Practical Skills in Chemistry

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John R Dean, Alan M Jones, David Holmes, Rob Reed, Jonathan Weyers and Allan Jones

Practical Skills in

Chemistry

Pearson Education

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Education lImiled 2002

The righh of John R. Dean. Alan M. JOl'leI. DIMd HolI'l1M, Rob Reed. Jooalt\8n Weyers and AIlBn ..Jones 10 t. ldenIifllld _llUltKln ollhla WOflc hlIVe "-" ~ by !hem in IIOOOtd_ wiItllhe Copyrighl. Desigll8lld PMem, AcI 1988.

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reproduced. IlOred fr1 8flV n,nillvlll SVlI1IIm. Of t"rlImilted Of by IIf"IV mBIIfl8, electrol'lie, mectlenlc,l, pholOcOpylrog. .eootdi!'lll. 01' otherwise wilhoU eil:hef"!he prior

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Contents Boxes Preface

vii ix x

AcktlOll'ledgemellts Abbreriations

xiii

1 2 3 4 5 6 7

Fundamental laboratory techniques Basic principles Health and safety Working with liquids Basic laboratory procedures I Basic laboratory procedures II Principles of solution chemistry pH and bulTer solutions Resources for fundamental laboratory tochniques

3 6 9 15 26 45 56 62

,

The investigative approach Making and recording measurements

9 10 11

SI units and their use Scientific method and design of experiments

12 13 14 15 16 17

Melting points Recrystallization

Project work Resources fOl" the investigative approach

65 70 75 80 '3

Laboratory techniques

18

Solvent extraction

Distillation Reflux Evaporation Inert atmosphere methods Resources for laboratory lechniques

87

92 102 107 116 121 125 131

Classical techniques

19 20 21 22 23 24 25

26 27 2' 29 30 31 32

Qualitative techniques for inorganic analysis Gravimetry Procedures in volumetric analysis Acid-base titrations Complexometric titrntions

Redox litrations Precipitation titrations Resources for classical tcchniques

Instrumental techniques Basic spectroscopy Atomic spectroscopy Infrared spectroscopy Nuclear magnetic resonance spectrometry Mass spectromelry Chromatogrnphy - introdoction Gas and liquid chromatography

135 139 141 14' 151 155 157 160 163 170 180 190 200 205 211 v

Contents

33

34 35 36

Electrophoresis Ek:cuoonalylical techniques Radioactive isotopes and their uses Thermal analysis Resoun:ai rOf" instrumental techniques

225

229 235 243 246

Analysis and presentation of data

37 38 39 40

41 42 43 44

Using graphs

Presenting data in tahles Hints for solving numerical problems De!icripti\'e statistics Choosing and using statistical tc:su Dra~;ng chemical SlrUClUI'e$ Chemomd.rics

Computational chemistry Resources for ana.!yl;is and presentation of data

45 46 47 48 49 50

51 52 53 54 55 56

Infonnation technology and library resources The Internet and Work! W~ Web Internet resowa:s for chemistry

299

Using sprea~

302 307

Word processors, dalabllscs and other padc:agt$ Fmding and citing published information

317

Communicating information Genenl.l aspects of scientifIC 'Hiring

311

Writing~ys

325 330

Reporting prada! and project work

332

Writing literature surveys and re\iews

339 341 344 348 354

Organizing a po;lcr display Giving an oral presentation Examinations Resources for communicating information References

Index

vi

251 256 259 264 271 280 285 291 294

355 357

Boxes 4.1

27.3

How to make up an aqueous solution of known concentmtion from a solid chemical How to make up an aqueous solution of known concentration from a solid chemical for use in quantitative analysis How to make up a linear dilution series for use in quantitative analysis How to weigh Olll a sample of a solid for use in quantitative analysis How 10 nute a filler paper for gravity filtration Isolation of a solid by suction filtration How 10 dry a solution over magnesilUll sulphate Useful procedures for calculations involving molar concentrations How 10 convert ppm into mass of chemical required An example of complex fonnalion The use of oxidation numbers 10 identify reinL~/infomlation below the ruler a~ you draw.

ClllclIl,,'OI'S These range from basic machines with no pre-programmed functions and only one memory, to sophisticated programmable minicomputers with many memories. The following m;IY be helpful when using a c:llculutor. •

•

Using calculators - take particular care when using the exponential key 'EXP' or 'EE'. PressIng this key produces l()8"""tth,ng. For example if you want to enter 2)( 10 ., the order entry is 2, EXP, -,4 nOt 2, x. 10, EXP, -.4.

Presenting graphs and diagrams - ensure these are large enough to be easily read: a common error is to present graphs Of' diagrams that are too small, with poorly chosen scales.

4

Fundamental laboratory teChniques

•

•

Power sources. Choose a battery-powered machine, rather than a mainsopenlted or solilr-powered type. You will need one with basic m:Hhematical/scientific operations including powers, log:uithms (p. 262), roots and parentheses (brackets), together with statistical functions such as sample means and standard deviations (Chapter 40). Mode of operation. Calculators fall into two distinct groups. The older system used by, for eJmmple. Hewlett Packard calculators is known ,IS the reverse Polish notation: to C'1 be recorded and appropriate safety informalion must be passed on to those at risk. For most practical classes. risk assessmcnts will have been carried out in advance by the person in charge and the information ncttssar}' 10 minimi7.C thc risk.lnmetlt

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Protective clothing is worn as a first barrier to spillage of chemicals on to your

body. lab coats are for protection of you and your clothing. Eye protection special spectacles with side pieces to protect you from your own mistakes Bnd those of your colleagues. If you wear spectacles, eye protection with

prescription lenses Bnd side pieces is available from your optician, an expensive but worthwhile investment. Otherwise goggles can be worn over spectacles.

Contact lenses should not be worn in the laboratory. Chemicals can get under the lens and damage the eye before the lens can be removed. It is often very difficult to remove the contact lens from the eye after a chemical splash. Shoes should cover the feet: no open-

toed sandals, for example. long hai, should be tied back and hats le.g. baseball caps) should not be worn.

3. SignaIlII"

Student:

Solpervisllr:

Oae:

Fig. 2.2 An example of a laboratory haUird assessment form.

names and telephone numlx.>J"S of safety personnel, lirst aiders. hospitals, etc. Make sure you read this and abide by any instructions.

Basic rules for laboratory work •

• • •

We-dr appropriate protective clothing at all times - a clean lab coat (butlom.>d up). eye protection, appropriatc rootwear and ensure Y0LU' hair does not constitute a haY..ard. Never smoke, eat or drink in any laboratory, because or the risks of contamination by inhalation or ingestion (Fig. 2.0. Ncvcr work alone in a laboratory. Make sure that you know what to do in case of lire, including exit routes, how to raise the alaon, and where to gather on leaving the building. Remember that the mosl important consideration is human safcty: do not altempt to light a lire unless it is safc to do so. Fundamental laboratory techniques

7

Health and safety

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biological hazard (biohazardl

Fig. 2.3 Warning labels for specific chemical hazards.

8

Fundamentill laboratory techniques

• •

• • •

• • • •

All laboratories display notices telling you where to find the first aid kit and who to contact in case of accident/emergency. Repon all accidents. even those appearing insignifiamt - your depRrtmcnt will have a reporting procedure 10 comply with safety legislation. Know the warning symbols for specific chemical hazards (Fig. 2.3). Never touch chemicals unless they are known to have minimal hazard: use a spatula to transfer and manipulate solids, and piiXtiCs for liquids sec p. 9. Never mouth pip:::tte any liquid. Use a pipette filler (see p. 10). Take care when handling glassware - see p. 13 for details. Use a fume cupboard for hazardous chemicals. Make sure that it is working and then open the front only as far as necessary: many fume cupboards are marked with a maximum op::ning. Always usc the minimum quantity of any hazardous materials. Work in a logical, tidy manner and minimi7.e risks by thinking ahead. Alway dear up spiJIages, espcci
Fundamental laborlltory techniques

Basic laboratory procedures I

Box 4.2

How to make up an aqueous solution of known concentration from a solid chemical for

use in quantitative analysis Example: Suppose you are to prepare a standard solution (250.00 mll of ammonium ferrous sulphate (approximately 0.1 M), which is to be used to determine the concentration of a solution of potassium permanganate.

is a quant;tative experiment so the ammonium ferrous sulphate must be of the highest purity available to you. Work out the mass of chemical that will give you the conC6ntnmon desired in the volume required. (a) Convert 1.0 M into mol L_1; concentration required = 1.0 mol L-1. (b) Express all volumes in the same units: therefore 250.00 mL = 0.25 L (c) Calculate the number of moles required from eqn [4.1J: 1.0= amount (mol) -;.. 0.25. By rearrangement, the required number of moles is thus 1.0 x 0.25 = 0.025 mol. (d) Convert from mol to g by multiplying by the M.. but note from the container that the compound is (NH t hFeSO•.6H20. Therefore the M r required must include the water of crystallization and M.=392.14gmol l . (e) Therefore, you need to weigh out 0.025 x 392.14 = 9.8035g of (NH t hFeSO•.6H zO. Place a clean. dry weighing boat or appropriately sized sample tube onto a simple two-decimalplace balance (see p. 22) and zero (tare) the balance and weigh about 9.809 of the chemical. Carefully transfer the sample tube plus chemical to a four-decimal-place analytical balance (see p. 231 and record the accurate mass: say 11.9726g. Remove the sample and container from the balam:e and tip the contents into a clean, dry beaker 1400mU. ensuring that there is no spillage outside the beaker. Do not attempt to wash out

1. This

2.

3.

4.

5.

the sample tube with water.

6. Immediately reweigh the sample tube on the analytical balance: say 2.1564g. This is the mass of the container together with a few crystals of the chemical which have remained in the container. However, you now know exactly the mass of the chemical in the beaker: 11.9726 - 2.1564 = 9.8162g of (NH.)2FeS04.6HzO.

7. Add deionized or distilled water (about 100 ml) to the beaker and stir the mixture gently with a dean Pyrex'lJ. glass rod until all the solid has dissolved. Do not splash or spill any of this solution or you cannot calculate its concentration. Remove the glass rod from the solution. rinsing it with a little distilled water into the solution. 8. Clamp a clean volumetric flask for support (see p. 26) and place a clean, dry filter funnel in the top supported by a ring. Carefully pour the solution into the volumetric flask. ensuring no spillage of solution by using the technique illustrated in Fig. 4.1 and pouring slowly so that no air-lock is formed and no solution runs down the side of the beaker. When the addition is complete, do not move the beaker from its position over the funnel.

9. Rinse the inside of the beaker several times with a distilled water wash bottle to transfer all of the solution into the volumetric ftask, paying particular attention to the 'spouf and glass rod. Then place the beaker aside and lift the funnel from the flask while rinsing it with distilled water. You have now achieved a quantitative transfer. Swirl the liquid in the flask to prevent density gradients. 10. Make the solution up to the mark using distilled water. stopper the flask and mix thoroughly by gentle inversion (10 times) of the flask while holding the stopper in place. You now have a solution (250.00 mL), which contains (NH,il2FeS04.6HzO 19.81629). The concentration of this solution is expressed as: 250.00 mL of the solution contains 9.8162 9 of (NH.hFcSO...6H20 Therefore: 10oo.00mL of solution contains

The concentration of the solution is 39.2648gL- 1 =39.2648-;-392.14= 0.1001 moll- 1 = 0.1001 M

Alternatively you can usc a weighing boule as shown in Fig. 4.2. The liquid is placed in [he bottlc, weighed 1:andard solution for each member of the series and pipette or syringe the calculated volume ioto an appropriately sized volumetric flask as described above. Remember to label clearly each diluted SOlution as you prepare it. since it is ~ to get oonfusal. Hie process is oulliDed in Box 4.3.

~

Make all the dilutions from the working stock solution to the required solution. Do not make a solution of lowet'" dilution from one already prepared: if you have made an elTor in the first dilution, it will be r-epeated for the second dilution.

