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Federal!Register- b Vol:..45; No.,21Z /;Tli`ur'sd'ay, October,30,?1980. / Prbpo'sed-Rules 72033, be exercised not to take the pH below 3.If 1.1.3 Pipet tips have beeivrep-orted to be a the pH is inadvertently lowered to<3,10% possible source of contamination. NH40H(7,6)should"be used to readjust the 1.1.4 The method of standard addition pH to 3.5-ro.3. should be-used iii aceordance'*ith the , 9.2 Pipet it 10 mi aliquot-of the adjusted " general methods given in the manual=`Test sample into a centrifuge tube(6.1.2).Add 100 Methods for Evaluating Solid Waste,"SW– ul of the lead,nitrate solution(7.1),stopper 846. the tube,mix the sample and allow to stand for 3 minutes. BILLING CODE 6560-3Q--M 9.3 After the formation of lead chromate, to help retain Cr(III)complex in solution,add 0.5 ml glacial acetic acid(7.5),stopper and . mix. 9.4 To provide adequate lead sulfate for _ coprecipitation add loo ul of ammonium sulfate solution(7.2),stopper and mix. 9.5 Place the stoppered centrifuge tube in the centrifuge,making sure that the tube is . properly counterbalanced.Start the centrifuge and slowly increase the speed to 2000 rpm in small increments over a period of 5 minutes. - Note.-The speed of the centrifuge must be increased•"slowly to insure complete coprecipitation 9.6 After centrifuging remove the tube and _ withdraw and discard the supernate using the apparatus detailed in Figure.l.As the pasteur ; pipet'is lowered into the,tube the supernate is sucked over into the-filtering flask.With care the supernate can be withdrawn to within approximately 0.1 ml above the precipitate. 9.7 To the remaining precipitate add 0.5- _ mi concentrated HNO,(7.4);100µl 30%KO - (7.7)"and 100µi calcium nitrate solution(7.3). - Stopper the tube and°mix using a vortex mixer to disrupt the precipitate.and solubilize the lead chromate.Dilute to 10m1,mix and analyze in the same manner as the calibration standard(8.2). 10.Verification 10.1 For every sample matrix analyzed verification is necessary to determine that neither a reducing condition nor chemical` interference affecting precipitation is present. This must be accomplished by analyzing a second 10 ml aliquot of!he pH-adjusted filtrate that has been spiked with Cr(VI)(7.7): = . The amount of spike added should double the concentration found in the original aliquot. _ Under no circumstance should the increase be of less than 30µg Cr(VI)/1.To verify the absence of an interference the spike recovery should be between 85%and 115%. 10.2 If the.addition of the spike extends 4' the concentration.beyond the calibration curve the analysis solution should be diluted with blank solution,and the calculated results adjusted accordingly. 10.3 If the result of verification indicates a suppressive interference,the sample should be diluted and reanalyzed. 11.Analytical Notes ,11.1 Nitrogen should not be used as,a purge gas because of possible CN.band interference. 11.2 The use of pyrolytic graphite should be avoided when possible,since in some situations an enhancement effect has been reported. 4 -