal T., Maity D.S., Analyst, 111, 49 (1986). 51. Beinrohr E., Hofbauerova H., Mikrochim. Acta, 1989 II, 119. 52. Mare6ek J., Singer E., Z. Anal. Chem., 203, 336 (1964). 53. Jones P.D., Newman E.J.,Analyst, 87, 66 (1962). 54. Marczenko Z., Kasiura K., Krasiejko M., Chem. Anal. (Warsaw), 14, 1277 (1969). 55. Fano V., Zanotti L., Anal. Chim. Acta, 72, 419 (1974). 56. Marczenko Z., Podsiad~o E., Mikrochim. Acta, 1976 II, 317. 57. Roy S.K., Kundu D., Anal. Lett., 24, 139 (1991). 58. Tiptsova V.G., Andreichuk A.M., Bazhanova L.A., Zh. Anal. Khim., 21, 1179 (1966). 59. Pilipenko A.T., Ryabushko O.L., Matsibura G.S., Ukr. Khim. Zh., 47, 751 (1981). 60. Pilipenko A.T., Ryabushko O.L., Matsibura G.S., Zavod. Lab., 48, No 5, 7 (1982). 61. Stryjewska E., Rubel S., Chem. Anal. (Warsaw), 26, 615 (1981). 62. E1-Sayed A.A., Bull. Chem. Soc. Jpn., 67, 3216 (1994). 63. Wu X., Liang S., Fresenius'J. Anal. Chem., 336, 120 (1990). 64. E1-Zawawy F.M., E1-Shahat M.F., Mohamed A.A., Zaki M.T., Analyst, 120, 549 (1995). 65. Zhou N. et al., Talanta, 37, 531 (1990). 66. Dobrowolski J., Szwabski S., Chem. Anal. (Warsaw), 15, 1033 (1970).

Chapter 47. Strontium and barium Strontium (Sr, at. mass 87.62) and barium (Ba, at. mass 137.33) occur in solution exclusively in the II oxidation state. The basicity and solubility in water increase from Ca(OH)2 to Ba(OH)2. Barium chromate and -sulphate are less soluble than the corresponding strontium compounds. The stability of the relatively weak complexes (e.g., with EDTA or tartrate) diminishes in the sequence Ca, Sr, Ba.

47.1. Methods of separation and preconcentration Before spectrophotometric determination, it is usually necessary to separate strontium or barium from the Analytical Group I, II, and III metals. Suitable methods are discussed in Chapter 14.

47.1.1. Ion exchange Mixtures of alkaline-earth metals are separated on strongly acidic cation-exchangers. The cations are retained on a cationite column, and then they are eluted selectively with appropriate complexants, based on the differences in stability of complexes formed by the alkaline-earth metals with suitable complexing eluents. Complexones such as EDTA (complexone III) [1-3] and DCTA (complexone IV) [4,5] are suitable eluents, but other complexing agents, such as citrate [3,6] and sulphate [7] are also applied. Barium has been separated from strontium and other metals by cation-exchange chromatography using mixed HCl-organic solvent eluents [8]. Strontium has been enriched and determined in sea water [5] and in milk [2]. Calcium, strontium, and barium have been separated by elution from a strongly basic anion-exchanger with citrate. Calcium and strontium have been separated by using a mixed medium, comprising 0.25 M HNO3 and methanol [9]. Barium is quantitatively retained on the chromate form of an anion- exchanger, while calcium passes through [10]. If barium and calcium are passed through an anion-exchanger column, in a medium containing HNO3 and methanol, only Ba is sorbed. It can then be eluted with 0.5 M HNO3 [ 11 ]. Strontium was sorbed from aqueous-organic media on a resin impregnated with Arsenazo I [12] and on a polyurethane foam [13].

47.1.2. Extraction. Precipitation Calcium, strontium, and barium have been separated by extraction with Azo-azoxy BN (formula 14.1) in CC14 + TBP [14]. First, Ca is extracted from 0.05 M NaOH, then strontium from 0.8 M NaOH solution; barium remains in the aqueous phase. Strontium is 95% extracted from 0.8-2 M NaOH, in the presence of a tenfold excess of Azo-azoxy BN. Strontium may be separated from other metals by extraction with HTTA in MIBK [15,16] or with HTTA and TBP in CC14 [17]. Strontium and barium can be extracted with BPHA in CHC13 [ 18]. Macrocyclic reagents, such as crown ethers, 18-crown-6 (formula 1.15), 15-crown-5 [19-21], and [cryptand-2.2.2] [20,22] give cationic complexes with Sr and Ba. When

400

47. Strontium and barium

associated with picrate ion, these complexes are a basis for separation of Sr and Ba [ 19,23], Sr (and Ca) [24], Sr [20,25], and Ba [21,26], using CHC13, 1,2-dichloroethane, and nitrobenzene as solvents. The ion associate with Erythrosin B has been used also for determination of Sr [24]. Strontium has been extracted with 18-crown-6 ether in the presence of trichloroacetate [27]. The effect of surfactant on Sr extraction with 18-crown-6 ether has been studied [28]. Benzoyltrifluoroacetone (in CHC13) has also been used for extraction of Sr and Ba (as well as Ca) [29]. In the separation of Ca(NO3)2 from Sr(NO3)2 by extraction (leaching) better results were obtained when acetone was used as the solvent instead of a (1+1) mixture of ethanol and diethyl ether [30]. Traces of Sr have been co-precipitated as the chromate with Ba [31] and as the oxalate with Ca [32]. Enrichment of strontium on MnOzaq. has also been utilized [33]. Barium has been precipitated as BaCrO4 from solutions containing Sr and Pb, by lowering slowly the pH of the solution containing the DCTA complexes of these metals. As much as 99.5% Ba precipitated, as the pH reduced from 10.3 to 6.7 when ammonia was expelled by heating the solution. The amounts of Sr and Pb co-precipitating with the Ba were

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