20

Fundamental laboratory techniques

Basic laboratory procedures I

Box 4.3

How to make up a linear dilution series for use in quantitative analysis

The experimental protc:x:cl states: 'Prepare a standard solution {250.oo ml; 0.01 MI of cadmium ions using cadmium nitrste and use this solution to produce 8 linear dilution series of solutions noo.oo mLl of accuratetv' known concentrations of approximately 0.0, 0.2, 0.4, 0.6, 0.8,. 1.0mrnoll-1"

,. Cak:ulate the amount of cadmium nitrate req,,*ed for the standard solution: cadmium nitrate is supplied 8S the tetrehydrate Cd(NOJ~.4H20; M = 308.47 g mol '. Therefore. using eqn [4.1l you should cetculate 250.00 mL of en 0.01 M solution of cadmium ions requires 0.01 x 0.25 = 0.0025 mol C: 0.0025 x 308.47 E 0.077129 of Cd(NO:Jh. 4H20. Remember: this solution will contain 0.0025 moles of 'cadmium nitrate' or 0.0025 moles of cadmium ions and 0.005 moles of nitrate

ions. 2. Make the standard solution by the Quantitative method described in Box 4.2 and calculate its concentration to four decimal places. 3. Calculate the vokJlTNt of soIutkM1 required for the dilution series using lCIJV, = fC:lJV:l, where VI = volume (mL) of standard s06ution required; C, "" concentration of standard solution; V2 = volume of diluted solution (1oo.oomL) and G.! = COl"'lCen-tration of diluted solution. 4. Express the oonc:enbaticMd In the serne units, the most oonvenient in this case being moIL-1. Therefore, Ct = 0.01 M = 1 X 10-2 moll- 1 and for the diluted solution of concentration 0.2 mmoll-'. ~ = 0.2 x 11)3 moll-1. By rearrangement V,

~

5. Transfer th. standard sokItIon ~ mLJ to the volumetric fIMk "00.00 DIU and Il\IIke up to tile mark with distilled water.

'01"

6. RepeIrt: the calculation each of the other diluted solutions _ required, bul note that 8 short cut is possible in Ihis C8/1r. :!linee you require stock solution (2.00mL) for the diluted solution of con'" centration 0.2 mmol L-1, you will need 4..00mL for the 0.4mmoIL-l soIutto"n· etc. Use pure distilled water for the solution of concentration 0.0 mol L_1.

7. Cakt*te the Pact concentrlltlona 01 the diluted solutions using tc.jV, = 1~]V2. since the c0ncentration of the standard solution Is most unlikelv to be exactly 0.0100 mol L-1 (see Box 4.2). For example, if the concentration of the st8ndard soMion IC,I is actually 0.00987 mol L-' =9.87 x 1()3 moa -1, then for the dilute solution of approximate concentration 0.2 mmol L 1, the actual concentr8tion {~) is:

fC2l =

{V, x

ICl l} .... V2

0.2 ml x 9.87 x 10.-.3 moll- l l00.00mL' , j.

,

-= 2 x 9.87 x 10-& moll-l = 0.1974mmoll-.1 . NOle: It Is much simpler to measure out whole-number volumes (2.00 ml. 4.00 mL. etc.) using 8 pipette and produce diluted so/udons of accuretely known concentrelions (but not necessarily whole, numbers) rether then to try to prod~ whole-nun;'lber co~ntTl'ltiOf'lS by measuring out non--whoie-1'lumber v06umes..

.

{Ic,!V,) +fC,]

..;;. 0.2 x ltJ3 moll _1 x 100.00 ml = 2.00 ml 1 x ffil mol L- 1

Sto";ng chemicals and sollilions

Seal[ng flasks - think carefully about the sealing system for a flask 10 be stored at low temperature. Cooling will reduce the volume of vapour (including air) in the flask and create a partial vacuum. Rubber bungs can be i"emovable aftar cooling (another good reason for never using theml and even ground-glass joints can seize up. Plastic stoppers, saewtops or lightly greased glass stoppers, as appropriate, are recommended.

Chemicals wlUch decompose easily (labile cht,'Illicals) may be stored in a fridge or freezer. Take special care when using chemicals which have been stored at low temperalure: lhe container and its contents must be allov.cd to warm up 10 room temperature before use, otherwise 'Nater will condense onto thc chcmical. This may render accurate wt,;ghing impossible and you mllY ruin the chemical. Chemicllis and solutions to be stored al low tcmpe.mturcs musl be in stoppercd or sealed vessels. Do nol store aqueous solutions below OCC since freezing can occur and, with the resulting expansion of the volume, the vessel may crack. Solutions containillg flammable solvents should only be stored in specialized 'spark-proor fridges: consult your laboratory instructor. You must be aware of lhe parlicular problems of storing solutions ill flasks with ground-glass joints. If you are using aqueous SOIUtiOllS you should Fundamental labor1ltory techniques

21

Basic laboratory procedures I

Greasing joints - if you are using solutions of NaOH Of KOH you must grease the ground-glass joint and stopper since the surfaces are anacked by strong alkalis.

lightly grease the joinl and stopper with pctrok:um jelly. since the water will not dissolve the grease as it is poured from the flask. The stopper can be rt.-moved easily and the solution will be uncontaminated. CO",'~I~'. if you are using solutions made up from organic solvents. you should 110' grease the joints since the orgnnic solvent will dissolve the greasc as you pour it from the Oask and contaminate the solution. Moreover, you should not allow the solution to come in contliet with the ungreaSt:d joints, since the solvent will evapor'd.te and leave the solute to 'weld' the stopper to the flask. Fill the flask with solution, using a filter funnel with the stem of the funnel positioned well below the joint. See p. 43 for use of ground-gl'L"5 jointwarc.

~ Always label all stored chemicals dearly with the following information; the name (if a solution. state solute(s) and concentration/sll. plus any relevant hazard warning information. the date made up. and your name,

Balances and weighing Electronic singlt,.... pan balance; with digital readouts lire now favoured over mechanical types and are common in most labor.:ttories, There are es..-.entially two types of ballince: I.

2.

Weighing - neverwcigh anything directly onto a balance's pan: you may contaminate it fOr others. Use an appropriate weighing container such as 8 weighing boat, sample tube, weighing

paper, conical flask, beaker.

'Weighing paper' - It is common practice to put 8 piece of paper onto the pan of general-purpose balances. The mass of

the paper is then 'Iared off before the weighing container is placed on the balance pan. The paper protects the balance pan from corrosion by spillages and also allows you to discard easily any malerial spilt without affecting the

weighing.

Both types are illustrated in Fig. 4.3 and you should familiarize yourself with their operation before use.

Genel'aJ-purpose balances The most useful fealure of this type of balance is the electronic lero facility (self-taring), which means the mass of the weighing container can be subtracted automaticdlly before weighing chemicals. To operate a standard self-taring balance: I.

2. 3.

4.

22

Fundamental laboratory techniques

General purpose balances which v..'Ci.gh to the nelirest 0.01 g with a capacilY of about 300 g. Chemicals may be dispensed for weighing, into a suitable weighing container, directly Ollto these balances. Analytical four-figure balances for quantitative work, which wcigh to the nearest 0.0001 g (0.1 mg) and have a maximum clipacity of libout lOOg. Chemicals must not be transferred onto the ballince at any lime and analytical balances must only be used for weighing by difference.

Check Ihal it is level, u."ing the adjustable feet to centre the bubble in the spirit level (usually at the back of the machine). For relatively accurate work or when using in a fume cupboard, make sure that the drought shield is in place. Ensure that Ihe balance is switched on: the display should be lit. Place an emp(y \\eighing container (see p. 24) centrally on the balance pan and allow the reading to stabili::t--C. If ,lie objet.:t is larger dum Ihe pall. take care ,hal no purl rests OIl the body of the lxlltUlce 0,. 'he draugh, shieltf as this wil/ illl'alidale the rfflding, Press the tare bar 10 bring the reading to zero. Place Ihe chemical or object carefully in the weighing vessel: (a) Solid chemicals should be dispensed with a suitably SiZL'. For ~mall masses. use a smaM weighir'lg containar (Fig. 4.41.

5.

6.

7.

(b) Non-volalilc liquids should be dispt'fls('' Fig.4.6 Transferring.ll solid 10.11 narrow-

necked flask.

Fundamental laboratory techniques

25

----------18

Basic laboratory procedures II Clamps and support stands When carrying oul experiments il is ,"ital that all aPPanilus is held in phi"'C securely during lhe procedure. It is essential that you know how to assemble supporting i1nd securing equipment to [he highest possible standllrds of safety. The most common types of supporting and securing equipment are coaled in plastic to provide the cushion between metal and g1a~.

Basic laboratory procedures II

If your support ring is metal. you can make a 'cushion' by finding a piece of thin-walled rubber tubing of the same bore as the metaJ of the ring, cuning it to a length equivalent to the circumference of the ring and then cUlling down the lenglh of the rubber tubing. You can then slide Ihe tubing around the ring to provide the 'cushion',

~ When using damps, support rings and support stands make sure that the clamped/supported apparatus is always in position above the base of the support stand lFig. 5.41 to prevent the stand toppling over.

OlO\llll:llejaw on TOP

~

Cork rings

fixed jaw undemealh 10 suPlJCll'\. e.g. a condenser

WRONG

Right and wrong ways of using suppOI1 stands, clamp holders and clamps. Fig. 5.2

·l1u:se are used 10 hold round-bouom nash on nat surfashing·. Some gravimetric analyses, such as the determination of sulphate as barium sulphate, require retention of the precipitate from gravity filtration sinee BaS04 is too fine to be collected on a vacuum filtration system. BaSO. is thermally stable up to 600 C, so the filter paper Can be bumed away,

-

"'' T Fig.

5.3 Clamping a parallel-sided funnel,

Filtmtion is the physical sepamtion of a solid from a liquid and is a process encountered in experimenlal procedures sUl;h as gravimelric analysis (p. 139), recrystallization (p. 92), and solvent drying (p. 41). In principle. the mixture of the solid and liquid is. passed through a porous material, filter paper or simered glass, and the solid is tmpped on the porous material willie the liquid passes through. "l1le type of filtration equipmt"tlt you select for use depends upon which of the Iwo components, Ihe solid or the liquid, you are Irying to isolate. Ln general: • •

If you wish lo isolate the liquid - lise gravi/}'jillra/iotl. If you wish 10 isolate the solid - lise suction (IYlCtlum) jil/ration

Gravity jiJI.-alion In gravity filtration you need to p pllrt> solVi'nt.

Suction filtration This technique is used for the isolation of a .~o/id from a suspension of a solid in a liquid and relies on prodUl..i ng a partial vacuum in the receiving Oask. The essential comp::ments of a SUClion fillmtion system are: •

28

Fundamental laboratory techniQues

A ceramic funnel containing a flat perforated plate: there are two types based on size and shape called Buchner funnels or Hirsch funnels. When you are filtering. the perforated plate is covered by a filler p
Using oil baths - oil baths should be used in the fume cupboard because prolonged heating may cause unpleasant and possibly toxic fumes to be released from the oil.

34

Fundamental laboratory techniques

These arc mostly used 10 heal round-boltom flasks at temperatures
A 1 normal solUlion (I f\,') is one that contains OIlC equivalellt maSS of a substance pcr litre of solution. Thc general fonnul~1 is: . mass of substance per litre normahty = equivalent mass

16.41

OSIII(}lm·ity Exa~.

Under ide&! conditions, 1 mol

of NaCI dissolved in water woukt give 1 mol of Na+ 101lii and 1 mol of Cl- tons. 9Quivalent to a theoreticel osmolarity of 201lmoll-1.

This non-SI expression is used to describe the number of moles of osmotically active solute particles per litre of solution (osmol L -I). Tht: need for such a tenn arises because somt: mok:eulcs dissociate to giv

(6.5J

If nccessmy, the osmotic coefficients of:l p.articular solutc can be obtained from tables (e.g. Table 6.2): non-ideal behaviour means that 1> may have values> I at high concentrations. Alternativcly. the osmolality of a solution can be measured using an osmometer.

Osmotic pressure This is based on the concept of a mcmbmne pcnncllble to water. but not to solute molecules. For example, if a sucrose solution is plaet:d on one side and pure watc:r on the other, then a passive driving force will be created and water will diffuse across the membranc into the sucrose solution, since thc cffective water con
When preparing a table for data collection. you should:

Designing a table for data coUeaioomake sure there Is sufficient space In each column for the values; if mdoYbt. err 00 the generous side.

Recording numerical data - write down only those numbers that can be justified by your measurement technique (significant figures).

