-R637 A Laboratory Manual For The Determination Of Metals In Water And .

(a)Chemical interferences occur wb' . the element being determinedcombines chemically with another reactive componentsample.The resultingcompoundin theinfluences the atonizationprocess in the flame, thts altering the number of free atomsavailable to absorb light.Chemical interferences can be con trolled by either:(i) Addition o a releasing agent;A releasing agent is achimical which reacts preferentially with either the elementbeing determined or with the interfering component.Acommon example is the addition of a solution of a lanthanumsalt to calcium solutions to overcome the depressing effectof phosphate on the calcium signal.(ii)Use of a higher temperature flame:This can overcome manyinterferences because more energy is available to break downcompounds which would be stable in cooler flames.Fo.example, the interference of phosphate on calcium observedin the air-acetylene flame is eliminated in the nitrousoxide-acetylene flame.(b)Ionization interferencesoc .yr when the flame temperature ishigh enough to ionize a significant fraction of the element beingdetermined.This lowers the number of atoms which can absorbradiation and reduces the analytical signal.Analytical error:can occur when the sample and standards exhibit different degreesof ionization.The simplest way to control ionization inter ference is to add a large excess of an oasily 'onizalle cation,such as potassium or caesium, to both samples and standards.Theelectrons provided by the more easily ionized element combinewith the ions of the element being determined, and increase thenumber of atoms which can ab?orb radiation.A common example Isthe addition of a solution of a potassium or caesium salt tobarium solutions to overcome the ionization of this meial in thenitrous oxide-acetylene flame.The use of a lower temperatureflame can also control ionization effects.Foi example, sodiumis partially ionized in the air-acetylene flame, but ionizationis negligible in the air-propane flame.5

(c) Non-atomic absorptionresults from the absorption of radiationfrom the hollow cathode lamp by materials in the flame other thanthe element of interest.It may be due to either:(i) Molecular absorption, which is caused by the presence ofmolecular species which absorb light at the same wavelengthas that of the element being determined.(ii)Light scatteringby particles in the flame, which occurswhen solutions containing high amounts of dissolved solidsare aspirated into the burner.Non-atomic absorption may be controlled by the use of a 'continuumsource corrector', '.?hich may be either a hydrogen-filled hollowcathode lamp or a deuterium arc lamp.(d) Matrix interferencesoccur when the sample matrix is so complexthat viscosity, surface tension and components ccunot be accuratelymatched with standards, resulting in the uptake rate, nebulizationefficiency and atomization processes in the fl .me being esmaybecontrolled by use of standard addition techniques or by means ofMIBK extractions with APDC, which are particularly useful withseawater samples, for example.1.4Sample handling and preservationFor the determination of trace metals, contamination and loss areof prime concern.Dust in the laboratory environment, impuritiesin reagents, and impurities in laboratory apparatus with whichthe sample makes contact are all sources of potential contamina tion.Sample containers can introduce either positive or negativeerrors in the measurement of trace metals by either contributingcontaminants through leaching or surfacedesorption or bydepleting concentrations through adsorption.Thus the collectionand treatment of the sample prior to analysis for metals requiresparticular attention.The sample container, whether borosilicateglass, polyethylene, polypropyleneor Teflon, should be tho roughly washed with detergent and tap water, then rinsed with 1:1

nitric acid, tap water, 1:1 hydrochloric acid, tap water, andfinally deionized distilled water, in that order.Samples shouldbe preserved immediately after sampling by acidifying with con centrated nitric acid to pH 2.(Addition of 10 mi of concen trated nitric acid per litre of sample will normally be suffi cient) .Afcer acidification, the sample should be stored in a refrigera tor at approximately 4 C . Under these conditions, samples withmetal concentrations of several milligrams per litre are utablefor up to 6 months (except mercury, for which the limit is 38days in glass and 14 days in plastic).For microgram per litremetal levels, the sample should be analysed as soon as possibleafter collection.Alternatively, samples for mercury determina tion may be preservd by the addition of 2 mJi of 200 mg/ potas sium dichromate solution (prepared in 1:1 nitric acid) per litreof sample.5Preparation of standard solutionsStandard solutions of known metal concentrations in water shouldbe prepared in water with a matrix similar to that of the sample.Standards which bracket the expected sample concentration and arewithin the working range of the method should be employed.If ispreferable to prepare stock solutions of standards in concentra tions above 500 mg/2. and store them in a refrigerator, dilutingas required.For samples containing high and variable concentra tions of matrix materials, the diluted standard solutions shouldcontain similar concentrations of the major ions in the sample.If the sample matrix is complex and components cannot be matchedaccurately with standards, the method of standardadditionsshould be used to correct for matrix interference effects.6Sensitivity, detection limits and optimuir concentration rangesThe sensitivity of flame atomic absorption spectrophotometry isdefined as the metal concentration that produces an absorption of% (approximately 0,0044 absorbance).7

