Introduction to
Voltammetric Analysis
Theory and Practice
F. G. Thomas and G. Henze
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National Libra ry of Australia Cataloguing-in-Publication entry Thomas, F. G. (Francis George),
1933-. Introduction to voltammetric analysis. B i b liography.
Includes index. ISBN 0 643
06593
1. Voltammetry. 2.
tl.
Electrochemical analysis. I. Henze, Gunter.
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Title.
Foreword
The subject of electrochemical analysis is essentially as old as the invention of the battery by Volta more than 200 years ago. In this 200 year history, the subject has been fashionable at times and under threat of extinction in other periods. For example, it has often been said that voltammetry, which is a very important sub-discipline of the electrochemical analysis, almost disappeared in the middle of the last century and then underwent a renaissance in the 1 960s when high quality com mercially available instrumentation became widely available at a low cost. This book reflects applications and concepts that have predominantly emerged since the 1960s renaissance period. The authors have a very acute sense of applications that are important and have produced a remarkabl e collection at the end of the book which covers the determination of organic, inorganic and biologically important materials such as pharmaceuticals. Applications using the classical polarographic method and the ultra-sensitive forms of stripping voltammetry are all included. The usc of the dropping m ercury electrode in polarography is presented to enforce the important historical aspect of this subject and also as an excellent tool for teaching the subject of voltammetry. Extensive data col lections, not available elsewhere, are also contained in a compact form in this book. The availability of this book should permit a teacher of the discipline to present a course on the topics described in the seven chapters and then immediately use the applications in a laboratory exercise, in order to enhance the teaching component of the subject. Voltammetric analysis is not widely taught in present-day university courses. In addition to assisting in rectifying this deficiency, it can be noted that a technician having to undertake practi cal voltammetric analysis could use this book as a self-help teaching guide. That is, undertake the experiments described in the book, whilst at the same time examine the theoretical principles on which they are based. A book of this kin d can only be produced by authors such as Henze and Thomas who have had wide experience in both the fundamental and applied aspects of the sub ject. Personally, I wi ll find this book on the introductory aspects of voltammetric analysis invalu able in both the areas of teaching and research. I am therefore delighted to be able to recommend this book as being a valuable addition to the literature on analytical aspects of voltammetry. Professor Alan M. Bond Head, School of Chemistry, PO Box 23, Monash University, Victoria 3800, Australia 27 February 200 1
v
Contents
v
F o reword
Preface 1
xi
Introduction 1.1 1.2 1.3 1.4
Defin ition The cell Electrode processes Volta mmetric cu rrents
References
4 7 7 10 12 14 17
Techniques
18
1.4.1 1.4.2 1.4.3
Diffusion currents
Kinetic an d catalytic currents Capacitive currents
1.4.4 Adsorption currents
2
1
2.1
Direct current polarography
2.l.l
The diffusion current
2.1.2 Reversible processes: the half-wave potential
2 . 1.3
2.1.4
2.2 2.3 2.4
2.1.5
Irreversible processes
Polarographic maxima
Selectivity and sensitivity limitations Amperometry Linear sweep and cyclic voltammetry Pulse techniques 2.4.1 Square-wave polarography
2.4.2 Sq uare - wave vo l tam m et ry 2.4.3
2.5 2 .6
Normal pulse polarography and voltammetry
2.4.4 Differential pulse polarography and voltammetry
Alternating current polarography and voltammetry
2.5.1 Tensammetry
Chronopotentiometry References
3 Stripping analysis 3.1 Electrochemical stripping techniques 3.2 Voltammetric stri ppi ng 3.2.1
Electrolytic accumulation 3.2.1.1
3.2.1.2
Anodic voltammetric stripping
Cathodic voltammetric stripping
3 . 2 .2 Adsorptive accumulation
18 19 22 27 27 28 31 33 38 39 39 43 43 46 51 52 56
58 58 60 60 60 70 72 vii
viii
Contents
3.3 3.4
Chronopotentiometric stri pping analysis 3.3.1 Data presentation Potentiometric stri pping analysis References
4 Practical aspects 4.1 4.2
4.3
4.4 4.5
5
Reference electrodes
4.5.2
Working electrodes
4.5.4
Modified electrodes
4.5.3
4.6 4.7
90 90 92 97 98 99 99
Sources of error Sample preparation Supporting electrolytes Voltammetric cel ls Electrodes 4.5.1
101 107 11 3 1 17 1 19 1 24
Micro-electrodes
Instrumentation and automation Evaluation and calculation References
Flow-through techniques 5.1
5.2
Am perometric and volta mmetric flow t hrou g h detection Flow-through stripping voltam metry -
References
6 Applications: inorganic species 6.1 6.2
6.3
7
Polarographic m et hod s Stripping voltammetry Amperometric methods Refe rences
Applications: organic species 7. 1 72 7.3 7.4 .
Pola rog raphic and voltam metric methods S tri ppi ng m eth od s Amperometric methods Flow-through detectors References
8
80 82 85 88
126
128
139
142
144 1 44 153
170
174
176
176 206
212 213 215
Appendices Appendix 1 Appendix 2 Appendix 3 Appendix 4 Appendix 5 Appendix 6 Appendix 7 References
Index
Inorg a nic species-water sam ples Inorganic species-biological and medical samples Inorganic species-agricultural, industrial and other samples Organic species-general Organic species-biological and medical samples Abbreviations Internet addresses of instrument supp l iers
220 225 227
230
233 237 241 24 1
247
List of applications
1
2
3
4
5
6
7 8
9
10
11
12
13
Determination of chromium in the presence of ni trate
Determination of s ulfide and sulfite P olaro graphic determination of cyanid e
Determination of cadmium, copper, lead and zinc after U.V. digesti o n using an HMDE
ASV determination of cadmium and lead u s in g an U l t r a tr a ce graphite electrode
Determination of antimony and bismuth by anodic s t ri p ping v oltammet r y ASV d e termina t ion of mercury on a gold electrode Determination of arsenic( III) and arscnic(V) by
cathodic s t rip p i ng vo l t am m etry
Determination of sel enium by cathodic st ri pp ing vo ltam me t ry Determination of iron
by adso rp t ive str ippin g v olta mmetry
Determination o f p l a t i num by adso rp tiv e stripping voltammetry of c h ro mium as its DTPA c omp l ex by a ds orp ti ve
Determination
st ripp ing volt a nune t r y
Determination of u r a niu m by ads orp t iv e stripping voltammetry
IS
Determination of aluminium as its alizarin-S comp l ex by AdSV Determination of cadmium and lead in high orga n ic content fluids by
16
Polarog raph ic determination of for mald ehyde
14
17
differential PSA
Polarographic
determin ation of as c orbic acid (vitamin C)
18
Pol a rogr ap h i c determination of nitrilotriacetic acid (NTA) and ethylenediamin-
20
Polarographic deter min at i on
19 21
22 23
24
25
26
27
etetraacetic acid (EDTA) [Din38 413, part 5]
Polarogr ap hi c determination of d i a zepam in body fluids and pha rmaceut i ca l p r od u c t s
Polarographic
of nicotine
dete rminat ion of cin ch o cain e
prep ar at ion s
(dibucaine)
Polaro grap hic determination of folic acid
Polarographic determination of riboflavin ( vi t a m in B2) of free styrene in p o l ys tyrene
1 53 1 57 159 1 61 162 1 62 164 164 1 66 166 168 169 1 80 184 1 86 194 1 97
in pharmaceutical
Polarographic determination of thiamine (vi ta m in B 1)
Determination
1 50 150 151
and mixed polymers
Determination of fenchlorazol-ethyl by adsorptive stripping vol tam me try
Determination of thiourea by cathodic stripping v ol t ammetry
198 200 201 201 204 207 209
ix
Preface
This text is a t ra nsl ation of the German text Polarographie und Voltammetrie: Grundlagen und Analytische Praxis ( Sp ringer 2001) by Gunter Henze. This E nglish lan g uage version, wr itte n con curren t ly with the German version, is a c o operat ive effort by the tw o authors i nc orp or atin g essen tially the same m aterial as in th e German text but p res e nt ing it in an alternative o r d er. The group o f a n alyti cal tech niques that are i n cl ud e d within the term vo l t a m me t ry are argu ably the most versatile of all analytical t echn i qu e s . Th ey may be used to determine sim pl e metal ions, complex m etal species, inorganic anions, and a wide ran ge of organ ic compounds at concen trations ran gin g over nine o rd e rs of magn it ude from 10-2 mol L _, (g L-1) down to t h e 1 o-11 molL-I (ng L-') r egi o n with recent deve l o pment s emphasisi n g analyses in the l ower concent r atio n ranges . S am p les from a vari e ty of sources-marine, p otab le, surface and waste waters, plant and animal tissue, b ody fluids, pharmaceuticals, foodstuffs, so i l s, agric ult u ral prod ucts and a range of indus trial products-m ay be an al ysed using vo l t a m metri c methods. V oltamm e t ri c techniques are of partic ular value to those involved in en viro nmen t a l studies-particularly i n studies of me t al spe ciation and of p o ll ut an t s and their metabolites, clin i cal analyses, and d r ug evaluation and metab olism st udie s. Furthermore, the eq u ip m en t r e qui re d for voltam metry is si mple a nd relatively inexpensive-in many cases b ein g an order of magnitude lower in cost than spectrophotometric equipment. Despite the many advantages of voltammetric tech niq u es, the authors believe that th ey are under-represented in many a n alyt i c a l laboratories. This is probably due to two factors-the lack, until re c e nt ly , of fully automated eq u ipmen t and the limited coverage of electrochemistry, p a r tic ularly polarograp hy and voltammetry, in many un dergradu a te courses and g e n era l an alytical chem ist ry texts. This text was written in order to try and overcome this latter d efi c i e ncy . It is aimed at students in final year undergra duat e courses and those b egin n in g research p r og ram s which req ui re the student to have a kno wledge of voltammetric t e chn i qu e s . The text has also been written for grad ua tes w o rk i n g in industrial and commercial analytical laboratories who have little or no experience of voltammetric methods and who wish to extend their p r ofes si o n al kn owledge and exper tise . The authors consider that good pra c t i c e in analytical c hemis t ry is based on a s o und under standing of the physical and chemical principles underlying the techniques used and so present the theoretical basis of polar ogr aphy and voltammetry before c o ve ri n g the practical aspects and applicati o ns . In ch apter 1, th e basic electrochemical c o n c e p ts relevant to an understan d in g of polarograp hy and vo l tamm etry arc introduced. I n chapter 2, the u n d erlyi n g th eo ry of the various voltammetri c t e ch nique s is presen ted . A detailed a cco unt o f the original technique, direct c urre n t polarography disc o ve red in Prague by H e yr ovsky in 1923 a n d put on a sound theoretical basis by llkovic ten years later, is prese n ted first. The various modifications made to this t ec hn iq ue in order to imp rove its sen s i tivity and s el e c tivity and dec r eas e the time r equ ired for a measurement are then discussed. Th ro ugho u t, a sem i-quantitative approach is adopted in presenting the theory in which the mathematical rel ati o n ship s between the relevan t physical q u an t ities and the analyte xi
xii
Preface
concentration are introduced for understanding the technique but with the derivation of the equations being omitted. In chapter 3, the sensitive stripping voltammetric techniques are discussed. Then follows a chapter dealing with the practice of voltammetric analyses. Sample pre-treatment and equipment including recent developments in electrode materials and construction are presented. In chapter 5, voltammetric and amperometric flow-through detectors are described. These detectors, which commonly have volumes in the f...LL range, are the key component for the development of minia turised and automated microprocessor controlled voltammetric equipment and procedures. Furthermore, the combination of chromatographic separation techniques with flow-through vol tammetric or amperometric detectors, results in powerful analytical procedures which are very sensitive and selective and which can be performed rapidly. In the remainder of the book applications of the various voltammetric methods are presented. An outline of the voltammetric behavio ur of the elements, interspersed with details of 15 applica tions to the determination of inorganic species in a variety of samples is the subject of chapter 6. In chapter 7, the voltammetry of organic functional groups is summarised and includes a selec tion of 12 applications of voltammetry to the determination of organic compounds. A series of appendices which list the important features of a selection of some 300 applications of voltamme try and polarography to the determination of i norganic and organic species in a wide range of samples concludes the book. The authors wish to thank Metrohm Ltd and their staff in Switzerland and Australia, espe cially Dipl. Ing. Uwe Loyall, Product Manager for voltammetric equipment, Herisau, Switzerl and. Francis G. Thomas MSc, PhD formerly Associate Professor of Chemistry, James Cook University, Townsville, Australia
Gunter Henze Dr.rcr.nat.habil. Professor of Analytical Chemistry, Technical University of Clausthal, Germany
Introduction
1.1 DEFINITION
to that group of electroanalytical t echniques in whic h the current (ampere) that fl ows t h ro u gh an electrochemical cell is meas ured as the poten tial ( volt ) applied to
The term voltammetry is appl ie d
the electrodes in the cell is varied. The term, first used by Kolthoff in 1940 [ 1), is derived from the units of the electrical parameters measured-volt-am (pere)-metry. It is im p o rt an t a t th i s p oi nt to distin guish between the term s voltammetry spelt with 'mm' and voltametry sp dt with on ly one 'm'. The term v olt amet ry has been used to describe the te c h n i qu e ge n e r al ly known as 'controlled current p o ten tiom e tric ti trat io n ' . The essential d iffere nce between voltammetric and other p oten tiodyn a mic tech n i ques, such as constant cu rren t co ulo metry, is that in vo lta m metry an elec tro de with a small surface area ( < 10-5 m2) is u sed to monitor the cu rrent p ro d u c ed by the spec ies in so lu tion reacting at this electrode in resp onse to the potential appli ed to i t . Because the electrode used in vol t am m etry is so small, the amoun t of material reacting at the electro de can be igno red . This is in c ont rast to t h e case in co ulo me try where large area el ect r o des are used so that all of a spec ie s in the cell may be oxidised or red uced . When mercury, flowing through a fi n e capillary so t h a t it emerges as droplets, is used as the small electrode in a vol tamm et ri c cell the technique has the sp ecia l name polarography. This nam e is derived from the fact that the electrode can be polarised . An electrode is said to be polar i sed when no direct current flows across its interface wi th th e solution even th ough there is a p otential difference across this interface. In his definitive w ork p u blished in 1922 [ 2) on 'Elect rolysi s with a dropping mercury electr ode ', J aro slav Heyrovsky referred to this p he n o men o n and called the recorded current-po te nti al polarisation curve s polarograms. The recommen dation of IUPAC [ 3] is that voltammetry is the gen e ral term to be used when current potent ial rel ati on shi p s are being in vest i g ated and th at o n ly when a flowing conducting liquid electrode ( such as a dro pping mercury e l ec t ro d e) is used as the wo rk ing electrode should the term polarography be u se d. P o larography, the o r igina l t ech n ique, is thus a s p eci al case of voltammetry.
1.2 THE CELL
cell u se d in vo lt a m m et ry is a multi-phase system in which electrical energy is used to bri ng about a che mical ch an ge ( el e ctr o lys is ) in species in the cell. In its si m p l est fo r m , su ch a cell consists of two electronic conductors called electrodes i m me rsed in an el ectrolytic conductor (ionic solution) co n tai ning the substance of analytic a l interest called the analyte. This ionic solution c o m pletes the el ec tr ical circuit. It is the processes which occur at the solution-ele ctro de interface involving the analyte and the factors that influence these interfacial processes that arc expl o i ted in electro-analytical ch e mis t ry. The application of a vo ltage or a current fro m an exte rna l source to the electrodes p r o d uce s an elect ri cal r esponse fro m the analyt e The electrochemical
1
2
Introduction to Voltammetric Analysis
in the cell solution. The nature and magnitude of this response may be used to both identify and quantify the analyte. The sm all electrode used to monitor the response of the analyte is known as the working elec trode (WE). Even though only a negligible amount of material is involved in the processes occur ring at the working electrode, its small size ensures that a high current density develops at its surface. Consequently, it is the processes which occur at this small electrode that control the cur rent flow through the cell. The small current-controlling WE may be constructed from a wide variety of conducting materials. The more common materials used include various forms of graphite and metals such as mercury, gold or platinum. These electrodes may be stationary, rotating or, in the case of mer cury, flowing with respect to the cell solution. They are usually named after the material from which they are made and in some cases after other physical characteristics. For example, the drop ping mercury electrode or DME; the mercury thin film electrode or MTFE; the ha ng ing mercury drop electrode or HMDE; the glassy carbon electrode or GCE; the carbon paste electrode or CPE; the rotat ing platinum electrode or RPE; a chemically modified electrode or CME; and so on. The second electrode in the simple cell, called a counter electrode (CE),-serves two purposes. It is used to control the potential applied to the working electrode and to complete the circuit for carrying the current generated by the processes occurring at the WE. In the former role it must act as a reference electrode (RE). The ideal reference electrode must be a completely depolarisable electrode, i.e. it must be able to maintain a constant potential at its interface with the cell solution irrespective of any cu rrent that may flow across this interface. This can only be achieved if the reaction that controls the potential of this electrode is very fast and if there are no significant changes in the ion concentration profile in the vicinity of the interface-conditions that cannot be realised in practice unless the current that flows th rough the cell i s extremely small and the composition of the cell solution does not change significantly from sample to sample. In the simple two-electrode cell the counter electrode must be much larger in area th an the working electrode so that ( i ) the current density at its surface is so small that the electrode processes occurring at the CE do not affect the current signal generated at the WE in any significant way, and (ii) the flow of current through the cell as a result of the processes at the small WE has a minimal effect on the concentration profile at the CE surface so that its function as a reference electrode is not seriously impaired. A simple arrangement for measuring the current that flows through a two- electrode cell as the voltage applied to the electrodes is varied is shown in Figure 1.1. Accurate measurements are only possible with this basic set-up when no voltage drop occurs in the cell and, as indicated above, the counter electrode is maintained in a constant-composition solution . The solution in the cell will have a resistance to the flow of current and as a consequence there will be a potential drop across the cell due to the current flow generated by the electrode process. Although this may be mini mised by adding a high concentration of a supporting electrolyte (SE) to the cell solution it can never be eliminated. The voltage applied to the cell, V•PP' then is given by: (l.la) where
£..11E and E,er arc the potential drops across the working electrode-solution interface and reference electrode-solution interface, respectively, when the reactions at both electrodes are wri tte tl as reductions. iceii is the current flowing through the cell. When the analyte is reduced electrons flow from the WE to the analyte and the current is called a cathodic current. When the an alyte is oxidised the electron flow is reversed and the current is called an anodic current. R is the cell resistance in ohms.
Introduction
3
Working electrode D. C. Potential source
(Potentiostat)
Cell
Ammeter Figure 1.1
Counter electrode
Two-electrode arrangement for measuring current-potential curves.
Since the absolute values of s ingle electrode potentials cannot be measured, it is normal to all working electrode pot ent ials to that of the reference electrode so that equation l.la
refer
becomes:
vapp where
=
E' WE
+ icell R
(l.lb)
E' wr is the potential of the working electrode relative to the reference electrode being used.
is only under zero current conditions when no reaction is occurring at the working elec that the voltage applied to the two-electrode cell is identical with the sum of the potential drops at the electrode interfaces. Since in a real measurement situation a current flows through the cell and Vapp differs from the sum of the two electrode potentials by iceuR-the so-called 'iR drop'-it is preferable to refer to the plot of icen aga ins t V,,PP ob tain e d from a two-electrode cell as
It
trode
Historical note. The pol arographic cell comprising of a UME and a mercury pool electrode was introduced by f. Hcyrovsky. In his book Polarographisches Praktikum [ 4] he describes the following si m p le arrangement tor mea suring current-voltage curves: 'Connect th!! ends A and B (Fig.l.2a) of a 10 to 20 ohm slide-wire e.g. a mea su r i n g bridge or Kohlrausch drum used for conductometry or a potentiometer wire used for compensation measurements, to a 2 or 3-volt lead batter y, C. The potential necessary for the electrolysis in the cell is obtained by connecting the end of the wire, A, and the slide contact, S, (fig.l.2b) to the (respective) mercury electrodes in the cell. The cell is made from a 5 to 20 em·' glass beaker in which the solution to be studied is placed. M ercur y to a hei gh t of 5 mm is poured into the beaker and the dropping (mercury) electrode p laced in the solution.' a
8
A z Figure 1.2
(a) Schematic arrangement for elect rol ys is with the dropping mercury electrode. (b) A simple practical apparatus [4].
4
Introduction to Voltammetric Ana lysis
a current-voltage curve. A very simple experimental a rr a nge men t for the two-electrode system is to use the m ercury c o l l e c t in g in t h e bottom of the cell as the counter electrode (i.e. a m e rcury pool electrode or MPE). However, as the c ell s olu tion comp osi tion varies so will the p ot e n t i al of th e MPE. For more pr ec i se control of E,'v"E it is necessary to use as a reference e l e ct ro de an el ectrode that can maintain a more constant poten t ial than the mercury pool electrode. In modern me a su ri ng systems the current carrying r ol e of the counter electrode is sep ar at e d from its potential control role by intr od u cin g a third electrode, th e auxiliary electrode, AE, to the cell to com p l et e the current carrying circuit. The a r e a of t his auxiliary electrode must be l a rge com p a red to t h at of th e working electrode for the reason (i) given above. The addition of the aux iliary elect ro de means that the counter electrode now is u sed only to control the potential of the worki ng electrode and so becomes a t ru e reference electrode. Since no current flows thro ugh t h e potential co n t rolli n g circuit, t he size of the reference electrode can be reduced and a co nducti n g salt br idge can be placed between the vari a ble compo s ition cell solution and the reference elec t ro de so th a t th e co m posit ion of its solution will remain constant. The in crease in resistance resulting from these ch anges does not affect the p otential control of the work ing electrode. Two electrodes wh ich ar e c o m monly used as reference electrodes for the p recis e control of the working electrode pot e nti al in aqueous media are the silver-silver chloride electrode in a solution of fixed chloride concentration and the saturated calomel electrode or SCE ( m ercury-mercurous chl o ri de electrode in a s atur ated KCl solution ) . Th e se ele ctro des are robust eas i l y constructed and main tain a constant po t enti a l over lo n g perio d s of time. The three-electrode system will be discus se d in more detail in chapter 4. ,
1.3 ELECTRODE PROCESSES
The t er m elect rode process includes a l l p he no m ena which, as a result of applying a voltage to an el e ctro de lead to a cu r re nt flow through that electrode. The course of an electrode process is g r e a t ly influenced by the nature of the phase b ound ary of t h e working electrode and its associated electrical double layer(§ 1.4.3). Th e importance of this re gio n cannot be over-emphasised s inc e the poten t ial appli ed to the c ell exists o nl y across the double layers of th e working and reference el ectrodes when there is no current flow throug h the cell; there bei n g no p otential drop across th e cell soluti on under zero current conditions. In cells wit h s u p port i n g electrolytes the double la ye r s are on ly a few nanometers thick y e t all the electrochemical reactions occur there. Most voltammetric and polarographic an a l yses utilise electrode processes in which electron s or ions are exchange d between the two phases. As a result of this the analyte will be either oxidi s ed or reduced d e p end i ng on the potential of th e WE. Such an electrode process is re ferred to as a charge transfer reaction a n d p rod u ce s a flow of charge that is a current, thro ugh the electr ode This is su m m a ri s ed in equation 1.2: ,
,
.
(1.2)
Ox +ne- �Red where
Ox and Red represent the analyte in its oxidised and reduced forms respectively, and n is t h e n umber of electrons, e-, that react with each molecular uni t of the analyt e .
and illustrated in Figur e 1.3. A c urrent flow that results from t he oxidation or reduction of a sub stance is known as a faradaic current, i F, a nd its mag n i tude depend s on the conc entratio n profile of t h at substance in t h e re gio n of th e electrode s ur face T his pr o fil e depe nds on the concentration of the substance in th e cell solu t io n and on the kinetics of all steps in the associated electrode pro cess. If the profile is controlled by the diffusi on of th e an al yte to the electrode, the faradaic current is known as a diffusion current, if controlled by the kinetics of a step in th e electrode process, as a kin e tic current, a nd if controlled by a cat alytic process, as a catalytic current. .
Introduction
5
Solution
Ox
Figure 1.3
Schematic representation of the electrode reaction (for a reduction) as an electron exchange at the electrode-solution boundary.
As a result of the elec t r od e process, the concentration of an a n alyte at the electrode surface drops until it r eac h e s an equilibrium value determined by the potential of the working electrode and the standard electrode potential, J!>oxireu' of the charge transfer reaction ( equatio n 1 .2). Once
equilibrium is reached it might be expected that the current would fall to zero. But this is not
observed. It is fo u n d
th at the charge transfer reac ti o n can be maintained even in a stationary solu
tion. Hence the decrease in concentration of the an alyte at the electrode surface due to the elec trode reaction must be countered by supplementation of the analyte from the bulk solution in the
cell1• In addition, any soluble electrode reaction products must be moving out into the bulk solu tion. This movement of mat e ri al to and from the electrode surface m a y be achieved by a number
of mechanisms.
The analyte in the test (cell) solution may be t r a nsp o rt e d to the electrode surface by convec This is fa c ili t ated by movement of t h e solution relative to the electrode, for example by stir ring or by the action of the mercury dr oppin g from the capill a ry of the DME. The most important process for transporting mo le c ule s through the thin laye r of solut i o n (10-100 f.lm) immediately in c onta c t with the electrode surface is diffusion. Thi s is the case for both the transport of analyte t o the electrode surface and the transport of the electrode reaction p ro du c ts away from the electrode surface into the bulk of the solution. Nernst called this impor tant region adjacent to the electrode surface the diffusion layer (figure 1.3 and§ 1.4). Another mechanism by which ionic analytes may be transported i n a cell arises when t h ere is a current flow th ro ugh the cell. The positive i o ns m ove towards the worki ng electrode w he n it i s negatively charged and the negative ions move in the opposite direction. This process is kn o w n a s migration and leads to the m igratio n current, im. All ions present in the cel l solution contribute t o the migration cu rren t in proportion to their ch arge , concentration and mobility. Because the conce ntrati o n of the supporting electrolyte is usually more than one thousand times that of the tion.
The electrodes use d for voltammetric and polarographic analyses have small surti1ce areas ( 1-10 x I o-� m2), and the faradaic current flow is normally in t he nAto f!A range. This is so small that even after repeated measurements there is no perceptible change in the analyte concentration in the bulk of the test solution in the cell. A small non-faradaic current can also be measured, even in an analyte-frec solution of the supporting electrolyte alone. Th i s s mal l cur rent is required to p ola rise the electrode, i.e. to e s t a bl ish the electrochemical double laye r and potential difference across the electrode-solution p hase boundary and is referred to as the charging or capacitive current (see § 1.4.3 ) .
6
Introduction to Voltammetric Analysis
analyte, the contribution of the analyte itself to the migration current can be neglected. In effect, in addition to reducing the cell resistance, the supporting electrolyte carries the current through the cell solution although it is not involved in the current-determining electrode process at the working electrode. Other processes which may affect the m agnitude of the current at the working electrode include: adsorption onto the working electrode of the analyte, its reaction products or other sur face active materials in solution; the de- solvation of solvated species; the formation of an insolu ble electrode reaction product; reactions with the electrode material (insol uble salt formation); and chemical reactions involving the analyte prior to the charge transfer reaction or its immediate reduction-oxidation products. In order to fully understand the electrode process, both mechanis tic and kinetic information about all the individual steps are required. An i m portant factor which influences the efficiency of a particular voltammetric analysis (limit of detection, sensitivity and/or resolution of adjacent current signals) is the degree of revers ibility of the electrode process. The reversibility of an electrode process will depen d on the kinetics and mechanism of the various steps in that process. An electrode process may be irreversible because of a slow charge transfer step, a slow chemical step, a slow adsorption step or a thermody nam i cally non-equivalent pathway for the reverse electrode process. If the charge transfer step is slow and rate determining the process is electrochemically irreversible, if a chemical step is rate determining then the electrode p rocess is chemically irreversible. However the experimentally observed degree of reversibility of an electrode p rocess not only depends on the kinetics of the electrode process i tself, but also on the particular voltammetric technique being used. The various voltammetric techniques differ in the dynamics of the poten tial applied to the cell and i n their measurement rates (time between individual current measure ments). Each technique has a characteristic time as has each electrode process. If we identify the half-life of an electrode process with its characteristic time, then if this is short (a fast reaction) compared to the experimental measurement interval equilibrium will be attained wel l within this time interval and the process will be seen to be reversi ble by this technique. On the other hand, if the half-life of the process is long on the experimental time scale, it may have little or no apparent effect on the measured response. For example, the characteristic time of a direct current polaro graphic experiment ( § 2.1 ) is related to the drop time of the DME, say about 1 s, whereas that of an alternating current polarographic experiment ( § 2.4) is related to the frequency of the super imposed a.c. voltage, say about 1 0 ms, so that an electrode reaction which appears reversible in d.c. polarography, may appear irreversible in a.c. polarograpy. The rate at which the charge transfer step ( oxidation or reduction of the analyte) occurs at a particular potential will determine whether or not an electrode process is electrochemically reversible. The time dependence of the electrode reaction may be estimated from the heteroge neous rate constants k,.ed or kox for the charge transfer step. For a reduction: k
red
ko
_
-
exp
and for an oxidation: k
ox
where
=
o
k exp
{-a.nF(ERT
l:,. o ) 1
f
{(1-a.)nF(E-�)} RT
( 1.3)
(1.4)
k" is the standard heterogeneous rate constant (em s-1 ) for the charge transfer reaction, i.e. the value of the rate constant at the standard electrode potential, E', for the charge
Introduction
7
transfer reaction at the electrode in usc. Often the polarographic half-wave potential , £112 , ( § 1.4) is used as t he reference potential instead of 1::.-o. a is the ch ar ge transfer coefficient and is that fraction of the total free energy change of the reduction step which is due to the p o tentia l difference at the W E-s o lut i on interface. 1 n is the number of electrons involved in the rate determining charge transfer reaction, E is the potential of the working electrode ( = E..v h ), F i s the F a raday constant ( F 96485 C mor- 1 ) , R is the molar gas constant ( R 8.314 J K- 1 mor- 1 ) , and T is the absolute temperature ( K ) . =
=
Using e quati o n 1 .3 or 1.4, t h e rate of a particular c h arge transfer reaction c a n be calculated from a k nowledge of its standard rate constant. In pola rography, c h a rge transfer reactions are 1 co nsidere d to be reversib l e when k0 > 0. 1 - 1 em s- \ as irreversible when k0 < 1 0-4- 1 0-5 em s- and as quasi-reversible for interm ediate values of k0• H owever as indicated above, the deg ree of reversibility also depends on the particular voltammetric techn i q ue being used . The in fl uence of the reversibility of the charge transfer reaction on the voltam metric response is dealt with in chapter 2.
1 .4 VOLTAM M ETRIC CURRENTS 1 .4 . 1 Diffusion currents
that flows through the working electrode depends upon the kinetics of the o vera ll and its magnitude will depend on the rate of the slowest individual step in that process. The r ate of the charge transfer step is potential dependent ( equations 1 .3 and 1 .4) so that if the w o rking electrode potential is such that this step is fast then diffusion of the analyte from the bulk solution to the electrode surface across the diffusion layer is the slow step which controls the current flow. This is the situation most often encountered in analytical voltammetry, and the cur rents that flow un d er these co nditi o n s are c a lled diffusion curren ts. The diffusion current depends on the limiting concentratiotl gradient of the analyte at the electrode surfa ce ( dc/dx)x=o• which is related to the d ifferen ce in analyte concentration at the surface of the electrode, c,, a n d in th e bulk solution, ca, and the thickness of the diffusion layer, 0 and ( c. l red > 0, th en equation 2. 1 5b becomes:
Eo::v!E where
=
{
' 0.0592 r ( id )cath - i ] l og [ . ( ) ] E112 + -n I l d an '
_
}
(2 . 1 5c )
( id lcath a n d ( id ) a n a re the limiting cu rre n ts for the forward cathodic ( reduction) and of equation 2 . 1 0, r es p e ctivel y, and !! 1 12 is the p o ten tial at w h ic h i ! ( id l cath + ( idlan l / 2
reverse anodic ( oxidation) r e a ct ions =
For a r eve rsible electrode process, I! 1 12 is the same for both the reduced and oxidised forms of the analyte, and when both the oxidised a n d reduced forms are present t oge th e r in so l u t io n, only a single wave is observed with thi s same !!112• The effect of the num ber of electrons involved in the electrode react ion , n, ( e q u ati o n 2. 1 Ob) on the slope of the polarographic wave is shown in Figure 2.3a. For a reve r s i b le electrode process the value of n m a y be d et e r m i n e d from a plot of EnME ve r s u s log { ( id - i ) / i ) which will be a straight line of slope 59.2/ n mV. Alternatively, n may be e s tim ated directly from the p ol a ro graphi c wave u s ing the Tomes [ 9 ] rel at i o n s hi p which for a reduction at 2 5°C is E3t4 - E 1 ;4
=
Ej 1 4
-56.4 -- mV n
(2. 1 6)
and £114 are t h e po t e n t ials where i 3 V4 an d i id / 4 re s pe c t iv e l y. other hand, i f t h e value o f n i s known from other experiments (e.g. co u l om et ry) , then either of the above two methods can be used to check wh e t h e r or not the electrode process i s reve rs ib l e . I t sho u l d be noted that for reversible electrode processes with more c om pl ex stoichiometries t h an that of e q u a t i o n 2 . 1 0 , the plot o f equations 2 . 1 5 m a y give slopes other th a n 59.2/ n mV ( see discussi on in r e fe re nce 5 ) . When the reduced form of the analyte i s a metal soluble i n m e r c u ry the wave i s also described by e q ua t i o n 2 . 1 5a . However e q u a ti o n 2 . 1 4 for E ;12 mus t be modified to allow for the diffusion of t h e reduced form i n t o the m e r c u ry instead of th e solution phase and eox/red m u s t be r epl a c ed with the equivalent e xp r e s s i o n fo r the m e t a l a m a l ga m [ 1 ] . I f t he reduction product forms an insoluble p h a s e o n the electrode su rface, the half-wave potential is n o longer independent of the analyte concentration but varies with log c. a c co r din g to t h e re l a t i o n s h ip [ 1 ] :
where
=
=
,
On the
E
o
oxtred
+
or Eo
ox/red
-
R T1
F n
R T1 F 11
n
{fux(c.Jox}
(kn) + -f
n T ) ox
( 2 . 1 7a )
2
R T1 ( . n ld n
.
- 1)
( 2 . 1 7b)
Tech n iques
25
a
- E
b
- 0.5 Figure 2.3
- 1 .0
E
- 1 .5 [V] (vs. SCE)
(a) The effect of the n umber of electrons involved i n the electrode process, n (=1 , 2 and 3, respectively), on the slope of the d.c. polarogram. (b) The effect of the degree of reversibility on the shape of the d.c. polarograms for two-electron reactions. The electrode reactions are, 1 , Cd1+ + 2e- � Cd(Hg)
and 2,
H,O,
+
2e - ---> 201 1
and the p o ten t ia l of the
( reversible )
( irreversible)
DME is give n by Emvt F
=
r
E l /2
+
RT nF ln
{ id --: i) } 2(
jd
(2.18)
The majority o f metal ion reductions which give metals insoluble i n mercury are not reversible. The above equations have been shown to describe the behaviour of the anodi c waves observed in the oxidation of the mercury electrode in the presence of low concentrations ( up to 1 mmol L-1 ) of halide ions. In these cases the electrode reaction (2. 1 9)
of the mercury ( I) halide on the electrode surface and the value of in equation 2. 1 7 is t ha t of t h e respective mercury-mercury(I) h alide electrode. �ox/red
produces an insoluble layer
26
Introduction to Voltam metric Ana lysis
In a d d i t ion to the small changes in the h alf- wa ve p o t enti a l due to the a ct i vi ty coefficient effect at high el ectrolyte concentrations, £" 1 12 may c hang e s i gnifi cantl y when the co mpo s i ti o n of the sup p o r t i n g electrolyte solution is varied. If a species, L, wh i c h can react with a m etal ion to form a complex is added to the solution, t h e value o f f!1 1 2 for the re d u c ti o n of t h a t metal ion may be s hifted markedly. Since the co mpl ex e d metal ion is a more stable species than the free metal ion it will be more difficult to reduce so that £" 1 2 is s h i fte d to m ore n e ga tive p ot en ti als 1 the str o ng e r the c o m p l e x , the greater the shift. If the c omp l exing agent, L, reacts with th e metal ion according to:
(2.20)
where
Kr is
the s tab i li t y constant for the for m at i on of the complex metal i o n MLp"+, and p is th e number of l i ga nd s bound to the m e ta l ion in the complex,
then the s h i ft in t: uz is g iv en by [ 1 ] : .:\ E �/2
whe re
=
( E �/2 h.1L - ( E�/2 )M
=
-
RT RT ln K- - p ln cL nf 1 nF
(2 . 2 1 )
( I:."' u2 ) M and ( Eruz ) ML is the reversible half-wave potential of the metal ion in the absence and presence of the li ga n d , respectively; C1 . is the bulk c o nce n t r at i o n of the l iga n d in the cell solution, and th e other symbols have their usual m eaning .