I. Use a concise title or a numbered code for cross·refcrencing. 2. Deckle on the number of variables to be measured and their relationship with ~eh other and layout the table appropriately: (a) The first column of your table should show values of the independent (colltrolkd) variable, with subsequent columns for the individual (measured) values for each replicate or sample. (b) If several variables arc measured for the same organism or sample, each should be given a row. (c) In time-coursc studies, put the repliattes as columns grouped according to t.reatment. with the row'S relating to different times. 3. Make sure the arrdllgemenl rcnccts the order in which lhe values will be collecled. Your table should be designed 10 make lhe recording procc!<s as stmighlforwHrd as possible, to minimizc the p~sibilily of mistakes. For final presentation, a different arrangemenl may be best (Chapter 37). 4. Consider whether additional columns are ft'Quircd for subsc::quelll eulculations. Creale a scpamte column for each mathematiC, and liOme parts can only be written after you have allihe rcsulL means that it ean be rushed - nOl a good idea beciusc of the weight attached by assessors to your analysis of data and thoughts about future experiments. It will help greatly if yOIl keep notes of aims, conclusions and ideas for fulure work (JS .1'011 go (Jlong (Fig. 11.1). Another useful tip is to make notes of compamble data and conclusion!> from the literature as you read papers and reviews. Acknowledgements Make a !>pedal place in your notcbook for noting all those who havc hc1pt:d you carry out the work, for lise when writing this section of the report. Refel'ences Because of the complex formats involved (p. 319). these can be tricky to type. To save time. process them in batches as you go along.

~ Make sure you are absolutely certain about the dead-

line for 5ubmttting your report and try to submit a few days before it. H you leave things until the last moment, you may find access to printers. photocopier.; and binding machines is difficult.

82

The investigative approach

Resources

Resources for the investigative approach

&oks Adams, MJ. (19IJ5) Cl1l'mol/l{'rric$ in Alit/lyrical Ch{'mi.wry, Royal Society of Chemistry, Cambridge.

Crdwforo, K. and Heaton, A. (1999) Problem Soli'ing in Analytical Che"lI:wr)'. Royal Socicty or Chemistry, Cambridge.

Massart, D.L., Vandcginstc, B.O.M., Buydens, L.M.C., de Jong, S.. l.ewi, Handbook of Chem()lll(>fric~' and Qllo/imet";cs: Port A, Elsevier Science B.V.. Amsterdam.

PJ. Clnd Smeyers-Verbcke, J. (1997)

Meier, P.e. (2000) S/ml.Wical k/l'I/uxl!; in John Wiley and Sons Lld. Chichester.

Ana~I'lica/

ChclIJi.stry, 2nd Cdn,

Miller, J.N. Clnd Miller, J.e. (2000) Slatis/ics and Chell/ome/des for AllolYliml ClIl'mislry, 4th Edn, Prentice Hall. Harlow. Essex.

The investigative approach

83

------8 Purity of solids - mahing points do not vary significantly with changes in atmospheric pressure. wnef6aS the boiling points of pure liquids are not constant: they vary with changes in atmospheric pressure and 1000/0. which is impossible. Therefore it must be 8 different compound to the one expected.

•

•

It is normal practice to rn",ke at least two measurements of the melting point, one

apprQKimate and at least one accurate reading.

•

•

TIle most common error is he:lting the sample too tjllkkl}'. Thcre is ortcn a lag between slowing the hc=atin8 rate and the rc=duction in tempentture increase. resulting in a reading of the melting point which is too high. If you do not know what Ihe compound is. you will nOl know its expected melting point. TIlCrcfore. you shoukl dete:rminc :1Il approxim:lte value and then repeat the procedure to give an accurate lllC 100 mll should always be carried out in cone-shaped separatory funnels supported by a ring, because there is then no danger of the funnel slipping from a clamp at its neck.

you arc carrying out specific experiments to determjne partition coefltcients. when you wilt be given specific instructions or references to lhe appropriate literalure: the solute has appreciable solubility in both solvents.

The reason calculalion is not necessary is that. in the overwhelming majority of extractions you will carry out, the conditions used are designed to ensure that the components will be aJmost totally soluble in one of the liquids and almosl insoluble in the olher, since complete separation is rcquin."d. The number of extraclions needed 10 extract a water-soluble solute into an immiscible organic phase can be calculaled from the following relalionship: 11'"

=

11'0

- -K ' - ' - )" ( Kv+s

[142)

wh(,...e K = partilion coefficient of the solute. I' = volume (mL) of aqueous solution of the solute. s = volume (ml) of immiscible organic solvent. 11'0 = w(,'ight of solule initially in the aqueous layer, II'" = weight of solute remaining in the aqueous layer after 1/ extractions. and n = number of extractions. Evaluation of this expression shows that, for a fixed volume of solvent. il is more effick"J1t to carry out many small extractions than one big one.

SeparatOl'y fiuJIlels These come in a range of !>1ZCS from 5mL to 5000ml and there are two general types: parallel sided and cone shaped (Fig. 14.1). Cone-shaped separatory funnel.. are made of thin glass and should be supported in a ring (p. 104). Small-volume cone-shaped funnels « IOOmL capacity) and parallel-sided separatory funnels should be damped at the ground-glass joint at the neck (p. 27). Separatory funnels will have glass or Tenon!.' taps wilh a rublx.-r ring and clip or screw cap on the end to prevent the tap slipping from the oorrel. or a Laboratory techniques

103

Solvent extraction

Rotaflo Ii tap. You must ensure that the tap assembly is in good condition by making the following ChL'Cks before starting work: •

For glass taps: disassemble the tap by first removing the dip and ring or cap from the lap (note lhe order of the comlX>nent parts for reassembling). Dry lhe tap and barrel with tissue, add a light smL-ar of grease to the tap (making sure you do not clog the hole in the tap) and reassemble lhe tap and fitlings. turning the tap to ensure free movement. Support the st.'paratory funnel in position and add some of the organic solvent to be used (2-3 mL) to the funnel, with the tap closed, to check that the tap does not leak. If the tap k-aks, disassemble and rcgrease. For Teflon \! taps: disassemble the tap, wipe the tap and barrel with dean tissue. reassemble without grease, check for free movement of the tap and for leakage as described above. When you have finished using the funnel, loosen the clip/cap on the tap since Teflon'li, will flow under pressure and the tap may 'scize' in the barrel. For a Rotaflo A tap: unscrew the tap from the funnel and ensure that the plastic locking thread is in place (Fig. 14.2). If it is not present, consult your instructor and obtain a replacement. Dry the barrel of the tap and the tap with a tissue and reassemble. Do not grease the Rotaf1o~[ tap.

•

•

The general proa.'"tiJlation This process can be used to separate water-insoluble covalent compounds

Fig. 15.7 A "bent' Pasteur pipette.

from crude reaction mixtures or to isolate volatile natural product'), e.g. terpencs from plant tissue. The crude mixture is heated wilh water and steam and the steam-volatile m.·uerial co-distils with the steam and is then condensed with the water and collected. You willthcn need to carry out an extraction (p. 102) to separate Ihe waler and the insoluble component.

700"0

Fig. 15.8 Nomograph for estimating boiling point at reduced pressure, To use it, draw a line between the recorded pressure and the boiling

100::

b.p!. corrected

10 160 rro-n Hg

poinl at 760 mm Hg. and then extend the line to the observed boiling point scale. This point is Ihe boiling point at the redlXed pressure. In

0-

this example. a liquid b.pt. of 250 C at 760 mm Hg will boil 81 118'C at 10 mm Hg.

112

Laboratory teChniques

100-

observed b.p(.

Jl'essure in rom Hg

Distillation

Box 15.4

How to carry out a reduced-pressure distillation using a water pump

1. Set up the distillation apparatus 85 described in Box 15.1, but use a two-necked distilling flask and include a short fractionating column between the lop of the distilling flask and the still-head or use 8 Vigreux flask (Fig. 15.2 I or use a Claisen still-head (Fig. 15.21. 2. Connect three Ouickflt,w round-bottom receiving ftasks to the receiving adapter Of' 'pig' using plastic joint clips and attach the 'pig' to the bottom of the condenser. Support the receiving flasks on a 'Iabjack" if they are 100ml size or larger. Rotation of the 'pig' allolNS three fractions to be collected without stopping the distillation - this is a major task when working under reduced pressure. 3. Insert the air bleed into the 'spare' rMK:k of the distilling flask Or into the lovver ioint on the Vigreux flask or Claisen head and make sure that the tip of the air bleed reaches the bottom of the distilling flask. If you are using boiling sticks as an ·anti·bumping' precaution, add the sticks, making sure that they reach to the bottom of the flask and do not float, and put stoppers in the unused joints. 4. Insert a Quickfit" thermometer into the remaining 'male' joint in the apparatus. Do not use an ordinary thermometer in a screwcap adapter: it may be sucked into the apparatus and break or crack the flask.

5. Using thick-walled rubber pressure tubing. connect the water pump (via the anti-suck-back trap) to the three-way tap and then to the Anschutl manometer. Connect another piece of pressure tubing to the manometer but do not connect it to the 'pig' for the moment.

6. To check the available vacuum, turn on the water pump fully, kink or seal the open pressure tubing on the manometer and turn the three-way tap so that a vacuum is created in the manometer. Slowly open the tap on the manometer and the mercury level should rise in one of the manometer 'arms'. When the mercury has stabilized, move the SC8~ so that the zero is level with the lowest mercury level. and read off the upper mercury level. The pressure (vacuum) is the difference in levels in mm. The reading should be between 10-20 mm. Now close the tap on the manometer, unkink or unseal the pressure tubing, and turn the three-way tap so that air is admitted into the system.

7. Connect the pressure tubing from the manometer to the ·pig'. Pressure tubing is heavy and it should be sUPpOrted with a clamp and stand, close to the 'pig', to prevent strain on the glassware.

8. Slowly turn the three-way tap to evacuate the apparatus. As the pressure decreases, bubbles will issue from the air bleed or the boiling stick and low-boiling solvents such as dichloromethane or diethyl ether will begin to boil. causing frothing in the flask. If this happens, stop applying the vacuum via the three-way tap and wait until the frothing has died down and then continue to lower the pressure until the three-way tap is fully open. 9. Check whether the air bleed is too coarse, producing large bubbles and vigorous $plashing in the distillation flask.. If so, place a piece of pressure tubing (about 2 cm long) on the end of the air bleed and constrict the tubing with a screw clip to reduce the air flow through the air bleed to an acceptable rate.

10. Slowly open the tap on the manometer, allow the mercury levels to stabilize and read the vacuum by moving the zero on the scale to the 1000000r mercury level: the reading should be 15-25 mm or better. If this pressure is not obtained, there must be leaks in the system, which usually occur at the groundglass joints or rubber-to-glass joints. Close the tap on the manometer, check. the sealing of the joints by rotating each one in turn and also check the rubber-to..glass joints by carefully pushing the rubber pressure tubing a linle further onto the glass. Open the tap on the manometer and recheck the pressure.

11. Gently heat the dlst.IIlng flask and carry out a fractional distillation as described in Box 15.3, ooUecting the fractions by rotating the ·pig'. Remember to record the temperature and pressure at which the fractions distil. If the liquid in the distilling flask bumps over into the oonden8e1'. you will need to clean out the apparatus with a solvent, evaporate off the solvent to recover the chemicals and restart the whole process.

12. When the distillation is complete, dose the tap on the manometer and allow the apparatus to cool. Support the 'pig' and receiving flasks (with your hand or a 'Iabjack'j and slowly open the three-way tap to allow air into the system. Disconnect the pressure tubing from the 'pig' and place the 'pig' and receiving flasks in a safe place. Disconnect all other tubing and apparatus for washing or storage, turn off the waler pump and finally. very gently, open the tap on the manometer to allow the mercury levels to equalize and then close the IBp.

Laboratory techniques

113

Distillation

_lor rtipIerosting

dimlaliDn Plll

-----I

--

---->{

Fig. 15.9 Apparatus for steam distiJIatiOll.

The steam n..'quired for a steam distillation can be provided by all external source, such as piped S1eam in the laboratory or steam generated by hctlling water in a flask. which is then piped inlo Ihe distilling flask. Steam is vcry dangerous and the safest way 10 generate steam is to heat Ihe compound wilh a vast excess of water in the distilling flask: steam is generated in silll. The equipment for steam distillation is illustrated in Fig. 15.9 and Ihe procedure is described in Box 15.5.

~

You must never attempt a distillation in 8 eompletllly

closed system. Always cheek that the expanding vapours can escape.