The detection limit is defined as the metal concentration thatproduces absorption equivalent to twice the magnitude of thebackground fluctuation.Sensitivity and detection limits vary with instrument, elementanalysed, and technique.The optimum concentration range usually starts from a concentra tion of several times the sensitivity and extends to a concentra tion at which the calibration curve begins to flatten.For bestresults, concentrations of samples and standards should be withinthe optimum concentration range.In many cases the concentrationrange may be extended downward by use of scale expansion andupward by rotation of the burner or use of a less sensitivewavelength.1.7Instrument operationBecause of the difference between makes and models of atomic sorption spectrophotometers, it is not possible to formulateinstructions applicable to every instrument.The manufacturer'sinstructions for each particular instrument should be followed.In general, hovever, the following procedure's employed (air-acetylene flame):(a)Install the hollow cathode lamp for the metal being measured androughly i,et the desired wavelength (see Table I).(b)Set the slit width and lamp current at the values suggested in "he manufacturer's instructions, and allow the instrument to warmup until the energy source becomes stable (1C-20 minutes).Readjust the current as necessary after warm up.(c)Optimize wavelength by adjusting wavelength dial until optimumenergy gain is obtained.Align lamp in accordance with manufac turer's instructions.(d)Install air-acetylene burner and adjust burner head position.Turn on air and adjust flow rate to that specified by manufacturer

to give maximum sensitivity for the metal being measured.Turnon acetylene, adjust flow rate to value specified, and igniteflame.(e) Aspirate a standard solution of the desired metal and adjustaspiration rate of the nebulizer (if variable) to obtain maximumsensitivity.Aspirate a standard solution near the middle of thelinear working range and adjust the burner vertically and horizon tally to obtain maximum response.(f) The instrument Is now r .ady for operation.(g) On completion of the analysis, extinguish flame by turning offfirst acetylene and then air.For determinations involving the use of a nitrous oxide - acetyleneflame, proceed as in (a) to (c) above and then continue as follows : -(d)Install nitrous oxide-acetylene burner and adjust burner headposition.Turn on acetylene (without igniting flame) and adjustflow rate to specified value, then turn off acetylene.With bothair and nitrous oxide supplies turned on, set the T-junctionvalve to nitrous oxide and adjust flow rate to specified value.Turn the switching valve to the air position and check that flowrate is the same.yellow flame.Turn on acetylene and ignite to a brightWith a rapid motion, turn switching valve tonitrous oxide.The flame should now have a red cone above theburner. If not, adjust the fuel flow to obtain a red cone.Afterignition, allow the burner to come to thermal equilibrium beforecommencing analysis.(e)Aspirate a standard solution of the desired metal and adjustaspiration rate of the nebulizer (if variable) to obtain maximumsensitivity.Aspirate a standard solution near the middle of thelinear working rai.ge and adjust the burner vertically and horizon tally to obtain maximum response.(f)The instrument is now ready for operation.

On completion of the analysis, extinguish flame by first turningthe switching valve from nitrous oxide to air and then turningotf the acetylene.This procedure eliminates the danger offlashback occurring on direct ignition or shutdown of nitrousoxide and acetylene.Actual determination and standardization proceduresfor thevarious metals are described in Sections 2 to 5.Scheme of analysisPresented in this technical guide is a scheme of analysis,currently employed by the Water Quality Division of the NationalInstitute for Water Research, for the determination by atomicabsorption of the metals most commonly analysed for in water andwastewater.Flame and vapour generation techniques have beenpreferred to flameless atomization methods, mainly for reasons ofsimplicity, speed and accuracy.Throughout the scheme, it hasbeen assumed that all samples have been preserved by the additionof 10 mi of concentrated nitric acid per litre of sample.In thecase of calcium, magnesium, potassium and sodium, an automated aswell as a manual method has been described.This technique can,of course, be adapted for use with the other metals determined byflame absorption, should the need arise.A list of recommendedwavelengths and flame gases for each metal determined is shown inTable 1.