Using e q u a t i o n 2 . 2 1 , the stability constant of the metal ion c o mp lex formed can be calcu lated from t h e shift in E r 1 12 on adding t h e ligand to the metal ion solution. The effect of com p lex i ng agents on e l /2 can be used to improve th e selectivity of a voltammetric or polarographic analysis. Two m etal ions w h os e polarographic wave s overlap beca use th ey have similar half-wave p o ten ti al s may be sep a rate d if a c o m p le xi n g agent which forms a strong com plex with one of the m etal ions but not the o th e r is added to the solution. The reduction wave of the for mer is shifted to a m u c h more negative p o t en tia l while that of t h e latter is only slightly affe c te d so that two distinct waves a re now observed. Note that equation 2 . 2 1 ass u m e s that the effects of the activity c o e ffi c i e n t s and diffusion c o e ffic i e nts of the various species do n o t con tribute sign i ficantly to E r 1 1 2 • Another situation where changes in the solution composition c an cause significant c h a nge s in the half-wave p o t en tial arises wh en the a n alyte is an o rganic compound, Org, wh ich reacts at the electrode acc o r di n g to the equation:
( 2.22)
In such cases the half-wave p o tential is pH depe n de n t and for a reversible process at 2 5°C is g i v e n by [ 6 ] E'1 /2
=
Eo oxired - --n
(0. 0 59m)
PH
Again , it i s a s su m ed that the diffusion coefficients of Org and OrgH111{ 111 that their activity coefficients do n o t significantly affect E r 1 12.
(2.23)
n)+ are similar and
Techn iques
2 . 1 . 3 I r revers i b l e processes
27
When the cha r ge
transfer reaction i s slow th e s h ap e of the wave is de t e rm i ned by the rate of the charge transfer reaction and not by diffusion o f the ana lyte to the electrode surface. As eq u ili b rium cannot be attained at the electrode s u rfa c e d u ri n g the t i m e of the measurement, the Nernst equation does not a p p ly and i nst e ad the appropriate rate expressions must be used to obtain the relationship between th e current and surface c on c ent r a t i o n of the a n a lyt e . Since the rate constants are p ote nt i al d e pe nd ent ( e qu at i ons 1 .3 an d 1 .4) an i n c r ea s e in the po tential ( more ne ga ti ve for a reduction and more p os it ive for an oxidation) causes the re a c ti o n rate to i ncrease, so that eventu ally a diffu sion controlled limiting current re gio n is reached where id k1 L c• . For the case of an i rreve rs ib le reduction, the potential-current relationship is given by [ 4, 5 ] : =
E
•
=
E
o
oxir< d
+
(0. 9 16R!1) o. n . F
-
In
{ } ( RT ) {
and E
_
E J /2
+
( ir i ) i
+
0.9 1 6 R o. n . F
�
---
(
( t0)112}
I n 1 . 3 5 9 kred D
o. n . F
{(id - i)}
ln --.1
( 2 .24)
(2.25)
w it h
( 2 . 26)
where
n. is t h e n umber of electrons in vol ve d i n th e ra t e d e t e r m in i n g step ( n. :S: n ) . (These equa tio n s apply fo r the c u rrent measured at t0, t h e end of the drop life . )
Th e sha p e o f t he wave is affected b y both the t ra n s fe r co e fficient , o. , an d the r a t e constant, k,.d· The effect is to r edu c e the slope of the wave so th a t it is s p r e a d over a wider p ote n t i al range than in the case of a reve rsib l e wave ( Figu re 2.3h ) . The half-wave potential now contains a kinetic term in addition to the thermodynamic t e r m E0 ox/rod and also depen ds upon the d rop time. Because o f these fac to rs it is i m po rt an t when stu dyin g irreversible systems t o re p r o d u ce all exp e ri m e nt a l con ditions carefully if £112 i s to be u se d t o id e n ti fY the analyte.
2. 1 .4 Po larographic maxima When r e co r d ing d.c. p ol a rogra ms , the current is oft en observed to rise st e e p ly ini ti a lly to a maxi mum value a t the head of th e wave before dropping back to t h e limit in g c urrent p l a te au ( F i gu r e 2.4). This p h e n o m e n o n is the result of a d d i t ional analyte b e ing brou g h t to t h e electrode surface by the movement of the s ol u t i o n past the drop surface. The movement is attrib uted to the m ot i o n of the double l ayer as a re su lt o f t h e a sym m et ric electrical field around the drop. Such m axi m a are referred to as maxima of the first kind and in t e r fer e with the determination of the analyte. Substances w h ich arc adsorbed a t the electrode interface h i n der this m o t i o n and can be used to suppress these u nwa nte d maxima. Good maximum suppressors in co mm o n use include Triton X- 100 and ge l at ine at concentrations of 1 0-2 to 1 0-.lo/o. The second type of maxi m a appear on the limiting current p l a t e au and also interfere wi t h the measuremen t of id . They are referred t o a s maxima of the second kind and arise from too strong a flow of mercury fro m the c a p i ll a ry durin g dr o p growth. Such maxima are fo u nd to be m o r e prevalent at high supporting e le c tr ol yt e concentrations. They c a n be red u c ed or eliminated by
28
Introduction to Voltammetric Analysis
Figure 2 . 4
- E
[V]
Polarographic maxima. A d . c . polarogram ( 1 ) with a maxi mum o f t h e fi rst k i n d ( 2 ) a n d with a maximum of the second kind (3) .
lowering the mass t1ow rate of mercury from the capillary and by working with supporting elec trolyte con centrations below 1 mol l .- 1 where possible. 2 . 1 . 5 Selectivity and sens itivity l i m itations
With DCP, inorganic and organic analytes may be determ i ned at concentrations as low as 1 0- 5 to 1 0-6 mol L- 1 • When more than one analyte is present in solution the currents are additive and pro vided that their half-wave potentials are separated by at least 1 20 mY they can be resolved and the concentrations of the various species determined. Figure 2.5 shows the polarogram obtained for Pb 2+ , Cd2+ and Zn 2+ ions in admixture in oxygen-free 0 . 1 mol L -I potassium chloride solution. The concentrations of the three m etal ions can readily be determined from the heights of each individual wave in the polarogram since the waves with E'u2 values of -0.40 V, -0.60 V and - 1 .00 V ( relative to the SCE ) respectively are well separated. The d . c. polarogram of an oxygen-free solution of the supporting electrolyte alone consists of three parts. When the working electrode is anodically polarised, a large anodic current is observed. As the potential is scanned towards more negative values, the anodic current rapidly rises to a small value. Then follows a potential region where the small current increases gradually passing through i 0 to become a small cathodic current, until at a sufficiently negative potential a steeply rising cathodic current develops. The large anodic current is due to the anodic dissolu tion of the mercury of the electrode and the potential at which it occurs is determined by the anion of the supporting electrolyte, whi l e the cathodic current arises from the reduction of the cation of the supporting electrolyte. Between these two limits is the polarograph ically usejiJI pote n tial range of that parti cular supporting electrolyte-solvent system and the small current that t1ows in this potential range is called the residual current, i,« ( Figures 1 .7 and 2 . 5 ) . The useful polaro graphic potential range of some common supporting electrolytes for the DME in various media are listed in Table 2. 1 . The residual current is important because it is the 'noise' level and as such determines the sensitivi ty of the polarographic measurement. The residual current is the sum of the capacitance or charging current, i,, ( § 1 .4.3 ) and the faradaic currents produced by the reduction or oxidation of electroactive impurities. The latter contribution is made insignificant by using high purity solvents, high purity salts as supporting electrolytes, and removing oxygen from the system by passing high purity nitrogen or argon =
Techniques
29
i cath
-----
-0
.
2
- 0.6
Figure 2.5
_ _ ....
/
/
/
I
I
Residual c u rrent
- 1 .0
E [V] (vs. Ag/AgCI,
M e rc u ry dissolution
----
/
- 1 .4
3 M
KCI)
i an
D.c. polarogram for the reduction of a mixture of Pb2 + ( £1 1 -0.40 V), C d 2 + ( £1 12 -0.60 V) and 2 - 1 . 00 V) in 0.1 mol L-1 KCI. Potentials are relative to the saturated calomel electrode. zn 2 + ( £1 1 2 The polarogram h a s been displaced vertical ly for clarity. The potentia l at which t h e residual current is zero, Em• occurs at ca. -0.3 to -0.4 V, i .e . at the foot of the Pb2 + wave. =
=
=
through the cell an d solution pri o r to, and over the solution wh il e , re co r di n g t h e p o l a ro gra m . Oxygen is a nuisance in polarographic s tu d ies and m ust be removed from the cell so lu t io n because it is redu c e d in two steps, first to H202 and then to hydroxide ions. In air-saturated 0. 1 M KCl solution, th e two waves each have a h e igh t of ca. l .9 1-1a and have half-wave potentials of -0.05 V a nd -0.85 V (versus SCE ) , respectively. On t h e other h a n d , exce p t at the potential of zero cha rge, Em, t h a t is the p o t e nt i a l rel ative to the reference electrode at which the c ha r g e on the electrode is zero ( F i g u r e I . 7) s o that i, 0, th e c a p ac i t ive current will always have a finite v a l u e . When the contri b u t i o n from c l e c troactive impurities is insignificant, iros can be iden tified with ic ( Note: I t i s only when the contribution from impurities is z er o at E111 that the p o t en t i a l at w h i c h i,•• 0 will be Em. ) The capacitive c u rr e nt depends on the electrode p o t e n t i a l and the rate o f change of the area of the dro pping mercury electrode. It i s given approximately by equation 1 . 1 5 which at constant po t e nt i al b e c o m e s : =
=
(2.27) O n combining e q ua t ion s 2.2 a n d 2 . 2 7 one ob t a i ns : .
lc
( t)
=
0 . 5 6·s cdl m 213 t-11 3 ( E' m - EDME )
( 2.28)
In a quantitative polarographic measurement, the a n aly t ic all y relevant p a r t of the c u rr en t is the diffusion controlled fa r a d a i c current, ir ( id ) , arising from the ch arge transfer reaction o f t h e analyte. The c ap ac itive current, i,, is the in terference or noise signal (§ 1 .4 . 3 ) . A c om p a r is o n of equatio ns 2.4 and 2.28 sh ows that d u rin g drop g rowth , iF an d ( cha n ge i n o p posi te directions, iF =
30
Introduction to Voltammetric Analysis
Table 2 . 1
Polarographically useful potential ranges o f some supporting electrolyte - solvent systems.
Solvent
Anodic limit
Cathodic limit
l .O M KCl
+ 0. 1
-1.9
l . O M HCl
+ 0.1
-1 .3
-0.4
- 1 .0
Electrolyte
( Vvs SCE)
( Vvs SCE)
Water
1 2 M HCI * I . O M NaOH
0.9 M NaF
1 . 0 M N H 1 + 1 .0 M NH4Cl l . O M LiOH
1 .0 M HCI04
0 . 5 M Na 2Tartrate ( pH 9) l .O M KCN
*
-0. 1 5
+ 0. 2
-1.85 -
1 .85
-0. 1
- 1 .9
-0. 1 5
-2. 1
+ 0.4
0 -0 .6
-1.3 - 1 .85
- 1 .9
0 . 5 M Na2S04
+ 0 .3
0. 1 M Et4NOH
-0. 1
-2.4
+ 0.8
-1 .9
- 1 .85
Dichloromethane n-Bu4NCI04 n - B u4NI
-o.s
Acetonitrile NaCI04
+ 0.6
Et4Ncto.
+ 0.6
NaCI04
+ 0.5
Et4NCI04
+ 0. 5
n - Bu.1CI04
+ 0.5
- 1 .7
- 1 .7 -
2.8
D i methylformam ide
•
From
[ reference 1 0 ) .
-2 .0
-3 .0
-3 .0
increasing with time while ic decreases with time as shown in Figu re 2.6. With decreasing analyte concentration, iF becomes smaller until ( for the case of a 4 s drop time) when if < 2 ic it becomes no longer visually detectable. Note that the small amounts of oxygen and any trace impurities that may be present in the cell solution will not be noticed because their combined faradaic currents are smaller than i,. The iF : i, signal- to- noise r a tio is the determining factor for the sensitivity ofDCP. If more sensitive polarographic determinations are to be performed, this ratio must be increased by using electrical circuitry or techniques in which either the faradaic signal is enhanced or the charging current is minimised or where both can be achieved. Inspection of Figure 2 . 6 shows that if the current is not measured over the whole drop life as in normal DCP, but over a short period of time towards the end of the life of a drop when the rate of increase of surface area is least, then the current averaged over the sampling interval, tm, should have an improved iF : ic ratio. This is the basis of one of the earlier attempts to improve the signal to- noise ratio, namely, tast or sampled polarography using a linear potential scan [ l l ] . In this method, the current is read over the sampling interval (typically 5-20 ms ) , recorded, and 'held' by the circuitry until the next current sample is taken towards the end of the subsequent drop. The result, shown in Figure 2 . l b , is an 'envelope' of a normal d.c. polarogram without the usual oscil lations and is described by the same equations, 2.4 and 2. 1 5, and so on. There are two developments which have resulted in a major reduction in the charging current contribution to the sampled current. The first is the replacement of the DME with the static
Techn iques
31
D rop fall
I -c ! ..
u :I
measu rement t i m e
T i me
Figure 2.6
�-- D rop l ife -----.!
The va riation of iF and i, during the l ife of the drop. In tast polarography the current is sampled over the me a s u rem e n t interva l, tm, nea r the end of the drop life where the ratio iF : ( is a maximum .
mercury drop electrode, SMDE.
In this electrode the mercury drop is extruded rapidly over 40 to
200 ms and its growth then stopped, after which the area of the mercury electrode rem ains con
stant. The area-time plot for the SMDE and a typical s am p ling pe riod is sh own in Figure 2 .7a. This el ec t rod e h as the advantage of the constant area of a solid electrode with the lower charging current baseline while still retaining all the advantages of the DME such as a renewable clean sur face b ein g ge n e ra te d with each d ro p . An other advantage of thi s electrode is that the voltage range can be covered mo re rapidly since the several second wait for each new drop of the DME to grow to a suitable size has been replaced by the b rief extrusion period. The second has come with the introduction of modern digitally controlled instruments. The linear d.c. potential ramp applied to the working electrode has been replaced with a stepped or so-called staircase vo ltag e waveform in which the applied vo ltage is ch an ged in s m a ll discrete steps at re gular intervals ( Figure 2 . 7 b ) . Th e voltage steps are syn c hro n i sed with the dro p formation and the current is usually sampled over the last 5 to 20 ms of the drop life. Since the voltage is constant (a E/a t 0) when the current is being sampled, there is no contribution to the ch a rging current from this source (first term of equation 1 . 1 5 ) . Because the current i s now measured at constant potential and con stan t electrode surface area, a major reduction in charging current is achieved resulting i n an i m p rovem e n t in sensitivity over DCP to ca. 5 x 1 0- 7 mol L- 1 • However there is no im p ro vement over DCP in the resolution of adj a ce nt w ave s . =
2.2 AMPEROMETRY
The d i rect proportionality between th e an alyte concentration and the current that flows when the potential of the working electrode is h eld constant at a value in the limiting current regi on of the analyte ( Figure 1 . 5a, region 3) is the basis of the technique kn o wn as amperometry. Amperometry, which might be c on s i de red a s vo l ta m me t ry at constant potential, has been utilised in a number of ways. First, in amperometric titrations as an i ndi c ato r to m o nitor the course of the titration. In this app l i cat i on e i t he r the a n a lyte, A, or the titrant, T, or both must be electroactive. The working
32
Introduction to Voltammetric Analysis
a
I -I I
I
:-
D rop life
E
Initial voltage
' ' ' ' -+-:
: ,
tstep
:'
- - - - - - - - - - - - - - - 1'
' ' ' :+--
b
c
Figure 2 . 7
-E
The variation of surface area with time for the static mercury drop electrode, SMDE, (a) and the potential-time profile for the stepped or stai rcase voltage applied to the worki n g electrode (b) and the resu ltant polarographic wave (c). tm, measuring interval; �rep• time interval of the voltage step (= to): !1 f,1•P' voltage step of the applied stai rcase waveform (adapted from [ 1 2]).
electrode is normally a rotating platin um or carbon electrode. The current signal is used to follow the disappearance of the analyte during, or the appearance of excess titrant o n completion of, the chemical reaction used for the titration . A
+
T � Products
(This may be either a precipitation, complexation or redox reaction . ) A plot of current versus titrant volume will give two linear sections and the end point of the titration is the volume corre
sponding to the intersection of the extrapol ation of these two linear sections.
Techniques
33
Second, two current-carrying working electrodes, usually platinum wire or sheet, may be used in a technique referred to as biamperometry or dead stop titrations. The current flowing through the two electrodes (one an anode and the other a cathode) is the same. A plot of current versus titrant volume is of the same form as that for an amperometric titration and the current at the end point is a minimum. An important biamperometric method is the Karl Fischer titration for the determination of low levels of water in organic solvents. The titration is carried out in anhydrous methanol. The titrant is a methanolic solution of pyridine, iodine and sulphur dioxide. The reac tion involved is
(2.29) so that each mole of water reacts with one mole of l2• Since l2 is the only species that can be reduced at the cathode a sudden flow of current occurs when, on reaching the end point, the first excess of l2 is added to the solution. Third, the proportionality between the current and concentration is used to determine the analyte concentration directly after calibration using standard solutions. The determination of oxygen in a variety of waters and biological fluids using the Clarke cell ( Figure 6. 1 7) is an example of direct amperometry. The probe has a gas-permeable membrane separating the test solution from an inner electrolyte solution. A fixed potential is applied to the two electrodes, usually a plat inurn wire cathode and a silver chloride coated silver wire anode, in the inner electrolyte solution. Oxygen diffuses through the membrane and is reduced at the platinum cathode. The fourth and increasingly important application of amperometry discussed in chapter 5 is in the detection of analytes in flowing liqu ids after they have been chromatographically separated from the matrix and other analytes in the original sample. Detectors based on hydrodynamic amperometry using flow-through cells are important in high performance liquid chromatogra phy. Applications of amperometry are given in chapters 6 and 7.
2.3 LINEAR SWEEP AND CYCLIC VOLTAMMETRY
Another approach to measuring the faradaic current under conditions where the ip : i, ratio is increased is to apply a rapid potential scan to the DME over the last seconds of the drop life when the rate of increase of area and hence i, is lowest. The potential profile is shown in Figure 2.8a. In this technique, referred to as single sweep polarography, SSP, a current peak is observed in the i-E plot rather than a wave for the reasons outlined in section 1 .4. In developing the theoretical description of the current peaks it was assumed that because of the rapid potential scan rate (v = SO to 500 mV s- 1 ) , the DME behaved as a stationary electrode over the duration of the scan (0.5 to I second) . However, this was not a good assumption. For example, if the fast potential scan is applied at 3 seconds after drop birth, the electrode area increases by approximately 20% during the next second. This results in a corresponding increase in the charging current above that required to change the potential during the period of the scan (equation 1 . 1 5 ) . The result is that SSP has a base line with a significantly large curving slope (Figure 2 . 8a) which limits its analytical applicability. With the introduction of the SMDE, the HMDE and the use of solid electrodes all of which have a constant area during the voltage sweep, SSP has been superseded by linear sweep voltam me try, LSV. For the case of a reversible process, the peak current in a linear sweep voltammogram is given by the Randles-Sevcik equation [ 4, 1 3- 1 5 ] ( 2.30)
Introduction to Voltammetric Analysis
34
a
t
b
t
-E
-E
icath
-
Figure 2 . 8
where
--
�
- - -
-
---
-E ian -E
, .- s7 mV I
I
Ep[ox]
Linear sweep (a) and cyclic voltammetry (b). Top: variation of the potential applied to the working electrode with time. Bottom: resulting current-potential curves .
k contains all the physical and mathematical constants and is constant at a given temper ature, Dux is the diffusion coefficient of the analyte in its oxidised form, 1 v is the potential scan rate in V s- , and the other symbols have their usual meaning.
This equation also applies for an oxidation except that ( ip )an is negative. The peak potential, EP, is independent of scan rate and is related to the DCP half-wave poten tial, 1!112 for a cathodic process, by [ 5 ] (2.3 I a)
Tech niques
35
and for an anodic process, by . mv at 2soc E r112 + -28 5 n
while th e hal f- peak potential process by
( Epdcath
EP12, i.e. th e potential at which i
=
E�1 2 +
1 .09 R T
nF
=
=
(2.3 1b)
ip/2' is rela ted to l! 112 for a ca th o d ic
E�12 + 2 8 · 0 mV at 25°C n
(2. 3 1 c)
and for an a no d i c pro c ess, by 0 Er1 12 - -2 8 . m v at 2soc
n
( 2 . 3 1 d)
The differe nce between EP and EP 12 is th us 56.5/ n mV for a reversible process at 25°C. Th e p eak current is considerably larger than the diffusion current for the s a m e system because the rapid ra te of ch a n ge i n vol ta g e means that th e diffusion layer is much th i nne r and the concen tration grad ien t correspondingly greater. The an alyt i c a l detection limit is m uch im p roved over DCP being about 5 X 1 0-7 m o l L- 1 • In addition, since LSV p r o d u ces peaks in the i-E p l o ts , better resolution is obtained a n d p e a ks separated by as l itt le as 50 mY can be r ea d i ly resolved. The peak current increases with the square root o f th e s c an rate , v , and so greater p eak cu rr e n t s a n d h ence greater sensitivity might be e xp e ct e d a t h i g he r scan r a t es . However the charging current increases linearly with v ( equat i o n 1 . 1 5 ) , that is at a greater rate tha n does th e p e ak current. In o rder 112 to optimise the ir: ic r ati o in LSV, sc an rates are l im i t e d to the range wh ere v > v, that is to v < 1 .0 V s-1 • For an irreversible electrode reaction, both the ch arg e transfer coefficient and the rate con stant of the c h arge transfer reaction influence the peak c ur re n t which is given by: ( 2.32) where
n.
is the number of electrons involved in the rate determining ch arge transfer step ( n. � n) , k ' c on t a i n s all t h e mathematical and physi c al constants and is a fu nc tion of t h e rate con stant of the e l ec t r ode reaction, and the other sym b o l s h ave their usual meaning.
The reader is refe r re d to referen ces 4, 5 or 15 for a detai led discussion. In su m m a ry, irreversibility decreases iP. For exa m pl e , for an irr evers ib le one-electron process wi th a. 0.5, (�);,..)Cip),• • 0 . 78 [ 5 ] . This con trasts with DCP where the l imiting diffusion current, iu, is not affected by the reversibili ty of the electrode process. Because of the dependence of current h eigh t on the reversibility of the e lectrode process, care m ust b e t ake n wh en us i n g LSV for analytica l purposes. Solution conditions and electrode s urfa ces must be reproduced as closely as possible for the sample and standard. Also, for an irreversible process t he ma gnit u d e of the difference between El/2 a nd EP is a function of scan rate and the rate constant of th e electrode reaction. The difference i ncr ea s e s with increasing scan rate and d ec reas i n g values of k,ed (or k0x) . T hi s results in a s p read i n g of the current peak shape so that as the el ect ro de process becomes more irreversible, t h e resolution between peaks decreases. Experimental c o ndi tio n s should be adjus ted so t hat the electrode process is as close to =
=
36
I ntroduction to Voltammetric Analysis
c
b
A
-E
a
ii
8
b
Figure
2.9
-E
[V]
-E
[V]
(A) Cycl ic voltammog rams for a reversible (a), a quasi -reversible (b) and a n i rreve rs ibl e (c), electrode p rocess . (B) T he effect of (i) the number of ele ct ro ns involved, and (ii) t he reversibility, on the sh a pe of a l i near sweep voltammogra m . I n cre a si n g i rreversibil ity (a) � (c) .
reversible as can be achieved when using fast scan methods for voltammetric a nalysi s The effects of reversibility and n o n the shap e of a linear swe ep volt amm o gram are shown in Figure 2.9B. If the p otent i al is first scanned in one direction ( negative) as in LSV and then returned to the initial p otenti al by a scan at the same rate in the o pp o s i te direction (positive ) , the technique is known as cyclic voltammetry, CV. F i gure 2 .8 compares the variation of potential wi th time and the responses fo r LSV a n d CV. Cyclic votammetry is used mainly for studying the mechanisms and reversibility of elec t r o de p rocesses and only sometimes used for a n alytica l purposes. The current-voltage cycle illustrated i n Figure 2 . 8b is obtained when the reaction products, formed during th e forward ( cathodic) s ca n as a result of the electrode reaction that gives rise to the peak at ( Ep ) calh' are oxidised on the revers e scan as shown by the peak at ( Ep )an · From equ ations 2 . 3 l a an d b, for a reversible redo x process at 25°C: .
(2.33)
Tech n iques
37
1
E p [red]
i cath
- E
2
Ep[ ox]
ian Figure
2. 1 0
1
E p [ox]
Cyclic voltam mogram for a n electrode reaction involvi ng a n E C mechanism i n which a reversible charge transfer step is followed by a chemical reaction to give a second electroactive species.
a n d LSV, the p o sit i o n of th e current peak is i nd ep e nd en t of the vo ltag e s ca n r a t e , v . In a d di t io n the ca t h od i c and anodic peak c u rre n t s are e qu a l . For quasi-reversible s ys t e m s , that is t h o s e which appear reversible in DCP but give an irrevers ible response when faster m easuring t ec hn i qu e s are u s e d , the difference in the p e ak potentials is about (60/ n) mV at low s can rates of 10 to 20 mV s-1, b ut becomes greater as the scan r a te is increased. Also, tlt.� i n c re a s e s as th e irreversibilty of th e electrode p rocess i n cre a s e s , that is as kred and as for SSP
and k0, ( equations 1 .3 , 1 .4) decrease.
For a totally i rr eve r s i b l e process, that is o n e in which the electrode r e act i o n pro d u ct s cannot be oxidised b ac k to the initial a n alyt e because kox is ext r e m el y small or because th ey have u n d e r gone a subsequent i r r eve rs ible chemical c han ge to compounds that are no t electroactive, no anodic c u r re nt peak is observed ( F i g u re 2.9A ) . F o r th e case where a reversible c ha rge transfer i s fo ll o we d b y a c he m i c a l reaction in which electroactive p rod uc ts are formed, a CV of the type shown in Fi gure 2 . 1 0 may b e obtained . In the first cat h o di c sweep the peak at ( E/red is observed. On the reve r s e (oxidation) s c a n a p e a k is observed at (t.� ) 10,. However, a l t h o u g h th e peak sep a r at i o n is the p re d i c t e d 57/n mV for a reve rs ible e lec t r o d e process, the ratio of the peak heights is n ot as e x p ec t ed . Rather ( iP) 1 ox i s smaller than ( iP) red b e c au s e part of the electrolysis pro d u c t has been c h em i c al ly converted to co m p o u n d 2 and is therefore no l ong e r available for oxidation a t ( E/ ox· The ratio of t h e s e peak h e i gh t s de pe n ds on the rates of the s ub s eq u e n t chemical reactions and is a fun c t io n of these ra t e constants and the p ote n t i a l scan ra t e . If th e anodic scan is c o n t i n u e d a second oxid at i o n peak a ppe a r s at (E/ox if the pro duct of th e following c h em ical reacti o n is electroactive. If a fte r the second anodic peak the po t e n t i al scanning c o n t in ue s into a s ec o n d cathodic sweep, then a new c a thod ic peak appears at ( E/ red due to the reduction of the oxidised product of th e chemical step that fol l ow e d the initial reduction { at ( EP ) 1 red } . If the s ec o n d redox system is r ev e r sib l e then these two p e aks will also be separated by 57/ n mV. Cycl ic voltammetry gives inform ation on the re do x behaviour of electrochemically active spe cies, on the kinetics of el ectr o de reactions, an d in many cases enables the identity of re ac t i ve inter mediates an d subseq u e n t r e ac t i o n p r o d uc ts to be ascertained. CV also g ives q u al i ta t ive
1
1: l
38
I ntroduction to Voltammetric Analysis
t
...
t
Principle of pulse methods showing the decay of the faradaic, iF, and capacitive, ic, currents fol lowing the application of a potential pulse, A fpl• to the working electrode.
Figure 2 . 1 1
indications of more complex electrode processes incorporating adsorption and as such i s a valu able diagnostic tool in the development of analytical procedures.
2.4 PULSE TECHNIQUES Following on from his earlier work with square-wave polarography, SWP, in which a small a mpli tude alternating sq uare-wave voltage was superimposed on the slow d.c. potential ramp, Barker [ 1 6 ] found that applying a short duration rectangular or square potential pulse to the WE also proved to be a very effective means of reducing the unwanted capacitive (noise) current and achieving a marked improvement in the sensitivity of polarographic and voltammetric determi nations. A number of polarographic and voltammetric methods have been developed from the original SWP [ 1 7 ] , however only the more widely used techniques will be presented here. In these techniques a square-wave potential pulse (or alternating square-wave voltage in SWP) with either a constant or an increasing amplitude, is periodically applied to the working elec trode. Following the initial potential step at the beginning of the pulse, the faradaic and capacitive currents that flow in response to this potential change decay in different ways. The faradaic current, 12 ip decreases with t- 1 in accordance with the Cottrell equation (equation 1 . 1 0 ), whi le the capacitive current, (, decreases according to the relationship for charging a capacitor:
AEP1,
.
where
R
l
c
_
(AEP1) [ J
- R
exp
-t
R Cd (
is the ohmic resistance of the polarographic circuit, and 1 Cd 1 is the double layer capacitance of the working electrode, WE. *
(2.34 )
Techniques
39
The variations of iF and i, over the duration, tpt• of the square pulse, A Ept• are illustrated in Figure 2 . 1 1 If the current is measured towards the end of the pulse, essentially only ir is recorded since ic has decayed effectively to zero. Methods based on the application of square voltage pulses to the working electrode use pulses of differing frequen cies and magnitudes and utilise a range of measurement techniques. Pulse methods may be used with the whole range of working electrode types, the DME, the HMDE and the SMDE, as well as with solid electrodes and the different types of modified electrodes. Except for the case of SWP, normally only one pulse is applied to each drop in analytical studies with the DME. This pulse is synchronised with the drop period and is applied in each case towards the end of the drop life. 2.4. 1 Square-wave polarogra p hy In the original square-wave polarographic method, the rectangular alternating voltage superim posed on the linear d. c. ramp potential usually has a frequency of 225 Hz and an amplitude in the range AEsq 5-30 mV. The current is sampled over a measurement interval, tm, near the end of each square wave half cycle. This current is then rectified, amplified and recorded as a function of the applied d.c. voltage. Sampling the current near the end of each half cycle ensures that the charging current arising from th e sudden step in potential at the beginning of the half cycle has decayed away and does not contribute to the measured current. However when using the DM E , there is still a charging current flowing during tm as a result of the slight in crease in area of the mercury electrode during tm ( equation 1 . 1 5) . This is minimised by ( i ) measuring the current towards the end of the drop life when the rate of increase in electrode area is least, and ( ii) by using a tilted square wave as shown in Figure 2 . 1 2 . The p ulse tilts are omitted if a static mercury drop electrode with its constant surface area is used instead of the DME. A further reduction in the contribution of charging current can be achieved in modern instru ments with digital control when the linear voltage ramp is replaced by a stepped voltage ramp (staircase waveform ) . The square-wave pulses can then be applied to the plateau (constant volt age) region of the staircase waveform. The current-potential plots obtained in SWP are peak-shaped ( Figure 2 . 1 3 ) with the peak potential corresponding to the half-wave potential of DC P . For reversible electrode processes, the peak current is dependent on the amplitude and frequency of the superimposed rectangular volt age and is given by [ 5 ] : =
(2.35)
where k i s constant a t a given frequency. The square-wave polarograms shown in Figure 2 . 1 3 highlight the influence o f n and A�,q o n the height and width o f the peaks. For reversible pro cesses, sensitivities of the order of 1 0-8 mol L- I can b e achieved with SWP and peak resolution is 40-50 mV. The peak height and hence the sensitivity decreases rapidly with increasing irrevers ibility of the electrode process. 2.4.2 Square-wave voltammetry The characteristic feature of the technique described by Osteryoung [ 1 8) and known as square wave voltammetry, SWV, is that the whole measurem ent process can be carried out using a very rapid voltage scan rate. The method thus enables very short analysis times to be achieved and is
The product RC of a circuit comprising a resistance, R, and a capacitance, C, in series is known as t h e t i me constant of that circuit, t,. S ince after the potential is stepped in such a circ u i t ( will decay i n an exponential manner ( equa tion 2.34), and after 12 t, seconds it will be only 6 x 1 0-4% of ils initial value. The time constant of a typical cell with an electrolyte concentration of 1 mol L-1 and a mercury drop work ing electrode is - 40 IJS so that 12 t, is reached in - 0.5 ms, which i s much less than the duration of the p ulses used in practice.