114

laboratory techniques

Distillation

Box 15.5

How to carry out a steam distillation

1. Set up the apparatus for distillation as described in Box 15. 1 with the following modifications: (s) Use a Bunsen burner with tripod and gauze as the heat source. fb) Use a large three-necked flask as the distilling

flask. usually 250 ml or 500 ml capacity for most laboratory procedures or a single necked

flask with 8 Claisen head (Fig. 15.9). (e) Use a splash-head instead of a still-head since there is no point in recording the distillation temperature and it is more important not to cOntaminate the distillate bv splashing from the distillation flask. Idl Insert an addition funnel (8 separating funnel with a ground-glass joint on the stem) into the distilling flask and fit a stopper in the remaining neck of the flask. (e) Use a single-surface condenser. it is easier to (f)

unblock than a double-surface condenser. Use a simple take-off adapter leading into a large conical flask since you will be collecting a large volume of water.

water, add a large portion of 'anti·bumping' granules and fill the addition funnel with water. 3.

Heat the flask until steady boiling commences and ds'tillatkm of an oily emulsion begins. Then continue the distillation adding water from the addition funnel

to maintain the level of water in the distillation flask. 4. K the distillate is a solid. it may clog and block the condenser. If this occurs. turn off the condenser water, take the condenser tube off the tap to let the water drain out of the condenser and allow the steam to heat up the condenser and melt the solid, which will then flow into the receiving flask. Then carefully restore the water flow. 5. The distillation is complete when no more oily emulsion is condensing (check by collecting a few drops of distillate in a clean, dry test tubel. Turn off the heat and allow the apparatus to cool before dismantling.

6. Separate the product from the water by extraction into a suitable organic solvent (p. 1021.

Z. Add the compound to be distilled to the flask via the spare neck, half-fill the distillation flask with

Laboratory techniques

115

-------10 When carrying out reactions using volatile or dangerous chemicals or solvents - use a reflux system to keep the vapours in the apparatus even though the

mixture is not heated to its boiling point.

Refluxing overnight - you mus! have the approval of your instructor for these operations and complete the necessary

documentation for the night-staff. A special laboratory may be available for 'overnight' reactions.

Since the water pressure may change during the night, you must fix the coolant tubes in place, on the condanser and the

tap, using copper wire twisted round the rubber tUbing or plastic cable ties.

Reflux Reflux is one of the most common techniques you will encounter in your chemistry laboratory c1as..es. Since many reactions between covalent compounds are slow processes rather than instantaneous reactions. prolonged heating forces the equilibrium to give an acceptable amount of product. In the reflux process, the reactants are dissolved or suspended in a suitable solvent, the solvent is boiled and then condensed so that it returns to the reaction flask. Onre set up. a reaction carried out under reflux can be run for minutes. hours or even days to promote the required rea
• The colour is often obliterated by trace impurites of sodium present lsodium gives an intense yellow roIourl. You can overcome this by viewing the colo"," through cobalHwue

glass which aJIolNs the lilac coIouration from potassium to be seen. •• VIeWing through cob&h-b1ue glass l!Ilso allows calcium and strontium to be distinguished. In this case, calcium is light green in colour while strontium appears purploe.

US

Classical techniques

Fig. 19.3 Holding a nichrome wire in a !lome test.

Aametests Simple flalM tC'its can be carrii:d oul on soM samf*$. Place a little of Ihe solid on a watch-glass and moisten with a drop of concentratOO hydrochloric acid 1k purposco of the hydrodllonc acid is to prodoc:c metal chlorides which arc volatile at the temper-llure ofthc Bunsen bumer. Pre-dcan a platinum or nichrome wire by holding it in the houest pan of the Bunsen flame (just above the central b1uc cone) until there is no coloured flame from the "ire. Cool.. then dip the cleaned wire into the moistened solid sample. Plaa: the ",ire at the edge ofthc Bunsen flame (Fig. 19.3) and record the colour of the flame from the sample (sec Table 19.2).

--------0

Gravimetry Gmvimc:tric analySis is the process of convcrting an clement imo a definitive compound, isolating this compound from other con.'.tilucnfs in a swnplc and then weighing the compound (Box 20.1). The weight of the dement eM thell be caJculau..-d from lhc formula of the compound and Ihe relative atomic masses ofthc clements involved. You rJ(:ed to be able to weigh accurately, by difference. a substance 10 four decimal places (sec p. 23).

~ The essential component of gravimetric analysis is

the transformation of the element of interest into a pure stable solid

compound. surtable for weighing.

'Inc most common approadl for isolating the clement is by pn.'Cipitalion from 11 solution where it is prt.'$enl in lonK: form (SL'C p. SO). Ideally, the consliIUl..'tl1 under investigation is precipitated out of solution as a wmerinsoluble compound. so Ihal 110 IOS&-'S ()(:cur when the precipitate is sepamtoo by filtmlion. wasll':d rree or soluble impurities and then wLighed.

Box 20.1

How to carry out gravimetric analysis

Suppose you wanted to analyse the amount of metal in an alloy. For example, you might want to determine the nickel content,. as a w/w percentage (p. 471, in a particular sample of steel. 1.

Select an appropriate solvent for your sample: in this example, you could dissolve the steel in aqua regia. a combination of concentrated nitric acid and concentrated hydrochloric acid in the volume ratio of 1: 3 respectively.

2. Choose an appropriate precipitant and carry out the precipitation reaction Here, an alcoholic solution of dimethylglyoxime could be used to precipitate nickel from a hot solution of aqua regia. by adding a slight excess of aqueous ammonia solution, forming a red precipitate of nickel dimethylglyoximate (Fig. 2O.3J. 3.

4.

5. 6.

Alter the precipitate through a pre-weighed Gooch crucibte. This should have been previously dried in an oven at 120 C and stored in a desiccator until required. Wash the precipitate: in this example, the nickel dimethylglyoximate can be washed with cold water until qualitative testing shows that the wash solution is free of chlOride ions.

Dry the precipitate in an oven, for example, at 120 C, and allow to cool in a desiccator. Determine the weight of the precipitated compound. Suppose in this example that the nickel had

been precipitated from steel (2.0980 g) using dimethylglyoxime (H 2 DMGI. giving a precipitate of nickel dimethylglyeximate tNi(HDMG)2) weighing

0.23709· 7.

Write out the equation for the reaction and perform the calculation. In this example. you need to check the relative atomic masses of the elements involved. The relative molecular mass of nickel dimethyl. glyoximate is 288.91 9 mol 1 (for structure see Fig. 20.3; A,.. H = 1.01; 0 "'" 16.00; C = 12.01; N = 14.01; Ni 3' 58.69). The equation for the reaction can be summarized as: Ni2 -t- + 2H 2DMG '" Ni(HDMGlz + 2W According to the above equation. for each mole of Ni in the steel sample, 1 mote of precipitate will be formed. Therefore. 0.2370 9 of precipitate carre· spends to: 0.2370g NilHDMGh + 288.95g mol- T Ni(HDMGlz = 8.20 x 10-· mol Ni(HDMGh The amount of nickel in the steel sample must therefore be: 8.2 x 10- 4 mol NilHDMGlz x 58.69 9 mol- T Ni = 0.0481gNi

The percentage weight of nickel in the steel sample is therefore:

0.0481 g Ni -:- 2.0980 9 sample x 100 = 2.29% w/w

aassical techniques

139

Gravimetry

Precipitation Table 20.1

Common precipitants IonlsJof

Interwt

.~

Interl_ms Pd 2.,

Dimethylglyoxime

~',

Bi:J.o

and Au'J+ Cu 2 • and Pb2 +

Cupferron a-Hydroxyquinoline (oxine)

Many metals

CH,-C-OOH I CH 3-C-NOH £.Wne!h';tolllyomle

Inorganic ions C'dn be sepamtoo from mixtures using organic reagents (precipitants), with which they form sparingly soluble, often coloured, compounds. The precipitants usually have high molecular weights, so a small quantity of the ion will producc a large amount of precipitate. IdeaU)', the precipitant should be specific for a particular ion. though this is rarely so. Examples of conunon precipitants and their target ions are shown in Table 20.1 and their structures are shown in Fig. 20.1. DimethyIglyoxime is only slightly soluble in water (0.40 g L-I) and it is therefore used as a 1"'/", w/v solution in ethanol. Cupferron, the anunonium salt of N-nitroso-N-phenylhydroxylamine, is used as a 5-10% wlv solution in hydrochloric add or sulphuric acid. Oxine (8-hydroxyquinoline) is almost insoluble in water and is USL'd as either a 2% or a 5% wlv solution in 2mol L-[ acetic acid. When precipitating a compound:

,NO @""N'O-NH;

•

"Jl the standard solution and the tcst substance is just complete is calk'd the t-"quivalencc point OT the tht-"OTctical (or stoichiometric) end-point. This is nonnally detected by a visible change. dther of the standMd solution itself or. more commonly, by the addition of an indicator.

SWntlm'tl !J·oluliolts A stand~rd solution can be prepared by weighing out the appropriate amount of a pure Tctlgenl and m~king up the solutioll to a particular volume, as descrilx.'CI on p. 24, The concentration of a st:mdard solulion is expressed in terms of molarity (p. 19). A substance used in a primary standard should fulfil the following criteria: • • • •

• •

It should be obtainable in high purity (>99.9%). II should rt-main unaltert->d in air during weighing (i.e. it should 1I0t be hygroscopic), It must not dt-'Compose when dried by heating or vacuum. It shouJd be capable of !x'ing tested for impuTitit'S. It should be readily soluble in an appropriate solvent. It must re-dct with the test substance stoichiometrically and rdpidJy.

P,'eparillg II Slltnlltw,1 soll/Iiolt Thc molarity of a solution is the concentration of (he solution exprcsS(:d as mol L-I. If x g of a substance of mok'Cular weight M r is dissolvl.'d in y mL of distilled watcr. the moles of substance dissolved = ~ = m. Therefore, Mf IJ'IxlOOO molarity (molL-I) = (21.11 )'

Classical techniques

143

Procedures in volumetric analysis

Primary standards prepared from solid compound.. should be weighed out using the 'weighing by dilTerence' method itS dt-'SCribed in Chapter 4, and accurately made up to volume using a volumetric flask. Complete transfer of the substance from the weighing vt--ssel to the volumetric flask is best achieved by inserting a funnel into the neck of the flask (Fig. 21.1). As much of the solid as possible should be tmnsferred I'fa the funnel. The funnel should be wdshcd with distilled water prior to removal. Distilled water is then addl-d to the flask, with occasional swirling to help to dissolve the solid. lllis is continued until the meniscus is about I em below the volume mark. At this point a stopper is inscrted and the flask is invcrted several timt:s to ensure the solid is completely dissolved. Finally, using a Pasteur pipette. distilled water is added lip to the volume mark. The solution should be thoroughly mixed before use. If the solid is not readily soluble in cold water it may be possible Lo dissolve it by stirring in warm water in a beaker. After allowing the solution to cool to room temperature. it can be transferred to the volumetric flask using a glass rod and filter funnel (Fig. 21.1) followed by several rinses of the glass rod/ftiter funnel. Finally, the solution is made up to the mark with distilk-d water.

Filling a burette For accurate readings - always remember to positton your burette vertically.

•

Clamp a ck'an 50.00mL burette (p. 10) in a laboratol)' stand. Place a beaker on a white tile immediately below the outlet of the burette (Fig.

21.2). •

• •

Place a small filtt-... funnel on top of the burette and, with the tap open. carefully pour in the standard solution (or titrant) until it starts to drain into the beaker. Gose the burette tap, and fill the burette with the standard solution until the meniscus is about 1-2cm above the zero mark. Remove the funnel. Open the tap and allow the solution to drain until the mcniscus falls to the zero mark. The burettc is then rC'ddy for the titration.

Note that to avoid contamination the solution in the beaker should be discarded, rather than rt-'Cyck'(juently will be undiluted and uncontaminated by any residue or liquids in the pipette.

Procedures in volumetric analysis

"", ""

,...,

,otIJ1 stand

-

~"-,-

if

~....:

rinse_

\

"\ f

a

~

~ ~

Fig.21.1 Quantitative transfer of a solid 10 a volumetric flask.

•

Fig. 21.2 Apparatus for 8 titration.