TABLE 1:Recommended wavelengths and flame gasesElementWavelength (nm)Flame gasesAluminium309,3Nitrous oxide-acetyleneBarium553,6Nitrous ,7/239,9Air-acetylene or nitrousoxide-acetyleneChromiumS57.9Air-acetylene or ene or etyleneZinc213,9Air-acetyleneArsenic193,7Hydride generationSelenium196,0Hydride generationMercury253,7Cold vapour generationPotassium

2.DETERMINATION OF CALCIUM, MAGNESIUM, POTASSIUM AND SODIUM(direct flame absorption)2.12.1.1Manual methodPreparation of reagents(a)Nitric acid:(b)Caesium-lanthanum interference suppressant solution:ofUse concentrated AR grade.hydrochloricacid(concentrated,ARAdd 400 milgrade) slowlyandcarefully (fume cupboard) to 117,0 g lanthanum oxide (AR grade)in a 2 Í. pyrex beaker.Stir the mixture until the lanthanumoxide is completely dissolved, then add 25,3 g caesium chloride(AR v a d e ) and again stir until dissolved.Cool the solution andadd it to 800 mil of deionized distilled water.with deionized distilled water.Dilute to 2 This solution contains 10 g/í, Csand 50 g/i, La.(c)Standardstock solutions:(1 000 mg/2.) toratomicUse standardabsorptionsolutions of metalsspectrophotometry(BDH'Spectrosol'- or equivalent).2.1.2Preparation of standard solutionsAdd from a burette, the following quantities of standard stocksolutions to 500 mil standard volumetric flasks, each containing5 m nitric acid and 50 mil caesium-lanthanum ium(mil)Sodiui(mil)MSI0000MS 0,0MS850,050,020,0100,0"Mention of trade names is for information purposes only and doesnot irply endorsement by the National Institute for Water Research.12

Dilute these solutions to 500 ml with deionized distilled waterand transfer to 500 mi. polythene bottles.The concentrations ofthe solutions are as follows:StandardNo.Calcium(mg/ S7707029.140MS810010040200Pretreatment of samplesAdd 5 mi of caesium-lanthanum solution to a clean dry 50 mistandard volumetric flask and dilute to 50 mi. with the sample orsample aliquot solution.2.1.4ProcedureAspirate the standard and sample solutions under the conditionsstipulated for the particular instrument being operated (referalso Table 1) and note the absorbance values obtained.Choice ofeither the air-acetylene or the nitrous oxide-acetylene flame forcalcium and magnesium determinations will be dependent on thesample constitution.Calculate the concentration of each metalion by reference to the calibration curves obtained by plottingconcentrations of the standard solutions versus the correspondingabsorbance readings.(For instruments equipped with directconcentration readout, this step will be unnecessary).rection factor ofA cor /9 should be applied to each result in orderto allow for the addition of the caesium-lanthanum solution.2.22.2.1Automated methodPreparation of reagents(a)Nitric acid: Refer 2.1.1 (a).13

(b)Caesium interference suppressant solution (potassium andsodium determinations):Dissolve 15,0 g caesium chloride(AR grade) in a solution of 50 mil hydrochloric acid(concentrated, AR grade) and 450 m deionized distilledwater, and dilute to 1 Í with deionized distilled water.This solution contains 11,7 g/i. Cs, equivalent to 1 g/iinthe aspirate.(c)Lanthanum interference suppressant solution (calcium andmagnesium determinations): Add 400 mi. of hydrochloric acid(concentrated, AR grade)slowlyandcarefully(fumecupboard) to 68,8 g lanthanum oxide (AR grade) in a 2 Ipyrex beaker.Stir the mixture until the lanthanum oxide iscompletely dissolved, cool, and add it to 400 mil deionizeddistilled water.water.g/i(d)2This solution contains 58,6 g/i. La, equivalent to 5in the aspirate.Standard stock solutions: Refer 2.1.1. (c).Preparation of standard solutionsRefer 2.1.2.3Dilute to 1 I with deionized distilledOmit addition of caesium-lanthanum solution.ProcedureSet up the manifold as shown in Figure 1 (sodium and magnesium)and Figure 2 (calcium and potassium).times and fill the sample plate.Set sampling and washingAspirate the standard solutionsand samples under the conditions stimulated for the particularinstrument being operated (refer also Table 1) and record theabsorbance values obtained as peaks on a chart recorder.Calcu-late the concentration of each metal ion by reference to thecalibration curves obtained by plotting concentrations of thestandard solutions versus the corresponding peak heights.