40
- �- -
Introduction to Voltam metric Analysis
-
i
�E
�J p i
1
t
i
I
� � tm
Figure 2 . 1 2
a
b
c
iF
I
I
I
I
I
I
I
c
I
�! !+---
�! !+---
�! �
tm
tm
tm
Alternating square-wave voltage and current transients in square-wave polarography: (a) waveform with pulse tilt, (b) capacitive current, (, (c) faradaic current iF . � measuring interval. =
X A6
- E
Fig ure 2 . 1 3
Square-wave polarography. The dependence of the peak height on I'>.E,q, the amplitude of the square-wave voltage, and on n, the number of electrons involved in the electrode reaction. (1 )-(3) n = 1 (thallium); (4)-(6) n 3 (bismuth); (1 ) and (4) M,q 40 m V; (2) and (5) M,q 20 mV; (3) and (6) l'>. f,q 1 0 mV. =
=
=
=
Techniques
41
Meas u ring point 1
_j
_ _
� Esq
-1--T �I 1
I
1+--
I
tstep
Figure 2 . 1 4
Meas u ri n g po i nt 2
Potential-time profile and current sampling program i n square-wave voltammetry a s proposed by Osteryoung [1 8] . The duration of each square-wave cycle, �. is the same as that of each voltage step of the stai rcase waveform ( -) . �Este = voltage step of the staircase waveform, p �Esq amp l i t u de of the supe r imposed square-wave voltage ( -). The current is measured at ti me s 1 and 2 . =
particularly important as a detection method i n flow systems. In this technique, the potential applied to the working electrode consists of a relatively large rectangular wave of amplitude .1.E,q = -50 mV being superimposed onto a stepped voltage ramp (staircase) with voltage steps of -10 mV ( for the case of a cathodic scan ) . The duration, t,q, of the rectangular wave cycle is equal to that of the staircase voltage steps and is usually within the range of 5-1 0 ms. figure 2. 1 4 illus trates the voltage-time profile applied to the electrode and the current sampling times within this profile. Two values of current are measured in each rectangular pulse cycle; one at point 1 at the positive end of the pulse and one at point 2 at the negative end of the pulse. The difference between the two current values, Lli ( i2 - i1 ) , is plotted against the potential of the working elec trode and gives a peak shaped current as in SWP. The pulse times being in the millisecond range enable the potential to be scanned very rapidly at rates up to 1 200 m V s- 1 • Only one mercury drop is required for each measurement sequence. Because of the rapidity of the measurement, irrevers ible processes do not give a significant current signal. This enables measurements to be made in the presence of oxygen. This is of particular advantage when making rapid measurements in flow ing systems as it is often difficult to remove oxygen from such systems. However the rapid analy ses are achieved at the expense of sensitivity, since the use of short pulse times results in a lower iF to i, ratio. An alternative version of square-wave polarography developed for digitally controlled instru ments is based on Barker's SWP. The linear d.c. potential ramp is replaced by a d.c. staircase potential and a SMDE replaces the DME. (If the DME is replaced by an HMDE or a solid elec trode the technique is square wave voltammetry. ) The drop life is synchronised with each step in the staircase potential. The constant potential of each voltage step is modulated with a small amplitude, L1E,q, alternating square-wave voltage of frequency f.q, toward the end of the step as shown in Figure 2. 1 5 [ 1 2 ] . The current, in I ' inz• is measured over a number of cycles, n, twice in each cycle for a short period, tm P t012, just prior to each polarity change of the square-wave voltage cycle. The average of the differences between in1 and i112 for each measurement cycle: =
42
Introduction to Voltammetric Analysis
I
A o Ill G) li G) u
1
I
I
_. I tm 1 +-
a.. 0 ... "D � ::s
I
I
a
�::1 !::!G)
0 E
I
1
_. I
Meas u ring point 1
Drop life
I 1 +I
t
E
b
Sqm
c
Figure 2 . 1 5
Digitally control led square-wave polarography using the static mercury drop electrode. A number of square-wave cycles is superimposed on each step of the staircase waveform towards the end of the voltage step. (a) The variation of the mercury drop surface area with time. (b) The voltage applied to the electrode as a function of time. (c) The resulting cu rrent-potential curve. � Estep = voltage step of the staircase waveform; � fsq amplitude of the alternating square-wave voltage modulation, sqm, n cycles of frequency fsq; tstep duration of each step (same as the drop life, to); �,. tm 2 , etc, = current measuri ng intervals (only two labeled); (adapted from [1 2]). =
=
(2.36)
is plotted against t he potential of the working electrode to give a peak as in SWP. W ith digital con trol, the parameters can be v a r ie d over a wide range of values. Typical settings are: t,tep t0 0.3 s; �Estep = 6 mV; �Esq = 20 mV; .fsq = 50 Hz; n = 4; tm 1 tm2 2 ms. These settings give a voltage scan =
=
=
=
Techn iques
43
20 mV s- 1 , but m u ch higher scan rates up to 1 000 mV s- 1 can read ily be programmed m a k in g the method ideal for fast voltamm etric detection in fl ow - thro u gh cells. Because of the high fr equen cy of t h e square wave mo d u l a t i on , the technique is only suitable for use with reversible systems. As w it h a.c. polaro gr ap hy ( § 2.5) i rreversi b l e electrode processes are unable to k e e p up with the voltage changes of a 50 Hz cycle (a voltage s tep every 10 ms) . The square wave modulation of the pote n t i al has virtually no effect on the concentration pro fil es established by the d.c. potential at the el ect ro d e interface for an irreversibly r e d uc e d sp e c i es a nd so tli - 0. Ad v an tage can be taken of this fe at u r e when a sample contains two electroactive s p ec i es in its ma t r ix which are reduced at similar p o te n t ia ls . If the e l e c t ro de reaction of one of the species is reve rsi b l e and that of the other irreversible, then the former can be rea d il y detected in the pres ence of the latter which gives no signal in SWV. The state of development and ca p a bilitie s of SWP and SWV have been well reviewed [ 1 9, 20 ) . rate of
2.4.3 Normal pu lse polarography and vo ltammet ry In normal pulse polarography, NPP, the vo l tage of the workin g electrode is c h a n ge d, not by me a n s of a d.c. ram p or staircase vo ltage as in SWP, b ut by a seri es of rectangular pulses of continuously increas ing magnitude superimposed on an i nit i a l c on sta n t vo ltage as sh own in F igur e 2. 1 6b. The application of each p u l se is synchronised with the drop-life of the SMDE and o cc u r s just before the drop is dislo dg e d . Only one voltag e pulse with a duration o f 3 0-60 ms i s a pplied to e ach drop. The am plitude, �EP'' increases from one p u l s e to t h e next up to a maximum of about 1 000 mV. To minimise i,, th e current measurement is made in the i n t e r v al tm at about 10 ms b efo re the completion of the p uls e ( F i gure 2. 1 6a). N ote : �EP1 i s negative for a cathodic and positive for an anodic process. The measure d current is plotted again s t the p u lse am p l i t u d e and the r e s u ltin g i-E plo t ( Figure 2. 16c) i s a wave similar to that obtained in sampled po l aro gra p hy ( F i gure 2. 1 b ) . The height of t he wave is proportional to the analyte concentration, a n d th e h a lf- wave p o t e nti a l c o rre spon d s to that for DCP. Peak - sha p e d curves are o b tain ed when the difference in current between s ucc e ssiv e pulses is p lotte d ag a in st potential. A favourable feature of the normal p ul se te c h n ique is that it can be u sed with solid electrodes, that is it b e c om e s normal pulse voltammetry, without th e p ro b l ems that arise from the ac cu m u latio n of e lec tro d e reac tion p ro d u cts on such electrodes. This is because the electrode pot ent i a l is returned to an initial valu e which is in th e range where t he reverse el e c tro de reaction occurs, so that any electrode re a cti on p roduc ts acc u m u l a ted d u rin g the short pulse will be cleaned from the electrode when it is at the initial pot e nt i a l for the tim e between pulses. S en sitivi t i es are reported down t o 1 o-7 mol L- J and wave resolution is of the or der of 1 00 mV. The s ens i t ivi ty is i m pro ved and the time of ana l ys i s decreased by u si n g a SMDE in place of th e DME. 2.4.4 Differential pu lse polarography a n d voltammetry The most im po rta n t and sensitive of the puls e p ola r ogr a p h i c methods is differential pulse polarog raphy, DPP. In this t e chn ique , a linear ramp ( older i n st r u m ents ) or staircase ( m o de rn instru ments) voltage rise is used ( F igure 2 . 1 7b) an d a rec tan g ul ar vol t a ge p u l s e with a co n s tant amplitude, tlEpl> of 1 0- 1 00 mV is ap p l i ed to each mercury drop (DME or SMDE) j us t prior to the end of its life, tD. T h e pulse d u ra t ion , tpi• is in the range 40-60 ms and drop times of 2-6 s are used. Synchronisation between the d ro p life and time of application of the pulse is a ch i eve d by means of the electronic c ircui try. If an HMDE or a solid el e c t ro d e is used instead of the DME then the te ch nique is differential pulse voltarnmetry, DPV. In DPP, two current measurements are m ade : the first, i" in the measu rement interval, t m P immediately before the a p p l i ca t io n of the vol t age p ul s e and the second, iz in th e interval tm2 n e ar the end of the pulse ( F ig u r e 2. 1 7 ) . Both m eas u reme n t s are made on the same m erc ury drop.
44
I ntroduction to Voltammetric Analysis
tm
I �I 1 I
A
I �I I
Drop life
I 1 +I I
a
I I 1 +I
E
t
f
L
Starting potential
b
6 EP1
t
c
-E
F i g ure 2 . 1 6
Cu rrent sampling progra m (a) and potential-time profile (b) for normal pu lse polarography using a S M DE. Resulting normal pu lse polarogram (c) . 6fP1 = pulse amplitude; tP 1 = pulse duration; tm current sampling interval .
=
( Because of the small pulse duration these are a t almost the same electrode area when using a DME, thus minimising charging current contributions from this source . ) In DPP with the SMDE and DPV with the HMDE the surface area is the same for both measurements. ln both DPP and DPV, the difference, 11i i2 - il' is plotted against the applied d.c. voltage ( ramp or staircase) and gives a peak-shaped polarogram/voltammogram ( Figure 2. 1 7c) as !1i reaches a maximum in the region of R 12• 1 For a reversible process, the peak current is proportional to the an alyte concentration and is given by =
( 2 . 37 )
and the potential at which the peak occurs is (2. 3 8 )
Techn iques tm1
tm2
-+l :.--- : : : : -+j �
A o a.. ca
45
o
I!! ca "C Q) � u :II
a
...
� e m E :II
Q)
'
-+ ;
D rop
l ife
:+' '
f---� - - - - - -
t
b
di
c
Figure 2. 1 7
Drop surface area-time plot and cu rrent meas u ring program (a) potential-time profile for differential pulse polarography (b) using a SMDE and the resulting differential pulse polarogram (b) using a SMDE and the resulting differential pu lse polarogram (c) tm1 and tm2 cu rrent sampling i ntervals; tP1 = pulse d urati on ; tstep staircase voltage step duration ( drop life time); dfstep voltage step of the staircase waveform and the d£P1 = pulse amplitude; EP peak potential . =
=
=
=
=
The peak current also de p e nds on the amplitude o f the voltage pulse, �B i • ( F igure 2. 1 8 ) and p pulse duration, tri· However, not only does the p ea k height increase with the p ulse a mp l i tude so a lso does the peak wi d th , thus decreasing the resolution which is undesirable. For small values of �EP1, the peak half-width, th at is the width at i i/2, is 90.4/ n mV but for larger p ul s e amplitudes it approaches the pu lse amplitude, � EP1• The peak p ote n tia l is also dependent on the pulse amplitude, being sh ifted in a positive direction for a reduc t i o n process (�£ 1 i s n e g a t iv e for 11 a reduction). on the
=
46
Introduction to Voltam metric Analysis
I
Cd
50 nA
2 1
- 0.3 - 0 .4 Figure 2 . 1 8
- 0 .5 - 0 . 6 - 0 . 7 - 0. 8
E [V] (vs. Ag/AgCI , 3 M KCI) I nfluence of the pulse ampl itude o n the differentia l pu lse polarograms of Pb(l l) and Cd(ll) 1 0 mV, (2) MP 1 -2 5 mV and (1 0 mg L-1 ) i n acetic acid solution at pH 2 .8. (1 ) MP 1
(3) MP1
= -
=
=
-75 mV.
Determinations by DPP or DPV are in general ten times more sensitive than determinations with NPP with the limit of determination for the differential pulse methods being about 1 0-8 mol L- I for a reversible process. For an irreversible process it falls off only slightly to 5 x 1 0-8 mol L- 1 • The loss of sensitivity due to irreversibility is thus smaller for differential pulse methods than it is for other pulse methods. On the other hand, catalytic and other complications of the electrode process which lead to non-linear iP versus c. plots in DCP have a similar effect in both the normal and differential pulse methods. A review of developments in p ulse voltammetry is given in reference 2 1 .
2 . 5 ALTERNATING CURRENT POLAROGRAPHY AND VOLTAMMETRY When the slow linear potential scan ( analogue instruments) or staircase potential ( digital instruments) applied to a SDME or DME is modulated with a small amplitude sinusoidal voltage (!'.E_ = 5-2 0 mV) of low frequency ( 5-500 Hz) then the technique is called alternating wrrent polarography, ACP. After filtering out the d.c. component of the current, the alternating current, i_, is plotted against the d.c. voltage, £d e> to give the peak-shaped a.c. polarogram as shown in Figure 2 . 1 9. Figure 2 . 1 9 shows diagrammatically why the p l o t of i_ versus Edc is peak- shaped with a maximum value ( iJ P at £ 1 12, the d.c. half-wave potential. ( With a DME the peak will have the oscillation s due to the drop growth and fal l . ) In digital instruments, /'.E_ is applied towards the end of each voltage step ( Figure 2 . 20 ) . I n a.c. polarography, the concentration profile at the electrode surface established b y the d.c. potential is modulated by the small a.c. voltage. For a simple reaction the surface concentrations of the reduced and oxidised forms of an an alyte are the sam e at £ 1 12 • These concentrations fluctu ate in response to the a.c. voltage modulation-in the positive half-cycle the oxidised form increases and in the negative h alf-cycle it is the reduced form which increases. When Edc £1 12, =
Techniques
47
i_
i_
- Ed. c. Figure 2 . 1 9
Waveform a nd response in a.c. polarography. !::!. £_ peak t o peak ampl itude o f t h e a .c. modulation voltage; i_ a.c. current; EP_ d.c. potentia l of the a . c. current peak; b 1 12 peak half-width {when ;_ = V2).
these concentration changes are gre a t e st so t h a t i_ ;which refle c ts these c h ange s , has i ts m aximum
value at this p o t en t i al .
If a so lid electrode or HMDE is u s ed instead of the DME, the resultant a.c. vo lt am mogra m has essentially the same s h a p e as the a.c. p o l aro gra m obtained with the DME but without the oscilla tions due to drop gr o wt h and fall. An alternative way of prese n ti n g the information that may be obtained by mo dula t in g the d.c. potential wi t h a small a.c. vo lt age is to plot th e cell i mp e d a n ce as a function of p o t e n t ial . This latter approach is val uable when i nfo r m atio n about the kinetics of the electrode process is being sought b u t as it i s experimentally tedious it is not used a s an an a lyt ical tool. Very good accounts of ACP wh i ch give a full discussion o f this and related t e c hn i qu es are presented el sewh e r e [ 22 , 23, 24, 25 ] . Because an electrochemical cell is a c o m p l e x, non -linear component in an electric circuit, the current response obtained in ACP has, in ad dit i on to the fundamental harmonic current, iw of th e same frequency, m, as the a pp l i e d a . c . v o lt a ge , c o m po n e n t s at fre q u e n c i es that are integral multi ples of the fundamental fr e qu e n cy ( i2ro, i3ro, ) as well as at zero fre q u e n cy (d. c. current) . These • • •
48
I ntroduction to Voltammetric Analysis
-.I 1 I I
A
tm
I 1+-
I I
a
-.:
'
D rop life
t
: +I
Meas u ring
E
I
Starting
- - - - - -
potential
I
I I I
+-
p e riod
b
�Estep
- - ·
t
c
Figure 2 . 20
- E dc Variation of electrode su rface area with time and current measuring program (a), potential-time profile (b ) and the resulting a.c. polarogram (c) for a.c. polarography using a staircase voltage with the SMDE. � = current sampling period, acm a .c. voltage modulation with a peak to peak potential of b.E_; � Estep and t,1•P the voltage step and step duration of the staircase voltage; EP_ d. c. potential of the a .c . polarographic peak. =
=
=
components c a n be s e p a r ated by mean s of tuned ci rcuits in the measurin g equipment. Funda menta l harmonic ACP is the most common techni que used, although phase selective second har monic ( i20) ACP is gaining wider use as suitable instrumentation becomes m o re re ad i l y available. The theory of ACP [ 2 3 ] is d eveloped usi ng the same a pp r o a c h as for DCP with the m odifica t io n that the potential ap pl i e d to the working electrode contains an a.c. term:
(2 .39) where
oo
is the frequen cy of the a.c. component of the applied voltage in rad s- 1 ( oo = 2rcfwhere fis expressed in Hz { cycle s-1 } ) .
Tech n iques
49
reversible electrode process, the peak po t ent i a l of t h e fundamental harmonic a.c. polarogra m is identical with th e h a lf- wave p o t en t i a l of the d. c. p ol a r o gr a m ( EP = I! 1 2 ) . The fu n 1 damental harmonic peak current, ( ioo) , is give n by r 4 1 : p For a
( 2 .40) where
the area of the e l ect ro de , D. is the diffusion coefficient of the analyte, and the other symbols h ave their usual meaning. A is
The width of the peak at i00 = ( i00 )/ 2, the peak half-width, b 1 12, is 90/ n mV at 25°C when m V a n d a plot of ( i,)p ve r s u s w 112 is linear th ro ugh the origin. These are tw o of the c r i te ria wh i c h characterise a reversible electrode process. Since b 12 90/ n mV a n d ( i00) P i s propor 1 tional t o n2, the more electrons involved in t h e electrode reaction, the higher and sharper is the peak in the ACP o r ACV. The continual ch a rgi n g and discharging of the do ub le l ayer b y th e a p pl ied a l tern at ing vo l t a ge results in a high c apa c it i ve c urrent component i_, in ACP. This l i m i t s t h e se n si t i vi t y of t h e tech nique to ab o ut 1 0-5 mo l L-1 • F ro m equation 2.40 it is seen that increasing the frequency w i ll increase the m agn it ud e of ( i00)P s ince the l at t er increases with w 112 • It m i gh t be expected then that increasing the fre qu e n cy would increase the se n s i t iv i ty of A C P . Unfortunately, the chargi ng cur rent i_c, wh ich is given by
dE_ < 8/ n
=
( 2 .4 1 )
a faster rate wit h w ( to the first power) t han does the faradaic current ( i00) P ( to the half so that the faradaic to charging current ratio decreases with increasing frequen cy, leadin g t o a decr eas e i n sen si t ivity. It i s for this reason that in pr a ctice ACP measurements are usually made at frequencies of about 5 0 Hz ( w 3 14. 1 6 ra d s I ) The se n si tivity can be improved by u sing one of two variations on the basic ACP t ec h n i q u e . In the first, phase selective a. c. polarography, PSACP, a dva n t age is taken of the fact that when an alter nating cu r re n t flows t hro u gh a res i s to r , the alternating vo l t age drop across the resistor ( E_ = i_R) is in phase with the current. However, when a n al t e r n at i ng current flows through a c a pa c i to r , E_ is no l on ge r in phase with i_ but lags behind i_ by 90° (rt/2 radian) . The d i ffe r e n c e in phase between the c u r r e n t and voltage i n a n a.c. circuit is called t h e phase angle, 8 (90° i n the above case). In an el ectro c h e m i c al cel l the fa ra d ai c curren·t, i_, is found to lead the a . c . vo lt ag e by 45° (1tl4 radian) or less, while the cha rgi ng current, i_c, leads AE_ by 90°. By using phase-selective detection of th e current and s el ect in g only the i n - p h as e (8 = 0) c om p on e n t of (, the c a p a c i t ive com p on ent b ec omes i nsi g nifican t while the faradaic component is only re du ced at most by a factor of 1 1,[2 . The net result is a m a rk e d increase in sensitivity over the b a s i c ACP by some two , orders of mag ni t u d e to about 1 o-7 mol L_ _ The se c ond va ri a t i on is to use t h e p h a se sel ective s eco n d harmonic a.c. r es po n s e . The m agni tude of the second h a rm o n ic alternating current, i2 w is given by [ 23 ) :
increases at
power),
-
=
i zw
{ =
n3 F A c. ( 2 wD. )
l l2
A £ sinh
.
G) (
sin 2 w t -
�)} ( 2 .42 )
50
Introduction
to
Voltammetric Analysis a
b
t
ip -
� l p :;C I .
Figure
2.2 1
a
l p
.,
l
p
Cu rrent-potential profile and eva l uat io n of phase selective second harmonic a.c. polarograms; (a) for horizontal and ( b) sloping residual currents.
j ( nFI R ] ) ( Edc - E'1 12 ) and the o th er sym b ols have the same s ignifi ca n c e as in e qu a tion 2.40.
where For
.,
I
=
reversi b le process there arc two peaks in th e s e c ond h a r m o nic p o l aro gra m which o cc u r at 34/ n mY and the peak current is given by
E ' 1 12 ±
. ( lzwl p
=
3 ,.,3
112
2
{ n t' A c. ( 2 ro D. ) L1 E_ } 2 ....2 41 .57 R 1
( 2 .43 )
In p h a se selective second harmonic ACP it is t h e total peak current, il'l' = ( i + + iPJ , that is p u s ed for a n a lyt i c a l purposes ( Figure 2 . 2 1 ) . L a rger a m p l itu d es of L1 E_ in the range 25-30 mY, are used because of the de pende nce of ( i2ro)P on L1E_ 2 • Unfortunately th e gain in sensitivity is to s o m e extent countered by t h e peak broadening associated with the l ar ge r values of � E_ . The use of the s ec o n d instead of t h e fundamental harmonic further reduces i_, so that the main s o u rce of noise in th e signal output is often instrumental in o r i gi n . A lt h o ugh ( �o) is m uch smaller than ( iro)p, the p iF to ic rat i o is very favourable a n d the detection limit is lowered to the range 1 0- R to 1 0-7 mo l L- I . Detailed analysis o f the case for a quasi- reversible charge t ran s fer reaction, that is a reaction which is reversible in DCP b u t not in ACP corresponding to reactions wit h k,ed val ues ( e q u atio n 1 .3 ) i n the range 1 0- 2 to 1 em s- 1 , shows that fo r the fundamental harmonic p eak current [ 23 ] :
( iro)p C irolp.rcf( a, kred' Da , ro ) , wh e r e C irol p,re• i s t he value of ( iw)r for the reversible case (equation 2 . 40 ) and F( a, k,ed, D., ro) is a complex function of the four p ara m e t e rs , is less than u n ity and decreases as k,.d decreases; (ii) the p e ak p otenti a l va ri e s wi th ro fo r all values of a other t h an a = 0.5; ( iii) ( i0,) P is no lo nge r a l i ne a r function of ro 1 12 ; and ( iv) the peak is b ro aden e d and no longer symmetrical except when a = 0 . 5 .
(i)
=
Slight changes i n solution composition s uc h as changing the c o ncentr at io n of the supporting electrolyte or the presence of tr a c e s of organic surfactants may have an effect on k,.d an d a, wi th a c o n seq u e n t marked effect on the s h a p e and height of the a. c. p e ak . Therefore the a p p lic atio n of a.c. methods to the determination of a nalyte s whose electrode reactions a re q u asi - r ever s i b l e must be undertaken wi th ca ution. This, when c o n s i d e red with the lower s e n s it ivi ty obtained with
Techniques
51
qu as i revers i b l e systems, means that ACP and ACV are not r e c o m me nde d fo r d e t erm i n in g th e conce n tra tion of anal yt es whose electrode reactions are quasi-reversible. For those a n a lytes whose el e ct ro d e reactions are irreversible, DCP or other te ch n iq ue s should be used sin ce the ACP or ACV s i gn a l is so small that it is of no a n al yt ica l use. This s e le c t ivity with a.c. m ethod s is s i m i l ar to t h at of SWV a nd may be exp lo ited in practice. An a nalyte which is reve rs ib ly reduced or oxidised can be determined in the presence of a large excess of ano t he r com ponent in the test s ol uti o n which i s involved in an irreversible process a t the p ot e n t ia l at wh i c h the analyte re a ct s since the anal yt e giv es a good an a lyt i cal s i gn a l in ACP while t h e i r r evers ib ly reduced component giv e s virtually no a . c . current at a l l . The i n fl u e n ce of oxygen (irreversibly reduced) in the c e l l solution is n owhere ne a r as c rit i cal as i t is in DCP an d DCV a l th o u gh for b e s t results it i s still recommended that so l u ti o n s be deaerated p r i or to making the a.c. measurement. The current in ACP and ACV is more strongly i n fl u e n ce d by t he components of the t e st solu tion such as the s u p p o r ti n g ele c t ro lyt e solvent and surfactants, than is the case in the other polarographic metho ds. The effect of s urfactants becomes m o re noticeable as th e number of elec trons i nvo l ve d in the electrode reaction increases. For exa m pl e even the minute traces of surfa c tants that are present in de i o ni s e d water can interfere with t h e determination of Bi(lll ) , ln(III) and Sb ( I II ) . -
,
,
,
-
2.5.1 Tensa m m etry Tensammetry is a vari an t of ACP (or ACV if a fixed area el e c t ro d e is used ) . In this t e ch n iq ue [ 24 ] , the cap ac it ive current component, i_c, whose value is d et er m ine d by the double l aye r cap a c i t ance (equation 2.4 1 ) is measured ( preferably at a phase angle of 90° ) i nste a d of the faradaic a l ter n a ti n g current, i.p The ad so r p t io n or d e sorp t i o n of s u rfacta nt molecules at the electrode interface causes changes in the d ouble l ayer c apa c i tance and le ad s to two almost sym m et ri ca l c u r re n t p e aks in th e a.c. p o l ar og r a m at th e potentials where the c omp o u nd is adsorbed and desorbed. S uch peaks are referred to as tensam metric peaks and the co mp o u n d is a ds o rbe d in the p o t e n t i a l re g io n between the peaks. An a dso rb e d surfactant m o l e c u l e replaces the small s o l ve n t molecules on t h e surface thereby i ncreasing the width of the do u b l e l aye r and lowering the double l aye r capacitance. As a result, at p ote n t i al s where the c o mp o u nd is adsorbed t h e ca p a cit ive or base cu rre n t is s m al ler than the base current i n a s o l u ti on of the s up p o r ti n g el e ct ro lyt e a lo n e ( F igur e 2.22 ) . A t concentrations o f s u rfac t an t b el ow th at re qu i r e d t o produce a m o n o l ayer on t h e surface, the he ight o f t h e p e a ks d e c re as e s wi t h d e cr e as i n g concentration and the pe aks move closer together ( E+ more negative and E_ m o re positive ) . The depr e s s i o n of the base c u rrent A4,asc> also decreases with de cr ea si ng concentration of t h e surfactant. In favou ra b l e cases, A4,asc may be us ed to d ete rm in e the concentration of the s u r fa cta n t [ 26 ] . The s hift in the p o t entia l of a peak in an a.c. polarogram wi t h the co n cen t r at i on of surfactant in dic � te s that it is a tensammetric peak and i s one of the features t ha t d i stin g u i s h it from a fa r ad a i c peak, the p o te n t i a l of which is independent of analyte concentration. Ten sa mm et r i c pe a ks are obtained in the a.c. p o l a r o gr a m s of both electroactivc and e le c t ro i n active s u rfa c tan t s Th ree main types of peaks are ob s er ved : ( i) those caused by the a dso rp t i o n or desorption of a n electroinactive species, ( ii ) t h o s e in which an ad s or b e d species with an asym m et ric sh a pe u ndergoe s a re or ie n t a ti on on the surface, and (iii) t h o se in which the s orp t ion process is ac co m p a n ie d by an e l e ct r ode reaction of the surfactant, t h at is they ar e mixed tensammetric and faradaic peaks. These can be distinguished by s t u dyi n g the d.c. polarogram obt a i ne d in the ,
.
-
same solution.
When t h ere is no wave i n the d.c. polarogram at the p o te nt i a l of the a. c. p o l a ro g raphi c peak, the a.c. p eak is p urely tensammetric and arises either from the a d s o rptio n or desorption of a species (base l i ne depressed on one side of the p e ak) or fr o m the re-orientation of an adsorbed species (base line depressed on b o t h sides of the peak), which is not clc c t roa c t iv e at that po t e ntial . On the o t h e r hand, when a peak in the a.c. p o l a ro g r a m occurs at the same pot e n t i a l as a wave in
52
I ntroduction to
Vo lta m m etric
Ana lysis
E
�
I�
Potential rahge of adsorption -E
0
Figure 2 . 2 2
[V]
- 1 .0
Tensammogram: a.c. polarogram o f the supporting electrolyte alone (-----) a n d after t h e addition of a surfactant showing the resultant adsorption-desorption or tensammetric peaks and the depression of the basel ine current over the potential region where the surfactant is adsorbed. i_, capacitive current; E+ the anodic and E_ the cathodic potential at which the surfactant is adsorbed or desorbed.
=
=
=
the d.c. pola rogra m and ( i ) th e base current in the a.c. pola rogram is de p r essed on only on e side o f t he pe a k then a c h a rge transfer reaction is ass o c i a te d wit h t h e s o rpti o n process; (ii) there is no d ep r ess i o n of the base c urre nt in t h e a.c. polarogram, th e n there is no s orp t i o n process occurring and the peak is pure l y faradaic in n at ure and (iii) th e base current is depressed on both sides of th e p e a k , t h e n t h e peak i s faradaic and both t h e reduced and oxidised forms of the surfactant are adsorbed. An alyte s for which the electrode process is irreversible and hence are n ot expe c ted to give any s i g n i fi c an t fa ra d a i c p e a k in th e a.c. p olarogram, may gi v e rise to a peak when the irreversible charge transfer process is accompanied by the adsorption of either the oxidi s ed or reduced form of the analyte. The plot of ( U p ve r su s c. has t he c h a r a cte ri sti c s h ap e o f a n a d s o rp t ion isotherm wit h a l i m i t i ng value of ( j_) r wh en the adsorbate reaches a m o n ol aye r coverage of the electrode surface. Such systems are useful for analytical p u rp oses at analyte concentrations wh e r e the ( i_ ) P v ers u s c, plot i s approximately li n e a r th at i s at concentrations below t h a t re qu i re d t o gi ve approx i m a t e ly half-monolayer coverage of th e adsorbate on the e l e ctro de Th e m a in app l i c at i on of t e n s am m e t ry is to the characterisation of adsorption phenomena which may i n ter fe re with the vo l ta mm et ri c determination of an a n a lyt e (§ 1 .4) an d fo r the analy sis of some el ectro inactive su rfa c tan ts The detection limit in these cases is abou t 1 0-6 mol L- 1 and is dep en d e n t on drop time. The sensitivi ty can be improved by u sing a stat i on a ry e l ec t r o de . ,
,
,
.
.
2.6 (HRONO POTENTIOM ETRY In voltammetry, th e current flowing t h rou g h the working e le c tro de is m e asu r e d as a function of the a p pl ie d vo lt a ge and the rate at which that voltage varies with ti m e . In chronopotentiometry, CP, the cu r re n t is the e lectri ca l variable that is c o n t ro l l e d by m e a ns of a galvanostat wh i l e the variation of the w o rki ng e l ectro d e p ot e nti a l wi th t i m e i s m o n it o r e d C h r o nop o t e nti o metry has also been referred to as galvanostatic voltammetry an d therefore can be groupe d with the m ethods d es cri b e d in this chapter. Th e t h e ore ti c al basis of chronopotentiometry was developed ea r ly in the 20th .
Tech niques
53
Constant current source (Galvanostat)
Working electrode
Recorder
Figure
2.23
century
Simple experimental arrangement for chronopotentiometric measurements.
[ 27, 28] but it was not until the late forties that Ruis [ 29 ] used it as an a nalytical tool and
that Gierst and Juliard [ 30 ] showed it to be a useful procedure for studying electrode kinetics.
When a controlled current is passed through a cell between the working and auxiliary elec trodes, initially the potential of the working electrode wi ll change rapidly as charge builds up a t the electrode-solution interface until a potential is reached at which t h e electrolysis of a n electro active component of th e solution (e.g. the analyte) begins. I f the analyte is present in solution in its oxidised form, aox' then with the onset of electrolysis it will be reduced and its concentration at
the electrode surface, ( c,) ox, will decrease. At the same time electrode reaction products will form an d ( c,\ect will increase from its initial value of zero . This will cause the potential of the electrode to change, but at a relatively slow rate u ntil ( cJ ox becomes effectively zero. At this point th e poten tial of the working electrode will again change rapidly until it reaches a value where the electrolysis of another component of the cell solution, for example a second analyte or an ion of the support ing electrolyte, commences. The basic experimental arrangement for obtaining chronopotentio grams is shown in Figure 2.23. When the current is controlled at a constant value by means of a galvanostat, the potential of the working electrode is found to vary with time as shown in Figure 2.24. The time between the steeply rising portions of the potential-time plot, that is the period over which ( c.)ox changes from ( c. )ox to zero, is called the transition time, 't , which for a diffusion controlled process is given by the Sand equation [ 27 ] : 't
where
l /2
1/?
=
{ n FA ( 7t D0x) - c c. ) ox }
( 2.44)
2i
i is the constant current in amperes, ( c.)ux is the cell concentration o f the analyte in mol em
Dox
is the diffusion coefficient of the analyte in cm2 A is the electrode area in cm 2 •
s-1 ,
\
and
54
Introduction to Voltammetric Ana lysis
E
Time Figure 2 . 24
Chronopotentiometric (potential-time) curve for a reversible electrode process . time; Ett4 potential when t t/4. =
=
t
= transition
When the electrode process is reversible ( equation 2. 1 0 } , the surface concentrations of a0, and { ( c,)ox and ( c,) '"..J will control the potential of the working electrode in accordance with the Nernst equation ( equation 2. 1 1 ) . Expressing ( c, ) 0, and ( c,) ,.d in terms of time using equation 2 .44 [ 5 , 2 8 ] leads to the equation which describes the shape of the reversible chronopotentiogram obtained at constant current ( Figure 2 .24), namely: a,.d
(2.45)
where
E,,4
=
E
o
oxtre d
+
RT ln nF
{ (D"d)1 '2 (fox )} Dox
(2 .46)
J;cd
is the potential of the working electrode when t = 't/4 and is identical wi th the reversible polaro graphic half-wave potential I! 1 2• 1 Equation 2.44 shows that the square root of the transition time, which is proportional to the concentration of the analyte, is the analytically significant parameter. From equation 2 .45 it is seen that a plot of E.vE versus ln{ (t/ t) 1 1 2 - 1 } will be a straight line with a slope of 59.2/n mV at 25°C for a reversible electrode process. The shape of the chronopotentiogram for an irreversible process is determined by the rate of the charge transfer reaction. In this case equilibrium between aox and a,cd on the electrode surface cannot be maintained and so the Nernst equation no longer applies. Instead the rate expressions (equations 1 .3 and 1 .4 ) are used to obtain the relationship between E.., E and time, namely: EwE
=
Eo oxtrcd
+
RT ln anF
{nFA(c.)0,k,cd } i
.
+
RT in anF
{(•)1 12 1 } t
-
( 2 .47)
Techniques
55
In this case, a plot of Ewr versus ln { ( 't/ t) 1 12 - l } enables the transfer coefficient, a, and the rate constant, kred• to be evaluated from the slope and intercept, respectively. As is the case for d.c. polarography, an irreversible wave is shifted to more negative potentials for a reduction and more positive potentials for an oxidation, and is more drawn out along the potential axis than is a reversible wave. Eventually at a sufficiently high potential, the electrode reaction becomes fast enough for diffusion control to be achieved. This means that the transition time, 't, like the limit ing diffusion current, id, in DCP, is not affected by the degree of reversibility of the electrode process. On the other hand, when two analytes are present in solution the transition time of the second analyte is enhanced by the presence of the first analyte. This arises because when the second ana lyte is being reduced, the first analyte is still diffusing to the electrode surface and being reduced, so that only part of the applied current is available to reduce the second analyte. This is in contrast to the case for DCP where the limiting currents of the two analytes are simply additive ( § 2 . 1 . 5 ) . The transition time of the second analyte i s related t o i t s concentration b y the expression: ( t 1 + t 2 ) 1/2
_
l/2
t1
=
n
Z FA { ( D ) 1t
ox
'
�
} I/2 ( cJox2 . 2t
( 2.48)
where the subscripts l and 2 refer to the analyte that appears first and second, respectively, in the chronopotentiogram. If ( c.)ox l = ( c3)0x2; ( D0x) 1 = ( D.,.) 2 and n 1 n2 ; then t2 3'tp Thus the consumption of part of the current by Ox 1 when Ox is being reduced results in a marked increase in the transition time 2 for the second analyte over that which would be observed if it alone were in solution. This enhancement effect has been used to increase the sensitivity of CP analyses. If instead of a constant current a time-varying current is applied to the cell, equation 2.44 no longer describes the relationship between transition time and analyte concentration. For th e gen eral case of a steadily increasing current: =
=
( 2.49 )
where � is the current-time proportionality constant. The transition time is given by ,._('{ + •
1/2)
=
k
(y) Ca
( 2 . 50)
where k(y) is a constant which depends on y.
When y = 1 /2 , a linear relationship between the transition time and the analyte concentration is obtained [ 31 ] .
The main problem with chronopotentiometry has been the difficulty o f accurately measuring the transition times, particularly the short times obtained at low analyte concentrations. More accurate transition times may be obtained if the first derivative dE/dt or, better still, the second derivative, d 2 E!d r , is plotted as a function of time rather than plotting the potential of the elec trode directly against time [ 32 ] . The beginning and end of the transition arc indicated by peaks in the first derivative plot and by the times at which the second derivative passes through zero (crosses the time axis ) as it changes from a positive to a negative value. Figure 2.25 illustrates the advantage of the second derivative plot compared to the simple chronopotentiometric E versus t plot in the analysis of a mixture of copper, cadm ium and zinc. Because it is easy to electronically detect the time at which d2 E/d r changes from a positive to a negative value, the second derivative tech nique can be readily automated to just record the transition times. It is also the best way to
56
I ntroduction to Voltammetric Analysis
E -
[V]
1 8 .
- 1 .4 - 1 .0 - 0.6 - 0.2
0
------ Zero l i n e
I
2
3
1
1
Fig ure 2 . 2 5
:+-- 0.32 s �:I 0.,06� S 1
�0.26 s -.:
t [s]
4 Position of i nflection poi nts
N orma l and second derivative chronopotentiog rams for a m i xt u re of 1 x 1 o-3 mol l-1 Cu(ll), 1 x 1 0-4 mol l- 1 Cd ( l l ) and 5 x 1 0-4 mol l_ , Zn(ll) i n 0 . 5 m o l l- 1 KN03 recorded at a current density of 0.858 rnA cm-2 [32] .
a n a lys e t h e chronopotentiometric signal when s m a ll amounts o f an analyte are t o b e determined in the p rese nc e o f l arg e amounts of other electroactive species. C hro n opo ten ti o m etry , d i re c t or derivative, is not an i mp o rta nt a n a lyt ic a l t e c h n i que . Detec t i o n limits are in the range 1 0-5 to 1 0-4 mol L-1 for si m p l e c h ronopotetiometric analysis and about 1 0-7 mol L- 1 a t best for d er i v ative chronopotentiometric a n alys i s . However c h r o n o p o t e n ti om etry is i mportant as the basis of t h e very sensitive stripping t e c h n i q ue s , derivative and differential s tr i ppi n g c h ro n o p o tenti om e t ry and of po t e nt iom e tr i c stripping analysis, used in trace and ultra trace analysis (see ch apter 3 ) . Referen ces 1
2 3
4 5
6
7 8
L. Meites, Po la rograp h ic Tec h niques, 2nd edn, Interscience,
J. Heyrovsky and J. Kuta, Principles
New York, 1 96 1 . of Polarography, Academic Press, New York,
1 966. D. Ilkovic, Co llection of Czechoslovak Che m ica l Com munications, 1 934, 6, 498. A.J. Bard and L . R. Faulkner, Electrochemical Methods, Jo h n Wi ley, New York, 1 980. A . M . Bond, Modern Polarographic Methods in Analytical Chemis try, Marcel Dekker, New York, 1 91!0. l.M. Kolthoff and J.J. Lingane, Polarography, 2nd edn, ln tersci cncc, New York, 1 95 2 . J. Kuta and I. Smoler, Progress in Polarography, Vol. 1 , P. Zurnan and I . M . Kolthoff, editors, Interscicncc, New York, 1 962, p. 4 1 . J. Heyrovsky and D. Ilkovic, Collectio n of Czechoslovak Chemical Comnumica tions, 1 935, 7, 198.