Refill the pipette until the meniscus of the titrand is above the graduation mark (Fig. 2I.3a). Remove the pipette filler and block the hole at the top of the pipette with the index finger of your right hand (if right-handed; Fig. 21.3b). Carefully raise the pipette to eye level, and allow the tilnmd to drain olll inlo a beaker by lifting your finger slightly from the top of the pipette. Continue until the bottom of the meniscus is on the graduation mark. Classical techniques

t45

Procedures in volumetric analysis

•

Wip: the outside of the pipette with a tissue (Fig. 2I.3c). Be careful not 10 touch the point of the pipette with the tissue otherwise solution will be

by capillary aClion. Allow the pipetle's contents to drain into a conical nask. Finally, touch the end of the pipette on the wall of the flask (Fig. 2I.Jd). :lnd rinse the inside of the neck of the flask with distilled water. This will ensure that exnctly 25.00mL of the test solution has been delivered by the pipette. 1051

• •

Note Ihat it is nonnal for n small (IUantity of solution to remain in the pipette tip. This volume is tnken into account when pipcltes are c:l!ibmted. so do not attempt to ;blow ouC this Liquid into the conical n:lsk.

Performing a titration Add one or two drops of indicator to the titmnd contained in the conic:!1 nask. For a right-handed person the process is as follows: 1.1

1'1

•

•

•

•

"1 Fig.21.3

I.

Using a pipene.

Washing the test solution - any distilled water used for washing the test solution from the walls of the conical flask has no effect on the titration or the calculation.

Volumetric analysis - calculating the concentration of a test substance The calculation should be carried out in a togical order as follows: I. 2.

Reading a burette - your eye-line should be level with the bottom of the meniscus. Then. record the volume used to one decimal place. Rough titration - always carry oul an initial rough titration to determine the approximate volume of litrant required for the end-point. This allows you to anlicipate the end-point in subsequent titrations to determine the accurate volume.

146

Classical techniques

Hold Ihe conical flask containing the litf'cwd and the indicator in your right hand, and control the tap of the burelle 'flith your lef! hand. The bureue should be arranged so Ihat the tap is on the opposite side of the burette to your palm. In this way, your len hand also suppom the body of the burette (Fig. 21.4). Add the titrant by opening Ihe tap. and simultaneously swirl the contents of the conical Oask. This may take a bit of pT'dCtice, so do not worry if you cannot do it stmight llway. The ,itraOl can be added quickly al first, but as the end-point approaches. additions should be made drop-wise. The end-point is indicated when the :lppropriate colour change takes place. When the end-point is reached, one drop of titr:mt should be sufficient to cause the colour change. You should note the volume used for the titration to the nenrest O.I-o.05mL. This is done by reading orr the volume of titrant used (Fig. 21.5). Refill the burette 10 the zero mark using a funnd ready for the next titralion.

Write the balanced equation for the reaction bet\vecn the standard and the test substance. From the stoichiometry of the reaction, detennine how many moles of the test substance react with I mole of the standard substance. For example, in the reaction behwen an H2SO4 standard solution and nn NaOH lest solution:

12I.2J 3.

Therefore 2 moles of NaOH react with I mole of H2S04 . Calculate the nwnbcr of moles of standllrd substance used to reach the end-point of the reaction. This can be detemtined from knowledge ofille concentration of the standard solulion (mol L-I) and the volume of tilrant used (mL). Rem('''ll1ber to take cnre with units - in this instance division by a factor of 1000 is required to convert mL to L:

Procedures in volumetric analysis

•

Ii

Fig.2J.5 Reading a burette. Place 8 white card b610w the level of the meniscus. This alloYJS an accurate reading to be made.

Fig.21.4 Performing a tltration.

For consistency of resuhs - alw8Ys perform two or three titrations (or until consistent resuhs are obtained, Le. titre values within 0.1-0.5 mL).

N 4.

S.

um

be

f Ics concentration (mol L -I) x volume (mL) ro mo = 1000

The number of moles of lest substancc present in the litnl.nd is then obtained from knowledge of the equivalences. In the example given above (point 2) the number of moles of test substance is twice the number of moles of standard substance. Therefore, if X moles of H 2 S04 are used (as calculated in point 3). 2X moles of NaOH were present in the initial volume of test solution. Finally, the concenlnllion of the lest solution C:ln be cdlculated using the fomlula: lOOOxamount of test substance (mol) Concentration of test solution (mol L -I) = initial volume of rCSl solution (mL) Again, the factor of 1000 is used to converr mL to L.

Classical techniques

147

-------0

Acid-base titrations The titrcHion of an acid solulion with a slandard solution of alkali will detennine the amount of alkali which is equivalent to the amount of acid presenl (or vice versa). The point al which this occurs is called the equivalence point or end-point For example, the titration of hydrochloric acid with sodium hydroxide can be expressed as follows: [22.11

safetv note Never pipene by mouth.

_...

Ifbolh the acid and alkali arc strong electrolytes, the rcsullam solulion will be nculml (pH 7). If on lhe other hand either (he acid or alkali is a weak electrolyte the resultant solution will be slightly alkaline or acidic, respectively. In either case, detection of the end-poinl requires accur:lte measureml>J11 of pH. This can be achieved L'ither by using an indicator dye, or by measuring the pH with a glass eleclrode (described in Chapler 7).

Acid-base indicators Typical acid~base indicators are organic dyes that change colour at or near Ihe equivalence or end-point. 'Iltcy have Ihe following characteristics: • •

,.

•

.....

l pH 11 (HvnJ-1 Solochrome ~ < pH5 (Hzln-) pH7-11 (Hln Z ) >pH11.5\1n J-j

Colour of free indicator

Colour of metal·ion complex

Red-violet Violet Blue

orange (Cu 2 'I, yellow (Ni 2 ' and Co 2 ') and red (Cahj

Red

In pH range 7-11 colour change is

Blue Orange

blue-red (Mg, Mn, In, Cd, Hg, Pb, Cu,

Calmagite pH 11.4 (tn J-)

Blue Red-orange

Pyrocatechol viok1t < pH 1.5 (H.lnj pH 2-6 (HJln I pH7 (H zln2-j >pH 10 fln' j

Yellow Violet Blue

Red

Red

AI. Fe,

n. Co, Ni and Pt metalsl

Same colour change as solochrome blad< bu! clearer and sharper

In pH range 2-6, yellow to blue (Bi and Thl; pH 7 violet to blue (Cu 2.., ln 2 ', Cd h , Ni z , and Coh )

Practical considerations • •

• •

For a metal-ion indicator to be useful it must be less stable than the corresponding metal-EDT A compklx.

•

pH adjus!ment is crilical in EDTA lilr:nions. The pH is monilorcd with a pH meter or pH test paper. The metal ion under investigation should ideally be approxima~ly 0.25 roM in a volume of 5tJ.-.-150mL of solulion. Dilution of Ihe melal ion may be m..'Cessary 10 avoid end-point deu.:ction problems. Do not add excess indicator. as too intense a colour can lead to problems. e.g. masking of !he colour change. It is somelimcs difflCull to delcel Ihe end-point because Ihe colour change can be slow to develop. Slirring is t\.'Commended 10 assisl colour ll"dnsformation. The use of meTal-ion indicators 10 indicale lhe end-poinl of complexomclric titralions is based on a specific colour ch:mge. Some individuals may find it difficult to delt.'ct a particular colour change (e.g. Ihose with colour blindnes.1

"""""" ,D" Fig. 26.1 The electromagnetic spectrum.

Light is most strK.1Jy defined as that pan of the spectrum of electromtlgnctic radialion detected by the human eye. However, the term is al~o Hpplied to rddialion just outside that visible range, e.g. ultraviolet (UV) and infrmed (lR) 'light', Electromagnetic radiation is cmined by the sun and by other sources (e.g. an incandescent lamp) and the electromagnetic spectrum is a broad band of radiation, ranging from cosmic rays to radio waves (Fig. 26.1). Most chemical experimenlS involve measun:menlS within the UV, visible and IR regions (generally, within the wavclt.:ngth ranJ,'e 200-IOOOnm). Rl:ldiation has the char.:lClcristics of a particle and of a vibrating W(lve, travelling in discrete particulate ulliL~. or 'packets'. termed photons. A quantum is the amount of energy contained within a single photon (it is important not to confuse these two tenns., although they ~lre sometimes used interchangeably in I.hc literature). In some circumstances. it is appropriate 10 measure light in tenns of the /lumber of photons. usually expressed directly in moles (6.02 x Ion photons = I mol). Alternatively, the energy content (power) may be measured (e.g. in Wm- 2). Radiation also behaves as a vibrating electrical and magnetic field moving in a particular direction, with the: magnetic and ekctrical compont:l1ls vibrdting perpendicular to one anotht.T and perp(:ndicull:lr to the direction of trd\'el. The wave nature of radiation gi\'es rise to the concepts of wavelength (J.. usually measured in nm), frequency (l', measured in S-I, but often recorded in hert,.. Hz). speed (c, the speed of electromagnetic radiation. which is 3 x l()llms- I in a vacuum), and direction. In other words., radiation i~ a vector quantity, where:

[26.IJ

c=J."

, l

.--

3

,0'

Basic spectroscopy

Sometimes, it is necessary to fe';:'16mL ofthi' stock the pre-deaned bea~

~n

into one of

3. Ouantit:lrliwlly bMISfer 10.00mL of thestoek solution Into a 100.00 mL volumetric flask. Then, dilute' to 100.00ml Vllith 1 % v/v HNO:J (high purityl.

10000pg 100ml You now have a 100~gml-' 'working' stock solution of Pb.

S. Tren. . . s::::.15 ml of the working Itodl solution

Into the other pre-dNned b8IIker~

6. Then. quentitdvety tJensler 2.00 mL of the Mlfudon into a 100.00 ml vaIumettic .... and dilute to 100.00 ml with 1 % v!v HN~ (high purity). label the ftesk as the 2J1g mL ' Pb calibration soltltion.

7. Similarly bwlsfw O....00. 0.00. 8.00 and 10.00 mL

4. What .. the c:oncentnItion at this new IIOIution1 Remember that we started with 8n initial 1000~ mL-, Pb stock solution.

8. Talc. the 0, 2, 4, &. 8 end 10 JIQ ml~' Pb caHbratioo solutions for FAAS analysis.

,:~g X 10ml= 10000JIgPb 10000jlgPb

~as

~ into ~ yohanetric flasks end dilute to 1oo.00ml vvith the nitrk acid and label as O. 4, 6, 8 end 10 p.g ml-1 Ph calibration solutions.

p'aced in 8 100.00mL volumetric

flask. so:

heater unit, capable of wanning up sc\'eml HCLs simultaneously. or by inserting the HCl in the AAS instrument .md switching on the current. The lamp is typically operated at an electric currenl between 2 and 3OmA.

A tonlizUl;On cell Safety note Gaution is needed when using strong (concentrated) 9cids. When using concentrated acids always wort in a fumo cupboard. Wear gloves to protect your hands from 'acid bums'. Always rinse affected areas with copious amounts of water.

Sc\!cml lypcs of atomi7...lIion cell arc available: flame. graphite furnace. hydride generation and cold vapour. Flame is the most common. In the premixed laminar flame. the fuel and oxidanl g."lses arc mixed before they enter the burner (Ihe ignition site) in an expansion chamber. The more commonly used flame in FAAS is the air-llcct),lcnc flame (temperature, 2500K), while the nitrous oxide-acetylene name (temperature. 31SOKJ is used for refractory c1eml.Tlts. e.g. AI. Both arc fanned in a slot burner positioned in the light path of the HCl (Fig. 27.4). In the grnphite furnacc atomizer, a small volume of sample (5-I00pL) is introduced onlo the inner surface of a graphite tube (or onto a platform placed within the tube) through a small opening (Fig. 27.5). 'nle graphite tube is armnb'Cd so that light from the HCl passes din.-ctly through the centre. Passing an electric current through the lube allows the operator to program a heating cycle. with scveral stages (Fig. 27.6) including the elimination of water from the sample (drying), removal of the sample matrix (ashing),
4. Adjustthe 98" control until theenergv meter reading roaches & maximum.

5. Adjust the wavelength 'elec:tcll' for maximum signal reading. You are now ready to ignite the airacet0ene flame. &. Turn on 'It. fume extreetion hood. Thifl aHows toxic gases to be safely removed from t~ laboratory environment.

7. Tum on the air suppty soch that the oxidant Row meter Is 8t'the desired setting. 8. Tum on the ec:etyIene supply such that the fuel flow meter is at the desired setting.

..