AUTOMATIC[oSAMPLER' RATE' 40-1 2mt/min.TO SAMPLE9«3.90,6SUPPRESSANT SOLUTIONSAMPLEs5,901STILLED WATER2.0AIR s— 28 TURNS-». WASTEAA - NEBULIZERSodium/magnesium manifoldO /OISTILLED WATERJFigure 1.cf.0,42MIXINGCOILO

roAUTOMATIC[o o olSAMPLERRATEi40-l 2 j0,3/mt/mln.3,9TO SAMPLE)0,42DISTILLED WATERSUPPRESSANT SOLUTION0,6DISTILLED WATER3,»SAMPLE2,0AIRMIXINGCOIL »"2 8 TURNS-M» WASTEAA - NEBULIZERFigure 2.Potassium/calcium manifold

3.DETERMINATION OF ALUMINIUM, BARIUM, CADMIUM, CHROMIUM, COBALT, COPPER,IRON, LEAD, MANGANESE, NICKEL, SILVER and ZINC(directfl«.jeabsorption)3.1Preparation of reagents(a) Nitric acid:Refer 2.1.1 (a).(b) Caesium-lanthanum interference suppressant solution: Add 400 miofhydrochloric acid(concentrated, AR grade) slowly andcarefully (fume cupboard) to 47,0 g lanthanum oxide (AR grade) ina 2 i pyrex beaker.Stir the mixture until the lanthanum oxideis completely dissolved, then add 12,7 g caesium chloride (ARgrade) and again stir until dissolved.Cool the solution and addit to 800 mi. of deionized distilled water.deionized distilled water.Dilute to2 I withThis solution contains 5 g/i Cs and20 g/i La.(c)Standard stock solutions:3.2Preparation of standard solutions3.2.1Refer 2.1.1 (c).Cadmium, chromium, cobalt, copper, iron, lead, manganese, nickeland zinc standard solutionsSecondary stock solution 1: Transfer, by pipette, 100 mi. of thestandard stock solutions(1 000 mg/i) of chromium, cobalt,copper, iron, lead, manganese, nickel and zinc and 20 mi of thestandard stock solution (1 000 mg/i) of cadmium to a 1 I standardvolumetric flask, and dilute to I I with deionized distilledwater.This solution contains 20 mg/i Cd and 100 mg/i, Cr, Co,Cu, Fe, Pb, Mn, Ni, Zn.Secondary stock solution 2:Transfer, by pipette, 100 mi. ofsecondary stock solution 1 to a 1 51 standard volumetric flask,and dilute to 1 i with deionized distilled water.This solutioncontains 2 mg/i Cd and 10 mg/i Cr, Co, Cu, Fe, Pb, Mn, Ni, Zn.Add the following quantities of secondary stock solutions 1 and 2to 250 mi standard volumetric flasks, each containing 2,5 minitric acid and 25 mi caesium-lanthanum solution, and dilute to250 ml with deionized distilled water:17

StandardNo.Secondarystock solutionN O'Volume of,. .stock solutionadded (m )TS13.2.2Concentration (yg/ 8004000TS912,510005000Barium standard solutionsSecondary stock solution:Transfer, by pipette, 100 m of thestandard stock solution (1000 mg/Jl) of barium to a I standardvolumetric flask, and dilute to 1 I with deionized distilledwater.This solution contains 100 mg/Jl Ba.Add the following quantities of secondary stock solution to100 mi. standard volumetric flasks, each containing 1 mil nitricacid and 10 mil caesium-lanthanum solution, and dilute to 100 milwith deionized distilled water:StandardNo.3.2.3Volume of secondarystock solution 01000Ba45,05000Ba515,015000Ba625,02500PSilver standard solutionsSecondary stock solution: Transfer, by pipette, 10 mil of the18

standard stock solution (1000 mg/&) of silver to a 1 standardvolumetric flask, and dilute tc 1 Í with deionizedwater.distilledThis solution contains 10 mg/2, Ag.Add the following quantities of secondary stock solution to 100mil standard volumetric flasks, each containing 1

means of atomic absorption spectrophotometry. This technique involves the study of the absorption of radiant energy (usually in the UV and visible regions) by neutral atoms in the gaseous state. In an atomic absorption analysis, the element being determined must be reduced to the elemental state, vaporized, and