Techniques 9 !0 11 12 13 14 15 16
17
19 20 21 22 23 24 18
25 26 27
28
29 30 31 32
57
) . Tomes, Collection of Czechoslovak Chemical Com m un im tions, 1 9 37 , 9 , 1 2. D .I. Sawyer and J.L. Roberts, Experimel'ltal Electrochemistry for Chemisls, Wiley, New Yor k , 1 9 7 4. E. Wahlin and A. Bresle, A c ta Chem ica Scarrdinavica, 1 956, 1 0 , 935. Metrohm VA Processor 693 Instruction manual, 1 995. ) . E . B . Randles, Faraday Society 1 ransactions, 1 9 4 8 , 4 4 , 3 2 7 . A. Sevcik, Collectiorr of Czech os l o va k Ch emical Commun ica tions, 1 948, 1 3 , 3 4 9 . P.T. Kissinger and W.R. Heineman, Laboratory 1 edmiques i n Hlectroa nalytical Chemistry, Marcel D e kke r, New York, 1 984. G . C . Barker and A.W. Gardner, Fresen ius Z Analytisclze Ch e m ie, 1 960, 1 73 , 79. G.C. Barker and l.L. Jenki ns, Analyst, 1 95 2 , 77, 685. J.G. Osteryoung and R.A. Ostcryoung, Analytical Chemistry, 1 985, 57, l O l A. G.C. l1arker and A .W. Gardner, Analyst, 1 9 8 2 , 1 1 7, 1 8 1 1 . E.}. Zuchowski, M . Wojciechowski and ). Osteryoung, Analytica Chim ica Acta, 1 986, 1 83, 47. ).G. Ostcryoung and M.M. Schrei ner, C.R. C. Critical Reviews i n Analytical Ch e m is t ry, Supp. 1 , 1 988, 19, 1 . B. B re yer and H.H. Bauer, Alternating Current Polarography and Jensammet1y, lnterscience, New York, 1 96 3 . D. E. Smith, Electroanalytical Chemistry, Vol. 1 , A . } . Bard, ed i to r, Marcel Dekker, New York, 1 966, p. 1 . M. Sluyters- Rehbach and J . H . Sluyters, Electrocmalytical Ch e m is t ry, Vol. 4, A.J. Ba rd, editor, Marcel Dekker, New York, 1 970, p. l . D. E. Smith, C R. C. Critical Reviews in Analytical Chem istry, 1 97 1 , 2, 247. S. Sander and G. Henze, Electroanalysis, 1 997, 9, 243 . H.J.S. Sand, Philosophical Magazine, 1 90 1 , I , 45. Z. Karaoglanoft� Z. Elektrochemie, 1 9 06 , 1 2 , 5 . A. Rius, J. Llopis a n d S . Polo, Annales de Ia Sociedad Espanola de Fisica y Quim ica, 1 949, 4 5 8 , 469 . L. G ierst and A.L. Juliard, ( a ) Proceedings of I he Second Mee t i ng of the In ternational Com mittee on Electrochemical Thermodynamics and Kinetics, Tamburini, Milan , 1 9 5 0 , p l 1 7, p279; ( b ) f. Physiwl Ch em ist ry, 1 9 53, 57, 70 1 .
G. Henze and R. Neeb, Fresenius Z. A n a ly tis ch e Chem ie, 1 98 2 , 3 1 0,
R.W. Murray and C.N. Reill ey, }. Electroanalytical Chemistry, 1 962, 3 , 64.
I l l.
3
Stri p p i n g a n a lysis
3 . 1 ELECTROCH EMICAL STRIPPING TECHN IQUES Electrochemi cal stripping techniques are among the most sensitive of instrumental analytical techniques. Because of their high sensitivity and selectivity they are important in trace analysis and speciation studies. With detection limits generally in the 1 0- 1 1 to 1 0-9 mol L- 1 range and in a few cases as low as 1 o-1 2 mol L- I , they are three or four orders of m agnitude more sensitive than the simple polarographic methods. The high sensitivity and selectivity of stripping techniques arises because two distinct controlled electrochemical steps are involved. In the first step the analyte, initially present in the test solution, is transferred to the electrode s urface in one of two ways namely: ( i ) by electrolytic oxidation or red uction which results in the analyte forming a deposit on the working electrode ( when the analyte is a metal ion, its reduction onto a mercury electrode may produce an amalgam rather than a deposit) , or (ii) by the adsorp tion of the analyte directly or in combination with other reagents onto the electrode surface. This first step, which is a pre-concentration, enrichment or accu mulation step, is carried out while the working electrode is held at a constant potential. Constant area mercury ( hanging drop or thin film ) , carbon, noble metal or modified electrodes m ay be used. The transfer of the analyte to the electrode surface is usual ly enhanced by stirring the solution or in some cases by rotating the elec trode. In the accumulation process a fraction of the analyte is transferred from the test solution matrix and forms a phase on or in the electrode in which its concentration is orders of magnitude higher than it is in the test solution. On completion of the accumulation step, a rest period of 1 5 to 30 s is observed so that the test solution can come to rest before the second step is commenced. In the second step, the amount of analyte accumulated at the working electrode is determined by monitoring the electrical response produced as this accumulated material is either oxidised or reduced from the electrode. This process is referred to as the stripping step since the material accu m ulated duri ng the first step is stripped from th e electrode and normally passes back into solu tion. The stripping is carried out in one of three ways namely: (i) varying the potential on the working electrode and monitoring the current response (voltammetry), (ii ) passing a constant current through the cell and m easuring the potential that the working electrode develops as a function of time ( chronopotentiometry), or ( iii) chemically and monitoring the potential of the working electrode during the stripping process ( potentiometry). In the voltammetric methods the potential applied to the working electrode in the stripping step may be in the form of a linear scan, stai rcase, a.c. modulated linear scan or staircase, square wave modulated linear scan or stair case, or a pulsed linear scan or staircase voltage. Because the different accumulation and determination procedures may be combined in a variety of ways, a whole family of electrochemical stripping techniques h as been developed many of which are listed in Table 3 . 1 A comprehensive account of stripping analysis h as been presented by Wang [ 1 ) and Brainina [ 2 ] . In discussing these techniques it is convenient to group them into three main categories based on the stripping process with sub-categories determined by the accu mulation process.
Stripping analysis Table 3 . 1
Classification of electrochemical stripping techniques.
Stripping step
- - ·· · · · · -··· ·
Accumulation step
Measure
Te chniq u e
Electrolytic reduction
i vs E
Anodic Stri pping Voltam metry
------
Linear or stai rcase anodic vo lta ge
59
sc an Adsorp ti o n
Pulsed anodic voltage scan
Ele c t roly t i c red uct io n
Ads orptio n Li near or s t airc a s e
El ectrolytic oxidation
ca t h o dic vo lta ge sca n Adsorp t i o n
i vs E
� i vs E � i vs E j VS f.
i vs E
Adsor ptive Anodic Stripping Voltamm etry Differential Pulse Anodic S t r i p p i n g
Voltammetry Differential Pulse Ads o rpt ive Anodic Stripping Voltammetry Cathodic Stripping Vol ta m m e t ry
Adsorptive Cathodic Stripping Voltamm etry
Pulsed c a t h od i c vo ltage
Electrolytic ox i d a tio n
scan
Adsorption
Constant anodic c u r re nt
Constant cathodic current
reduction*
Ele ct rolyt i c oxi d a t i o n
Adso r p t i o n
Constant current*
Chemi cal ox i dat i o n
Electrolytic reductio n
or
El ect ro lyti c reduction or oxida t i on
�i vs H � i vs E H vs t
Anodic Chronopotentiometric S t ri p p i n g Analysis ( ACPS)
d t/ d E vs E
Differential ACPS
E vs t
Cathodic Chronopotentiometric Stripping Analysis ( CCPS)
dE/d t vs t
Derivative CCPS
d t/ dE vs E
Differential CCPS
E vs t
Ad s o r p tive C h ro n o p ote n t io m e t r ic St r i p p i n g Analysis (AdCPS)
d E/d t vs t
D e r ivat ive Ad C P S
d t/d E vs E
Differential AdCPS
E vs
t
E vs t dE/dt vs t
reductive stripping often
Differential Pulse Adsorptive C ath o d ic Stripping Vo l ta mm c t ry
De r i vative ACPS
d t/ d E vs E
' Oxidative and
Volt a mm e tr y
dE/d t vs t
dE!d t vs t
Ad s o rp t io n
Differential Pulse C a t ho d ic Stripping
d t/d E vs E
not distingu ished in the techn ique name.
Potentiometric St rippi ng A n alysis Derivative Potentiometric Stripping A n alysis
Differential Potentiometric S tri p p i n g
An alys is
Adsorptive Potentiometric Stripping A n alys i s ( AdPSA ) Der ivative AdPSA Di ffe r e nt i a l Ad PSA
60
Introduction to Voltammetric Analysis
3 . 2 VOLTAMMETRIC STRIPPING 3 . 2 . 1 E lectrolytic accu mulation 3.2. 1. 1 Anodic voltammetric stripping Zbinden [ 3 ] had demonstrated in 1 93 1 th a t small amounts of copper could be qu a n tifi e d by plat ing the copper onto a plati n u m electrode and then m e a s u ri ng the current that flowed during the electrochemical stripping of the copper from this electrode. However it was not until the late 1 950s that t h e high sensitivity of s t ri p p i n g analysis and its po t en t i a l for the a na lysis of trace m et a ls at the s u b p a rt per billion level was fully recogn ised [ 4 , 5 ] . During the 1 960s and 1 970s the theo retical principles of stri pp i ng voltammetry were established [ 6 , 7] and t h e meth od was success fully applied to the an alysi s of metals at the trace and ultra-trace level [ 8 , 9] and to the environmentally i m por tant pro blem of trace metal sp e ciat i o n in natural waters [ 1 0- 1 2 ] . The two pro ce ss e s involved when the a n alyt e i s accumulated by electrolytic reduction and d e te rm in ed by voltammetric o xid a t i on are summarised in e q uat i on 3 . 1 : -
M e "+ + ne- + (Hg)
Acc umulation
Determination
(3. 1 )
Since the determination step is the inverse o f the ac c u mu l atio n processes, the term Inverse Voltammetry was at one time used to describe the stripping technique [ 1 3 ) . After a variation on this technique was d ev elo p e d in wh ich the en riched analyte was determined by a reduction rather than an oxi da ti on step, th e above technique became referred to by th e more clearly descriptive name anodic stripping volta m metry or its ac ro ny m ASV. In those cases where the analyt e accumu lated on the electrode is measured by a re d u ct io n step, the technique is r efer re d to as cathodic stripping voltammetry or CSV (§ 3 . 2 . 1 . 2 ) . The steps a n d re s u l tant current-potential profile for a n a nalysis b y ASV are depicted i n Figure 3. 1 . The l e ft side of the diagram represe n ts the accumulation step while the right side depicts the determination step. D u ring the time interval , a, referred to as the accu m ula tion time, tacc ' the ana ,
lyte is transported to the working electrode surface by convection and diffusion and d ep o s i t e d on the electrode by electrolysis at a constant potential , Eacc' The convective transport is achieved by st irri ng the sol ution or ro t a t in g the electrode at a con stant rate so that a constant di ffu si o n layer thicknes s is m a in t ained (see § 1 .4. 1 ) . During ac cu m ula t i on , the electrolysis is not exhau st ive and on ly a sm al l fraction of the analyte is transferred fr o m the test solution to th e working e lectro d e Typically, when the test so lu t i o n volume is 25 mL only about 0.25% of the analyte is transferred to the electrode after 5 minutes a c c um u l a ti o n [ 6 ) . With smaller sample volumes, a higher propor tion of the analyte is accum ulated for a given tacc· In o rder to obtain reliable analytical results with high precision it is essential that the same fraction of a n al yt e be transferred to the electrode in each experiment. Therefore it is n e c e s s a ry to c a re fu lly control all those factors which affect the transport of the anal yt e to and its depositi on on the w o rk i ng electrode. In addition to holding the temperature and electrode potential c o ns tant , s p e c i a l attention must be paid to th e reproducibi l i ty of t""' the test solution volume, t h e el ectrode position, t h e surface area of the electrode, and the mechanical c on d iti o n s , such as the shape, size and p o si tio n ing of the stirrer, and the solution stir ring speed or electrode rotation rate. The time interval, b, a ft er stopp ing the sti rring or rota tion, must also be c a r efu l l y controlled. D u r ing this ti me called the rest period, the w or ki n g electrode is held at the accu m ul at i o n poten tial, the motion of the solution ceases and the cathodic current drops to a sm all value as the con tribution from the forced co nvecti o n falls to zero. In an a lyses where a m etal amalgam is p rod u ced in a mercury drop electrode, the rest p e r i o d also allows time for t he metal concentration to become u n i form t hr o u gh o u t the drop. At th e end of the rest peri od, a linear or staircase anodic voltage scan is applied to the working el ectrode from an i n i t i a l value o f Eacc' thus i ni t i a ti n g the determ ina tion or stripping step, c, .
,
Stripping analysis
..--- a � Figure 3 . 1
b
61
+-- c � . +-- d --
Potential-current-time relationshi ps dunng an anodic stripping voltammetric analysis: (a) accumulation time, (b) rest period, (c) determination or stripping step, (d) anodic dissolution of mercury.
The recorded voltammogram exhibits a peak as a result of the oxidation and transfer of the enriched analyte from the working electrode back into the cell solution. Ideally, this transfer should be complete but, particularly with an HMDE, it may be incomplete because the metal dis solved in the amalgam may not have completely diffused from the centre of the drop during the time of the potential scan. This is indicated by 'peak tailing'. The peak is characterised by the peak potential, Ep, and the peak current, ip , which are determined by the nature and concentration of the analyte, respectively. The rise in the current in the fourth section, d, is due to the anodic dis solution of the mercury working electrode. Following the introduction of the hanging mercury drop electrode [ 4 ] , anodic stripping volta mmetry was applied to the trace analysis of those metals which dissolve in mercury to form amal gams, such as antimony, bismuth, cadmium, copper, gallium, indium, lead, manganese, thallium, tin and zinc. In addition to the mercury drop electrode, the mercury thin film electrode, MTFE, which has a much higher surface area to volume of mercury ratio than the HMDE, was also devel oped for the determination of the amalgam-forming metals. The thin film of mercury is plated onto an inert supporting material for which purpose glassy carbon is particularly well suited. The mercury is either plated onto the glassy carbon prior to the accumulation of the analyte or, more commonly, generated in situ to produce the MTFE. In the latter method a small amount of mer cury( I I ) nitrate is added to the test solution and the mercury film (thickness ca. 50- 1 500 nm) is formed on the GCE surface simultaneously with the deposition of the analyte. In order to deter mine mercury itself and the more noble metals, such as Ag and Pt, by ASV it is necessary to use an inert solid electrode such as a carbon electrode. As is the case with most polarographic and voltammetric analyses, it is necessary to remove oxygen from the cell solution in stripping voltammetry. The reduction of the dissolved oxygen produces a current which affects the background current and produces hydroxide ions at the elec trode surface. These may react with analyte ions in the diffusion layer. Furthermore, if oxygen is present in the test solution, it may chemically oxidise the amalgam in competition with the elec trochemical oxidation in the stripping step. This can lead to a decrease in the peak current in the anodic stripping voltammogram and unreliable results.
62
Introduction to Voltammetric Analysis
Linear Sweep Voltammogram
-E Anodic Stripping Voltammogram
Figure
Comparison o f t h e linear sweep voltammogram with t h e anodic stripping voltammogram o f a n analyte.
3.2
The linear sweep voltammogram for the reduction of an analyte in solution and the anodic stripping voltammogram for the oxidation of the reduced analyte back into solution are shown in juxtaposition in Figure 3 . 2 . The shape of the stripping voltammogram depends on the nature of the electrode reaction and on the surface area to volume ratio of the electrode. The sharpness of the peaks increases with the number of electrons involved (Table 3 . 2 ) and with the reversibility of the electrode process and depends on the voltage waveform used to strip the analyte from the electrode. By using smaller mercury drops or lower voltage scan rates, sharper peaks are obtained so that a better separation of closely lying peaks becomes possible. Even better resolution of the current peaks results from using a MTFE rather than an HMDE because of the sharper and more symmetrical current peaks obtained with the former electrode as a consequence of its larger sur face area to volume ratio. Better peak resolution and symmetry means that analytes with smaller differences in their peak potentials can be successfully resolved in a single run. The accumulation potential, Eacc' used in the second step may be determined from the linear sweep voltammogram (Figure 3 . 2 ) or from the DP or DC polarogram or voltammogram of the analyte in the same supporting electrolyte as that to be used for the analysis. The value of Eacc for the case of an electrode reaction where n = 1 should be at least 0.2 V (for n = 2 at least 0. 1 V) more negative than the half-wave potential or peak potential for the reduction of the analyte, that is the value of Eacc should be in the potential region of the diffusion limited current ( Figure 1 .5 ) . Ta b l e
3.2
Peak widths at half peak height, b 112, for different ASV techniques [ 1 4 ] . HMDE
TFME Metal ion in 0. 1 M HCl
Linear scan b112 ( mV)
Differential pulse b112 ( mV)
Fundamental a. c . b 112 ( mV)
Second harmonic a. c. b112 ( mV)
Tt
74
86
94
84
Pb2+
34
44
56
44
33
44
38
In3+
25
Theoretical basis
The development of the theory is treated in detail elsewhere [ 1 , 6, 1 5 ] and only a summary is pre sented here. For those metals which form amalgams with mercury, the concentration of the metal in the mercury working electrode is governed by Faraday's law and given by
Stripping analysis
63
(3.2)
where
carna l is the concentration of the reduced analyte, Me0(Hg), in the mercury electrode, i.,, is the current which flows during the electrolytic accumulation of the analyte, t.,, is the time of accumulation, v is the volume of the mercury working electrode ( HMDE or MTFE ) , Hg n i s the number of electrons p e r mole o f electrode reaction, and F is the Faraday constant.
For an HMDE with a drop radius, rd, v H g (4/3 )7tr/, and for a MTFE of thickness, l, and sur face area, A£> vH g Arf. It should be noted that the MTFE obtained by depositing a thin film of mercury on a glassy carbon substrate is really a series of micro-droplets rather than a true contin uous film (Figure 4 . 1 5 ) [ 1 ] . Such a deposit however behaves as a thin film. During the accumulation step when the solution is stirred, the analyte is transported to the diffusion layer by forced convection and then diffuses through the diffusion layer to the electrode surface. For the HMDE in a stirred solution, the accumulation current is given by [ 1 6 ] : =
=
(3.3)
and for a MTFE in a stirred solution or for a rotating MTFE by l ace =
.
where
k2 n FA f D
ro
213 1 12
vk
1 /6
c.
( 3 .4)
k1 and kz are constants, D is the diffusion coefficient of the analyte, rd the radius of the mercury drop ( HMDE), Ar is the surface area of the mercury film (TFME ) , ro i s the rate o f rotation o f the electrode or of stirring the solution, v k is the kinematic viscosity of the solution, c. is the concentration of the analyte in the cell solution, and the other symbols have their usual meaning.
The second term in the brackets of equation 3.3 is the sphericity correction which allows for the effects of the curvature of the electrode. Since this term is only significant when working with micro-electrodes, it can be ignored when using normal-sized mercury drop electrodes ( rd 0.2-0.5 mm ) . O n combining equation 3.2 with 3.3, o r with 3 .4, it can b e seen that by using a constant stirring rate, the concentration of the enriched analyte in the amalgam is proportional to both the accumu lation time and the concentration of the analyte in the cell solution provided that the latter is not sig nificantly depleted during the accumulation period. This is the case under the experimental conditions normally employed in ASV. When using a mercury drop electrode, only a few tenths of one percent of the analyte is deposited out of a 20 mL test solution after 1 5 to 20 minutes electrolysis. With micro-cells this is not the case and the product ( i.,J.,,) in equation 3.2 must be replaced with the integral Ji.,,( t)dt. In the case of micro-cells with volumes < 1 00 j.!L all of the analyte in the sample may be deposited after 1 0 to 1 5 minutes electrolysis. A high efficiency of deposition is an advantage when carrying out continuous voltammetric measurements in flow-through cells. The flow channel in such cells is designed with a very small cross-section area so that complete electrolysis of the ana lyte may be achieved in the time it takes for the sample solution to pass across the electrode surface. =
64
Introduction to Voltam metric Analysis
Wh e n workin g with a MTFE, the GCE disc o n t o which the thin mercury film is plated m ay be rotated at constant sp ee d instead of s ti rrin g the solution about a stat i o n a ry electrode. Such an e l e c t ro d e , referred to as a rota ting disc electrode, or RDE, re s u lts in an im pro ved rate o f transport of the analyte to the electrode surface and gi ves greater reproducibility than that obtained with a s t at io n ary electrode in a stirred sol u ti on . In an an alo gous way to linear sweep voltammetry (LSV) and cyclic voltammetry (CV) ( cf. e q u at i on 2 . 30 ) , the p e a k current, iP, in anodic stri p pin g v olt am m et ry using a l i n e ar pot e nt ial scan a n d an H M DE is gi ve n to a good approximation by the Randles-Sevcik e qu at i on [ 1 7, 1 8 ] , the dif fere n c e being t h at the solution parameters in e quation 2.30 (for LSV and CV) are rep lace d in ASV b y the corresponding parameters for the enriched analyte i n the amalgam to give (3.5) where
v
is the v o l t age scan rate in V s - I , k3 c ontain s all t he relevant numerical and physical constants and the mass transfer terms ( stirri ng rate, vi sc os i ty) , Dam al is the diffusion coefficient of the metal in the am a l g a m, and th e other sym b o l s have their usual meaning.
No t in g that from equations 3 . 2 and 3 . 3 or 3.4, carnal is proportional to both tacc an d ca and that for the HMDE, AHg k4 rd2 , equation 3 . 5 leads to the following expression for t he s tr i p pi n g p eak current when using a n HMDE ( ignoring th e s pheri c i ty c orr ec t i on ) : =
( 3 .6a)
and when u s i n g a MTfE [ 1 9 ] : ( 3 .6b) where
k5 and � are constants.
From these equations it can b e seen that for both electrodes the peak current i s proportional to tacc ca , so that the concentration r ange over which iP versus c. is linear may be extended s im p l y by varying tac c. It follo ws from this that th e detection l im it for a given determination, which de pend s on the lowest value of iP t ha t can be meas ured above th e noise level, wi ll a l so d epe n d on tacc I t should be noted that the peak current should vary w i t h v 1 12 and rd fo r the HMDE but with the first p o wer o f v a n d the area of the film, and be i n de p e n de n t of t he film thickness fo r the MTFE. Comparison of mercury drop and film electrodes
tive than l!112
With the HMDE, th e peak potential is independent of t he scan rate and is 28.5/ n mV more posi for a reversible pro c es s at 2 5°C i n ASV and 28.5/ n m V more negative in CSV (cf. LSV in § 2.3 ) . On the other hand, for the MTFE, the p eak potential varies with the scan rate and the film thickness, and the d i ffe re n c e between EP and F! 1 1 2 is a p p roxim a t el y four times that for the HMDE. Ano t her s i gn ifi c a n t difference between t h e d rop and film is t ha t the value of iP d e p e nd s on the concentration of the metal in the ama l ga m for t h e HMDE but on th e amount of m et a l in t h e amalgam for the MTFE. The differences in the performance of the mercury d ro p and thin film e l e c t ro d e s when us e d in the determination of trace metals by ASV arise m a i n ly fro m th e l a rge difference in their surface area to vo l u me ratios [ 1 4, 1 5 ] . Typically a MTFE has a s u rfac e area s o m e ten times th a t of an H M D E or SMDE, while the volume of mercury in t he former is less th a n 0.5% of t h at in the latter
Stripping ana lysis
65
two electrodes. This makes the MTFE in h e r en tly more sensitive but it s la rger surface area means that the blank ( ch a rging c u rrent ) s i gn al is larger a n d it is more prone to interference from surface active i m p ur i ti es in the sample. The mercury d ro p electrodes (HMDE or SMDE) h av e some disadvantages c o m p a red to the MTFE:
The accumulation efficiency is l o w because of the s m a ll s urfac e a r e a . The stripp ing p e a k s are broadened as a result of the lar ger volume of mercury (the the oretical peak half-width, b1 12 , 1 08/n mV for t h e HMDE c ompared to ( 3 8-50 ) / n mV for the MTFE [ 1 ] ) . This peak bro ade ning arises because of the finite time required for t he metal to diffuse from the i n te rior of th e drop. L on g accumulation times, which allow the metal to diffuse up into the mercury column, result in further peak broadening. (iii) In order to maintain drop s t ab i l ity only low solution stirring rates ( and hence lower accumulation rates) can be used. (iv) The rest period prio r to s tripping must be ke pt at about 30 seconds since it takes this time for the deposited m e ta l to reach a uniform concentration t hrou gh out t h e d ro p co m p a r ed to a few seconds to achieve u n ifo r mi ty in a thin mercury film.
(i) (ii)
=
Lon g e r accumulation times may be used to counter (i) and ( iii ) , however, as alre ad y noted, this leads to enhanced peak b road e n i n g and extends the time required for each analysis. P e a k broaden ing, as a result of the lon ger accumulation times r e qu ir e d to obtain a measurable peak current in the analys i s of ultra-trace metal ion concentrations, is one of the factors that deter mines the limit of detection in ASV. While the M T F E requires sh o rte r accumulation t i mes because of its large surface area, gives sh a rp e r , m ore easily resolved s tri pp i n g peaks because of the small volume of mercury, and is stable at h ig h st irri n g rates, there are a few problems that arise from the fact that only a sm a l l amount of mercury is present in a film electrode.
Erratic behaviour i s observed with very thin films, h o we v e r films thicker t h a n 1 00 nm g ive reproducible behaviour ( de scrib e d by eq u ation 3 .6b ) and th e p e ak currents observed for a given amount of metal in th e a m alg a m are found to be independent of film th ic k n ess for l 1 00 to 2 0 00 nm [ 2 0 ] for both linear and pulsed voltage stripping t e ch niques . (ii) Metals with a low solubility in mercury may quickly reach sat u ra t i o n concentration during the a cc u m u la tion step. The excess m et al is d e p os i ted on or th rou g h the mercury film thereby c re ati ng a second phase on th e electrode and a lte r ing t h e p h ysi c al and elec trochemical properties of t h e electrode. The pro bl e m metal may come from t h e reduc tion of a metal ion of no analytical inte rest in the s amp le matrix or be the reduced analyte itself. For ex a mple , it ha s been c alculated that for a MTF E , 6 m m dia. and 250 nm thick, being rotated at 60 rps in a 25 mL solution co n t ain i n g 6 ppm Cu2+ , t h e mer cury film will become saturated in c opp e r after only 2 m in utes electrolysis [ 2 1 ] . (iii) Sub strates o n wh ic h the mercury film is st ab l e and which do n o t contaminate the mer cury are limited. The m o st suitable is glassy carbon, however while thin m e rc u ry films on glassy carbon are q u it e stable in n e u t ral and alkaline s o l u tions they are much less stable in acid solutions, failing after a fe w hours in s ol uti o n s at pH 1-2 [ 22 ] . (iv) Several metals when dissolved t o geth e r in mercury form inter- metallic comp ou n d s in the amalgam. This problem b eco m es more pronounced as the metal concentrations in the amalgam increase a nd s o affe cts anal yses u s i ng the MTFE to a m uc h greater extent than analys e s u si n g a drop electrode. Inter-metallic c o m p ou n d formation may occur in the amalgam between the m etal s from two a nalytes or an analyte and a reducible matrix
(i)
=
66
Introduction to Voltammetric Analysis
component. It leads to do ubl e pea ks changes in peak pot e n t ial s or the comple t e disap pearance of a peak from the stri pp i ng volta m m o gram Metal combinations which have been reported to form int e r me tallic co m pound s in mercury include Co with Zn; Cu wi t h Cd, Ga, I n , Mn, Ni, Sb, Sn, Tl a nd Zn; F e with Mn; and Ni with Mn, Ga, Sb, Sn and Zn. Platinum also forms inter-metallic compounds with many other elements in m er cury and for this reason is not suitable as a substrate for the MTFE. ,
.
-
The determination limits attainable with anodic stripping voltamm e try using the HMDE gen the range 0.5 to 1 �g L- 1 • This c an be i m p rove d by up to a fac to r of 1 0 when a MTFE formed on a glassy carbon or graphite substrate is used. More sensitive and reproducible mea surements can be obtained when the usual graphite or glassy carbon substrate is replac ed by the re ce ntly developed ultra-trace g r a p hite el ectrode of Metro h m (§ 4.5.2). The surface of t h i s elec trode, which displays micro-electrode-like properties, can be c o ated wit h m e rc u ry dropl ets and used to a ccumulate a nalytes from rapidly stirred solutions. In g e n e ral more reproducible results are obtained with hanging d r o p electrodes at the h igher c o ncen t r at i o n s while at very low concen trations more precise result s are obtained when a mercury film el e c t rode is used. In anodic stripping voltammet ry the differential pulse mode (DPASV) is p refer re d for strip ping the analyte from the electrode. Under identical exp erim ental conditions (supporting electro lyte, Eacc' tacc a n d c. c ons t a n t ) a h i g h e r s e n s iti vi ty is obtained us i n g DPASV than that obtained with linear scan stripping ( LSASV ) . T h e a dvantage of using LSASV is that sur fac e active substances cause less interference. On the other hand, interference from i rreversible processes can be dimin i s hed by us i n g an a.c. m odu l ated waveform for the s trippi n g process (ACASV). When a staircase waveform is used instead of a linear (DC) scan to stri p the metal from a MTFE, the faradaic cur rent p e a k i s s i gn i fic a ntly reduced for thin films with l = 0.25- 1 .5 �m. The decrease in p e ak current amounts to ca. 28% for .1.Estcp 2 mV and to ca. 50% fo r .1.Estep = 1 0 m V [ 20, 23] The p eak current decreases with ste p h e igh t (.1.E51,P ) at constant scan rate (.1.Estep/.1. tstcp ) but increases with .1.Estep at constant .1. t,1•P ' that is with increasing .1.E,1./.1. t,1ep · However the staircase waveform discriminates against the c h arg i n g current ( 24 ] , so t h a t t h e ba c kg ro u n d current is much less than and not ste e ply rising as is the case when a linear scan is used for the stripping step. The n et result is an i m prove m ent in sensitivity when a staircase waveform is used. The sensitivity of staircase strip p i ng is s i m il a r to that of di ffe re n tia l pu l se s tr i p p i n g, h owever much h ighe r scan rates can be used with staircase waveforms resulting in much reduced analysis times. The stripping vo l tammo gram s obtained for the determ ination of zi n c i n aci d so l u t i o n shown in Figu r e 3 . 3 illustrate the differences in the data obtained by using different stripping vo lt age waveforms [ 25 ] . The rise in the b ase current produced by the i rreversible reduction of the H+ ion (begins ca. - 1 .0 V) is much lower in the AC-voltammogram than in t h e DP-voltammogram. By far the greatest effect however, was on the LS (DC) -voltammogram where t h e formation of the p ea ks was s evere ly h i n de re d e rally lie in
,
=
.
Resolution of current peaks in ASV
In trace an alysis it is desi rab l e that the treatment of the s a m p le prior to th e quantitative measure ment step be kep t to a minimum. It is p referable if the t echn ique used to obtain the q uan ti tat ive signal is able also to differentiate between the various analytes in a mixture so that th e s a mple does not have to be s ubj e cte d to a s epa rati o n p rocedu re which m ay h ave a po o r recovery efficiency or may introduce contamination. A number of experi m en tal strategies h ave b e en develop ed in ASV which utilise both the c hem i c a l and electrochemical p rop e rties of the analyt es to facilitate the res olution of their stripping peaks so that all desired co mp o nent s of the s am pl e may be a nalyse d in a single run with the m in im um of pre- treatment. Since the accumulation ele ctr o lys i s is c a rr ied out potentiostatically, i t is p o s s i b l e ( i n p ri n c i p l e ) to adjust the a cc u mula t i o n potential so that a nal yte s that have stripping p eaks at si m ila r p ot en -
Stripping ana lysis DC
a
0.6 0.7 0.8 0.9 1 .0
1 .1
1 .2 1 . 3
AC
,.1 I I I I I I I I I I I I
b
:
•
I I I I I I ' I ' ' '
_ _ __
Figure
3.3
I I I I I I I I
-
E [V] (vs. SCE)
' I I I I • I ' I I I I ' I I
I
I
0.7
67
' I I I I • I ' I I ,'
I
I I
/
,.,. , '
... _ __ ,
0.8
0.9
1 .0
1 .1
1 .2
- E [V] (vs. SCE)
Comparison of t h e anodic stripping voltammog rams obtained for the dete rm i n ati on of zinc in 0.1 mol l-1 HCI solution using an H M D E and th ree d i fferent stri pping techniques: linear scan (DC}, differentia l p u l se (DP} and an a.c. modulated l i near scan (AC). face = -1 . 2 5 V (vs SC E); t,,, 3 m i n . ( a ) 1 00 IJ9 L-1 Z n 2 + a n d (b) 1 0 j.Jg L-1 Zn 2 + . =
tials m a y b e s eparat e d duri ng the a ccumula ti o n step. Simultaneous det e r m i n at io n s r eq ui r e a dif fe r ence in the peak p o t en t ia l s of �EP > 1 00 mV for good resolution. The s it u ati o n is illustrated in Figure 3.4 for the determination of P h , Cd and Zn wh e r e it can be seen that by the ap pro pr i ate choice of Eacc' separation of the ana lyt e s can be ach i eve d . The resulting stripp i n g vol t am mo gra ms sh ow pe a ks for Ph, for P h a n d Cd, or fo r all th r ee metals t o gether depending on the p o t e ntia l selected for the accumulation step. Another way to analyse mixtures of metal ions with poorly resolved stripping p e aks is to st op the poten t i al sweep at a suitable value during the course of the st rippi n g step. While the electrode is held at this co n s t a n t po ten tial , those m etal s with more n egative stri p p i n g potentials are stripped out of th e a m a l gam . On co n t i n u i ng the potential sweep, the metals with the more positive peak p ote n t i a l s can be analysed without interference from the more easily oxidised metals, even when the fo rm e r are p r e s en t at much lower concentrations i n the test solution than the latter.
68
Introduction to Volta mmetric Analysis
a
- 0.4
- 0.8
- 1 .2
E
[V] (vs. SCE)
b
Figure
3.4
Se l e ctivity i n anodic stripping voltammetry by the choice of the accumulation potential: (a) d. c. polarogram of lead, cadmium and zinc (each 1 0-3 mol L- 1 ) in 0 . 1 mol L- 1 KCI, (b) a n odi c stripping voltammograms of lead, cad mium and zinc ( ea c h 10-6 m o l L-1 ) i n 0. 1 mol L-1 KCI after accumulation at various potentials, (i) E'.,,. stripping vo l ta mmogr am of lead alone, (ii) E2ac" stripping voltammogram of lead and cadmium, (iii) E\cc , st i p p in g voltammogram of lead, cadmium and zinc.
For those metals that form inter-metallic compounds in the a m a l ga m , t h e p eak s h i ft i n g and elimination caused by the inter-metallic co m po un d formation can be t u r n ed to an advantage. For example, the presence of copper markedly interferes wi t h the ASV determination of zinc. The problem can be o ve rc ome by the a ddition of Ga3 + to the s olu t io n and co-depositing gallium with th e zinc and copper. The Cu-Ga c omp o un d is much more stable than the Cu-Zn c o m p o u n d so that the copper combines wi th th e g alli u m , freeing the zinc which can now be determined free fro m interference [ 26 ] . The selective determination of lead or thallium in the p r e s e nce of b i s mu th or indium i s a ch i e v e d us i n g t h e same p r incipl e by c o - d e po s it i n g copper or gold on the w o rkin g electrode w i th the an alytes [ 27, 28 ] . Metal- ion peaks that lie close together in the s t rip ping vo lt a mmogr am may also b e separated from each other by complexing the metal ions in solution. There are two possibilities. In the first case , an appropriate c o m p lexin g agent, wh i c h shifts the half-wave and stripping p e ak potentials of one of the metal io ns to a more neg at ive value, is added to the sample so l u tio n. Both metal ions t h e n can be j o int ly accumulated at the appropriate more negative pot e n ti a l and give separate peaks in the stripping voltammogram. Alternatively, by using a less n e ga tive accumulation poten tial only the metal ion which has n o t reacted with the complexing agent may be accumulated.