Table 27. 2 Standard operating conditions for Ieed

283.3 217.0 205.3 202.~

261.4 368.3 364.0

0.7 0.7 0.7 0.7 0.7 0.7 0.7

c:. III bMion ~1,..~1, , , 0.45 0.19

6.'

7.' 11,0 27.0 87.0

Nato: recommended tIame: alr-acetytene, Ol&iling (1e80, bluel.

9. Press the

9Wte button lor fteme button).

flame should light instantaneously with

8

The

'pop'.

10. Aftsr establishing the flame, insert the aspirator tube into distilled water. Allow the flame to stabilize for up to 1 minute by aspirating distilled water, prior to analysis. 1 1. AIt.... completing your allIItysIs. shut off the acetYlene first Iby closing the cylinder valvel.and vent the acetylene gas line while the air is still on. Then. shut off the air compressor and allow the air line to

vene 12. FUUtlly, switeh off the fume extriKtion hood.

Matrix interferences arc oOen associated with the sample introduction process. For example, pneumatic nebulization ean be affected by the dissolved-solido; content of the aqueous sample, which affccts the uptake rate of the nebulizer and hence the sensitivity of the assay. Matrix effects in the plasma source typically involve the presence of easily ionizable clements (EIEs), c.g. alkali metals, within the pla~na source.

Decomposition techniques for solid inorganic samples Conversion of a solid matrix into a liquid matrix involves the decomposition of the sample. One of the major problems in preparing solid samples for trace clement analysis is the potential risk of contamination. Contamination can arise fmlll several sources: the grade of reagents used: the vcsscls used for digestion and the subsequent dilution of the sample: and human involvement. In order to minimize the risk of contamination you should take the following measures: • •

Use the highest purity of reagents and acids. including the water used for sample dilution. Use sample blanks in the analytical procedure, to identify the base level of impurity in the reagents. Instrumental techniques

177

Atomic spectroscopy

•

• •

Soak sample vessels in an acid leaching bath (e.g. 10% v/v nitric acid) for at least 24 hours., followed by rin.'iing in copious amoulllS of ultrapure water. Store e1eaned volumetric nasks with thcir stopper.; inserted: cover beakers with Qingfilm R or store upside down to protect from dust. In addition to the wcaring of a labor-dlory coat and safety gLasses. it may be ooccss.ary to wear 'oontamimmt'-frec gloves and

•

I'

for C-C

as the massc v for C-D: v for 0 H > \. for S-H

IR absorption bands - since the frequency of vibration of a bond is a specific value you would expect to see line spectra on the chart. However, eact1 vibration is associated with several rotational motions and bands (peaks) are seen in the spectrum.

Bonds can also bend. but Ihis movement requires less energy than stretching and thus the bending frequency of a bond is always JOJ1'er than the oorresponding stretching frequency. When IR radiation of the same frequency as the bond interacts with Ihe bond it is absorbed and increases the amplitude of vibration of the bond. This absorption is detected by the IR spectrometer and results in a peak in the spectrum. For a vibration to be detected in the IR region the bond must undergo a change in dipole moment when the vibration occurs. Bonds with the greatest change in dipole moment during vibration show the most intense absorption. e.g. C=O and C--o. Since bonds betwecn specific atoms have particular frequencies of vibralion. IR spectroscopy provides a means of identifying the type of bonds in a molecule, e.g. all alcohols will have an O--H stretching frequency and all compounds containing a carbonyl group will have a c=o slretching frequency. This propeny. which does not rely on chemiml tests. is extremely useful in diagnosing the functional groups within a covalenl molt."Cule.

IR spectra A typical IR spectrum is shown

III

Fig. 28.1 and you should note the

following JX)ints:

"

100.00 , - - - - - - - - - - - - - - - - - - - - ,

\I ,

~

,

,

~

M

g , ~

nooL-~-

The use of wavenumber - this is an old

established convention, since high wavenumber"" high frequency", high energy = short wavelength. Expression of the IR range, 4OOOcm-' to 650cm- 1 , is in 'easy' numbers and the high energy is found on the left-hand side of the spectrum. Note that IR spectroscopists often refer to wavenumbers as 'frequencie5', e.g. 'the peak of the C=O stretching 'frequency' is at 1720 em-I •.

4000

Fig. 28.1

•

IR

_ _- ~ ~ - - - _ - - _ - _ _ _ J

3500

3lXXl

2500

2lXXJ

1500

IlXXJ

6OOcm"

spectrum of ethyl ethanoate CH 3 COOCH zCH 3 as a liquid film.

The x-axis, the wavelength of the radiation. is gi\'en in waveflumbers (v) and expressed in reciprocal centimelres (em-I). You may still see some spectra from old instruments using microns (p, equivalent to the SI unit 'micromelres·. j./m, at I x 10-6 m) for wavelength: (he conversion is given by eqn [28.2): wavenumber (em-I) = I/wavelength (em) = IOOOO/wavelength (pm) [28.2] Instrumental techniques

181

Infrared spectroscopy

•

•

The y-axis. expressing the amounl of rndialion absorbed by the molecule. is usually shown as "I" trnnsmittance (p. 164). When no radialion is absorbed (all is trnnsmiued through Ihe sample) we have 100% transmittance and 0% tmnsmiuunce implies all radiation is absorbed at a particular wavenumber. Since the y-axis scale goes from 0 to 100% IrnnsmiUallce. the absorption peaks are displayed (fOlln from Ihe 100% line; this is opposite 10 mosl other common speclra. The cells holding the sample usually display imperfeclions and arc not completely transparent to IR ntdiation. even when emply. Therefore Ihe base line of the spectrum is rarely set on 100% trdilsmiUance and quantitative applications of IR spectroscopy arc more complex than for UV-vis (p. 166).

JR spectl'Ometen· Therc arc two general types: monochromelOr detet:\QI'

(t~~~~-----,

~W ....

chlppel"

grating

slits

't ---

~

1.

SlUre

lWTl

Fig. 28.2 Schematic diagram of a doublebeam IR spectromeler.

(a) scan speed: this is lhe rate at which the chart moves - slower for greater accuracy and sharp peaks: (b) wavelength range: thc full s!XCtrum or a part of the IR range may be selected; (c) 100% conlrol: this is used to set the pen at the 100% transmiuance line when no sample is present Ihe base line. It is usual practice to sel Ihe pen 1.11 90% transmittancc at 4000 em-I when the sample is present. 10 give peaks of the maximum deneclion.

Using double-beam instruments - you can identify the sample beam by quickly placing your hand in the beam. If the pen records a peak, this is the sample beam, but if the pen moves uP. then this is the reference beam.

Using the 1000A, control- if you use this contralto set the base line for the sample, you must turn down the 100% control when you remove the sample, otherwise the pen-drive mechanism may be damaged in trying to drive off the top of the chart.

Double-beam or dispersive instrumenls in which the lR radiation from a single source is split into two identical beams. One beam passes through the sample and the other is used as a reference l=!.

l=!.

are not used on a routine basis.

""18... field

110 =

.-

I.'

For a given value of the applied field (80), nuclei of different elements hallc different values of the magnctogyric ratio (;') and will give rise to resonance at various radio fl\.'<juencics. The plincipal componenl" of an NMR spectromcter are shown in Fig. 29.2.

htHHHHH+

'.d

}

"'" tttttPttttttt~

t>E~

[29.11

"'IBo/2rr

pole of magrEt

hv

field COO\JoIer

Ibi

-.

.,

Fig. 29.1 Effect of an applied magnetic field. 80. on magnetic nuclei. (a) Nuclei in magnetic

field have one of two orientations - either with the field or against the field (in the absence of an applied field, the nuclei would have random orientation!. (bl Energv diagram for magnetic nuclei in applied magnetic field.

Example

For an external magnetic field

of 2.5 T (lesla), !::.E for 'H is 6.6 x 10 2fl J and since l:lE = hi', the corres~nding

frequency M is l00MHz; for same field, b.E is 1.7

X

25 MHz.

190

1

C in the

10-2ti J, and v is

Instrumental techniques

I~'"

Fig.29.2 Components of an NMR spectrometer.

For maBnetic nuclei in a given molecule, an NMR spectrum is ber1 increase with increasing acidity of the 0-1-1 group: thus fJ = 1~6 ppm for alcohols, 4--12 ppm for phenols and 1(}....14 ppm rorcarboxylic acids. Hydro~cns bound to nitrogen (10 and 2"' amines) are round al fJ = 3-8 ppm. The approximate chemical shin regions are shOwn in Fig. 29.5.

I"teg,.atio" ofpeak m·eas Integration of coupled peaks - the area under the singlet, doublet,. triplet,. quartet, etc., is still thai of the type of hydrogen being considered. For example, if the peak for the three protons of a methyl group is split into a triplet by an adjacent methylene group, the Brea of the triplet is

The area of each peak gives the relative number of protons and is produced directly on the spectrum (Fig. 29.6). On CW-NMR spectrometers the height or the peak area integration line must be measured using a ruler, whereas on FT-l'.'MR machines the area is calculated and displayed as a number. You must remember that: •

three.

•

154

Instrumental techniques

The areas are ratios, not absolute vaJ~, and you must find a peak attributable to a specific group to obtain a reference area. e.g. a single peak at (; = 1.0 ppm is likely to be a CH) group and thus the area displayed or measured is equal to three protons. You must ensure thaI you include integrations from all the fine-structure (coupling) peaks in the peak area.

Nuclear magnetic resonance spectroscopy

13 12 11 10 9

B 1

6 5

4 J

2 1 0

Fig. 29.5 Approximate chemical shift positions in the 'H-NMR spectrum.

~

• b

,

CH,CtylCH3

, •

-

TMS

b

•• Fig. 29.6

•

3.0

2.0

1.0

'H-NMR spectrum of melhoxyethane.

Do not expect the pt:ak area integrations to be exact whole numbers, e.g. an area of 2.8 is probably three protons (CH 3). 5.1 is probably five protons (e.g. a Ct;H s group), but 1.5 is probably 7.27 ppm (Fig. 29.IOc);

•••

(b) para disubslitutcd aromatic compounds, which are of two types. If the SubslilUcnls are lhe same. then all the rill!! protons arc identical and a singlet. of relative area four. is ~n (Fig. 29.IOd). If the substiluents are different, Ihen the paiN; of hydrogells orlhf) to each substituent are different and ol'/ho-D

'.0

'0

,.,

II

no

'.0

Jif

.,

~"

J.2 1.0

,--..,.....-.-

"

6lppnt

Ibi

•

•

• 'OCl
~

r"

,:1'

10

1114 11

10

'"

mI'

"

"

Fig. 30.3 Mass spectrum for methanol; rrizmass-to-
Chromatography is sub-divided according to the mechanism of interaction of the solute with the stationary phase.

Adso,pt;on c/".omatog,'aplty This is a form of solid-liquid chromatography. The stationary phase is a porous, finely divided solid which adsorbs molecules of the mixlure on its surface by dipole-dipole interactions, hydrogcn bonding and/or \'an der Waals' in(f.Taclions (I-.g. 31.1). The range of adsorbents is limited 10 polystyrene-based resins for non-polar molecules and silica, aluminium oxide and calcium phosphate for- polar molecules. Most adsorbenlS nUL'll be activated by heating to I 1(}'-120°C before use, since thcir adsorptive capacity is significantly decreased if water is adsorbed on the surface. Adsorption chromatography can be carried out in column (p. 217) or thin-layer (p. 216) form, using a wide range of organic solvents. Fig. 31.1 Adsorption chromatography (polar stationary phase).

Par/ition ch,'omalOgl'apltJ' This is based on the partitioning of a substance between two liquid phases, in this instance the stationary and mobile phases. Substances which are more soluble in the mobile phase will pass rapidly through thc system while those which favour the stationary phase will be retarded (Fig. 31.2). ]n nonnaJ phase partition chromatography the stationary phase is a polar solvent, usually water, supported by a solid matrix (e.g. cellulose fibres in paper chromatography) and the mobile phase is an immiscible, non-polar organic solvent, For reversed-phase partition chromatography the stationary phase is Instrumental techniques

205

Chromatography - introduction

....

-

~=~'J-­ ,~ "o

--

lheewrm

I flow.

Fig.31.2 Uquid-liQuid partilion chromatographV, e.g. reversed-phase HPLC.

Maximizing resolution in IEC - keep your columns as short as pOSSible. Once the sample components have been separated, they should be eluted 8S quickly as possible from the column in order to avoid band bf"Oodening resulting from diffusion of sample ions in the mobile phase.

sellllle cations oisplIal ndlie lXUllOOons Ie.g. H' IllI'CIlIre retllinBd twllfllI cd.ml stlllil;ray p/lasIl is B eatiorr~

,,,;,

Fig. 31.3 IOfHIXI:hange chromatography lcation exchanger).

a non*polar solvent (e.g. a elM hydrOC'.ubon, such as octadcx:ylsilanc) which ill chemically banded 10 a porous support matrix (c.g. silic'd), while the mobile phase can be chosen from a wkle rlsotope solution required: for example, a manufacturl'!r's stock solution of I·e_labelled mannitol contains SO/ICi of radioisotope in 1 ml of 90% vlv ethanol; water. Using Table 35.3. this is equivalent to an activity of 50 x 37 = 18SOkBq. So, the volume of solution required is 250/1850 of the stOCk vOlume, i.e. 0.1351 ml f1351Ill. 3. Calculate the amount of non-radioactive substance required as for any calculation involving concentration (see pp. 17, 146), e.9. 50 mL (0.05 L) of a 25 mmol L 1 (0.025 moll 1) mannitof (relative molecular mass 182.17) will contain 0.05 x 0.025 x 182.17 = 0.2277 g. 4. Check the amount of radioactive isotope to be added. In most cases, this represents a negligible amount of substance, e.g. in this instance, 250kBq of stock solution at a specific activity of 14.8)( 1()6kBqmmol- 1 (converted from 0.4Ci mmol 1 using Table 35.3) is equal to 250/14800000 = 16.89 nmol, which is equivalent to approximately 3119 mannitol. This can be ignored in calculating the mannitol concentration of the experimental solution. 5. Make up the experimental solution by adding the appropriate amount of non-radioactive substance and the correct volume of stock solution.

Liquid scintillation counting of highenergy beta emitters - beta particles with energies greater than 1 MeV can be counted in water lCerencov radiationl, with no requirement for additional fluon; fe.g. 32 PI.

238

Instrumentat t~hniques

6. Measure the radioactivity in a known volume of the experimental solution. If you are using an instrument with automatic correction to Sq, your sample should contain the predicted amount of radioactivity, e.g. an accurately dispensed volume of l00,ll of the mannitol solution should give a corrected count of 100 x 5 = SOOBq (Or 500 x 6030 000 d.p.m.). 7.

Note the specific activity of the experimental solution: in this case. l00/ll (1 x 10-4 11 of the mannitol solution at a concentration of 0.025 moll 1 will contain 25 x 10-7 mol (2.5pmoll mannitol. Dividing the radioactivity in this volume 130000d.p.m.) by the amount of substance (eqn 135.21) gives a specifIC activity of 30000/2.5_ 12000d.p.m.llmol 1, or 12 d.p.m. nmol I. This value can be used: lal To assess the accuracy of your protocol for preparing the experimental solution: if the measured activity is sUbstantially different from the predicted value, you may have made an error in making up the solution. lb) To determine the counting efficiency of an instrument; by comparing the measured count rate with the value predicted by your calculations.

Ie) To interoonvert activity and amount of substance: the most important practical application of specific activity is the conversion of experimental data from counts (activityl into amounts of substance. This is only possible where the substance has not been metabolized or otherwise converted into another form; for example, a tissue sample incubated in the experimental solution described above with a measured activity of 245 d.p.m. can be converted to nmol mannitol by dividing by the specific activity, expressed in the correct form. Thus 245/12 = 20.417 nmol mannitoL

Many liquid scintillation counters treat the first sample as a ·background', subtracling whatever value is oblained from the subsequent measurements as part of the procedure for converting to d.p.m. If not you will na-d to subiraci Ihe background count from all olhcr samples. Makc sure thai you use an appropriate background samplc, identical in all respects to your radioactive samplc but with no added radioisotope, in the con-CCI position within the machine. Check that the- background reading is reasonable (1530c.p.m.). Tips ror preparing samples for liquid scintillation counting are givcn in Box 35.2.

Radioactive isotopes and their uses

80x 35.2

Tips for preparing samples for liquid scintillation counting

Modern scintillation counters arc very simple to operate; problems are more likely to be due to inadequate sample preparation than to incorrect operation of the machine. Common pitfalls are the following: • Incomplete dispersal of the radioactive compOund in the scintillation cocktail. This may tead to underestimation of the true amount of radioactivity present: lal Water-based samples may not mix with the scintillation cocktail - change to an emulsifierbased cocktail. Take care to observe the recommended limits. upper and lower, for amounts of water to be added or the cocktail may not emulsify properly.

based cocktails by adding a specific amount of water. • Chemiluminescence. This is where a chemical reacts with the f1uors in the scintillation cocktail causing spurious scintillations, a particular risk with solutions containing strong bases or oxidizing agents. Symptoms include very high initial counts which decrease through time. Possible remedies are:

lb} Solid specimens may absorb disintegrations or scintillations: extract radiochemicals using an intermediate solvent like ethanol (ideally within the scintillation vial) and then add the cocktail.

lei Particulate samples may sediment to the bottom of the scintillation vial- suspend them by forming a gel. This can be done with cenain emulsifier-

la} Leave the vials at room temperature for a time before counting. Check with a suitable blank that counts have dropped to an acceptable level. lb) Neutralize basic samples with acid (e.g. acetic acid or Hel). lcl Use a scintillation cocktail that resists chemiluminescence such as HiconicfluorJ:'. ldl Raise the energy of the lower counts detected to about 8keV - most chemiluminescence pulses are weak (0-7 keY). This approach is not suitable for 3H.

rra)' spect"ometry

This is a method by which a mixture of j·-my-cmitting radionuclides can be resolved quantitatively by pulse-height analysis. It is based on the fact that pulse heights (voltages) produced by a photomultiplier tube are proportional 10 the amOlUllS of I'-ray energy arriving at the scintillant or a lilhium-drirted gennanium detector. The lithium-drirted gelmanium detector, which is abbreviated to GL-(Li) - pronounced 'jelly' provides higher resolution (narrov.-er peaks), essential in the analysis of complcx mixtures.

Anroralliography This is a method where photographic film is exposed to the isotope. It is used mainly to locate mdioactivc tracers in thin sections of all organism or on chromatography papers and gels. but quantitative work is possiblc. The radiation interacts with the film in a similar way to light. silver grains heing fonned in the developed film where the particles or rays have passt.'" '-.......... ~ Iltin; CllIllIitions

monohydrdte is fonned quantitatively. After washing the precipitate with ethanol it can be anaIYSL-d. A typical TG curve, for calcium oxalate

As well as inorganic compicxl.'S, thermal .lltullysis is applicable to a wide mngc of substance". e.g. pol)mers. drugs, soils and coals. It can also be applit.'(] to mixture
a'/" = V'8 ir" = '/11'

a b x a" = a(b.
the ~ key, "'1th x = log value;

•

the inverse then the log key; or

•

thelL:]kcy. withy = 10 and x = log valuc.

I

I

Hints for solving numerical problems

Wilh log fables, you will find complemelltary antilogarithm tables to do this. There are many uses of logarithms in chemistry, and in paTlicular physical

chemistry. including pH ( = -log[H+]), where [H+] is exprcsS(;,,,l in mol L- 1 (p. 56), and chemical kinetics, e.g. ratc constants, where a plot of In(reactant) against time produces a straighl line if the reaction is first order.

Hints for some typical problems CalclJlations im'o/t'ing proportions OJ' ratios Example A lab schedule states that 59 of a compound of M, 220gmol- 1 are dissolved In 400 ml of solvent. For writlng up your Experimental section, you

wish to express this as moll 1. 1. If there are 5g in 4OOml, then there are 5/400 9 in 1 ml. 2. Hence. l000mL will contain 5/400 x 1000L "" 12.59. 3. 12.5g=12.5j220mol=O.0568mol.so [solution] = 56.8mmoIL- 1

(=

56.8molm- 3 J.

The 'unitary method' is a useful way of approaching calculations involving proponions or ratios, such as those required when making up solutions from stocks (see also Chapter 6) or as a subsidiary part of longer calculations. • •

if given a value for a multiple. work out the eor~sponding value for a single item or 'unit'. Use this 'unitary value' to ctandard deviation of the mean. 95.45% within ±2 standard deviations of the mt:"dn. and 99.73% within ±3 !>t.andard deviations of the me-an. Some chLmical examples of dala likely to be distributed in a normal fashion arc pH of natural walers; melting point of a solid compound. To check whether data come from a normal distribution. you can:

•

•

•

c

Use the rule of thumb thaI the distribution should be symmetrical and that nearly all the dala should fall within ±3s of the mean and about t\\·o-thirds within ±Is of the mean. Plot the distribution on norm:!1 probability graph paper. If the distribution is normal. the dala will lend 10 follow a straight line (sec Fig. 41.5).

.,

.,

:; ~

deflllOOI!fW variobls

Fig. 41.4 Examples of normal frequency distributions differing in mean and standard deviation. The horizontal bars represent population standard deviations for Ihe curves, increasing from fal to lel. Vertical dashed lines are population means, wtlile vertical solid lines show positions of values ±1, 2 and 3 standard devintions from the means.

274

Analysis and presentation of data

Choosing and using statistical tests

..

Deviations from linearity rf.'Vcal skewness and/or kurtosis (Sl.'C p. 269). the signiricance of which can be tcsted ~;tatistiC'dlly (see Miller and Miller,

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For dala satisfying the ANOVA requirements, the least significant difference (LSD) is useful for making planned comparisons among several mean... Any two means that differ by more than the LSD will be significantly different. The LSD is useful for showing on graphs. The chief non-parametric tests for comparing locations are the MannWhitney V-test and the Kolmogorov-Smimov test. The fanner assumes that the frequency distributions of the data selS are similar, whereas the latter makes no such assumption. 10 lhe Kolmogorov-Smirnov lest, significant differences found with the test may be due to differences in 1000.Hion or shape of the distribution, or both. Suitable non-parametric comparisons of location for paired quantitative data (sample size ~ 6) include Wilcoxon's signed rank test, which assumes that the distributions have similar shape. Analvsis and presentation of data

277

Choosing and using statistical tests

Non·pantmetric comparisons of location for three or more samples include the Kruskal-Wallis II-test. Here. the two data selS can be unequal in size. but again the underlying distributions are assumed to be similar,

Comparing dispersiom; (e.g. I'ariances) If you wish to compare the variances of two sels of data that arc normally distributed. use the F·lcst, For comparing marc than two samples, it may be suffICient to use the F......·test. on the highest and lowest vanan The Royal Society of Chemistry: Bibliographic Databases and Reference Databanks In addilion to the BIDS RSC service, The Royal Society of ChemistI)' (http:// www.rsc.org/isfdatabase/sec_serv.hlm) also offers a range of bibliographic dawbases and reference databanks. These are (excluding the ones available via the BIDS RSC service) described below. Information technology and library resources

305

Internet resources for chemistry

Anal.I·lical ."emislry

• • • •

Analytical Abstracts Chromatography Abstracts Mass Spectrometry Bulletin Window on Chemometrics.

Chemical business

• • • • • •

Chemicals, Fonnulatcd Products and their Company Sources Focus on Catalysts Focus on Pigments Focus on Polyvinyl Chloride Focus on Powder Coatings Focus on Surfactants.

Health. safelY and loxh'Ology

• • • • •

Chemical Hazards in Industry The Dictionary of Substances and their Effects Environmental Chemistry. Health and Safety Hazards in the Office Laboratory H37.1l.rds Bulletin.