Stripping analysis
Sn2 + Tt
2+ Pb
1
69
\ lil T Tl
Sn
Pb
2 Tl
II
3
Tl
Sn
Pb
I
Sn
Pb
4
- 0.4
- 0.5
- 0 .6
- 0. 7
- 0. 8
- 0. 9
- 1 .0
- 1 .1
E [V] ( vs. SCE ) Figure 3 . 5
The effect of changing the supporting electrolyte composition on the separation of the potentials of the anodic stripping voltammetric peaks. Supporting electrolytes: (1 ) 1 . 0 mol L"1 HCI; (2) 1 .0 mol L-1 HCI + 2 . 0 mol L-1 ethylenediamine; (3) 1 .0 mol L- 1 HCI + 2 .0 mol L- 1 N a O H + 0.2 mol L-1 ethylenediamine tetraacetic acid; (4) 1 . 0 mol L-1 HCI + 2 .0 mol L- 1 NaOH + 0.2 mol L- 1 sodium potassium tartrate.
A n o t her s e p aratio n t e c h n i qu e which makes use of c o mplexa tio n is t h e so-called solution or mediu m excha nge method. Th e p r i n c iple of this method is that after the electrolytic a cc u m u l a tion step, the cell s o l uti o n is r e p l ace d by a second solution. Often, in order to o b t a i n the best s e p a rat i o n of the desired a n a lytes fro m the sample matrix, the electrolysis m u s t be c a r ri e d out using a s up p o rt i n g e lect r olyt e so l u tio n which is un s ui table fo r o b ta i n i n g goo d p e ak separations in the s tri p p in g step . By changing the cell solution to o n e c o n ta i n i n g an a pp rop ri a te complexing agent t he d es ire d se pa ra t i o n can be achieved. This is illustrated by the data presented in Figure 3.5 [29] . T h e a c c um ul a t i o n of tin, lead and thal l i um is ca r ri e d out in 1 mol L- 1 h yd roc h l ori c acid solution. However, the peak p o ten t i a ls in t h e stripping vo l t ammogram of these three ions in 1 mol L- 1 hydrochloric acid are too close to ge t h e r and unsuitable for the simultaneous determina tion of each of t he th ree ions ( Figu re 3.5, l i ne 1 ) . When, after the electrolysis, t h e hydroch loric acid solution is re plac e d with a solution which contains either ethylenediamine, EDT A, or tartrate as complexing agent, the p e ak shifts as indicated in F i g u re 3 . 5 ( l i n e s 2, 3 and 4) are o b t a i n e d . In the si m pl est case, the solution exchange is a c hi eve d by the rap i d exchange ( < 5 s) of the el ec tr olys i s cell with another vessel c o n t a in ing the deaerated s ec o n d s u p p ort i n g electrolyte. This method of solution exch ange, however, yields unsatisfactory results with base metals such as z i nc because on contact with the air the amalgams of th e s e metals can be ra p i d l y oxidised by atmosph eric o xyge n . A quick, complete and con tinu o u s solution exchange can be achieved best wi t h a tl ow -th ro ugh c e ll ( ch a p t e r 5 ) . Alternatively, t h e supporti n g electrolyte may be modified by sim p l y a d d ing the appropriate oxygen-free reage n ts to the cell s o l u t i on used for the accumulation step. For example, in th e ASV det e rm in at i o n of l ea d and thallium the metals are best accumulated from 0. 1 mol L- 1 HCl. However in this electrolyte the stripping peaks are too clos e fo r goo d reso lution. By changing the pH o f the cell solution to 1 3 with deaerated 5 m ol L-1 NaOH, well s e p a ra t e d p e aks are obtained in the st r i p p i n g voltammogram. exchange
70
Introduction to Voltammetric Analysis
In summary, a n odi c st ripp i n g volt a m me t r y u s in g mercury electrodes can be u se d for the trace analysis of antimony, bismuth, cadmium , c opp er , galliu m , germanium, indium, lead, manganese, thallium, tin and zinc especially in a range of water samples ( d ri nki n g , surface, sea, i n d ustr i al effluent, sewage) and biological fluids. Many of these s a mp l e s will c o ntain organic material which will adsorb onto the w o rki n g electrode and interfere with the ASV determination. In these cases it is necessary to d es tro y t hi s or g an i c material by U . V. i r ra dia t i o n or oxidative microwave diges tion in the presen ce of H202 b e fo re proceeding wi th the ASV determination. For samples with a high organic c o n t ent such as sludge or tissue material, wet acid digestion u si ng aqua-regia will be n ec essary (chapter 4). ASV in c o mb i n a ti on with U.V. d i ges tion is also used to determine the concen trations of the various forms in which a t o xic heavy metal may be present in a water sample (metal ion s peciation ) ( c hapter 6). 3.Z. T . Z Cathodic vo/tammetric stripping Ca th o d ic s t r ip p i ng vo l t a m m et ry is used for t h e indirect determination of inorganic and organic anions which form sp ar in gly soluble s a l ts wi th Hg(I) or Ag(I ) . When a mercury (or silver) elec trode is anodically polarised, Hg2 2 + (or A g +) are produced and react with the analyte to form the sparingly soluble Hg(I ) { or Ag( I ) } sal t which a c c umu ates on the electrode surface. The accumu lation potential d e pen d s on t h e supporting electrolyte, the solubility p rod uc t of the Hg(I ) or Ag( I) sal t , and the co n cen tra t ion of t h e an a lyte in th e test solution. For mercury electrodes E.cc is usually in the range +0.4 V to -0.2 V ve r su s Ag/AgCl, 3 M KCI) . In the d ete rm inat on step, a cathodic p ote n tial scan is a pp l i e d to r educe the H g ( I ) o r Ag( I ) in t h e sparingly s olu ble salt accumulated on the elect ro d e surface which gives rise to the voltammetric current peak. CSV can b e used for the determination of trace levels of halides, pseudo-halides or sulfide ( m e rcu ry [30] or s i lve r [ 3 1 ] electrodes) and a number of o xom e ta l a t e ions such as vanadate, ch ro ma t e tungstate and m o yb date and orga ni c anions ( mercury electrodes) . The analytical pro cess is summarised in the fo llowing eq u a t ion s ( for a mercury elec trod e ) .
l
(
,
i
l
l
Hg �+
Accum ulation:
+
2Hg � Hg;+ + 2e 2A- � Hg 2 A 2
(3.7)
.!
Determination:
The current peak height d u e to the electrolytic d iss o l ut ion of the insoluble l ayer is propor tio n a l to the amount of anion accumulated on the electrode surface, which under c o nst a n t accu mulation and stripping c o nd i t i on s is in turn pro p o rt i o na l to t h e concentration of the analyte in s o lution The peak potential is determined m a i nl y by the solubility product of the acc u m u l at ed salts. The differences in s o l ub i l i t i es between the Hg(I) halides and b etwee n Hg(l) ch ro mate , molybdate, tungstate and vanadate are not ve ry large so that the strippi n g peaks occur a t similar potentials and the r eso luti on of adjacent p eak s i s limited [ 32 ] . This in direct p r oc e d u re can b e used to d et e rmin e a number o f organic substances which are capable of forming in s ol uble Hg( I I ) compounds on an anodically polarised mercury electrode. These are mainly sulfur-containing s ub s tanc es such as thiols, t h i o am ides thiobarbiturates a n d dithiocarbamates and some s ulfo n i c acids [ 3 3 ] . Al th o u gh there is still s o m e uncertainty about the m echanisms involved, it is generally believed that the cat h o d i c s tri pp i ng t e chn i q u e for t h e deter mination of thiols is based on the reaction: .
,
accu m u lation
2RSH + Hg
cathodic stripping
(3.8)
Stripping analysis
- 0 . 77
V
71
(As)
a
- 0. 1 8
- 0 .04
+ 0.02
Figure
3.6
V
V
V
b
(Cu) (As) (Hg2CI2)
The differential p ulse cathodic (a) and anodic (b) stripping voltammograms of arsenic. The cel l solution was 0 . 1 mol L-1 HCI containing 2 x 1 0- 3 mol L-1 Cu2 + and 1 0 j.Jg l-1 As. Voltage scan rate 1 00 mV s- \ face = -Q. 5 5 V ; tacc 1 min. =
while for primary thioamides the accu mulation rea ction is [ 3 4 ] : RC ( = S ) NH 2 +
Hg --� HgS + RCN
+
2H+ + 2 e-
(3.9)
and the cathodic stripping peak arises from the red uction o f t h e HgS layer o n t h e mercury elec trode. Secondary thioami des also give rise to a layer of HgS o n oxidative accumulation but the ter ti ary compounds appear not to accum ulate oxidatio n products on the electrode and so can not be determined by CSV. A typical application is the determination of thiourea, H2N ( C=S) NH2, in
1 mol L_, N aOH solution, which can readily be determ ined at the 1 jlg L-1 level using differential 2 7 , chapter 7 ) . A n umber o f organic compo unds which d o not contain sulfur such a s various flavins [ 3 3 ] , phenylhydrazine, uracil derivatives and n u cleic acid bases [36] also form well developed cathodic pulse cathodic stripping voltammetry [ 3 5 ] ( see Application
stripping peaks after oxidative accumulation. Many
of these compounds arc known to form in sol
uble mercury compounds and the electrode processes involved in th eir analysis by stripping vol
tamm etry can be summarised in an analogous way to that for the thio compounds. However, there is evidence that in some cases a dsorption of the analyte onto the electrode surface plays an importan t if not domi nant role in the accumulation step. Metal ions such as a rsenic ( III ) , selenium and tellurium ( Mea" ' ) may be determ i n ed by strip ping voltammetry after adding a second metal , such as copper
(Mcb"'+ ) , to the test solution and
72
Introduction to Voltammetric Analysis
two metals onto the surface of the HMDE. Th e copper acts as a co - d ep osition agen t a nd fa c il i tates th e d eposition of th e a n alyte , Mea, on the electrode surface as an inter- metal lic com po u n d . T h e ana lyt e may t h e n be s t rip p e d from the e l ectr o d e either b y oxidation (ASV) or by further reduction (CSV) t o an anionic s p ec ie s accordin g to th e fo l l owi ng r eact i on s ch em e : c o- ele ct ro lys i n g the
A cc u m u l ation :
St r ipp i n g: Arsenic, selenium and tellurium are th ree such elements which m ay be determined by cat h o d ic stripping volt a m met ry a fte r h aving b een re d uct i ve l y co-deposited with copper [ 3 7 ] . A characteristic of the cathodic stri pp i n g vo l t a m m o grams of these three elements is that o n ly a si n gl e c urre n t peak which arises from the further red uc t io n of th e depo sited a n alyt e to As3-, Se22 or Te - , respectively, is o bs erve d . In this case t h e d ete r mi n ati on b y CSV is more selective than by ASV, s inc e in th e a n od ic di s so lut i o n addi ti on a l cu rrent signal s are obtained: these arise from t h e oxidation of the copper an d po ssib l y also of mercury [ 3 7 ] . Typ i c al stri pp i n g voltammograms for the de t er m in atio n of arsenic by both ASV and CSV after de p os i t i o n from a co pp er - con tain in g solution are shown in F i g u re 3 .6. The determination limits for the CSV a n a lys i s of As, Se or Te in th e presence of copper were fo u n d to be 0.5 fig L-1 fo r As a nd 0.2 flg L- for Se and Te. Note that o nl y As (Ill) is det e rmi n e d since under t h e s e exp er i m en t a l conditions As(V) is not electroactive. The small amount of arsenic present in water s amples i s m ain ly in the +5 oxida t io n state as a result of o xi dati on by oxygen . T h i s must fi rst be reduced to As(III ) with hydrazine or sodium s ul fi te before pr o c ee di n g with the voltammetric determination [ 38 ] . Recently it has been found tha t if the test s ol ut i on contains D-Mannitol, As(V) can be reduced ele ct rolyti c all y to As( O). This is the basis of a m eth od for determining As(V) in the presence of As (III) by cathodic st rip p i n g vol ta mm e t ry [39] (chapter 6 ) .
1
3 . 2 . 2 Adsorptive accumu lation A wide range o f orga n ic m o l ecul es a n d m etal c hel a te co m p l exe s are surface active and can be accumulated on the surface of a mercury e l e c trod e by adsorp tio n processes rather than by a c h a rge t ra nsfe r ( o x i d a t i o n o r r e du c t i o n ) process. When the adsorbed compounds can be oxi dised or reduced, they may be e s t i m a t e d by stripping t h e m from th e el ectrode surface using linear sweep, staircase or differential pulse voltammetry. T h e technique, referred to a s adsorptive stripping volta m m etry, AdSV, is a powerful method for deter m in i n g ultra-trace levels of a v a ri ety of m e t al ions ( as their chelate complexes) , organo-metallic and organic compounds [ 1 , 40-43 ] . It has grea t ly exte n d ed t h e a ppl i ca b i l i ty of st r ippin g voltammetry to the a n a lysis of any el e m ent o r co mp o u n d that ca n be adsorbed onto an electrode surface and which can be reduced or oxi dised within t h e potential range a vail abl e wit h th e p a rt i cul a r working electrode-supporting el e c trolyte and solvent combi nation used in the an a lys i s . This te chni q ue is particularly important in environmental studies for the determ i nati on o f trace and ultra-trace levels of m eta l ions that do not form a m alg am s with mercury or are n o t re a d ily d epo sit ed as the e l em e nt on a m e rc u ry elec trode. Adsorptive accumulation has also been used with t e n s amm etri c s t ri ppi n g for the d et e rmi nation of some n o n - e l ectroactive but su rfa ce active orga nic c o m p ounds . Two ap p ro a c he s have been u se d to effect the adsorption of metal ions onto the el e ctro d e sur fac e as the metal chelate. The simplest is to add an excess of a s u ita b l e comp lexi ng agen t t o th e t e st so l u t i o n p r i o r to the accumulation step. This is t h e most common ap pr o ac h and is used with m er cury, gold or p l at i n um working electrodes. A s ele c t i o n of frequently used c o mpl ex i ng agents is listed in Table 3 . 3 . An alte rna t ive approach is to m od i fy the s urfac e of the working electrode wit h
Stripping analysis
73
Comp l exing agents used for t h e determi nation of metal ions by a d sorptive stripping voltanunetry.
Table 3 . 3
Complexing agent
Elements
Reduction of the central metal ion
1 ,2-dihydroxybenzene (catechol)
1 ,3- b utanedionedioxime ( dimcthylglyoximc)
8- hydroxyquinoline ( oxi ne) 2-hydroxy-2,4,6-cycloheptatriene (tropolone) 2 , 5 -dichl oro - 3 , 6 - di hydroxy - I ,4-benzoquinone ( chloranilic acid)
U,
Cu, Fe, V, Ge, Sb, Sn, As
Co, Ni, l'd Mo, Cu, Cd, Pb, U
Mo, Sn
U, Mo, Sn, V, Sb
N-n itroso- N-phenylhydroxylamine ( cupferron )
U, Mo, Tl
(OCP)
Cc, La, Pr
Reduction of the ligand
o-cresolphthalexone
5-sulfo-2-hydrobenzeneazo- 2 -naphthol ( soloch rome violet RS - SVRS )
AI, Fe, Ga, Ti, Y, Zr, V, Tl , Mg, alkali
1 ,2 - d i hydroxyanth raquinone-3 -sulfonic acid ( alizarin red S)
AI
Chrom azurol B ( M B9, mordant blue 9 )
Th, U
and alkaline earth metals
Catalytic hydrogen evolu tion
Pt
Formazone
the complexing agent. The metal ion is then accum ulated by reaction with this modified surface. Electrode surfaces have been modified by physi cally coating the electrode wi t h a layer of an insol uble complexing agent (carbon electrodes), by chemically attaching a complexing agent to the electrode surface (platinum or carbon electrodes) , by incorporating a suitable complexing reagent into the polymer matrix of a polymer coated electrode, or by mixing the complexing agent into the matrix of a carbon paste electrode. Such electrodes often exhibit a high degree o f selectivity for a particular analyte. A metal-ion analyte is adsorbed onto the electrode as its complex at a fixed potential as for electrolytic accumulation and determined voltammetrically by the reduction or oxidation either of the central metal ion or of the ligand complexed with the metal ion . Organic analytes can be adsorbed in an analogous way and determined via the oxi dation or red uction ( as appropriate) of their electro active functional groups. The magnitude of the peak current, iP, obtained in th e linear scan voltammogram is deter mined by the accumulation time, tacc' the amount of the adsorbed species on the electrode surface, 2 r.d A (where r.d m ol em- is t h e surface concentration of adsorbate and A cm2 is the electrode area), and on the scan rate, v, V s- 1 . Furthermore, r.d is proportional to the concentration of ana lyte in the test solution, c., provided that the surface coverage is small (less than 50% ) . Assuming for the sake of simplicity that the electrode processes are diffusion controlled, then for a spherical electrode of radius, r, th e peak current in the stripping voltammogram is given by [ 44, 45 ) :
(3. 10)
where
k
=
n2 P v 4RT
For mercury drop electrodes, for which r i s approximately 0 . 5 m m, the first term i n equation 3. 1 0, the sphericity correction, is small for t"'" < SO s and has only a slight effect. Thus, iP is propor-
74
Introduction to Voltammetric Analysis
ti o na ! t o
the square root of tacc · This is not the case whe n 1 0 11m) are used as the first term then becomes the dominant term and iP bec o mes p rop ort ional to the first power of tacc S i nce the accumulation process involves the for mation of a film of adsorbed a na lyte on the e lectr od e surface, the surface concentration of analyte will reach an e q uilibrium value, (r.d)e ' determined by the so l uti o n conc e ntrat i o n , c If the solu q tion c on centra t i o n of the ana lyt e i s s u fficiently h igh , the val ue of r.d wil l be t h at for the s u rface be i n g saturated ( c o m p le tely covered) with the analyte, (r.d)sat· Correspondingly, the peak current should increase l ine a rly with tac c 1 12 fo r a gi ve n value of c. unt il the time is reached when r.d has attained its equilibri um value o r the su r fa c e becomes saturated. At this time, iP reaches its maxi mum value, ip( max)' which now remains con stant with acc u m u lation time and is given by c.
and approximately prop o rtional to
micro - e le ct rode s ( r -
•.
ip(m ax )
=
kA rad (eq )
(3. 1 1 )
If th e increase of i with c. at constant tacc is measured, a similar situation is observed. The peak P current increases l inearl y with increasi ng c. un t il the concentration that produces complete coverage in the chosen tacc is reached, at which point ip(max) ( corresponding to cr.d)sat) is rea ched and no longer increases with further increases in c• . Knowing the stripping voltage scan rat e and the value of ip (max)' rad(rnax) may be calc ulated and used to determine the surface area occupied by each an alyte molecule. This may g ive useful information about the structure of the layer of a n alyte adsorbed onto the electrode surface particularly when c. is such that surface saturation is achi eved [ 44 J . 112 Th e li n e a r re l a t i o n sh ip between ip and c. at constant tacc ( and b etwe e n ip and tacc for constant c.) fo r all val ues up to ip(max) i s an ideal situation. In practic e most iP versus c. calib rat ion curves have a shape similar to an ad s orption isotherm, that is they are e ss e ntially linear at the lower con centration ranges but become non-linear ( decreasing slope) at higher c o nc e ntrations as full s ur face c over a ge of the electrode by the adsorbed analyte is approached ( see Figure 3. 7). Al though these c u rve s are u su al l y qu ite reproducible under fixed accumulation conditions, it is pre fe rab l e to carry out a n a l ys e s usin g the linear region of the ir versus c. pl o t . D e p a r t ure s from li nea ri ty observed at the high er analyte concentrations are overcome by us in g shorter accumulation times, by dilution of the sample solution or b y either using lower s ti rr in g rates or not stirring the solu tion at all du r i ng the accumulation step ( i . e . allowing the diffusion laye r thickness to increase) 1 • All these procedures either s i ngly o r i n combination lower the electrode surface coverage o f the adsorbed analyte from a given sample so that measurements can b e made in the linear range of the i versus c. p l o t . I n general, in AdSV short accumulation times in the range 20-60 s are used P I for the det e r m i nat i on of analytes in th e 0. 1 to 1 0 11g L- range. Too lo n g an accumu lation tim e n ot only re sults in a non-linear peak current-concentration p lo t but also affects the p e a k p o te n t i al s . This is illustrated by the voltammograms obtained in the presence of dim ethylglyo xime [47] as comp l exin g agent for the AdSV d eterm i nati o n of nickel and cobalt at different concentrations, usi n g different accumulation times. When a 30 s acc um ulat io n time is used t he p ea k p o tent i a l s are co nst ant and linear iP versus c p lo t s are obtained. H oweve r when a 1 20 s accumulation time is used, the nickel peak moves to more n ega tiv e po t en ti al s and iP for the cobal t peak only increases slightly on the addition of standard amounts of each m e tal io n (Fig ure 3 . 8 ) . Th e a dso rp tio n of neutral m olecules i s governed b y the adsorption energy o f the surface active molecule at the e l e ct r o d e-s o l u tion interface. In electroanalytical chemistry, it is the s p e cific adsorption of the analyte wh i ch takes p l a ce within the compact part of the do uble l aye r (see Figure 1 .6, § 1 .4.3 ) that is imp o rtant . The adsorption process involves the re placem ent of solvent In many ca ses the determination can be carried out a l higher concentrations in the und iluted solution a nd without accumulation using l i near sweep voltamrnctry at an HMDE or d ifferenti al pulse polarography a t a DME or SMDE ( chapter 2 ) .
Stripping analysis i
1 00
75
[nA] c
80 60
b a 0
Figure 3.7
20
40
60
1 00
80
1 20
1 40
1 60
1 80 t
[s]
Relationship between accumulation time, analyte concentration and peak cu rrent for the determination of 5-fluorouracil in a supporting electrolyte of pH 7.8. 5-fl uorouracil concentration a - 5 x 1 0 7 M ; b - 1 0-6 M ; c - 5 x 1 0-6M [46). -
1 20 nA
I
Accumulation t ime: 30 s
Co
1 20 nA
I
Accumulation ti m e : 1 20 s
Ni
- 0 . 85
Figure 3.8
E
[V]
- 1 . 2 35
- 0 . 85
E
[V]
- 1 . 235
The effect of different accumulation times on the adsorptive stripping volta mmograms of cobalt and nickel u sing a H M DE. Sample solution: 2 0 119 L-1 Ni and 20 11g L-1 Co in 0.1 M N H3-NH4CI (pH 9.3) + 5 x 1 0 -4 M dimethylglyoxime. Standard additions: each 200 ng Ni plus 200 ng Co.
molecules on the electrode surface by analyte molecules. The amount of adsorbed analyte that is required to fully cover the electrode surface to give a monolayer coverage depends on the size of the adsorbed molecules and their orientation with respect to the surface. The extent of adsorption of the analyte and, in th e case of asymmetric molecules, its orientation, is related to a range of fac tors such as the solubility of the analyte ( low solubility often enhances adsorption ) , the hydropho bic nature of the analyte, electrostatic interactions between ionic analytes and the charged
76
I ntroduction to Voltammetric Analysis
el e c t r o d e ,
di p ol e interactions between polar po rt i on s of the analyte and the electric field o f the double layer, and t h e ch emisorption of certain con s tit u e n t atoms of the a n a lyt e onto the surface of the electrode mater i a l . The s ign an d magn it u de of the c h arge on t h e electrode surface, which is r e l at ed to th e el ect ro de p o t e n t i a l b y eq u a t i o n 1 . 1 4a, d e term i nes the mag nitude of the a d s o r p t ion energy a n d degree of ads orpti o n of the analyte. This h a s been the s u bj ec t of s t udy by m a n y w ork ers [48, 49 ] . In ge ner a l , neutral molecules tend to be a d so rb e d at p o t e n ti a ls in the re g io n of the p ot en tia l of zero charge, Em, ( see § 2 . 1 . 5 ) , that is when t h e el e ctrode c h ar ge is s m a l l and the electric field i n the double layer is r ela t iv e ly weak. Anionic molecules and those con tai ning high electron d en s i ty centres such as a r om a t ic rin g s , are general ly ad so rb e d at potentials wh ere the electrode is p o s i t ively c h a rge d while cationic molecules are more likely to be adsorbed at p o te n ti al s mor e negat i v e than Ezc· P o l a r centres in a m o lec u l e also infl uence the p o te nt ia l range over wh ich a molecule is adsorbed and in many cases the o rie n tat i on the adsorbed mo l ecule adopt s at a p a r t i c ul a r po te n tial [ 50, 5 1 ] . When m et al ions are be i n g accumulated as th e i r complexes, the metal-ion c omp l exes nor ma lly d is p l ace th e co m pl exi n g ag ent , HL, fro m the surface of the ele c t ro de , that is ML11 is m ore strongly adsorbed than H L. There are several po ss i b le mechanisms by wh ich a m e t a l may be accu m ul ated on the el e c tro de surface as its complex: 1
The anal yte forms an uncharged co m p l ex with the ligand L- in solution
and the c o m p l ex is adsorbed onto the electrode surface. M Ln(soln) � ML n(ad+ 1 + e -+ M + ( n + ! ) OW
nOH
" ML, ,. 1 1 + e- -+ M' + ( n + l ) L-
ML, -+ M 1 " ' 1 " + e- + nLM EX + n e- -+ M E " + x"-
M.,"Mh"
An 1 c1"'"
+ xe- -+ M,x-
+
+
Fe( JJJ)
Co + 1 -nitroso-2-naphthol
Sn + d i cthyldithiophosphatc
c o m p o und s ( M E" = Ag or Hg
Mh"
ne
Oxidation or reduction of either the met M e t r o h m , Herisau.
27
26
28 30
29
31 32
33
M . G . Pa n cl i and A. Voulgaropoulos, Elect roa n a lys is, 1 993, 5 , 355.
C.M.G.
v an den Berg, Analytica Chi mica Acta,
1 99 1 , 250, 265.
M . L . Te rc i e r a n d J. Buftle, Elec tro ana lysis, 1 993, 5, 1 8 7. M . Es te r ba n and E. Casassas, Tre nds in Analytical Chemistry, 1 994, 1 3, 1 1 0 . M. Kolb, P. R a ch , J, Sc h a fe r and A. Wi l d , Frese11ius ' /. Analytical Chemistry, 1 992, 342, 34 1 . M e tro h m Application Note No. V 35. M e trohm Appl ication Note N o . V 39. G . Henze, Me tal Speciation i n the Environment, S . Giicer and F. Adams, Editors, NAl'O AS! S e ri es G 23, 1 990, H. W. N ii rnb e rg , P. Valen ta, L. M a r l , B. R a s p o r and L. Sipos, Fresen i us
p. 3 9 1 .
34
T. M . Florence and G . E . Batley, C. R. C. Crit ical Rev iews in Analyti ca l
35
T.M . Florence, An a lyst, 1 986,
36
H . W. N iirnberg, Fresen i us Z. A n a ly tisch e Clzemie, 1 983, 3 1 6, 557.
37 38 39 40
41
42 43
44 45 46
47
48
50
49 51
52 53
688.
J . Wan g , P. Tu z h i a n d T.
1 1 1 , 489.
Z Analytische Ch e m i f, 1 976, 282, 357.
Che m is t ry, 1 980, 9, 2 1 9 .
Mart i n ez, Analytica Ch imica Ac ta , 1 987, 20 1 , 43.
No. 2 3 1 / l d. Chevallerie-Haaf a nd G . Henze, Fresenius Z. Analytische Chemie, 1 987, 328, 565. Met r o h m Ap p l i cation Bulletin No. 24 1 / l d . I T . - P. Nimai cr, M . La u e r and G. Henze, Electroa nalysis, 1 998, 5, 326. Me t r o h m Application B u l l e t i n No. 7412d. Me t r oh m Application B u l l et i n No. 9614d. M. Kolb, P. Rach , j . Sch afer a n d A . Wild, Fresenius' /. Analyt ical Chemistry, 1 99 2 , 344, 283. Mctrohm Appl i c a t i on N o t e No. V 36. Metrohm Appl i c a t i o n Note No. V 4 2 . M e t r o h m Application Bulletin No. 226/ l d . U . Greulach a nd G . H e n ze , Analytica Chi m ica Acta, 1 995, 306, 2 1 7. G. Henze, W. Wag n e r and S. San der, Frese 11 i u s ' f. Analytical Ch e m is t ry, 1 997, 358, 7 1 4. G . Henze, P. Monks, G . Tolg, F. Uml a nd , a nd E . WeBling, Fresenius Z. Analytische Ch em i e, 1 979, 295, I . G . Henze, M ikro ch e m ica Acta [ Wie n ] , 1 98 1 , 2, 343. C. Arlt a n d R. N a um an , Fresen ius 7.. Analytische Chem ie, 1 976, 282, 463.
Mctrohm Application Bulletin
A. Meyer, U. de Ia
Applications: i norganic species
54
1 75
R. Kalvoda Imtrumelltation in A n a ly tica l Chemistry, ) . Zyka, Editor, Ellis H o r woo d , New York, 1 994, Vol. 2 ,
W.F. S myth, In o rgan ic A ds o rp ti ve S tripp i ng A n alys is, Mc tro hm M o n o g ra p h, Herisau, 1 99 1 . Ch . 3 .
55
56 57
58 59
C.M.G. van den Berg, Ana lys t, 1 989, l l 4,
1 527.
Z.Q. H u a ng and C.M.G. van den Berg, f. Electroanalytical Ch e m is try, 1 984, 1 77, 2 6 9 . C.M .G. va n d e n B er g, Analytical Proceedings, 1 988, 25, 265. Mctrohm Application Bulletin N o . 1 2 3 / 2 e .
60
C.M.G. va n den Berg an d G.S. jacinto, An a ly tica Chimica Acta, 1 988, 2 1 1 , 1 29.
61
Metrohm A p p l i ca ti o n Bulletin No. 220/ 2d.
62 63 64 65
66
67
68 69
70 71 72 73
74 75
76 77 78 79 80
81 82 83 84
85
87 88
86
R9
90
91 92 93
) . G o l im o ws k i , P. Va l e n ta and H .W. N ii r nb erg , Fresenius /:. Analytische Chemie, 1 98 5 , 322, 3 1 5. N .A. Malakhova, A.V. Chernysheva and Kh.Z. Brainina, Electroanalysis, 1 99 1 , 3, 803. N.A. Malakhova, A.V. Chernysheva and Kh.Z. Bra i n i n a , Electroanalysis, 1 99 1 , 3, 69 1 . S. San de r, W. Wagn e r and G. Henze, Analytica Ch im ica Acta , 1 995, 305, 1 54 . S. Sander and G. Henze, Fresenius' f. Analytical Ch em is t ry, 1 994, 349, 654. W. Wagn e r, S. S an d e r and G. Henze, Fres e nius' f. Analy tica l Chemistry, 1 996, 354, 1 1 . M. Ka ra kap la n , S . G ii C = C< -C
=
C-
,
7C - X
ha l o ge n substituted aliphatic and aromatic
hydrocarbons with the
exception of fluoro compounds
aliphatic and aromatic aldehydes, a ro m at i c ketones, qui nones
> C = O -O-O-
aliphatic an d aromatic peroxides a n d hyd ro pc rox i d es n i t r ate e s te rs
- O - N02 - N02
aliphatic and a romatic nitro compounds
- NO
al i p ha ti c and aromatic nitroso compounds
- NH - OH
hydroxyIa mines
-N= N-
azo com p oun ds
- NH - NH -
hyd raz o co mp ou n d s
> C = N-
benzodiazepines, pyri di n e s , qu inolines, acridincs, pyri m idi n es, triazines, oximes, sernicarbazones
- NCO
isocyan atcs
-S-S-
disulfidcs
>C=S
th iobenzophcnoncs
- S0 -
diary! and alkylaryl sulfoxides
- 50 2 -
sulfones
- S0 2 NH -
sulfonamides
7 C - Me
Oxidisable groups
organometallic compounds
- OH
phenols
- NH 2
aromatic arnines
- CO - N
I mg kg- 1 ) in s ta r c he s can be re ad i ly determined by differential p ulse p o la ro graphy [ 1 6 ] . .
,
Application 1 6
Polarographic determination o f formaldehyde [ 1 7 ]
Sample: Supporti ng e l e ct ro l yt e: Form a l d e h yd e sta ndard :
Wa ste water. Ga l va n i c b a th s pla st ics textiles, a i r, fi lm materi a l , f i s h . Dissolve N a O H (8 g) a n d Na2E DTA. 2 H 20 ( 7 . 44 g) i n water and make u p to 1 L. Vol u metrica l ly a na lyse concentrated (ca . 3 7 % ) for ma l d e h yd e sol uti o n and d i l ute to g ive a s t a n d a rd sol ution c o n t a i n i n g 1 g L- 1 formaldehyde . The solution has l i m i t ed sta b i l ity and m u st be made up d ai l y 1 Wast e waters a n d galvanic baths can be d i rectly a n a l ysed . 2 P l a st i c s and texti les. Red uce to sma l l fragments a n d extract by shaking with 0 . 0 5 M N aO H . Fi lter the extract. ,
,
.
Pre-treatment:
Applications: organic species
1 81
Acetone - 1 . 35 v
I50nA - 1 . 00
- 1 . 25 E
Figure 7 . 2
- 1 . 50
[V] (vs. Ag/AgC I , 3 M KCI)
DP-pol arograp h ic determ ination o f forma ldehyde, acetaldehyde and acetone i n a s a m p l e of methanol .
3 Air s a m ple s Forma ldehyde is a bsorbed by pass i n g the air t h ro u gh 0 . 0 5 M NaOH. 4 S o l i d s a m p l e s . Blend 1 -5 g of sa mple i n 20 m l water, add 1 m l 3 0 % H2S04 and ste a m disti l . C o l l ect t h e forma ldehyde i n 0.05 M NaO H . Deaerate 1 0 m l o f s u p po rt i n g electrolyte with n i t ro g e n a n d a d d 1 0 m l of sample sol utio n . After further dea erati n g , record the pu lse polarogram betwee n - 1 .4 V a n d - 1 . 8 V. The pea k potentia l of formalde hyde occurs at -1 . 6 5 V (vers u s Ag/Ag C I , 3 M KC I ) . Determ i n e t he concentration by standard addition, de a e ra t i n g with n i t ro g e n after each addition of the sta n d a rd solution . Because of the toxicity of formaldehyde and its suspected ca rci nogen i c proper 3 t i es, a maxi m u m workpl ace conce ntrat i o n of 0.6 m g m- has been set . DPP can measure forma l dehyde i n the sub-�g L-1 range and so is well su ited for deter m i n i n g the levels of forma l de hyde enco untered in envi ron menta l sa m p l e s such as r oo m a i r, b u i l d ing materi a l s, t ob a c co smoke etc. The formaldehyde, aceta lde hyde and a ceto ne content of m e t h a n o l ca n be determ i ned di rectly using a citrate- phosph ate buffer (p H 6. 5) conta i n i ng hydra zi ne s u l p hate . The DP-pola rogram, reco rded between -0 . 8 5 V and - 1 .60 V (versus Ag/AgC I , 3 M K C I ) i s shown in Fi g u r e 7 . 2 . The c u rren t peaks a re at - 1 . 0 5 V ( H 2CO), -1 . 2 V ( C H 3 C H O) and - 1 . 3 5 V (CH3hCO. The determi nation l i m its a re i n the mg L-1 ra n g e . .
Analysis:
Notes:
In contrast to al ip h ati c aldehydes, wh ich can only be red uc ed in n e u t ral or alkaline media, aro matic aldehydes are polarographically active over t h e whole pH range. The pH of the supporting electrolyte d et e rmi n es whether one- or two-wave p o l a r o g ra m s are obtained. The half-wave potentials for a s e l e c t i o n of aldehydes, sugars and ketones are listed in Table 7.2.
1 82
I ntroduction to Voltammetric Analysis
Table 7 . 2
Polarographic half-wave potentials of selected aldehydes, ketones and sugars [ 1 4] .
-------
Compound
Supporting electrolyte
Formaldehyde
Buffer solution,
Aldehydes
Ellz( V)
-
pH 10.7
pH 8.0
pH 1 2.7 Acetaldehyde
0. 1
M
.
1 4 6 ( vs SCE ) - 1 .59 -1.7 1 - 1 . 89
LiOH
Propionaldehyde
0. 1 M LiOH
Acrolein
Buffer solution, pH 8.7- 1 1 .0
(1) (2)
- 1 .04 - 1 .44
Crotonaldehyde
0.2 M TMAOH in 50% ethanol
(1) (2)
- 1 .37
- 1 .92
Benzaldehyde
0. 1 M LiOH
Phthaldialdehyde
Acetate buffer, pH 5 w i t h 1 .5% ethanol
Nitrobenzaldehyde
-1.51
0.092 M H3P04 + 0. 1 96
M TMAOH
in
ethanol-dioxan
Hexahydrobenzaldehyde
- 1 .80
(1)
-0.72
(2)
- 1 .09
(l) (2 ) (3)
-0 .8 ( vs Ag/AgCl )
-1 . 1
- 1 .7
- 1 .9
0.25 1\rl LiOI I
Ketones
NH,-(N H .1 )2S04 buffer
Acetone
2.5 M
Cyclohexanone
0.05 M TBAOH in 50% isopropanol
Acetophenone Benzophenone
1 M NHrNH4Cl buffer with 1% ethan ol
Buffer (pH
7) plus 0 1 M KCl in 25% ethanol .
- 1 .62 ( vs Ag/AgC]) - 1 .65
-!.15 - 1 .35
Anthrone
Buffer solution, pi I 7 i n 40o/o dioxan
Benzylacetone
0.1
a- lonone
0 . 1 M LiCl in 50% ethanol
- 1 .60
Flavone
0.05 M TBAOH/CH J COOH in isopropanol
-1 .35
Alloxan
Phosphate buffer, pH 7
Bem:oin
BR buffer - pH
Sugars
Phosphate buffer - pH 7
Allose
M N H 4Cl
-1.41
-1 .06
- 1 .02
l .3
pH 7.0 pH 1 1 .6
-0.90 ( vs SCE) - 1 .39 - 1 .53
- 1 . 74 (vs SCE)
Arabinose
Phosphate buller - pH 7
- 1 .54
Fru ctose
0 . 1 M LiCI
- 1 . 76
Galactose
Phosphate buffer - pH 7
Glucose
Phosphate buffer - pH
7
-1 .55 -
1 . 54
Lyxose
Phosphate buffer - pH 7
- 1 .50
Maltose
0 . 3 M KCL I KOH
- 1 .60
Mannose
Phosphate buffer - pH 7
-1.51
Ri bose
Ph osphate buffer - pH 7
Sorbose Xylose
0 . 1 M I .i Cl
Phosphate bu ffer - pH 7
-
1 . 77
-1 .76
- 1 .50
Applications: organic species
1 83
Fumaric acid - 1 --. 1 .5 mg L
Maleic acid
- 1 .0
- 1 .2
1 . 5 mg L-1 __.
-
1 .4
- 1 .8
- 1 .6
E [V] (vs. SCE) Figure 7 . 3
DP-polarogram of 1 . 5 mg L- 1 mal ei c acid and 1 . 5 mg L-1 fumaric acid in a NaH2 P04-NH4C I/NH3 buffer (pH 8.2) [14) . Peak potentials (versus SCE) : ma leic acid -1 .35 V; fumaric acid -1 .60 V.
Keto nes, like ald ehyd es , are also pol arographically active. Satu rated aliphatic keton es are more diffi c ult to reduce than the unsaturated ketones. The reduction of the unsaturated ketones pro duces either a d ike ton e or a s at urat e d ketone which m ay then be reduced further at a more nega tive po tential . Th e d iketon e s yie ld enediols in a two - el ect ron reaction. The reduction of aromatic ketones is gen e rally complicated. The n umber a n d p ote n t i al of th e waves are strongly influenced by the composition and pH of the supporting e l ect rolyt e . In acid solu tion the first wave ( E 1 2 > 1 - 1 .0 V ) is d ue to a on e - ele c t ro n reduction to the free radical which then dimerises to give the cor respo nding pinacol. At more n egative potentials a second wave occurs which arises from the reduction t o the corresponding carbinol. At h igh e r pH the waves merge into each other. The car bonyl fu nc t ion in steroids is reduced to a carbinol and gives a po l a ro g r aphi c wave in alkaline solu tion at ca. -2.0 V [ 1 8 ] . M a ny ph arm aceuti ca l p r o duc ts contain electroactive arom atic ketone gro ups, the reduction of which forms the basis o f their voltammetric determination [ 1 9 ] . The carboxyl pa rt of the carboxylic acid gro u p is, in general, n o t polarograph ically active. Polarographic wa ve s observed b etwee n - 1 .5 V and -2 .0 V ( v e rsu s SCE) are usually due to the reduction of the di ssoc i a ted p roto n. Ca r b o xylic acids with reducible functional gro u p s in the mol e c u le give ris e to two -wave p ol a rogr ams ( see Table 7.3) . In t he case of unsaturated carboxylic a ci ds , the double bond is reduced to give the corre sp o n di n g saturated acid. For exampl e , maleic acid ( cis-form) and fumaric ac i d ( trans-form are redu c ed to succinic acid, a t potentials which are pH and structure dependent. At pH 1 , the DP -polarographic peaks of both acids occur at EP -0.6 V ve rsus SCE, however in alkaline s o l ut i o n at pH 8.2 ( s u p port in g elect rolyte: 0.2 M NaH2P04 + 1 M N H4Cl + NH1) the signals are well separated with EP 1 3 5 V for maleic acid and EP - 1 .60 V for fumaric acid ( Fi g u re 7.3) [ 1 4 ] . T hi s behaviour forms the basis of the p o l ar o grap h i c determination of fumaric acid in foods a n d c o n di m e nt s , in plasti cs as well as in fats and oi ls which may contain m ale i c acid as a preservative. =
=
=
HO - CH, -
OH
&
�
HO
-
.
o
OH
As co rbi c a c id ( vit a m in C) c a n be determined thr o u gh the oxidation of the enediol. In a Britton-Robinson buffer of pH 2.9, th e D P - polarographic p e ak poten ti a l fo r t h e oxidation o cc ur s
1 84
I ntroduction to Voltam metric Analysis
Table 7.3 -----
Polarographic half-wave potentials of sel ected organic acids [ 1 4 ] .
-----
-----
-----
-----
Compound
Supporting electrolyte
Acetic acid
0.2 M LiCl
Salicylic acid
0.05 M TEAl -----
----- -
Acetylsalicylic acid
0. 1 M Li Cl
Tartaric acid
0.05 M TMAHr
Oxalic acid
-----
----- ·
-----
----- · · -----
-----
-2.06
----- -
- 1 . 72
Ph o sphate buffer - pH 7 in 1 0% ethanol
( 1)
Phosphate buffer - pH 7 in 1 0% ethanol
(2)
- 1 .50
Ba(CH1C00 ) 2
--
-----
Acetate buffer - pH 4.64
0.05 M TEAl
-----
-----
----- ·
- 1 .65
0.1 M KCl
2 - Nitrobenzoic acid
Sulfanilic acid
-----
- 1 .89
Biphthalate b uffer - pH 4 with 0. 1 M
Qui naldinic acid
- --- .
- 1 . 70
Phthalic acid
3 - N itrobem:oic acid
---
E112(V) (vs Ag/AgCI)
-----
(1) (2) (1) (2)
-----
-----
-O.HO -1.10
-0.70
-1.10
-0.90 -1 . 1 0 - 1 . 5 !! (vs NCE)
at +0. 1 4 V (versus SCE) using either a d ro pp i n g mercury el ectrode or a carbon paste electrode as working electrode. As co rb ic acid can be determined in fruit and vegetables and in vitamin prepa rations with a determination limit in the mg L- 1 range [ 20, 2 1 ] .
Application 1 7
Polarographic determination of ascorbic acid (vitamin C) [22]
Sample: Pre-treatment:
Solutions: Standard : Determ i n ation :
N otes :
Vita m i n prepa rations, drin ks, fruit j u ice, foodstuff, etc. 1 L i q u i ds, fruit and vegetable j u ices. Fi lter if n ecessary. 2 Solid foodstuff (fru it, vegeta bles, etc) . Cut i nto small pieces, homogen ise with oxalic acid sol ution and filter. 3 Other vita m i n conta i n i n g foods or preparation soluble i n oxa lic acid. If neces sary add trich l o roacetic acid to p reci pitate prote i n and filter. Acetate buffer p H 4.64. Oxa l ic acid 1 g L- 1 • Dissolve 5 0 m g ascorbic acid i n oxa lic acid and make u p to 1 00 m l with the oxal i c acid soluti o n . Prepare a fresh sta ndard each day. Place 1 5-20 ml of acetate buffer i n the polarographic cel l , deaerate with nitro gen a n d add 1 ml of sample. After further deaerati ng record the a nodic DP polarogra m using a D M E o r DP-volta m mogram using a Pt working electrode from -0 . 2 V to + 0 . 2 V. The ascorbic acid peak occurs at ca . + 0 . 0 8 V. Determine the concentration using sta ndard addition s. By using various polarographic methods and su pporting electrolytes it is possi ble to determine s i m ulta n eously, a n u mber of electrochemically active vitamins i n a m u lti-vita m i n p reparatio n . An example is s hown in Figure 7.4. Using differ ential pulse polarography and an acetate b uffe r of p H 4.4, vita m i n s B2 and C ca n be determi ned together (Fig u re 7 .4a ) . Record i n g the a . c . polarogram of the same solution (Figure 7 . 4b) e n a bles the concentrations of vita m i ns K3 and 82 and fol i c acid (fP -0 . 58 5 V) to be determ ined. Addition of NaOH to the same sol ution and a ga i n recordi n g the DP-po l a rogram a l l ows n icoti nam ide ( fP - 1 . 7 6 5 V) to be determi ned (Figure 7 .4c). Amount of vita m i n s per ta blet d eterm i ned polarographica l ly - vita m i n B2 3 mg; vita m i n C 50 mg; folic acid 0 . 5 mg; vita m i n K3 0 . 5 mg and n icoti n a m i de 2 0 mg. ==
==
Resu lts:
==
=
==
==
=
Appl ications: organic species
-
Vitamin C
a
+0.065 v
�
1 . 32 v
�
Vitamin 8 2
- 0 .285 v
�
- 0.2
Vitamin �Vit K,
1
+
0.2
0
- 0.6
E
82
�
- 0 .045 v
�
- 1 .4
[V) (vs. SCE)
b
- 0 . 285 v
+0. 1 0 v
- 1 .0
Folic acid
- 0 . 585 v
�
+
0.2
0
- 0 .2
- 0.6
- 1 .0
Ni cotinamide
Vitami n C - 0.28 v
- 0 .2
Fig ure 7.4
- 1 . 765 v
c
�
- 0.6
- 1 .0
- 1 .4
E [V] (vs. SC E )
�
- 1 .4
- 1 .8
E [V] (vs. SC E )
Polarographic analysis of a m ulti-vita m i n preparation [ 1 4] .
1 85
1 86
I ntroduction to Voltam metric Analysis
Organic acids such as nitrilotriacetic acid (NTA) and other p o lya min oac ids such as ethylenedi a m i netetraacetic acid ( EDTA) wh ich form s ta b l e chelates with heavy metal io n s may be determined b y indirect polarographic methods. Because of t he i r ability to c o m p lex with metal ions th es e co mpo un d s are u s ed as water softening age n ts . Along with p h o sph at e they are often p r ese nt in detergents a n d cleansers and so may be present in wa s te waters. Their determination is b a s e d on th e formation of stable h eavy metal complexes which have polarographic p ro p er t ies dif ferent to those of the a n a lyte itself. Bismuth salts are commonly used a n d are added in a small excess to the s a mp l e [ 23 ] . For the a n a lysi s , use is made of the cur r e n t s i gn a l s that arc caused either by the excess metal ions or by t h e n ewly formed m etal ion complex. In ac c ord a nc e with DIN 384 1 3, Pa r t 5, both NTA a n d EDTA arc d ete rm i n e d after the addition of bismuth (I II) nitrate to the sam p le acidified to pH 2 wi th nitric acid as detailed below. Application 18 Polarographic determination of nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid ( EDTA) [DIN 38 4 1 3, part 5] S a m ple : Waste water, surface water. 1 Add 1 m l 69% H N 03 to 1 L of sa mple, adjust to pH 2 a n d f i l te r th ro ug h a 0.45 �m Pre-treatment: m e m bra ne filter. The sample may now be stored for up to one week at 4°C. 2 To rem ov e su rfacta nts and other i n t e rfe r i n g o rg a n ic i m p u riti e s , add 10 g KN03 to 1 00 m l of acid ified s am p l e , pass i t throug h a n ad so rpt i o n col u m n fil led with XAD 2, pre -co nd i t io n e d with methanol and d i scarde the fi rst 20 ml. Pass the re ma i n i n g 80 ml t h ro u g h a c a t i o n e x c ha ng e col u m n with t he res i n in the Na form . Di sca rd the fi rst 20 m l. The rem a i n ing 60 ml is rea dy for t h e polaro gr a p h i c dete r m i n a ti o n . 1 B i - NTA ( "" 1 g H3 NTA L-1 ) . (a) Di ssolve 4. 5 g B i ( O H h N 0 3 i n 3 0 ml 69% H N 03 Stock sol utions: and d i l ute to 400 ml w i th d i sti l led water. (b) D i s sol v e 1 g N T A in 20 m l 2 M NaOH a n d d i l ute to 400 m l with d isti l l ed water. M i x a w it h b and make up t o 1 L. Th e solution should have pH 0. 7 a n d is stable for 4 weeks . 2 Bi-EDTA ("" 1 g H 4 E D TA L-1 ). (a) Dissolve 3 . 1 g Bi(OHhN03 in 30 ml 69% H N 03 and d i l ute to 400 ml with d isti l l ed water. (b) D issolve 1 . 274 g N a 2 E DTA. 2 H 20 i n 20 m l 2 M N a O H a n d d i l ute to 400 m l with p u re water. Mix a wi t h b a n d m a ke up to 1 L. The so l uti o n is sta ble for 4 weeks. 1 B i ( 2 g L-1 ). D i ssolve 2.8 g Bi(OH)2N03 i n 2 5 m l 69% H N 03 a n d d i l ute to 1 L Sta n d a rd s : w i t h p u re wate r. 2 Bi- NTA ( "" 1 00 mg H 3 N TA L- 1 ). Mix 10 m l B i - N T A stock solution with 50 ml pure water a n d 1 5 ml 2 M H N 03 i n a 1 00 m l vol u metric f l ask and make u p to the mark. The sol ution is stable for a bo ut 1 week . 3 B i - E DTA ( "" 1 00 mg H 4 E DTA L-1). M i x 1 0 m l B i - E DTA sto ck solution with 50 m l p u re water and 1 5 ml 2 M H N03 i n a 1 00 ml volumetric flask and m a ke u p to the mark. The solution is sta ble for about 1 week . Determi nation: P lace 20 m l of pre-treated s a m ple i n the pola ro g ra p h i c c e l l , a dj ust the pH to 2, add 800 mg asco rb i c acid, deaerate with n itrogen fo r 1 0 m i n a n d record the DP- p o la r ogram between +0 . 1 V a n d -0.6 V. Th i s g ives the base cu rre nt w h i ch should h ave n o current pea ks, o t h e rw i se the s am p l e pre-treatment is i n ade q u ate and s h o ul d be c h e c ke d . Add 5 0 � L B i standard solution, deaerate a n d record the D P-po l a rogra m . N ote t h a t th e h e i g ht of the p u re Bi p ea k m u st be at l east twice that of either of the peaks due to the B i - c omple xe s (see F ig u r e 7. 5). If the B i peak is less than this, the B i concentration i n the sample solution m ust be increased and t h e D P - p o l arog ra m re-recorded . =
Appl i cations: orga n i c species
1 87
Bi3+
i [nA]
(E
700
p
= -
0 . 05
V)
4
600
500
B i - E DTA
Bi - NTA (E p
400
=
-
0 .25
( Ep
V)
=
-
0.48
V)
4
4
300
200
1 00
+0.05 0
Figure 7 . 5
- 0.4
- 0.2
E
- 0.6
[V] (vs . Ag/AgC I , 3 M KC I)
Polarographic determination o f NTA and E DTA: (1 ) residual current; (2) after additi o n of Bi 3 + sta ndard; (3) after first addition of standard B i -NTAIE DTA solution; (4) after second addition of sta ndard Bi-NTA/EDTA solution.
H 3 N T A a n d H 4 E DTA were eva l uated b y sta n d a rd additions ( 2 ) of the respective sta n d a rd B i - NTA and B i - E DTA s o l u t i o n s as i l l ustrated in F i g ure 7 . 6 .
Eva l uati o n :
T h e concentration o f
I n o rder t o c h eck t h a t the s a m p l e p re-tre a t m e n t d oes not i ntrod u ce erro rs i nto
Note s :
the a n a lysis, it is necessary to determ i n e the recovery rate of the p roced u re . T h i s is d o n e by s u bject i n g 1 00 m l a l i q u ots o f e a c h of the
N TA
and
E DTA
sta n d a rd
solutions to the s a m e p roced u re as t h e s a m p l e ( a dsorpt i o n col u m n , cati o n
exc h a n ge, etc . ) . The recove ry rate, R, m ust be > 90% a n d i s co n s i dered i n the eva l u at i o n of the resu l ts. R i s given by:
R (% ) = A X 1 00/8
w h e re A i s the m e a s u red concentrati o n and 8 i s the added concentrati o n of
H3NTA o r H 4EDTA .
The va l ue of R i s used to co rrect the m e a s u red concentrati o n of each a n a lyte i n the s a m p l e a s fo l l ows: Cfound X 1 00/ R cm rrected =
Peroxides ( R - 0 - 0 - R) and hydroperoxides ( R - 0 - O H ) arc polarographically reduced in a two-electron step as follows: R - O - O - R' + 2H ' + 2e(R
=
H, alkyl or
•
R - OH + R ' - OH
a cy
l ( R - CO -) ) .
Hal f-wave potentials for selected peroxides a n d h yd ro pero xi d es are listed i n Table 7.4. I n a homologous s e r i e s of acyl and al kyl hydroperoxides and of dialkyl peroxides the half-wave
1 88
I ntroduction to Voltammetric Analysis
i
[nA] 400
300 1 . Addition
7
6
5
8
9
10
Concentration NTA or EDTA
F i gure 7 . 6
Table 7.4
Concentration of N TA or E DTA in the sample in mg L- 1
added to the solution in
mg L-1
Evaluati on o f polarographic data by t h e method o f standard additions.
Pol aro g ra p h i c half-wave potentials of peroxides and hydropcroxidcs [ 1 4 ] .
E112(V vs SCE)
Co mpound
Supporting electrolyte
Methylhydroperoxide
0. 1 M LiCl
-0.75
Ethylhydroperoxide
0. 1 M LiCl
-0. 2 5
n-Hexy lh ydro p eroxi d e
0.3 M
n - Octylhydroperoxide
0.3 M LiCl
(I:I )
-0.02
t - Butylhydroperoxide
0.3 M LiCl in benzene-methanol ( 1 : 1 )
- 1 .00
n - A mylhydroperoxidc
LiCI in benzene-methanol ( 1 : 1 )
0 . 3 M LiCl i n benzene-m ethanol ( 1 : 1 )
in benzene-methanol
-0.20 -0. 1 2
-0. 7 1
Dimethylp eroxide
0 . 1 M LiCl
Diethylperoxide
0.1 M LiCl 0.3 M LiCl in benzene-methanol ( 1 : 1 )
0.00
D i l a u roylperoxidc
0 . 3 M LiCl in b e n z en e-m eth a n ol ( 1 : 1 )
-0.09
D ib en zoyl p e roxi de -- · - -
-0.25
p o t e nti a l s become more positive with increasi ng molecular weight. Because of the range of h al f wave pot e nt i a l s encountered it is possible to polarogroaphically determine the components i n a mixture of peroxides. for i nsta n ce , in a mixture of hydrogen peroxide, methyl hydroperoxide, t-butylhydroperoxide, acetylhydroperoxidc and diethylperoxide, each component can be deter mined polarographically in one run [ 24 ] . D e pendin g o n the pH, 1 , 4 - benzoq uinone i s r e duce d a t the m ercury electrode i n aqueous solu tions between +0. 1 and -0.8 V ( ve r sus SCE ) to give a single two-electron wave. The reduction
Applications : organic species
0.0
Figure
7.7
- 0.2
1 89
- 0.4
E [V] (vs. SCE)
Pola rog ra ms of 5 x 1 0-4 M vita m i n K1 : (a) d .c. pola rogram; (b) a . c . pola rogram; and (c) DP pola rog ram recorded at pH 7 using a D M E [2 5 ) .
actually proceeds via two one-electron steps through the intermediate sem iquinone radical (which is unstable in aqueous media) as follows: o
=C)=
o
+ e +
H
Quinone
+
._
�
0
=0--
0H + e + H
+
_...
HO
-o-
OH
Hydro q u inone
( S e m i q u i no n e )
In aprotic media such as acetonitrile o r dimethylformam i de, in which the semiquinone radi cal intermediate is stabilised, 1 ,4-benzoquinonc gives two one- electron waves. The half-wave potential for the reduction of qui n ones is influenced by the nature, position and number of sub stituents as can be seen from the half-wave potentials listed in Table 7.5. Two important quinones which have been studied polarographically are vitamin K 1 ( T ) (2-methyl-3-phytyl- 1 ,4-naphtho quinone) and vitamin K3 (II) (2-methyl- 1 ,4-naphthoquinone) . The d.c.- , a.c.- and pulse-polaro grams of vitamin K3 are shown i n Figure 7.7. Using pulse polarography, the determination limit for vitamin K1 was found to be 0 . 1 1 mg L -I an d that for vitamin K3 to be 0 . 1 7 mg L- I [ 25 ] . 0
0
H 0
(I)
(II)
1 90
Table
Introduction to Voltammetric Analysis
7.5
Polarographic half-wave potentials of selected quinones [ 26 ) .
--- ---
Compound
--- ---
---
---
---
---
---
--- - - --- · --- --- · - --- - - --
Supporting electrolyte
---
---
E112(V vs
O . l M TEAP in acetonitrile
-0. 3 1
1 ,4- Benzoquinone
0. 1
-0.5 1
M TEAP in acetonitrile
2,6-Dimethyl - 1 ,4-benzoq uinonc
-0.66
2,6- Di- isopropyl- ! ,4 -benzoquinone
-0 .70
2,6-Di -t-butyl- 1 ,4-benzoquinone
-0.74
2,6-Diphenyl- l ,4-benzoquinone
-0.34
2,3,5,6- Tetramethyl- 1 ,4-benzoquinone
-0.84
2 - Chloro- l ,4-benzoquinone
-0 .34
2 , 5 - Dichloro- 1 ,4-benzoquinone
-0. 1 8
2,3- Dichloro - 5 ,6- dicyano- l ,4-benzoquinone
I ,4-Naphthoquinone
-0. 5 1 Phosphate buffer - p H 3
+0.08
pH S
-0.07
pH 7
-0. 1 7
9 , 1 0 - Ant hr a qu i n o n e
0.1 M
9, I 0- Phenanthrcnequinone
0.2 M Me1NOH
9 , 1 0 - Naphthacenequinone
---
---
---
----
----
SCE)
--- - - --- - --
1 , 2 -Benzoquinone
Me4NOH in 40o/o dioxane
0. 1 M LiCl in 80o/o ethanol
----
---
---
---
-0.60
-0.40 0 6 3 -1 .28
-
.
---
,
---
In acid solution, aliphatic nitro compounds are reduced at relatively positive potentials in a single step to the corresponding hydroxylamine.
The further reduction to the amine
occurs at quite negative potentials and can only be observed polarographically in alkaline media. In acid solution the polarographic wave corresponding to this process is obscured b y the hydro gen evolution current . Aromatic nitro-compounds likewise are reduced through the hydroxylamine to the amine in two steps. In most cases both waves can be observed in acid solution. Some nitrophenols and nitroanilines are reduced directly to the amine in a single step; while in alkaline solution the reduction proceeds only to the hydroxylamine. Tetranitromethane gives only a single polaro graphic wave in both acid and basic solution, while 1 ,2,3-tri-nitrobenzene gives two waves and picric acid is reduced in three steps. Substituents in the benzene ring influence the reduction potential of the nitro group( s ) ; the m agnitude of the effect depending on the electrophilic charac ter and position of the substituent group. In all cases the half-wave potentials become more posi tive as the acidity of the solution is increased. Polarographic data for selected nitro-compounds are given in Table 7.6. The reduction potential range of both aromatic ( E 12 from ca. -0. 3 to --0.8 V) and aliphatic 1 (E112 > -1 . 3 V) nitro compounds, and the num ber of electrons i nvolved in the electrode reaction ( n == 4 or 6 compared with n = 2 for most other organic compounds) means that many important pharmace uticals, agricultural products and explosives can readily be determined by the polaro graphic reduction of their nitro groups with high sensitivity, for example < 10 8 mol L- 1 using DPP [ 19, 27, 28 ] . The presence of the reducible n itro group in the carcinogenic 4-nitroquinoline-
Ta ble 7.6
Polarographic half-wave potentials for selected nitro-compounds.
4) E11 2 [ 1 (V vs SCE)
Compound
Supporting electrolyte
pH
Nitromethane
BR buffer in 3 o/o methanol
1 .8
-0.79
4.6
-0.89
8.0
-0.94
1 1 .6
-0.94
BR buffer in 30o/o methanol
Tetranitromethane
1 .0 M NaOH/2.0 M NH4CI/ 1 0o/o methanol ( 1 : 5 )
Nitroethane
0.3M
1 - Nitro p ro p ane
BR buffer in 30o/o methanol
LiCI - methanol/benzene ( 1 : 1 )
1 2.0
4.6
-0.8 1
1 1 .6
-0. 8 9
4.6
-0.79
- 1 . 20
0.3 M LiCl - methanol/benzene ( 1 : 1 )
- 1 .20
-1 . 3 5
2,2- Dinitropro pane
-0 .90
1 -Nitrobutane
- 1 .26
2 -Nitrobutane
- 1 .35
I ,3- Dinitropropane
- 1 .20
Buffer solution
with d ioxa n
1 .0
-0.28, -0.88
7.2
-0.66
1 2.0 1 ,2 - D initrobenzene
) E112 [ 29
(V vs SCE)
-0.4 1
2- Nitrop ro p ane
Nitrobenzene
pH
)> "Q_ '0
-0.32, -0.58
2.5
-0. 1 2, -0.32, - 1 .26
I , 3 - D initrobenzenc
5.7
-(J.35 , -0. 5 1
2.5
-0 . 1 7 . -0.29
1 ,4-Dinitrohen zcne
5.7
-0 .32, -0.59
2.5
-0. 1 2 , -0 . 3 3
4. 1
-0. 1 9, -0 .3 1 , -0.45
1 ,3 , 5 -Trinitrobenzene
NaO H/ N H 4 Cl ! ! Oo/o meth a nol
2,4,6-Trinitro p h enol
Buffer solution
2,4,6-Trinitrotoluene
Phthalate bu ffer
1 2.0 1 1 .7
-
0 . 35 , -0.44, -0 . 6 1
-0.40, -0.60, - 1 .0
!':. c;·
;:; ·
-0. 8 7
5.7
KHPhthalate-NaOH buffer in 8o/o ethanol
::::J � 0
.a "' ::::J ;:;· "' '0 "' n "'
;;; ·
.. � ..
1 92
Introduction to Voltammetric Analysis
N-oxide enables it to be determined in the presence of 4-hydroxyami noquinoline-N-oxide and 4aminoquinoline- N-oxide using differential pulse polarography [ 30 ] . Nitrazepam, parathion, nitrofurantoin and the nitroimidazoles in blood, plasma or urine samples can also be determined via the reduction of the respective nitro groups. At pH 0-5
( 7.4a)
and at pH 9- 1 2 Alkyl
'\ � - N0 + 2e- + 2H+
"'-.
-H2C
N - H + .l N 2 0 + .l H , O 2 2 -
/
(7.4b)
-H 2C
C-rzitroso compounds are reduced to either the amine or the hydroxylamine. In weakly acidic to basic solution nitrosobenzene is reduced to phenyl hydroxylamine while 4-nitrosophenol is reduced to 4 -aminophenol. The mechanism for the reduction of N-nit roso compounds (nitro samines) is summarised in equations 7.4a and 7.4b. The determination of nitrosamines by differ ential pulse polarography is quite sensitive, for example, N-nitroso -N-methylan iline in blood, serum or albumin, is read ily determined in the jlg kg- 1 range [ 3 1 ] . The polarographic and voltam metric behaviour of the nitrosamines, quinones, steroid hormones and imidazoles is important in connection with cancer research [ 32 ] . A number o f nitra tes, that is compounds containing the -O-N02 group, used i n medicine can be analysed polarographically via the reduction of the nitrate group. For example, the determina tion of glycerol trinitrate in pharmaceutical products is carried out by dissolving the product directly in the supporting electrolyte (0. 1 M NH3 + 0. 1 M N H 4Cl in 20o/o ethanol) and recording the DP-polarogram [ 1 , 3 3 ] Similarly, isosorbide din itrate { 1 : 4, 3 : 6,-dianhydrosorbitol-2,5dinitrate-( I ) } and isosorbide-5-mononitrate ( I I ) can be determined in medical preparations and the endo and exo forms of the mononitrate distinguished polarographically ( E1 12(endoJ -0.45 V and E112(exoJ -0.38 V versus SCE) [ 1 9, 34] . .
o2N -b:J
=
=
(en do)
(I)
(exo)
(II)
In the absence of conjugated systems, aliphatic N-oxides are reduced at relatively negative potentials. Similarly the reducti on potentials of th e pyridine and quinoline N-oxides are in the
Applications: organic s pe c i es
1 93
Cl
0
- 0. 9 E
Step A:
+ 2e -
= N-
+ 2H
•
0
'
Step B:
Step C:
[ V] (vs. Ag/AgCI)
/
Cl
+ 2e
c=N-
J:X.
N=
O
H"
-
+ 2H C
C-N
, �
+
+
•
' /
CH - NH -
,. NHCH3
:
CH2
'H
Cl
�
Figure 7 . 8
D.c. polarogram of 0 .35 mM chlord iazepoxide i n a c eta te buffer (pH
4.2) [ 1 4, 35].
region of - 2 . 3 V t o -1.8 V ve rsus SCE. The presence of other polarographically active groups in the molecule facilitates the r edu c ti o n of the -N�O group. In the c a se of chlordiazepoxide (see Figure 7.8) the reduction of the -N�O group and the two azomethine ( >C==N-) groups occur at different potentials resulting in three wel l separated waves with that associated with the reduction of the -N �0 group occurring at the least negative potential ( E112 - 0 5 7 V versus Ag/AgCl) [ 3 5 ] . Chlordiazepoxide i s one o f a series o f 1 , 4-benzodiazepines which are extensively used as psy chotropic d r ugs , particularly as anti-depressives and tranquillisers. The parent compound, 1 ,4benzodiazepine contains two electroactive azomethine groups in the diazepine ring. These com pounds can be dete rmi n ed in the jlg L- 1 range by differential pulse polarography [ 3 5 , 3 6 ] in which the peaks arising from the reduction of the azomethine groups or, if present, other electroactive -
.
1 94
I ntroduction to Voltammetric Analysis
substituents may be used for the analysis [ 1 4 ] . Peak potentials associated with the reduction of the electroactive functions in several of these drugs are listed in Table 7.7.
o:-> 5
4
1 , 4- Benzodiazepine
The current peak arising from the following reduction which is common to all of these com pounds is the one normally used for their determination.
Cl
n
Application 1 9
CH 3 I
0
N-C
C=N
'
C H2
I
+
-
2e
+
2H
+
-----1•�
CI
6
0 It
f"""y N - C> I
C H3
It
� CH - N
H,
'
6"
Polarographic determination of diazepam i n body fluids and pharmaceutical products [37]
Sample: Pre-treatment:
Ta blets, seru m , u ri ne 1 Ta blets. Grind 1 0 weig hed tablets i n a mortar. Pl ace a known weight of the powde r in a beaker, sti r with 3 5 m l methanol for 20 m i n , rinse the m ixtu re i nto a 50 ml vol u metric flask with methanol : water ( 1 : 1 ) a n d make up to the mark. Store in a r efr i g erato r to a l l ow the insol u b l e material to settle pr i or to the anal ysis of the d rug . 2 Blood, serum, urine. Add 1 5 m l blood (hep a r i n added to prevent coagulation) to 40 ml n-penta ne in a separating fu nnel, shake 2 m in and sepa rate the phases. Extract the blood phase with two further 20 ml a l i quo t s of n-pentane. Combine the n - pe n ta n e e xtrac ts an d ce ntr ifuge for 1 0 min at 7 500 rpm . Sepa rate from any solid and disti l the n-pentane at 70°C. Dissolve the residue in 500 �L methanol and rinse into the polarographic cel l with 1 4 . 5 m l of supporti n g electrolyte . Use the same scheme to p re-treat the seru m a n d urine sa m p les B ritton-Robi nson buffer of pH 2 . 8 i n 20% methanol. Dry 0. 5 g d iazepam at 70°( for 24 h . Dissolve 1 00 mg of this i n 50 m l metha nol and m a ke u p to the mark i n a 1 00 ml vol u metric flask with pure water. Store in a cool dark place . Th is 1 g L -I stock sol ution is stable for a b ou t 1 week. Worki n g standards of 50, 1 00 and 200 m g L- 1 a re p r ep a re d by d i l u ti ng the stock solution with 50% methanol. They m ust be stored i n a cool d a rk place a n d a re stable for 2-3 d ays . Place 1 ml tablet extract with 1 9 m l s u ppo rt i n g electrolyte i nto the polaro gr aphi c cel l (or use the 1 5 ml of solution from the pre-treatment of a blood, serum o r urine sa mple) and deaerate with n itrogen for 5 m i n . Record the DP pol a ro g ra m from -0 . 5 V to -0. 9 5 V. The d iazepam peak a ppears at c a . -0. 7 3 V .
.
.
S u p po rtin g e lectrolyte: Diazepam standard :
D ete r m i na tion :
Table 7 . 7
DP polarograph i c peak potentials of selected 1 ,4 -benzodiazepincs measured at p H 4 a n d p H 1 2 i n Britton-Robi nson buffers ( �, V v s SEC) [ 36) .
Supporting electrolyte
Functional groups: Compound
-N02
=N-tOH+ -0. 3 7
Chlordiazepoxide
>C=NH+-
Medazepam
.
1 12
-0. 1 5
-0.60
-1 .22
-0. 1 6
-0.73
-0 .6 1
- 1 .50
Cl o n a ze p a m Flunitraze pam
Flurazepam
- 1 .24
-1 . 1 7 -
>C=N-
-0.73
-0.78
P r aze p am
-N02
-0.99
-0. 1 6
Lorazepam
-N=C
10 mg L _ , _ in order for the conce ntration to remain in the l i near iP vs c ra nge. 2 The reproducibility of the measurement is not influenced by the slight asym metry of the curve. 3 As w e l l as d i a z e pa m , 1 , 4-benzodiazep i n e and other members of the 1 ,4-benzodiazepine group (e. g . nitrazepam} may be determined by stripping voltam metry.