As an example. Chromatography Abstracts (http://www.rsc.orgjis/dawbasei chroabs.htm) provides an update of developments in separation science. It is arrallgt.>d into the following sections: general and miscellaneous techniques; gas chromatography; liquid chromatography; electrophoresis: and thin-layer chromatography. An example of a typical record is shown in Table 46.4. Table 46.4

Example record from Chromatography Abstracts

1854 Bovine whey fractionation based on cation-ekc;h,onge ctlromatography.

Hahn. R.; Shull, P. M.; Schaupp, C.; Jungbauer. A.J. Chromatogr., A, 6 Feb 1998, 795l2J, 277-2fJ1 Four callon"exchange resi05. namely Ii) S-HyperD-F. (ii) S-Sepharose FF.liii) Fractogel EMD S03-650151 and liv) Macro-Prep High 5 Support. were compared for the separation of proteins such as rgG, lactoferrin and lactoperoxidase from bovine whey. The chromatographic method was based on nushing .alpha.lactalbumin through the column and then eluting the proteins by a sequential step gradient with 0.1, 0.2, 0.3, 0.4 and 1M-NaC!. The collected fractions were analysed by GPC and 50S-polyacrylamide gel electrophoresis. The lactoperoxidase activity was measured by the oxidation of o-phenylenediamine. Resin iii exhibited the highest binding capacity for rgG of 3.7 mglml gel at a frow rate of 100crnfh but the lowest proteIn resolution. Resins i and ii exhibited slightly poorer dynamic capacities for IgG but better protein resolution. For i. ii and iii. the binding capacities were not dependent on flow rate for 1Q0....600cmjh and me binding capacities were not improved by removing low molecular weight compounds from the food solution. Resins i. ii and iii were considered suitable for the large scale purification of bovine whey proteins.

.

306

Information technology and library resources

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~ The spreadsheet is one of the most powerful and

()efIn;t;ons ~

Using spreadsheets

- a displ&y of a grid of (l&/Is

into which numbers, text or formulae can , be typed to form 8 worksheet. Each cell is uniquely ldentiflable'bv its column and row number combination (I.e. tts t\Nodimensional co-ordinate£1 and can contain 8 formula which makes 11

wide ranging of all microcomputer applications. It is the electronic equivalent of B huge sheet of paper with calculating powers and pt'"ovides 8 dynamic method of storing end manipulating data sets.

Statistical c:lkulations and graphical presentations are available in many versions and most have scienlific funclions. Sprcndsheetscan be used 10:

possible for an entry to Doe cell to aher the contents of one or more other cells.

•

Ternplate- 8 pre-designed spreadsheet withCIut data but: including all formul8e

•

necessary for (repeated) date an8lys;i&.

Macro - a sequence of user-defined instructions carried out by the spmadsheot. allowing complex repealed tasks to be 'automllted".

• • •

manipulate raw data by removing the drudgery of repeatL'd c.tlculations. allowing easy tr.lIlsformalion of data and calculation of st:ltistics; graph out your data rapidly to gel an instant evaluation of results printout can be used in practical and project reports; carry Qut limited statistical analysis by buill-in procedures or by al10winB construclion of formulae for specific task~ model 'what if siluations where the consequences of changes in data can be secn and evaluated; store data sets with or withoul statistical and graphical analysis.

The spreadsheet (Fig. 47.1) is divided into rows (identified by numbers) and columns (identified by alphabetic characters). Each individual combination of column and row forms a cell which can contain a data item. a formula, or a piece of lext called a label. Fonnuiae c.m include scientific and/or statistical fWlctions and/or a reference to other ce.lls or groups of cells (often called a range). Complex systems of data input and analysis can be constructed (models). The analysis. in part or complete. can be printed out. New data call be added at any time and the sheet recalculated. You ean constnlet templates. pre-designed spreadsheets containing the fonnulae required for repealed data analyses.

Men~ber

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Fig. 47.1 A screen dump of a typical spreadsheet, showing cells, rows and columns; 10010015 etc. Screen shot reprinted by permission from Microsoft Corporation.

Information

technology and library resources

307

Using spreadsheets

Templates - these should contain:

• a data input section, • data transformation and/or calculation sections, • •

a results section, which can include graphics, lext in the form of headings and

annotations, •

The power a spreadsheet offers is directly related to your ability 10 create models (hat are accurate and templates that are easy to use. The sequence of operations required is: I. 2. 3.

a summary section.

4.

Constructing a spreadsheet - start with a

5. 6. 7.

simple model and extend it gradually. checking for correct operation 8S you go.

8.

Determine what information/statistics you want to produce. Identify the variables yOli will need to use, both for original data that will be entered and for any intermediate ealculoltions Iholt might be required. Set up areas of the spreadsheet for data entry. calculation of interme(Jillte values (statistical values such as sums of sqll:\re5 etc.). calculation of final parameters/statistics and. if ncccs!;ary. a summary area. 8.1ablish the format of the numeric data if it is different from the default values. This can be done g1ob:llly (affecting the entire spreadsheet) or locally (affecting only a s~fied part of the spre.adsheet). Establish the column widths required for the various activities. Emer labels: usc extensively for annotation. Enter a test set of values to use during formula entry: use a fully worked example to check that formulae are working correctly. Emer the formulae required to make all the calculations. both intermediate and final. Check that results are correct using the tcst data.

The spreadsheet is then ready for use. Delete all the test data values and you ha\~ created your template. Save the template to a disk and il is then available for repeated operations.

Data entry Spreadsheets have built-in commands which allow you to control the layout of data in the reUs. These include number fonnat. the number of decimal places to be shown (the spreadsheet always calculates using eight or more places). the I,.'CII width and the location of the entry within the rell (left, right or centre). An autOo-entry facility assists greatly in enlering large amounts of data by moving the entry cursor either vertically or horizontally as data are entered. Recalculation default is usually automatic so that when a new data value is entered the entire sheet is recalculated immediately. This Colli dr.tmatic.llly slow dO\\'n data entry so select manual recalculation mode before entering new data sets if the spreadsheet is large with many calculations.

The parts of a spreadsheet

Labels These identify the contents of rows and columns. They arc text characters. and cannot be used in calculations. Separate them from the data cells by drawing lines. if this feature is available. Progmms make assumptions about the nature of the entry being made: most asswne that if the first character is a number. then the entry is a number or formula. If it is a letter. then it will be a label. If you want to start a label with a number, you must override this assumption by typing a designalcd character before the nwnber [0 teU the program that this is a label; check your program manual for details.

Numhe,'s Using hidden {or zero-widthl columnsthese are useful for storing intermediate calculations which you do not wish to be displayed on the screen or printout.

308

You can also enter numbers (values) in cells for use in calculations. Many programs let you enter numbers in more than one way and you must decide which rnethOO you prefer. The way you enter the number does not affect the way it is displayed on the screen as this is controlled by the cell fonnat at the

Information technology and library resourt:eS

Using spreadsheets

point of ently. There are usually special ways to enter data for percentages, currency and scientific notation for vcry large and small numbers.

Formulae These are the 'power tools' of the spreadshret berl technology and library resources

-0

Word processors, databases and other packages Word processors The word processor hots f:lcilitmed writing because of the ease of revising texl.

Word processing is a lr.msfemble skill valuable beyond the immediate requirements of your chemistry course. Using a word processor should improve

your writing skills :lOd speed because )'ou can create. check Hnd change your text on the screen before printing it as 'hard copy' on paper. Once entered and saved, multiple uses can be madeofa piece of text with littleeffort. When using a word processor you can: • • • • • •

refine material many times before: submission; insert material easily, allowing writing to take place in any sequence: use a spell
Example The book Inorganic Chemistry, 3rd edn, by D.F. Shriver and P.W. Atkins 11999; Oxford University Press) is likelv to be classified 8S follows: Dewey decimal system: 546 where 500 refers to natu ral sciencos and mathematics S40 refers to chemistry 546 refers to inorganic chemistrY UbrBfY of Congress system: 00146-191 where Q refers to science

00 00146-197

refers to chemistry refers to inorganic chemistry

Computer databases - several databases are now produced on CD-ADM for open

use in libraries le.g. Mcdline. Applied SCience and Technology Index). Especially useful to chemists are Chemical Abstracts and Analytical Abstracts. Some databases can be accessed via the Internet such as BIOS (Bath Information and Data servicel, a UK service providing access to information from over 7000 periodicals (usemame and password required via your library). Each of these databases usually has its own easy-ta-foIlOW menu instructions. It is worthwhile to consider key words for your search beforehand to focus your search and save time.

Sources of information

Fo,. essays and I'evi!,';on You are unlikely to delve into the primary literature for these purposesbooks and reviews are much more readable! If a lecturer or tutor specifies a particular book, then it should not be difficult to find out where it is shelved in your library, as most libraries now have a computerized index system and their staff will be happy to assist with any queries. If you want to find out which books your library holds on a spa;:ified topic, use the system's subject index. You will also be able to search by author or by key words. There are two main systems used by libraries to classify books: the Dewey Decimal systctn and the Library of Congress system. Libraries differ in the way they cmploy thcse systems, especially by adding further numbers and leiters after the standard classification marks to signify, say, shelving position or edition number. Enquire at your library for a full explanation of local u
The 'Kat";tzky· .(j'stelll A third system has been popularized in publications involving Professor Roy Katritzky (University of Florida, USA) and uses the best features of both the Harvard and Vancouver systems. For each reference in the text a code is written comprising the year of publication, the journal and the page of the journal. In the reference section the coded references are cited, together with the full reference - authors. journal. year. volume and page - in year order and a list of journal codes in alphabetical order can be provided. For example: ~.

Paper in journal: Smith, A. 8., Jones. C.O. and Professor, A JournaJ of New Results, 11 (2000) 19. BooIc Smith, A. B. 119981. Summary of My Ufe's Work. Megadosh Publishing Corp., Bigcity. C~

in edited book.

Jones. C. D. and Smith. A. B. 119981. Earth-shaUering research from our

laboratory. In: Research Compendium 1998 led. A. Professor), pp. 123-456. Bigbucks Press, Booktown.

Thesis:. Smith, A. B. (19951. In\/8Sligat~ns on mv fawurite topic. PhD thesis:Uniwrsity of life, Fulchoster. Note that underlining used h~e specifIeS italics in print; use an italic font if working with 8 word processor.

320

in the text ... Robinson and Watt (34JCSI536) found ... in the reference section 341C'S1536 R. Robinson and SJ. Watt. J, Chem. Soc., 1934, 1536. TIle advantage of this system is that it is easy to insert missed references into the text, a great disadvantage of the otherwise neat Vancouver system. For delails consull the reference seclion in Comprehensil'e Heleroc)'c/ic Chemistry, ed. A.R. Katritzky and C.W. Rees, Pergamon. Oxford, 1984.

How to list your citations in a bibliography Whichever citation method is used in the text, comprehensivc details are required for the bibliography so that the reader has enough information to rmd the reference easily. Citntions should be lisled in alphabetical order with the priority: first author. subsequent author(s). date. Unfortunately, in temlS of punctuation and layout, there an: almost as many ways of citing papers as there are journals!

Information technology and fibfary resoul'Ces

Finding and citing published information ~ Your department may specify an exact format for

project work; if not. decide on a style and be consistent - If you do not pay attention to the details of citation you may lose marks. Take special care with the following aspects:

• •

•

• • Citing websites - there is no widely accepted format at present. We suggest providing author namelsl and date in the text if using the Harvard system, while in the bibliography giving the above. plus site title and full URL reference (e.g. Hacker, A. (1998) University of Cybertown homepage on aardvarks. http://www.myserver.ac.uk/ homepage).

•

Authors and editors: give details of wI authors and editors in your bibliography, even if given as e( af. in the text. Abbreviations for journals: while there are standard abbreviations for the titles of journals (consult library Slam, it is a good idea to give the whole title, if possible. Books: the edition should always be specified as contents may change between editions. Add, for example, '(5th edition)' after the title of the book. You may be asked to give the International Standard Book Number (ISBN), a unique reference number for each book published. Unsigned articles, e,g. unatlributed newspaper articles and instruction manuals: refer to the author(s) in text and bibliography as 'Anon.'. Unread articles: you may be forced to refer to a paper via another without having seen it. If possible, refer to another authority who has cited the paper, e,g, '". Jones (1980), cited in Smith (1990), claimed thai ... '. AJternal.ivcly. you could denote such references in the bibliography by an asterisk and add a short note to explain at the start of the reference list. Personal communications: information rt'Ceived in a letter. seminar or conversation can be referred to in the text as, for example, ', .. (Smith, pefS. comm.)'. These citations are not generally listed in the bibliogrdphy of papers, though in a thesis you could give a list of personal communicants and their addresses.

Information technology and library resources

321

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General aspects of scientific writing Written communication is an e!i.'iCnlial component of all scicncl,.'S. Most courses include wriling cxerciso; in which you will learn to OCSl.:ribc ideas and resull~ accurdlcly, sUf...c inctly and in an appropriate style and formal. The following are features common 10 all (omlS of scientific writing.

Organizing time Making a timetable al the outsct helps ensure thaI you give each stage adequate attention and complete the work on time. To creale and uSC: a timetable: I. 2. 3.

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