No t e s :
In addition to the diazepi nes, other pharmaceutical products and pesticides which contain the reducible azom ethine group can be determined polarographically [ 3 8 ] . In oximes, the a zomethinc group is reduced in acid solution as follows:
>
>
>
C
=N-
N - OH + H + � > C = N - OH ; OH; + 2e- + 2H+ > CH - NH - OH;
C
=
CH - NH - OH� + 2e- + H '
� >
CH - N H 2 + H 2 0
A single pola rographic wave is obtained for th e reduction to the amine. Information on the biotransformation of drugs and the excretion of the drugs and their metabolites is important pharmacologically. Numerous methods for the polarographic and volta mmetric determination of pharmaceuticals and their metabolites have been developed for the assessment of such relationships. The determ ination is often preced ed by separa tion from the sample ma trix by liquid-liquid extraction with diethyl ether or ethyl acetate. As an example, the flow diagram for the polarographic analysis of flurazepam and its metabolites in body fluids [ 3 9 ] i s presented i n Figure 7 . 9 . Other pharmacollogically active 1 ,4-diazepines can b e determined polarographically via the reduction of their azomethine groups. In addition to flurazepam, the related drugs, bromazepam , chlorazepam, chlordiazepoxide, diazepam and lorazepam may be determined in th e flg L- 1 range using AdSV or DPP [ 1 4 ] . The po larographic behaviour of the azo compounds depends o n the p H and o n the nature and position of the substituents in the benzene rings. Azobenzene is reduced to hydrazobenzene in a si ngle step:
I n acidic solution, 4-aminoazohenzene, and 2 - and 4-hydroxyazobenzene a re reduced in a four-electron process to the respective amines, for example 4-hydroxyazobenzene gives aniline and 4-aminophenol. Nitro groups or hyd roxy groups in the 3 -position stabilise the hydrazo prod uct. In general, the presence of substituents shifts the h alf-wave potential to more negative values. Azo dyestuffs, used in a va riety of matrices, are commonly determined by pola rographic and vol tammetric methods [ I , 1 9, 40 ] . Nitrogen heterocyclic compounds, such as pyridine, are reduced in a two-electron step: A number of alkaloids which are derivatives of pyridine can be determined on the basis of this reaction. Nicotine ( 3 - ( l - methyl-2-pyrrolidinyl) pyridine) is one such alkaloid which can be easily determined in the Jlg L - 1 range. +
2 H
+
+
2e
Applications: orga nic sp ec i es Extract s a m pl e with 2
8
Fl urazepam, O H , COOH, N H , x
NH-3-0H
m l ethyl acetate
Organic phase
Aq ueo u s phase (COOH)
(fl u razepam , OH, N H , N H -3-0H)
2
residue
3, e xt ra ct with 1 2 ml d i eth y l et h er
Aqueous
phase
in 10 mL B R buffer, e xt ract 1 0 ml d i ethyl ether
Evaporate to d ryness, take u p
Adjust p H to x
1 97
with
2
x
ic ph a s e
Organ ic phase
Aqueous phase
O rgan
(COOH)
(fl urazepam)
(OH, NH, N H-3-0H)
2 ml (pH 4),
with 2
formate buffer
9, e xtract 1 4 ml diethyl
Adjust p H to
Evaporate solvent, take up residue in
ethe r
x
rec o rd polarog ram t o
Eva porat e s o lvent ,
1 0),
2 ml b u ffe r reco rd polarogram
take up residue in (pH
to dete rmine metabolites
d e te r m i ne COOH
Aqueous phase
Organic phase
r es i due in 2
Evapo rate solvent. take up m L formate buffe r
(pH 4 ) , record pol arogram to
fu
dete rmine l raze pa m
Figure
7_9
F low diagram for the polarographic determination of flurazepam and its metabolites in body fluids [39) . OH N-1 -hydroxy metabol ite; COOH N- 1 -acetic acid metabolite; NH N- 1 -dealkyl metabol ite; N H -3-0H N-1 -dealkyl-3-hydroxy metabolite. =
=
=
=
Application 20
Polarographic determination of nicotine [ 41 ] Sample:
Supporti ng electrolyte : Nicotine sta n d a r d : Dete rm i nati o n :
Tobacco, tobacco s m o k e . B ritton-Robinson buffer of pH 6 . Mix 40 m l each of 1 M H l 0 4 1 M C H l O O H a n d 1 M H 3 B 0 3 w i t h 425 m L 0.2 M NaOH a n d m a ke u p to the m a rk i n a 1 L flask. Di ssolve 1 00 mg n i coti n e i n p u re water a n d m a k e u p to 1 00 m l. A s n i cotine is l ig h t sensitive this solution should be made u p f r e q u e n tly . 1 I n tobacco . Place 1 g of toba cco i n a 1 00 m l vol u m etric flask with 50 m l p u r e water a n d 1 m l 2 M NaOH, s h a k e a n d let stan d for 1 2 h . Make up to th e m a r k with pure wate r and fil te r. Add 1 00 � L of filtrate to 2 5 m l of su pporti ng electro lyte, deaerate a n d record the D P - polarogram b e tw e e n - 1 . 0 V and -2 . 0 V: the pea k occurs at a b o u t - 1 .26 V (versus Ag/Ag C I , 3 M KC I ) . 2 I n to bacco s mo ke . Pass the s m o ke t h r oug h 50 m l su pporting el ectrolyte via a g lass frit (G 1 ) . De aerate 2 5 m l of this sol ution in the po l a ro gr aph i c cel l a n d record t h e DP polarogram as above . Calcu l ate the content by sta n d a rd a d d it i o n s . Q u oted determi nation l i m it is S �g l-1 .
The reduction of qui n o l i ne proceeds i n a similar way to that o f pyridine. The two-electron reduc tion is i rreversible, p roducing 1 ,2- or 1 ,4-dihydroquinoline in weakly acidic or basic solution. Acridine likewise is reduced by taking u p two electrons a n d gives two waves in the polaro gram.
1 98
Introduction to Voltammetric Analysis
Table 7.8
Polarographic half-wave potentials of some pyridines, quinolines and acridines
[14].
E1 12(V vs SCE)
Compound
Supporting electrolyte
Pyridine
Phosphate-citrate butTer
7.0
3-Acetylpyridine
BR buffer
2.0
-0.72, -0.87
6.0
-1 .07
1 0.0
- 1 .40
pH
-1 . 75
Pyridine-2-carboxylic acid
0. 1
M HCI
-0.89
Pyridine- 3 -carboxylic acid
0.1
M HCl
- 1 .08
Pyridine-4-carboxylic acid
0. 1
M HCI
-0.80
Pyridi ne- 2,3 -dicarboxylic acid
0. 1 M HCI
- 0 . 78
Pyridi ne-2,4-dicarboxylic acid
0 . 1 M HCI
Qui noline
0.2
8- Hydroxyquinoline
Acetate buffer
3.0
-0.89, -1 .08
Borate buffer
1 0.0
- 1 .48, - 1 .80
8.3
-0.79, - 1 .45
7.0
-0.65, - 1 .22
Acridine 1 -Aminoacridine
M Me4NOH in
-0.66
50% ethanol
Phosphate buffer in 50% ethanol
- 1 .50
Half-wave p o ten ti al s of some nitrogen h ete rocycle s are listed in Table 7.8. Q u in o l ine alkaloids such as the anti-malarial d ru g quinine, and the local anaesthetic dibucaine (or cinchocaine) (2-butoxy- N- ( 2 - ( d ie thyla m i n o ) ethyl ] -4-quinolinecarboxamide mono-hydrochloride) which un dergo two-electron reductions to the c o rre s po n di n g di hydroquinolines can r ea dily be determined polarographically. In an ac e t a t e buffered supporting electrolyte of pH 4.8, c inc h o ca i ne , give s two current peaks in the DP-polarogram. The first peak at -0.93 V is best suited for the concentration measurement. The second peak at about - 1 .25 V is distorted by im p u ri ties or hydrogen evolution. Because hydrogen waves may occur with these ni tro ge n bases it is best to use higher pH support ing electrolytes for their a na lys i s if p oss ibl e . Another problem that may occur is interference aris ing from the ad so rp t i o n of the analyte on the mercury electrode which may le a d to n o n - l i ne ar iP versus c pl o ts . Application 2 1
Polarographic determination o f ci n chocaine (dibucaine) i n pharmaceutical preparations [ 7 , 42]
Sample: Pre-treatment:
Su pporting e lectrolyte:
O i ntments, i njection sol ution . 1 I n j ect io n solution - none. 2 Ointments - v igo ro usl y stir 1 g of sample with 20 m l 1 M H C I for 1 5 min at 6 5°C to e xt ra ct the cinchocaine. Cool to room temperature, add 1 5 m l pure water, tra n sfer the solution to a centrifuge tube and centrifuge for 20 m i n at 7 500- 1 0000 rpm. Cool to 5°C, carefu l l y tra nsfer the aqueous phase with a p ipette to a 1 00 ml vo l u m et ri c flask. Vigorously rinse the pi p ett e and ce n t r ifug e tube (with 1 0 m l 1 M H C I), c e n tr i fu ge the washings as above. Withdraw the aqueous phase, comb ine it with the fi rst extract a n d make up to the m ar k with d i stilled water. A slightly turbid s o l u t ion will not affect the determination . Fi lter only if the solution is markedly turbid. Acetate bu ffe r , pH 4. 7-4 . 8 .
Applications: organic species
S ta n da rd s :
Dete rm i nation :
Disso lve 5 0 0 mg d ri e d cin choca i n e hyd roch loride i n p u re water, make u p t o the mark i n a 500 m l vo l u m e t r i c fl ask. T h i s stock solution contai n s 904 m g L-1 c i n c h oca i ne b a s e . The co n ce nt ra tion re ma i n s u n c ha n g e d ove r 4 w ee k s if t h e solu tion is stored i n the dark i n a refr i g e ra t o r . A wo r k i n g sta ndard is made b y d i l ut i n g 50 m l of t he stock w i t h p u re wate r t o 2 0 0 m l i n a vo l u metric fl a s k . This 1 conta i ns 2 2 6 m g L-1 cinchoca i n e (or 2 50 m g L- cin c hoc a i n e hydrochlori de) and should be sto r e d i n a cool dark place . Pl ace 1 m l o f o i nt me nt extract ( o r a su i t a b le vol u m e o f i n j e ct i o n solution) i nto t h e p o l a ro g ra p h i c ce l l wit h 1 9 ml su pporti ng e l e ctrolyte, deaerate a n d record t h e DP-po l a ro g ra m from -0 . 7 0 V to - 1 . 2 0 V. T h e cu rre nt p e a k occurs at ca . -0 . 9 3 V (versus Ag/AgC I , 3 M K C I ) . U se sta n d a rd additions to determ ine the
conce ntrati o n . Notes:
1 99
1 I n o rder t o rem a i n with i n t h e l inear iP ve rsu s
c range, t h e tota l a m o u n t of cin
choca i n base i n t h e 2 0 m l of solution i n the cel l ( i n c l u d i n g that i ntro d u ced by
the sta n d a rd additions) s h o u l d not exceed 3 4 0 �g (3 7 5 �g ci n c h o c a i n hyd ro c h l oride). 2
T h e dete r m i nation l i m it i s 2.8 �g c i n c h o c a i n base in 2 0 m l. ( 1 . 4 m g
L-1).
Quinine was used for a l o n g time a s a n anti-malarial dru g and i s now u sed a s a decongestant in numerous influenza remedies as well as a b itter substance in drinks such as tonic water. Q u in i n e in these products c a n be determined using DP-polarography. With a buffer at pH 7 as su p p o r tin g el e ctrol yte , the current peak occurs at about - 1 .03 V (versus Ag/AgCl, 3 M KCl) [43 ] . Other c om poun ds co n t a inin g nitrogen in a ring system, such as p i p e ri d i n e , indole and phe nothiazine, are n o t reduced at a mercury el e ct rode . Some of these, like piperidine, can be oxidised voltammetrically and the resulting anodic current peak can be used for their determination. Heterocyclic compounds with two an d three ni t ro ge n atoms in the ri n g, such as imidazole, pyrimidine, pyrazi n e and triazine, a re polarographically active and a re reduced to the corre sp on d ing tetrah ydro derivatives. Only a single wave is observed in the pol a r o gram s and the half wave p oten t ial is pH dependent. V i t a m in B 1 ( th ia m in e ) is a pyrimidine derivative and is present
+
+
2 H . 2e .,.
Tetrahydropyrimidine
in vitamin p rep arati o ns as the hydrochloride (1) or m o n o n i t r a t e (II) salt. The thiamine content can be de te rm i n ed at the f.lg level.
(I)
(II)
200
I ntrod uction to Voltammetric Analysis
Application 22
Polarographic determination of thiamine (v itamin B 1 ) [ 44, 45]
Sample: Pre-treatment:
Supporting electrolyte:
Th iamine sta ndard:
Determ inatio n :
N otes:
Vita m i n tablets a nd solutions. Weigh 1 0 tablets and grind to a powder. Wei g h 1 / 1 Oth of the powder (average weight of a tablet) i n to a beaker and stir with 30 ml 0.0 1 M NaOH for 20 m i n . F i lter i nto a 1 00 m l vol u metric flask and make up t o t h e m a r k . Vita m in solu tions can be a n alysed d i rectly. Disso lve 4 . 1 g an hydrous sod i u m acetate i n ca. 400 m l disti lled water, add 2 . 86 m l g l acial acetic acid and d i l ute to a bout 950 m l with d isti l led water. Adj ust to pH 6.4 to 6 . 6 with 2 . 5 M NaOH a n d make up to 1 L. For a 1 g L_ , stock solution of the th i a m i ne cation (C12H 1 7N40St, d issolve 636 mg of the hydroch loride (or 6 1 7 mg of the mononitrate) i n 500 m l of the supporting electro lyte . This solution is sta ble for a l o n g time if stored in a refrig e rator. M a ke u p sta ndard solutions daily by appropriate dilution with the sup porting e lectrolyte. Place 1 8 ml of supporting electrolyte and 0.4 ml of sam ple sol ution in the cell and deaerate with n itrogen for 5 m i n . Add 0.8 m l 1 % triton X- 1 00 and deaerate aga i n . Record the DP-polarogram using a DME between - 1 . 1 0 V and -1 . 50 V . The current peak of thiamine occurs at ca . - 1 .38 V (versus Ag/AgCI, 3 M KCI ) . Use standard additions to determine the concentration. I n order to remain in the l i near iP versus c range the a mount of thiamine i n the cel l should not exceed 5 !Jg. 1 The S M DE can not be used as work i n g electrode for this determ ination . 2 N i coti n a m ide and Fe( l l ) i nterfere with th is determi nation .
Another group of polarographically active nitrogen heterocyclic compounds are the pteridines which contain four ring nitrogens. Folic acid ( pteroylglutamic acid) and riboflavin (a benzo [ g ] pteridine) which are both members of the vitamin B group are pteridines and can be determined polarographically. Folic acid is reversibly reduced in a single 4-elcctron step to tctrahydrofolic acid while riboflavin is reversibly reduced to dihydroriboflavi n in a single two electron step.
+
Folic acid H
� � y� A Jl _)c OH
N
2NH
H
H2N
N
I
N
H
H
HOOC
Tetrahyd rofolic acid
'Q I
h
I
C
/0 /
, NH
� CH I
COOH
-
4e , 4 H
+
Appl ications: organic species
201
Application 23
Polarographic determination of folic acid [ 46] Sample:
P re-treat men t :
S u p po rt i n g e l ectrolyte :
F o l i c acid sta n d a rd :
I nject i o n sol u t i o n , vita m i n ta b l ets .
I nject i o n so l ution s ca n be a n a lysed d i rectly afte r a dj u sti n g the pH to 8 . 0 with N a O H . G ri n d 1 0
vitamin
t a b l ets to a powd e r . Add 200 m g of the powd e r to
3 0 ml wate r in a bea ke r , adj u st the p H to 8.0 with 0 . 1 M N a O H and st i r fo r 1 5 m i n . C heck the pH a n d if ne cessa ry re-adj ust to 8 . 0 . F i lter t h e s o l u t i o n i nto a
1 00 m l vol u metric fl a s k a n d m a k e up to the m a r k w i t h a l ka l i n e wate r of pH 8 .
D i s s o l ve 6.2 g bori c acid i n 1 00 m l p u re water with 2 g N a O H . D i l ute t o 9 5 0 m l w i t h p u re wate r, a dj u st to p H 1 1 . 1 to 1 1 . 2 with
vo l u m etri c f l a s k a n d
2M
N a O H , tra n sfer it to a 1 L
m a k e up to t h e m a r k with p u re wate r .
P l ace 1 2 7 . 5 mg 9 8 % fol i c a c i d i n a b e a k e r w i t h 8 0 m l p u re wate r a n d wh i le
sti rri n g a d d 0 . 1 M N a O H u n t i l the sol uti o n i s cl ea r a n d pH 8 . 0 . D i l ute to 2 5 0 m l i n a vo l u metric flask, t ra n sfe r t o a b rown bott l e a n d store i n a refrig erato r . T h i s stock sol ution s h o u l d be m a d e u p d a i ly. D i l ute a s req u i red to obta i n t h e sta n Dete rm i n at i on :
d a rd sol u t i o n s . Add 0 . 5 m l of s a m p l e s o l u t i o n to 1 9 . 5 m l of s u p p o rt i n g e l e ctrolyte i n t h e pol a ro g r a p h i c ce l l a n d d e a e rate. Record t h e D P - p o l a rogram between -0 . 7 V a n d - 1 . 2 V. Use sta n d a rd a d d itions to d ete rm i n e the fo l i c acid conce nt rati o n
from t h e c u rrent pea k at ca . -0 . 9 7 V ( ve rsus Ag/Ag C I , 3 M KC I ) . E n su re that the
conce n trati o n of fo l ic acid rema i n s with i n the l i ne a r iP v ersu s c ra n ge ( 1 . 5 �g to 1 7 5 �g in 20 m l) .
Riboflavin c a n be determined i n a similar manner [ 4 7 ) .
Application 24
Polarographic determination of riboflavin (vitamin B2)
Sample: P re-treatment:
Vita m i n p re p a rati ons, s o l u ti o n s and ta b l et s . S o l uti o n s ca n be a n a lysed d i rectly . Wei g h 1 0 tablets a n d g r i n d to a powde r. Wei g h 1 / 1 Oth of the powd e r i n to a 1 2 5 m i n i u m fo i l . Add 8 m l 0 . 2
M
ml
E rl e n meyer flask covered with a l u
KO H a n d pass nitrogen t h r o u g h t h e s o l u t i o n wh i l e
sti rri n g for 1 5 m i n . F i lter i nto a 1 00 m l vo l u m et r i c flask fi l led with n itrogen a n d m a k e u p to the m a rk wi th oxyg e n -free p u re water. C over t h e f l a s k w i t h a l u m i n S u ppo rti n g e l ect ro lyte : Ri boflav i n sta n d a rd :
i u m fo i l a n d store i n the refri g erato r .
Di ssolve 3 . 73 g K C I a n d 1 6 . 5 g K2C 0 3 . 1 . 5 H 2 0 i n 1
L oxyge n -free p u re wate r .
D i ssolve 1 0 m g i n 8 m l 0 . 2 M KOH b y b u b b l i n g n i troge n t h ro u g h t h e s o l u t i o n .
U s i ng oxyg e n -free p u re water, r i n s e t h e s o l u t i o n i nto a 1 00 m l vo l u metric flask covered i n a l u m i n i u m fo i l , m a ke u p to the m a rk and sto re i n a refrig e rato r.
Determ i n at i o n :
Deae rate 1 9 m l s u p p o rt i n g electro l yte i n a d a r k g l ass p o l a ro g ra p h i c cel l with
n itroge n for 5 m i n , a d d 1 ml s a m p l e s o l uti o n and deaerate fu rther. U s i n g a
D M E , reco rd the D P- p o l a r o g r a m betwee n -0 . 3 0 V a n d -0 . 7 0 V. The c u rre nt
p e a k occu rs at -0 . 5 5 V (ve rs u s Ag/Ag C I , 3 M K C I ) . The d eterm i n at i o n l i m it is ca .
N otes
1 . 3 mg L- 1 sa m pl e vol u m e . W h e n dete r m i n i n g t h e co n centration by sta n d a rd 1 a dd itio ns , the l i nea rity r a n g e of 1 - 1 5 mg L- m u st be born i n m i n d . 1 Al ka l i n e sol u t i o n s o f ri bofl a v i n a re ra p id ly d ecom posed by atmospheric oxy
g e n , l i g ht, a n d heat. The refo re it i s necessary to work u nder n itrogen in d a r k e n e d vesse l s .
2 Trace levels o f ri bofl avi n ca n b e determ i ned by a dsorptive stri p p i n g volta m me
try [48 ] .
202
Introduction to Voltammetric Analysis
E - 0 . 04 V P
Cyste ine
Cyst i n e EP - 0 . 4 V
- 0. 1
Figure 7 . 1 0
- 0.4
-
E [V ] (vs. Ag/AgCI , 3 M KCI) 0.8
+ 0 .2
0
- 0.2
E [V] (vs. Ag/AgCI, 3 M KC I)
DP-polarographic determination of cystine a n d cysteine i n 0. 1 M H C I04 [49) . Each standard addition 1 00 119 cystine or 5 0 119 cysteine. Sample contained 5.6 m 9 l-1 c yst i n e and 2.3 m9 l-1 cysteine.
Organic sulfur compounds such as the disulfides, sulfonamides, aromatic sulfonic ac i d esters, sul fones, sulfoxides, thiobenzophenone, thioethers , and thiocyanates c a n be p o l arogr aphically reduced. A se l ect i on of polarographically ac t i ve sulfur compounds is listed in Table 7.9. The polarograp h i c reduction of disulfides results in s plitti ng the - S - S - bond to produce th e c o r responding t hi o l s :
The determination of cyst i ne and the ox.idised fo rm of glutathione is based on this reaction . When the de gradat i on of bio l o gical s a m p l es leads to bo th cystin e and cysteine being formed it is often n ecessary to determine the ratio of these two amino acids. This may be car ri e d out by recordi ng the DP-polarogram between -0. 1 V to -0. 8 V for cyst ine and between +0.2 V a nd -0.2 V ( versus Ag!AgCl, 3 M KCl) for cyste i ne using 0. 1 M HC104 as s u p porting electrolyte [49 ] . Typical D P pol a rogram s arc shown in Figure 7 . 1 0 . Sulfones and sulfonamides can only be reduced polarographically when the - S02 - NH - or - S02 - groups ar c conj ugated with a d o u ble bond. The reduction of sulfonamides may lead either to the splitting of the - S - N - bond or, in the p r esence of electronegative substituents in the benzene ring, to the splitting of the - C - S - bond [ 50] .
Appl ications: organic species
Tab l e 7 . 9
Polarograph i c h a l f- wave potent ials of selected orga nic s u l fu r compounds [ 1 4 ] .
Compound
Supporting electrolyte
Methyl benzenes ulfonate
0. 1 M TEAl in 1 0% ethanol
E112
( V v s NCE)
-2.06
Phenyl benzenesulfonate Diethyldisulfide
203
- 1 .96
0.025 M TBAOH in water
+ isopropanol +
- 1 .82
methanol ( I : 2 : 2 ) (pH 1 2 . 3 )
Di- (n- propyl ) disulfide
- 1 .84
Di- (isobutyl ) - disulfide
- 1 . 84
Di-( tert-butyl) -d isulfide
-2.04
Cystine
D iphenyld i s u lfid e
Acetate buffer, pH 3 . 8
--0.72
Phosphate buffer, pH 7.0
--0 .83
NH3 - NH/:I
- 1 .05
Phosph ate buffer i n 50% ethanol, p H 7 . 0
--0.50
2,2' - D imethyldiphenyldis ulfide
pH 7 . 0
--0.48
3 , 3 ' -Dimethyldiphenyldisulfide
pH 7 . 0
4,4' - D im ethyldiph cnyldisulfide
pH 7.0
--0 . 5 1
Dibenzyldisulfide
pH 1 2 .3
-
--0 . 5 1
1 .46
D i t h i o gl ycol ic acid
Phosph ate buffer
pH 3.0
--0.4 1
Glutathione (oxidised form)
Soren sen buffer, 0 . 1 M KCl
pH 9.45
--0 . 7 3
Di p h e n yl s u l foxi d e
0 . 1 M TEAl in 50% ethanol
Methylphenylsulfoxide
0 . 1 M TEABr in 50% ethanol
-2. 1 8
Diphenylsulfone
0. 1 M TEAl in 50% e thanol
-2.24,
Methylphenylsulfone
0. 1 M TEABr in 50% ethanol
-2. 1 4
-2.24,
-
2 . 66
2 . 64
--0. 5 5
Cysteine Decanthiol
0.0 1 M H2S04 in 70% ethanol
-0.49
Thiophenol
0 . 0 1 M H2S04
--0.34
D i mercaptopropanol
Acetate buffer,
p H 4.64
--0.49
Na-Diethyldithiocarbamate
-0 .44
Thioacetamide
-0.09
Thiourea
-
0 . 1 M HC101
+
2H
+0 . 1 0
+
Sulfonamides give well formed waves in aprotic media which have been used for the determina tion of sulfonamide dr u gs in medications [ 1 9 ] . Thiobenzophenone is reduced to the corresponding thiol i n a two-electron process. Likewise the reduction of aromatic sulfonic acids is a two-electron process producing the corresponding
204
I ntroduction to Voltammetric Analysis
sulfinic acid. Benzyl and phenyl thiocyanates give polarographic signals which have been used analyt i cally Alkaloids, proteins and many organic sulfur compounds reduce the overvoltage of the hydro gen reaction on the mercury drop electrode and are determined by means of the resulting catalytic hydrogen wave. In native protein molecules, the thiol and disulfide groups are masked and there fore do not catalyse this electrode process. When the pro t ei n is denatured by the action of bases, acids, or U .V. radiation, etc., these groups become exposed and the catalytic waves appear. This phenom enon has been used to determ ine single-stranded DNA in the presence of closed circular DNA in pl asmid [ 5 1 ] . Organic compounds with oxidisable functional groups (see Table 7. 1 ) include catechola mines, sulfonamides, phenothiazines, dopami nes, estrogens and other hormones, tocopherols and various groups of insecticides. These may be successfully determined in solid and liquid bio logical samples, foods, plants and environmental and potable water samples in the m g L- 1 range using DPV with a carbon, gold or platinum electrode. For the lower concentration ranges anodic stripping after adsorptive accumulation, or cathodic stripping after oxidative accumulation, is preferred (§. 7 . 2 ) . Electrochemically in active compounds may be determined voltammetrically o r polarographi cally by first converting them to an e lectroactive derivative. Nitration, nitrosation, oxidation, hydrolysis or some other preliminary chemical reaction is used to form the derivative. Examples of this are the polarographic determination of styrene monomer in polystyrene and mixed poly mers after converting the styrene to the pseudo-nitrosite with sodium nitrite [ 52 ] , alkylbenzene sulfonates in natural water samples after extraction and nitration of the aromatic ring [ 5 3 ] and the determination of beta-receptor blocking agents in tablets after conversion to the correspond ing N-nitroso derivatives [ 54 ] . .
Application 25
Determination of free styrene in polystyrene and mixed polymers [52]
Sample: Pre-treatment Styrene standard :
Determination:
Polystyrene (e . g yog h u rt bucket) . Break the sample into sma l l pieces, wei g h out about 1 g, dissolve it i n 1 5 ml dioxa n, mix with 1 1 0 m l acetic acid and filter. Dissolve 1 00 m g styrene i n 3 m l d ioxa n in a 1 00 ml vol u metric flask, add 22 ml acetic acid and 1 . 2 5 m l 50% N a N 0 2 soluti o n . Al low to react for 20 m i n at room temperatu re, add 20 m l 50% N a O O C C H 3 sol ution and m a k e u p t o the mark with p u re water. This solution, conta i n i n g 1 g L- 1 styrene, is sta ble for a bout 1 month . To 1 0 m l of the fi l tered sam p l e solution in the polarographic cel l add 0. 5 m l 5 0 % N a N 0 2 • A l l ow t o react f o r 20 m i n and t h e n add 1 0 ml pure water a n d 5 m l 5 0 % NaOOC C H 3 . Deaerate t h e solution w i t h nitrogen and record the DP polarogram between 0 . 0 V and -0 .4 V using an S M DE. From the cu rrent peak at -0 . 24 V (versus Ag/AgC I , 3 M KCI) determ i ne the concentration of styrene by stan d a rd additions . The determi nation l i m it is ca . 5 mg kg-1 ( 5 ppm).
If the electrochemically inactive analyte is a surfactant then it may be determined by tensammetry [ 5 5 ] . The peak observed in the a.c. polarogram as a result of the changes in the double layer capac itance when the analyte is adsorbed on the electrode surface is useful analytically in favourable cases and is quite sensitive. Polarography assumes an important role in the analysis of organometallic compo unds in envi ronmental samples. In contrast to the spectroscopic methods (AAS, ICP-MS, etc.) which only give the total metal content in the sample, polarography is capable of characterising and deter mining the various chemical forms of the organometallic compounds as they exist in the sample
Appl ications: organic species
Table
7. 1 0
205
Peak po tentials of the first reduction step in the differen tial p ulse polarograms of selected organometal l i c compounds [ 56 ] .
Compound
Coumpound
� (V ) -
CH3Hg... CH3 C H2Hg+
(CH3)3Sb H
0. 35
( CoHs l l H g
( C6H , ) ;Sb 1 +
-0. 5 1
CH , ( C6H 5 ) 3Sb 1
( C H 3 ) 2 Sn 2 '
( CH)3Sn•
-0 . 8 0
-0.89
( C6H 5 ) 4Sb"'
-0.87
( CH , ) ,Pb+
-1.14
( C6H5)4Pb
-0.44
( C6H5 ) 2Tl '
-0. 7 7
-0.49
( C6H5 )3Sb
- 1 .07
CH3Sn 3-t-
-0 . 94
-0.33
-0.65
( C,.H3 ) Hg•
EP (V)
-0. 5 7
-O.X9
( C,H5 ) , Pb'
-0 . 50
( speciation) . Because there can be great differences in the toxicity of particular compounds formed by a given metal [ 2 8 ] , it is important to know the concentrations of these various species. The polarographic peak potentials of a few representative organometal lic compounds are listed in Table 7 . 1 0 . Organomercury compounds of the type RHgX generally are reduced in two steps via the following proposed mechanism:
pH < 7
RHg + + e . �
RH + Hg
RHg " + e-
J, (RHg)2
pH > 7
J, R 2 Hg + H g
RHg-
-"-" • H.:... -�
RH + Hg
Thiomersal (thimerosal, sodium-2 - (ethylmercurithio) -benzoate) , used in contact lens rinses and in various medications such as eye and nasal d rops, is readily determined polarographically, often directly without any pre-treatment. The electrode reaction on which the determination is based is [ 1 9 ] :
�COONa
�
S H g CH 2 CH3
-
+ e , +
H
+
�
CtCOONa �
SH
+
Organo- arsen ic, antimony and tin compounds can also be polarographically determined [ 93 ] . Mono-, di- and tri-alkyl tin compounds either singly o r i n admixture can b e analysed in the �g range by DP-polarography [ 57 ] . Trialkyllead( IV) compounds are reduced i n two one-electron steps whereas diethyllead( IV) is reduced in a single step process [ 1 4 ] . By mean s of OF-pola rography, Pb (II) can be determined in the presence of the organically bound trimethyllead( IV) as shown in Figu re 7. 1 1 [ 58 J .
206
I ntroduction to Voltam metric Analysis
0
- 0. 5
-
- 1 .0
1 .5
E [V] (vs. Ag/AgCI, 3 M KCI) F igure 7 . 1 1
DP-polarogra m of 1 . 0 sea water [58] .
x
1 o-5 M (CH3hPb+ a n d 1 .0
x
10
·5
M Pb2+ in fi ltered (0.45 1Jm membra ne)
7 . 2 STRIPPING METHODS The h igh sensitivity of adsorptive stripping voltammetry h as resulted in it becoming the preferred technique for the analysis of many organ i c compounds with electroactive groups at the trace and ultra-trace level in a wide variety of matrices. In m any cases it is the simplest technique capable of measuring the low concentrations dema nded by research groups or the environmental protection regulations. Pesticides in waters, soils and foodstuffs; toxic chemicals in industrial effluents, sew age, surface waters and sediments; medical preparations and pharmaceuticals in body fluids; and surfactants can now be readily determined in the sub-micromolar and n anomolar concentration ranges [ 59-62 ] . Other compounds which can be accumulated by adsorption and then determined voltammetrically by cathodic or anodic stripping i nclude dyestuffs [ 63 J ' benzodiazepines r 64, 65 ] , tetracycline [ 66 ] and i ndole [ 67 ] . I n many cases the cel l solution cond itions (solvent composition, supporting electrolyte, pH, etc . ) developed, and the electrode used ( usually a DME), for the determination of an analyte by differential pulse polarography at the 1 - 1 00 mg L- 1 level can be used for its determination at the ultra-trace level by adsorptive stripping voltammetry by simply changing the electrode to an HMDE, MTFE, GC, Pt, Au or modified electrode and including the accumulation step prior to the differential pulse cathodic or anodic voltage sweep. However care should be taken. While some adsorbable impurities in the sample may not interfere significantly with the DP-polaro graphic determination, they may interfere with the adsorptive stripping voltammetric determ ina tion because the analyte is present at much lower concentrations when using AdSV. The importance of AdSV in the trace analysis of adsorbable organic molecules has been summarised [ 68 ] . Potentiometric stripping has been used as an alternative to voltammetric stripping for the determination of adsorbed proteins [ 69] . Several compounds such as the triazines simazin and ametryn , the organophosphates guthion, diazinon, thiram and dimethoate, the nitro compounds DNOC and dinoseb can be determined in low organic content aqueous samples without any pre-treatment. For instance, the determination limit for DNOC in tap water is ca. 0. 1 flg L- 1 and in slightly contaminated river water ca. 1 flg L- 1 • The extent that impurities interfere may to some extent be overcome by chang ing the accumulation potential and/or accumulation time and using standard additions to evalu ate the concentration.
Appl ications: organic species
207
However, what is more common is that when a sample contains species in addition to the analyte which are adsorbed onto the electrode these substances interfere with the analysis, partic ularly if t hey are more strongly adsorbed or present in much higher concentrations than the ana lyte. The interfering species are preferentially adsorbed and prevent accumulation of the an alyte. In such cases the analyte must be separated from the sample matrix and the interfering species by high performance liquid chromatography, by solvent extracti on or by means of a suitable solid phase extractant. Substances which have been determined in the lower J.lg range by AdSV after separation from a solid or liquid matrix by solvent extraction include phosalone, carbofos, methamitron and isomethiozone in soil samples. The determination of the pesticide fenchlorazol- ethyl ( cthyl- 1 (2,4-dichlorop henyl ) - 5 - trichloromethyl - ( I H ) - 1 ,2,4-triazol- 3 - carboxylate) i s a n example in which solid phase extraction i s used to both extract and enrich the analyte ( 70 ] .
L-1
Application 26
Determination of fenchlorazol-ethyl by adsorptive stripping voltammetry [70]
Sa mple: Pre-treatment:
Solutions: Determ ination :
Dri n k i n g water Add aceti c acid to 1 L of sample until the pH is 2 . 8. Pass sam p l e at 5 m l m i n-1 through a 6 m l sol i d phase extraction col u m n ( Baker octadecyl or Su pelco ca r bopack) pre-treated by wash i n g with 2 x 5 ml C H 3 0 H , 2 x 5 ml pure water and 2 x 5 ml acetic acid (pH 2.8). E l ute a n a lyte with 2 ml C H 3 0 H . 0 . 1 M NaOH . Di ssolve 2 5 mg fen c h lorazol-ethyl i n 2 5 0 m l C H30 H . D i l ute this stock solution as req u i red to obta i n worki n g sta ndard solution s . Place 2 0 m l 0 . 1 M N a O H i n t h e voltammetric cel l a n d deaerate with n itrogen for 5 min then add the 2 m l of e l u ate a n d deaerate for 5 m i n . Using as l a rge a d rop as possi ble, accu m u late fench lorazo l-ethyl on the H M D E at -0 . 1 V for 3-1 0 m i n with stirri n g . Record the D P-stri pping volta mmog ram f ro m -0 . 1 V to - 1 . 3 V. U se the peak at -0. 9 V (ve rsu s Ag/Ag C I , 3 M KCI) and sta ndard addi tions to determine the concentration of fench lorazol-ethyl ( F i g u re 7 . 1 2). Deter m i nation l i m it 0 . 2 �g L-1 .
I [nA]
c
,.---,-....,..--.,----,--...,--, a
- 0 .2
Figure 7 . 1 2
---
- 1 . 3 - 8.8 E [V] (vs. Ag/AgCI, 3 M KCI)
--
---
8
16
c [n g
mL-1]
AdSV stripping determ ination of fench lorazol-ethyl after solid phase extraction from 1 L d rinking water and 1 0 m i n accu mulation at -Q.1 V. Eval uation by standard add itions: (a) 20 m l 0 . 1 M NaOH + 2 ml eluate, (b) a + 8 �g L-1 fenchlorazol, (c) a + 1 6 �g L-1 fench lorazol.
Tab l e 7 . 1 1
Adsorptive st ripping voltammetric dete r m i n ation of pesticides in various samples [ 72 ] .
Substance
Sample
Ametryn
River water
Bensultap
C ar h o fos
D i a z i non
Potato, river water Soil
Pre-treatment
E." -
Tol uene extraction \-\later/acetone extraction
2 ,4- Dichlorophenoxyacetic acid
Dimethoate
River water
Dinobuton
River water
DNOC
Fenitrothion
(V)
0.7
River water River water
lsometiazin
Nit ralin
Soil
-0.72
- 1 .20
H M D F.
+0. 1
0.0
HMDE
-0.70
HMDE
0.0
Paraquat
Water
Complex with
Phosalon
Water
CH2Cl2 extraction
Soil
Water/acetone extraction
B(C6H5)4-
MTFE
HMDE
-0.44
HMDE
0.0
-0.40
Hg!Pt
0.0
-
0 .70
1 .7
1 .0
0.6 0. 1 21
3 0.5
0.0
-0.56
-0. 3 -0. 1 -0. 1 0. 5
-0. 7 1
-0.51
-0.70
0.78
-
HMDE HMDE HMDE HMDE MTFE
River water
-LOS
Solid phase extraction
-0.2
-0.6
HMDE
Water*
Solid phase extraction
0.0
+0.6
Au
-0 . 8
-0. 3
1 .5
0.9
0.2
1 .5
22 2
-0.75
Water, soil
Anodic .> tripping
1 .2
Hg
Simetryn
•
92
Hg
Thiram
Complex with fe( I I )
3.5
�. 0
0.2
-0.32
HMDE
2,4,5-trichlorophenoxyacctic aci
§:
-0.64
-0.5
-
0. 4 5
Hg
HMDE
N 0 ao
(J.Ig l-I; !Jg lcg-1 )
-0 . 3
-
Simazin
-0.46
HMDE
Determination limit
-0.2
River water CI I1Cl1 extraction
HMDE
-0. 1
2nd peak more sensitive
Guthion
-1 .02
-0.4
-0.2
-
Electro de
-0 .93
-0.3
Note
EP ( V)
-0.6
Hydrolyse at pH 1 2
River water
Ethylenedithiocarbamate
· · · --
4
0.4
0.3
1 .5
0
0r:::
::>
....
0 < 0
3 3
iif
�
;::; · )> ::> Q>
-< "'
;;; ·
Applications: organic species a
EP - 0 . 75 V
I
209
b
i [nA] 800
ro nA
600
- 0.9
0
Figure
7.1 3
E
0
10
20
30
40
50
60
70
c [j.Lg L-1 ]
[V]
Determ ination of thiourea (6 . 5 IJg L-1 ) by cathodic strippi n g voltammetry showi ng the voltammogram and non-linear calibration curve.
Those compounds that form slightly so l u ble co mp o u nds with me r cu r y or silver ( Table 7 . 1 ) a re determined polarographically via the anodic wave o r by s t ri ppi n g voltammetry after a n o dic accu mulation. Among this gro up are agrochemicals, many of which containing thiourea or other thio compounds, which are often determined by c ath o d i c stripping with l i m i ts of determination in the sub-micro m o la r range [ 7 1 ] . A selec ti o n of pesticides that can be determined by ca t h o d i c strip ping vol tammetry after anodic accumulation is listed in Table 7. 1 1 [ 72 ] . Other sulfur-containing compounds which readily form insoluble Hg( I I ) co mp onds o n the surface of an a n o dic ally p o l a rised electrode (equ a t ions 3.8 and 3.9) are u su all y determined a t very low levels by cathodically s t r i pp i ng the Hg(I I ) c o m po u nd from the electrode. A s e le c t i on of sulfur-contai n i n g compounds which may be determined by CSV is g iven in Table 7. 1 2. Application 2 7
Determination of thiourea by cathodic stripping voltammetry [ 7 1 , 73]
Sa m p l e : Solutions: Determ ination:
Aq u e o u s s a m p l e s,
pharmaceutica l products, u r i n e . 2 M NaOH i n pure water. D issolve 1 00 mg thiourea in 1 L p u re water. Dil ute this stock as req u i red to prepa re fresh standard solutions daily. For thiourea concentrations between 0. 2-2 mg L- l . Add 10 ml sam ple so l uti o n to 1 0 m l 2 M NaOH i n t h e v o lt a m m e tr i c ce l l a nd dea erate with n itroge n . U s i n g a D M E, record t h e D P-pola rogram f r o m -0 . 4 5 V to -0 . 1 5 V. T h e a n o d i c pea k due to thiourea occu rs at ca. -0 . 2 6 V (versus Ag/Ag C I , 3 M KCI). Determ i n e the concentration by sta ndard additions.
Table -
7.12
Selected sulfur-containing compou nds which m ay b e determ i n ed by cathodic st ripping voltammctry.
Compound
Supporting electrolyte
Cysteine
0. 1 M Na2Bp7
Penicillam inc (2,2 -dimethylcysteine)
0 . 1 M Na 2 B407
0.02 M NaCI
Electrode
HMDE
HMDE
E"" (V vs SCE)
� (V vs SCE)
Linear
+0.2
-0.49, -0.68
Linear
-D.S l
6 - Mercaptoxanthine
HMDE
+0.2
-D.72
2 - Merca ptopurine
HMDE
+0 . 2
-
HMDE
+0.2
-0 .3
oxidiscd form redu ced form Thioamides ( various)
2- Thiobarbiturates
BR - buffer
(mol L-1 )
-D.59
+0.2
Glutathione
Determination limit
+0.2
HMDE
2 - Mercaptoethanol
Stripping waveform
0 .48 ,
-
l .O X 1 0-R
2.0
X
1 0'8
+0.2
HMDE
+0.05
HMDE
-0.90 (Ads)
p H 4.78
BR - buffer
pH S Lau rylsulfonic acid
pH 8 (with NaO H )
Dodecylbenzenc-sulfonic acid
pH 8
HM D E
-0.70 (Ads )
-0.46, -0.52
-D.3 - 1 .3 -1.2
to
-D.06
X
1 0'8
Linear
2.0
Dif[ pulse
ppm
Diff. pulse
w·6-w·8
Diff. pulse
8-. 0 "'
5'
::::! . ,.., )> "' "'
� v; ·
-0. 2
HMDE
c.. "C "C rtl :::J c.. )( ' �
N w "'
Appendix
5
Organic species-biological and medical samples
Element
Matrix Tiss u e
Ranitidine
Phar.
Riboflavin
Pre-treatment SEx
0.001 M HC1
Phar. Sh
(continued)
Working electrocle
Accumulation process
BR-pH 2.3
HMDE
Adsorption
SM DE
Adsorption
0.2 '
Ac-pH 5.0
HMDE
Acio;orption
-0. 1
0.01 M KCl
HMDE
Adsorption
-0.70
Su.pporting electtolyte 0.001 M N a OH
E.. (V)
Measurment technique
-0.56
DPCSV
' -0.56
PSACCSV '
E, (V)
DPCSV
DPCSV
-0.29
-
1 . 35'
Detection limit (fiBL-1) 0.22
0.36
Rufloxacin
Urine,blood
Streptomycin
Urine
O.oi M NaOH
s�mE
Adsorption
-1.2
LSCSV
- 1 .58
0.4 1
Water, Vet.
0. 1 M HCI
HMDE
Acio;orption
-0.30
SWCSV
-0.60
12
BR-pH 3.0
HMDE
Adsorption
-0.5
DPCSV
-0.77
17
SMDE
Adsorption --- ---
-0.8 '
DPCSV
Sulfadimetoxal
Tcmazepam
Testosterone
- ---
--- ·
Urine
Phar.
---
---
SEx · --- ·
0.005 !\-1 NaOH --- --- . . ---
---
·---
---
3.7
-J .35A 0.086 ___ . . ___ ---
Interferences
265
Other tlavins
266, 267
Plasma matrix Zn1-t
Creatinine,
Reference
uri< acid
243 268
269 270 271
Proteins
272
Other tctraq'Clines
273
--- --- · --- ·---
Tetracycline
Std.soln .
Bor-pH 5.5
HMDE
Adsorption
-0.6'
LSCSV
- 1 .28 '
O.Y
Theophylline
Phar.
RR-pH 7.5
HMDE
Adsorption
0 .0
DPCSV
-0.38
0.6
Cu(ll)
274
6- Thioguanine
Std.soln.
HMDE
DPV
-0.98'
3.3
Other thiols
275
Phos-pH 7.0
Pt
DPP
-0.64
1 04
276
0 . 1 M H2S04 in C2H10H:C,H,. J : 1
GCE
CP
+0.66/0.83
1 0'
277
BR-pH 2.5
HMDF.
Adsorption
-0.9
DPCSV
-1 .32
so
278
Phos-pH 9.0
GCE
Adsorption
0.0
DPASV
+0.74
4.1
Metabolites
279
HCI-pH 0.9
HMDE
Adsorption
+0.05
DPCW
-0.28
96
Proteins with S-S
280
CPE
Adsorption
Ox.;Ads.
+0.3
DP!I.SV
+ ! .01
0.2
-0. 1
LSCSV
-0.22
-0.64
Thiomersal
Eye drop s
Tncopherals
Veg.oil
Torasemide
Urine
Trimipraminc Trypsin
TC)rtophan Vitamin K I
Vitamin K3 ( M enadione;
Urine Std.soln. Serum Std.soln. Plasma Std.soln.
TIC
SEx SPEx
0.! M H 2S04
Ac-pH 3.5
SEx
0.5 M NaCIO,,pH 5
CuM FE
Ac-pl I 5, 50% ale
CPE
Adsorption
oc
LSCSV
0.3 M HCIO,
SMDE
Red.;Ads.
-0 . 1
SWASV
+0.1
2.0
CJ', CNS , r,
Pb"
281 23
130
282
0.022
283
� :;.... a
Q. c:
a 0 ::>
g
� 6f 3 3 �
::::! . n
)>
::::1 Qj
� v; ·
Appendix 6 Appendix 6
Abbreviations used in Appendices 1 -5 .
Abs
Ab sor pt io n
Ac
Acetate buffer
Ad s
Adsorption
AD
Acid d ig es tion
ale
Alcohol ( ethanol )
Am
Ammonium buffer
A mAc
Ammonium acetate buffer
AmCl
Am monium ch l o r id e buffer
Am Cit
Ammonium citrate buffer
AmFor/Phos
Ammonium formate plus phosphate mixed buffer
AN
Acetonitrile
APDC
Amm o n i u m py rrol i di n ed i th ioc a rb a m at e
aq
aqueous
BES
N,N bis ( 2 - hyd roxyethyl ) - 2 -aminoethane- sulfonic acid
B eryllon lil
4- [ ( 4-diethylamin o - 2 - hydroxyphenyl) azo] - 5 - hydroxynaphthalene-2,4-disulfonic acid
BEx
Back e x t racti o n
2-BIBH
2- Benzylideneiminobenzohydroxamic a c i d
Bor
Borate buffer
Br-PADAP
2- ( 5 - Bromo- 2 -pyridylazo ) - 5-diethyl -aminophenol
Br- PADN
4 - ( -2- ( 5 - Bromo pyridyl a zo ) - 1 , 3 ,- d ihydroxyn ap h t h ale n e
BR
Britton-Rob inson buffer
Br-TUO
7- Bromo- 5- ( 2 - chlorophenyl) 1 ,3 -dihydro- 2H- thieno- 2,3 -e- 1 ,4-diazepam - 2 -one
c
C a t a lyti c peak
Cd,Zn MT
Cd,Zn metallothionein
Cit
2- ( N- cyc loh exyl am i n o )ethanesulfonic acid bu ffer
CA CHES
Chloranalic acid { 2 , 5 - D i chloro - 3,6-dihydroxy- 1 ,4 -benzoquinone)
Citrate buffer
Cupferron
Ammonium nitrosophenylhydroxylamine
DA,AD
Dry ashing followed by acid digestion
DAB
3 , 3 ' - D iaminobenzidine
DASA
UBr-PADAP DCM
Der
1, 2 - D i hydroxyan thraquinone-3-sulfo n i c acid
2 - ( 3 ,5 - Dibromo - 2 - pyridylazo ) - 5 - diethylaminophenol Dichloromethane
D c ri vat isati o n
Di a z
D iazotisation and coupling
DMF
Distillation
DMG
D i m e t hyl gl yox im e
D i st
D i m e t hyl for m a mid e
DMSO
Dimethylsulfoxide
D MT D
2 , 5 - Dimercapto- 1 , 3 , 4 - thiadiazole
DPC
4,6-Dinitro - 2 - hydroxytoluene
DPG
Diphenylguanidine
DNC
237
Diphenylcarbazide
238
I ntroduction to Voltammetric Analysis
Appendix 6
Abbreviations used in Appendices 1 -5.
(continued)
DTADP
3 - (4,5-Dimethyl -2 thiazolylazo)-2,6-diam i nopyridine
DTPA
Diethylenetriamine-pentaacetic acid
EBBR
Erichrome Blue Black R {4- { 2 - ( 5 -bromopyridyl)-azo ] - 1 ,3 -dihydroxynaphthalene}
EBT
Erichrome Black T
EDTA
Ethylenediaminctetraacetic acid
F.n
Ethylenediamine
Extn
Extraction
Ferron Ha
7 -lodo- 8- hydroxyquinol i nc-5 -sulfonic acid
HCS
Hydrocarbon solvents
HE PES HNB
Hydroxylammonium chloride N- 2 -hydroxyethylpiperazine- N'- 2-ethanesulfon ic acid
Hydroxynaphthol Blue
Hom
Homogenised
HOx
Oxalic acid
HSA
Human serum albumin
Hyd
Hydrolysis
IX
Ion exchange
MES
2 - ( N-morpholino )-ethanesulfonic acid
MESep
Molecular exclusion separation
MGK 326
Dipropylpyridine-2,5-dicarboxylate
Mod
modified
Morin
2 · ,3,4' ,5,7 -pentahydroxyflavone
MPAQ
I , 2-Cyclohexanedione dioxime
Nioxime 1 N 2N
oc
5 - [ ( 4- Methylphenyl) aw ] -8-aminoquinoline 1 -Nitro- 2- naphthol
Open circuit
OCP
o-Cresolphthalexone
OD,MW
Oxidative digestion in a microwave oven
OF
Oxidative fusion
o-PDA
I ,2-Diaminobenzine ( o-phenylenediamine)
Oxine
8-Hydroxyquinoline
Ox
Oxidation
OxPr
Oxidation pro duct
lpl
Plating or pre-concentration supporting electrolyte
PAR
4-(2 -Pyridylazoresorcinoi)
Phar
Pharmaceutical preparations
Phos
Phosphate buffer
Phos/cit
Phosphate-citrate buffer
Phth
Phthalate buffer
PIPES
Piperazine- N , N ' -bis(2-ethanesulfonic acid)
pl.
Plating
ppt.
Precipitati on
QT
2 - Quinolin ethiol
Appendix 6
Appendix 6
Abbreviations used in Appendices l-5.
(continued)
RE
Rare earths
Red
Reduction
SATP
Salicylideneamin o - 2 - thiophenol
SDDS
Solve n t ext ra ct i o n
Is)
SEx
Stripping electrolyte
Sodium do de cyl sulfate
SPEx
Solid phase extraction
SSA
Sulfo sa l ic ylic acid
Std. soln
Solochrome Violet RS 1 5 -Sulfo-2-hydroxybenzeneazo - 2 - naphthol)
SVRS
Standard s o l u tions
TBAH FP
Tetra b utylammo niu m hexafluorophosphate
TBP
Tri-n-butylphosphate
TEA
T EAB F4
Triethanolamine
TEABr
Tetraethylammonium bromide
TEAP
Thin l aye r ch ro m a togra p hy
TLC
TP
Tetraethylammonium tetrafl u orob o rate
Tetraethylammonium p e rchlo rate
u.v.
Thymol p h th a l exon e
Var.ind
Various industrial ap plications
Vet.
xo
U.V. digestion
Veterinary preparation Xylenol Orange
Electrodes AgRDE
S ilver rotating disc electrode
AgTFE
Au RD E
Silver thin film e l ect ro de
AuT FE
Gold thin film elec t rod e
CE
Carbon electrode
CPE-mod
Modified carbon paste electrode
Gold rotating disc electrode
CRDE
Carbon ro t at i n g disc e l ectro d e
Cu -MFE
Cop p e r based mercury film e l ectro d e
DME
Dropping mercury electrode
GCE
Glassy carbon electrode
GCE-mod
Modified glassy carbon electrode
GE
Gr a p h ite el ect ro d e
HMDE
Hanging m e rc u r y drop electrode
M FRDE
Mer cur y film rotating disc electrode
MTFE
Mercury thin film el ect ro de
film e lect ro d e
MTFE-mod
Modified mercu ry thin
SMDE
Static mercury drop electrode
WIGE
Wax im pr e gn a te d graphite elec t r od e
239
240
I ntroduction to Voltammetric Analysis
Appendix 6
Abb revi ations used in Appendi ces 1-5.
Measuring techniques
ACASV
ACCSV ACP
(contin ued)
-------
Alternating current anodi c stripping voltammetry Alternating current cathodic stripping vol ta nunetry Alternating current polarography
ACV
Alternating c urrent voltammetry
AST
Anodic stripping tensamm etry
CP
Chronopotcntiometry
CPS
Chronopotentiometric stripping
DCP
Direct current polarography
DerPolar
Derivative polarography
DerPSA
Derivative potentiometric stripping analysis
DifCPS
Differential chronopotentiometric stripping
DifPSA
Differential potentiometric stripping analysis
DPCSV
Differential pulse cathodic stripping voltammetry
DPASV
Differential pulse anodic stripping voltammetry
DPP
Differential pulse pol arography
DPV
Differential pulse voltammetry
LSASV LSCSV
Linear scan cathodic stripping voltammetry
LSV
Linear scan voltammetry
oswcsv
Osteryoung square wave cathodic stripping voltammetry
PSA
Potentiometric stripping analysis
PSACCSV
Phase-sensitive alternating current cathodic stripping vo ltam m etry
SSP
Single sweep polarography
SWASV
Square wave anodic stripping voltammetry
SWCSV
Square wave cathodic stripping voltam m etry
Linear scan anodic stripping voltammetry
-------
Appendix 7
Appendix 7
Internet addresses of
241
i n strument suppliers
AD Instruments
HEKA Electronik
www. adinstruments.com
www.heka.com
Arne!
KH Design and Development www. kh des i g n . d e mon . co. uk
www. amelsrl .com Axon
Keithly
www. axonet.com
www. ke it hly. co m
Bank Electronik
M et ro hm
www. bank-ic.de
www. m e t r o hm. c h
BA S( B i o a nalyt i c al System s)
P i n e I n stru me n t Co m pa ny www. p in e i n st. com
www. bioanalytical.com B i o Lo gi c
Pro D i gital ( PAR)
www. bio-logic.fr
www. p rodigital.com.au
Cypress Systems
Radiometer
WW\v. cy p res sh o m c .co m
www. radiometer.com
Eco c h e m i e www. e co ch e m i e . n l
Solartron www. solartron.com
Electrosynthesis C o mp any
Sycop el
www.electrosyuthesis.com
www. syso p e l . c om
Gamry Instruments
Warner Instrument Co rp orat i o n WW\V. warnerinst.com
www.gamry.com
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254 j .C. Co rt in a Villar, A . Costa Garcia and 1'. Tunon Blanco, Analytica Chim ica Acta, 1 992, 256, 23 1 . 253
2 5 5 R . Kalvoda, J. Electroanalytical Chemistr;� 1 984, 1 80, 307.
256 M . Khodari, J.C. Vire, G.}. Patriarche and M.A. Ghandour, A nalytical Letters, 1 990, 23, 1 87 3 .
257 A . J. Barrio Diez- Caballero, L. Lo p ez de Ia Torre, J.F. Arranz Valetin a n d LA. A r r a nz Garcia, 'lalama, 1 989, 36, 50 1 .
246
I ntroduction to Voltammetric Analysis G. Stubauer and D. Obendorf, Analyst, 1 996, 1 2 1 , 35 1 .
259 A . Zapardiel, J.A. Perez Lopez, E . Bermejo, L . Hernandez and M . Chicharro, Analytica Chim ica Acta, 1 99 1 ,
258
E. Pinilla Gil, L. Calvo Blazquez, R.M. Garcia-Monco Carra and A . Sanchez Misiego, Fresenius Z Analytische Chemie, 1 988, 332, 82 1 . 26 1 M . Telting-Diaz, A.J. Miranda Ordieres, A . Costa Garcia, P. Tunon Blanco, D. Diamond and M .R. Smyth, 244, 49.
260
Analyst, 1 990, 1 1 5, 1 2 1 5 .
Metrohm Application Note No. V 5 5 . 263 M . J . Honeychurch a n d M . J. Ridd, Electroanalysis, 1 996, 8, 654. 264 D.V. Vukomanovic, D.E. Zoutman , G.S. Marks, J.F. Brien, G.W. van Loon and K.Nakatsu, ]. Pharmacological and Toxicological Methods, 1 996, 36, 97. 265 S. Altonoz, D. Ozer, A. Temizer and Y. Bayraktar, Analytical Letters, 1 992, 25, I I 1 . 266 J. Wang, D-B. Luo, P.A.M. Farias and J.S. Mahmoud, Analytical Chemistry, 1 9 8 5 , 57, 1 5 8. 2 6 7 A. Economou and P.R. Fielden, Elec t roanalysis, 1 995, 7 , 447. 268 S. Furlanetto, P. Gratteri, S. Pinzauti, R. Leard i, E. Dreassi and G. Santoni, ]. Pharmacological and Biomedical 262
Analysis, 1 995, 1 3, 43 1 .
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269 J . Wang and J.S. Mahmoud, Analytica Chi mica Acta, 1 986, 1 86, 3 1 .
J.A. Perez Lopez, E. Bermejo and L. Hernandez, Fresenius Z. A na lytis ch e Chemie, 1988, 330, 707. 2 7 2 j. Wang, P. A . M . Farias and J.S. Mahmoud, Analytica Chimica Acta, 1 9 8 8 , 1 7 1 , I 95. 273 J. Wang, T. Peng and M.S. Lin, Bioelectrochemistry and Bioenergetics, 1 986, 1 5 , 14 7. 274 R.M. Shubietah, A.Z. Abu Zuhri and A.G. Fogg, Analyst, 1 994, 1 1 9, 1 967. 275 J. Rarek, A. Berka, L. Dempirova and J. Zima, Collection of Czechoslovak Chemical Com m u n ications, 1 986, 5 1 , 2466. 276 Metrohm Application Note No. V 57. 277 Z.j. Suturovic and N.J. Marjanovic, Hlectroanalysis, 1 999, 1 1 , 207. 27!1 M. Fernandez, R.M. Alonso, R.M. Jiminez and M . J . l.egorburu, Analyst, 1 994, 1 1 9, 3 1 9. 279 J. Wang, M. Bonakder and C. Morgan, A nalytical Chemistry, 1 986, 58, 1 024. 280 J. Wang, V. Villa and T. Tapia, Bioelectrochemistry and Bioenergetics, 1 988, 1 9, 39. 281 H . Wa ng, H . Cui, A. Zhang and R. Liu, Analytical Com m u n ications, 1 996, 33, 2 7 5 . 28 2 J.P. Hart, S.A. Wri n g and I . C . M orga n , Analyst, 1 989, 1 1 4, 933. 283 J.C. Vire, N.A. El Maali, G.J. Patriarche and G . D. Christian, Talan ta, 1 988, 35, 997. 270 271
A. Zapardiel,
I n dex
A.c. polarography, 46 basic principles of, 46-48 base ( charging) current depression in, 5 1 charging current i n , 49 definition of, 4 6 effects of adsorption in, 5 1 fundamental harmonic current, 47 irreversible electrode processes in, 5 1 peak width, 49 phase angle, 49 phase selective, 49 phase selective second harmonic, 50 quasi-reversible e lectrode processes in , 50 reversible electrode p ro cess es in, 48 second h a rm onic, 50 theory, 48 time domain of, 6 with digitise d instrumentation, 46, 48 A.c. voltamm etry, 46 Adsorption, 1 4 current, 1 4 o f analytes on electrodes , 1 4 o f i mp ur ities o n electrodes, 5 1 waves in d.c. polarography, 1 4 Adsorptive stripping v oltammetry 72 catalytic re a ctions in, 77, 1 64 effect of accumulation time on the current peak in, 74 effect of stripping w aveform in riboflavin analysis by, 79 limits of detection in, 77 linearity of current peak height with concentration in, 74 of metals as complexes, 72, 76 of organic compo u n d s 206 of pesticides, 208 using mercury elect ro des 71 using modified electrodes, 1 1 4 ,
,
,
Aluminium determination, 1 68 A m p ero m etric, biosensors, 2 1 2 detection in, ion chromatography, 1 38 high performance capillary electrophoresis, 1 3 8 detection of organic compounds i n flow through detectors, 2 1 4 detectors, 33, 1 28, 1 7 1 factors affecting the sensitivity of, 1 30 gas sensors, 1 7 1 titrations, 3 1 Amperometry, 3 1 , 1 70 Analyte, 1 Anodic current, 2 Anodic stripping voltammetry, 60 ap p li ca t i o ns, 1 53 An tim o ny, determin ation, 1 59 Arsenic d eterminatio n by cathodic str ipp i n g voltammetry, 1 6 1 As hi ng, cold plasma, 96 high pressure, 95 Automated voltammetric analysis, 1 40 Auxiliary electrode, 4 Biamperometry, 33 Bismuth determination by anodic stripping voltam metry, 1 5 9 Butyl-tin c omp o unds , determination of, 2 1 1 Cadmium determination by anodic stripping voltamm etry, 1 53, 1 5 7 using the RAMTM electrode, 1 59 Calibration cu rves 1 67, 209 Capillary characteristics, 2 1 Capacitive current (see charging current) ,
247
248
I ndex
Catalytic currents, 4, 1 0 in the determination of trace metals, 1 1 , 1 49
Cathodic current, 2 Cathodic stripping voltammetry, 60, 70 insoluble m e rcury compound formation
in,
70
in, 72 of orga n ic sulfur c o m p o unds 2 1 0 C EC mechanism , 1 2 metal co- deposition
Cells, 1
flow-through, 1 26- 1 30
micro, 99 thin layer, 1 26, 1 29 tubular, 1 26 voltammcrtric, 98 wall-jet, 1 26 CE mechanism, 1 1 Charge transfer coefficient, 7 effect on polarographic wave shape, 27 Ch a rge transfer r e a c t io n , 4 rate of, 6 Charging current, 1 2, 29, 3 8 , 5 1 Chemically modified electrodes, 1 1 3-1 1 7 Ch r o m ium , determination , 1 49, 1 66 Chronopotentiometric stripping analysis, 80 comparison with anodic stripping voltammetry, 8 1 derivative, 8 2 differential, 82 peak width in 84 effect of surfactants on, 84 li mit of detection, 82 th eo ry , 80 Chronopotentiometry, 52 basic pri n c i pl e s of, 53 d e riva t i ve , 55 irreversible processes i n , 54 rev ers i b l e processes in, 54 with time-dependent current, 55 sensitivi ty of, 56
Clarke cell, 33, 1 7 1 Cobalt determination, 1 4 1 Complexing agents, 26 use in improving selectivity, 69, 1 48 u s e in trace metal adsorptive stripping analyses, 72, 76, 77 Concentration gradient at electrode surface, 7, 8
Convection , 5
Copper determ ination by a no di c stripping vo lt am m e t ry, 1 53 using the RAM"'"M electrode, I 59 Cottrell e qu at i o n , 9 Coulometric detectors, 1 30 Counter el e ct rode , 2 Current, diffusion, 4 faradaic, 4 faradaic to c h a rgi n g cu r r en t ra t i o, 1 4 peaks, measurement of, 1 2 0 vol tammetric, 7 waves, measurement of, 1 2 0 Current-sampled d.c. polarography, 3 0 C ycl ic voltammetery, 36 ap p l i c a ti on of, 37 influence of chemi cal reaction of the electrode product, 37 irreversible electrode processes in, 37 reve r s i ble electrode processes in, 36 theory of, 36 Cyanide, determination of, 1 5 1 Cysteine and cystine, determinati on of, 202 Data processing, graphical 1 I 9 digitally, 1 20 D.c. polarography, I 8-3 0 adsorption currents in, 1 4 catalytic currents i n , 1 1 charging current in, 29 detection limits in, 3 1 diffusion current in, 1 9-2 1 effect of capillary characteristics on, 2 1 effec t of concentration on, 20 effect of mercury column height on, 2 1 effect of temperature on, 2 I effect of drop time on, 20 potential dependence of, 20 invention of, 3 irreversible electrode processes i n , 27 kin e t i c a lly controlled currents in, 1 0 l imiting current in, 9 , 20 maxima in, 2 7 migration current in, 5 residual current in, 2 8 reversible electrode processes in, 22 selectivity of, 28 theory of, I 9 D.c ( linear scan ) stripping voltam metry, 66
Index
Derivitisation 1 38, 204 Desorp ti o n , 5 1 Detection limits, de t ermining factor of, 1 4 Diazep am, determination of, 1 94 Dibucaine, determination of, 1 98 Differential p ulse polarography, 43-46 current measurement in, 44 peak half-widths in, 45 peak resolution in, 45 sensitivity of, 46 theory of, 44 waveform used in, 43 with d igitised eq u i p ment , 43 D i fferenti al p u lse stripping voltam met ry, 66 Differential p ul s e volt am metry, 43 Diffusion, 5 coefficient s, 8 controlled c u r rents , 4, 1 9 current, average, 2 1 c u rrent constant, 2 1 Fick's laws of, 8 , 9 layer, 5, 1 27, 1 29 linear, 7, 1 9, 107 spherical, 1 9, 108 up to a dropp i n g m er cury electro d e , 1 9 Di gest ion, 93 microwave, 93, 96 U.V., 93 wet, 94 Digitised instrumentation, 1 1 7 Do u ble layer, 1 2 capacitance, 1 3 differential capacitance of, 1 4 D ro p ping mercury electrode, 1 , 1 9, 1 03 controlled drop time, 20 dro p time, 20, 3 1 potential dependence of gravity controlled, 20 EC mechanism, 1 1 Electroactive organic fu n ct i onal groups, 1 76 Electrocapillary maximum, 14, 29 Electrode potential, effect on electrode reaction rate, 6 Electrode processes, 4 catalytic, 1 49, 1 0 , 1 64 and coupled chemical r e a ct i o n s , 6, l 0, 3 7 d iffus i o n controlled, 5 , 7 irreversible, 6 ki net ically controlle d , I 0
249
quasi-reversible, 7, 50 and rate determining ste p s, 6 reversible, 6 Electrodes, 1 auxiliary, 4 calomel, 1 00 carbon paste, 1 06, 1 3 1 charge on, 1 2 , 1 4 c h e m i ca lly modified, 1 1 3 counter, 2 disc, mic r o , 1 09 dro p p i ng mercury, 1 , 1 9, 1 03 glassy carbon, 1 06 g old , 1 06, 1 3 1 , 1 6 1 gra p hi te, 1 06 gr ap h i te u ltra - t r ace, 1 1 2 hanging me rcu ry drop, 6 1 , 1 03, 1 40 mercury pool, 4 mercury t h in film, 6 1 , 1 0 5 micro, 1 0 7 construction, 1 1 0 rapid response time of in flow-through analysis , 1 36 response time, 1 09 sensitivity, 1 09 mic ro-array, 109, 1 1 2 multi-modeTM, 1 05 platinum, 1 06, 1 3 1 "" random assembly micro d isc array � M , 1 1 2 reference, 2, 4, 99, 1 02 ro ta t ing disc, 64 silver, 70 silver - silver chloride, 1 00 static mercury drop, 3 1 , 1 03 thick film gra ph ite, 1 1 2 thick film m odified grap h ite , 1 1 4 tubular , 1 2 7 wall-j et, 1 3 3 working, 2, 1 0 1 Electroinactive, 1 1 Errors, sources of, 90 Ethylenediaminetetraacetic acid d ete r m i n atio n , 1 86 Extraction p olaro g raph y, 1 5 1 Faradaic currents, 4 Fast sweep p ol a r ogr a p hy ( see single sweep polarogra p hy ) Fenchlorazol-ethyl, determination of by a dsorpt i v e stri p ping voltammetry, 207
250
Index
Flow-through amperometric detectors in the analysis of, phenols, 1 33 polyaromatic hydrocarbons, 1 34-1 37 Flow- through cells, 1 2 6- 1 30 Flow-through electrodes, 1 27 Flow-through stripping voltammetry, 1 3 9 electrolyte exchange in, 1 4 1 , 142 Folic acid determination, 201 Formaldehyde determination, 1 80 Galvanostat, 52, Gold electrode, 1 6 1 Half-wave potential, 22 definition of, 9 dependence on concentration of analyte, 23, 24 dependence on concentration of supporting electrolyte, 23 dependence on solution composition, 26 effect of metal -ion complexation on, 26, 1 48 influence of pH on, 26 irreversible processes and, 27 of metal ions in various supporting electrolytes, 1 46 ranges of classes of organic compounds, 178 relationship t o E0, 2 3 reversible processes and, 2 3 Heterogeneous rate constants, 6 Heyrovsky-Ilkovic equation, 23 High performance liquid chromatograpy, with amperometric detection, 1 29, 1 32 with voltammetric detection, 1 36 High pressure ashing, 9 5 Hydrodynamic voltammetry, 1 34 Hydrogen sulfide sensor, 1 7 1 Hydrogen waves, catalytic, 1 2 Ilkovi c equation, 20 Insoluble product formation, 24, 70 Instrumentation, 1 1 7 automation of, 1 1 8 , 1 40 computerised, 1 1 7 multi functional, 1 1 7 iR drop, 3 Iron ( III) determination , 1 64 Irreversible electrode processes, 2 7
Karl Fisher titration, 33 Kinetic current, 4, 1 0 Lead determination by anodic stripping voltammetry, 1 53, 1 5 7 using the RAMrM electrode, 1 69 Linear sweep voltammetry, 33 charging current in, 35 influence of scan rate in, 3 5 limits of detection i n, 35 irreversible electrode processes in, 35 peak potential in, 34 resolution in, 35 reversible electrode processes in, 34 shape of curves in, 36 theory of, 33 Mass transport, 5 Medium exchange, 69 Mercury, determination of, 1 6 1 oxidation i n the presence o f organ ic compounds, 70, 209 Migration current, 5 Molybdenum determination, 1 1 , 1 68 Nernst, diffusion layer, 7 equation, 22 Nickel determina tion, 1 4 1 Nicotine determination, 1 97 Nitrilotriacetic acid determination, 1 86 Non-faradaic current, 1 4 Normal pulse polarography, 43 charging current in, 38 selectivity of, 43 sensitivity of, 43 waveform used in,44 Normal pulse voltammetry, 43 Organic solvents, 98 Osteryoung square-wave voltammetry, 39 Oxygen, interference, 28, 6 1 reduction at a mercury electrode, 98 removal of from solution, 28, 98 Peak current, 1 0, 6 1 Peak potential, 1 0, 62 Platinum determination by adsorptive
I ndex
stri p pin g voltammetry, 1 64 Polarographic speciation, 1 45 Polarographic spectrum, 1 4 5 P olaro graphy, definition of, 1 hi sto rical aspects of, 3 of anions, 1 50 of 1 ,4 b en zodiaz epi n es, 1 9 3 of c arbo n yl co m pounds, 1 80 of carb ox yl ic a cids , 1 8 3 of hydrocarb ons, 1 78 of multivalent i ons, 1 48 of n itrates , 1 92 of n i tro compounds 1 90 of nitroso c omp o unds, 1 92 of N-heterocycles, 1 96 of N -o xide s, 1 92 of organ ic halides, 1 79 of organic peroxides, 1 87 of o rgan i c s u l fur c o mpou n ds, 202 of o rg a no - met a ll i c com p o unds , 205 of qui n ones , 1 8 9 po te n t i a l ran ge i n , 2 8 Potassium chloride, d.c. charg i ng current in, 14 Potential o f zero charge, 1 4, 29 Potentiom etric st r ip pi n g analysis, 85 diffe r ent i al, 86 Potentiostat, 3 Q uarter-transition time, 54 Randles-Sevci k e quatio n , 33 Rapid scan square - wav e v o l tam metry in flow-th ro u gh analyses, 1 37 Rate c onsta n ts , 6 Reference elect rodes, 99 p os i t ioni n g of, 98 Reversible electrode processes, 22 Reversibi lity, 6 Riboflavin determination, 2 0 1 Sample pre-treatment, 92 Sand equati o n , 53 Selen ium det e rmi n a tio n , 1 62 S i n gle sweep po l aro gra p hy, 33 Solid phase extraction, 96 Sphericit y effect, 20, Square-wave polarography, 39 charging current in, 38
251
d ig ita ll y con t r o l led, 41 sen s itivi ty a n d se l ec t i vit y of, 39 theory of, 39 waveform used in, 39 Square-wave voltammetry, 39 Osteryoung, 39 Staircase vo l t age, 3 1 Staircase v o lt a m metry, 39, 4 1 , 45, 48, 66 dis c rimi n a t ion against charging c u rren t in, 66 waveform used in, 4 1 Staircase s tripping volt am metry , 66 Standard addition, 1 24, 1 52, 1 6 1 , 1 88, 207 Standard redox pote nt i al, 23 Stationary electrodes in flowing solutions, 1 27 Stripp in g volt a m metry, 60 acc umulation step i n , 58 accumulation po te n tial , 60, 62 accumulation time, 60 comparison of thin film and mercury drop e l e ctrodes i n , 62,64 electrodes used in, 62 in envi ronmen ta l studies, 77 in flow-th rou gh analysis, 1 39- 1 42 i n fl uen ce of sur facta nt s in, 77 inter-metallic co m po un d formation in, 65 limits of detection in, 66, 67 m e tal compl exa t i o n in, 68 peak width i n , 62 resolution in, 66 rest period in, 58 stripping step summary, 87 theory of, 62-64 tra ce met al analysis using, 1 53-1 70 trace metal spec i a t io n u si n g, 1 55 waveform used in, 66 Styrene, determination following derivitisation, 204 Sulfide dete rm i natio n , 1 50 Suppo r t i ng el ec trolytes, 30, 97 T ast
p o l ar o graphy, 30 Tellurium, dete rmina t io n of, 1 63 Tensammetric p e a ks , 5 1 Te nsammet ry, 5 1 applications, 5 2 Tomes equation, 2 4 Thiourea determination, 209 Tributyltin d eterm i n a tio n , 2 1 1
252
I ndex
Titanium determination, 1 52 Transfer c o efficient 6, 7 Trace metal speciation 1 5 5 Transition time, 5 3 , 80
Voltage , applie d, 2 Voltammetric determination of trace metals in polluted waters, 1 56 Voltammetry definition, 1
Uranium determination by adsorptive stripping voltammetry, 77, 1 66 U.V. i rradiation, 93 effect on trace metal determinations, \ 53
Water analysis, 1 55 Wo rki n g e lectrodes 2, 1 0 1
,
,
Vitamins, polarographic determination of, 1 84
,
,
Zinc determination by anodic stripping voltammetry, 1 53 half-wave potential effect of supporting electro lyte concentration on, 23 ,
