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Federal Register f Vol. 45, No. 212 f Thursday, October 30, 19W ,* Proposed Rules 72033 he exercised not to take the pH below 3.If 11.3 Pipet tips have been reported to be a the p1l is inadvertently lowered to C3,10% passible source of contamination. NH.QH(7.6)should be used to readjust the 11.4 The method of standard addition pH to 3.5--0-3. should be used in accordance with the 9.2 Pipet a 10 nil aliquot of the adjusted general methods given in the manual"Test sample into a centrifuge tube[t3.1.2].Add too Methods for Evaluating Solid Waste;'SW– ul of the lead nitrate solution 17.1].stopper 845. the tube,mix the sample and allow to stand BALLING CODE 6560-3e-M for 3 minutes. 9.3 After the formation of lead chromate, to help retain Cr(lfl)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 100 ul of ammonium sulfate solution(7.2),stopper and mix. 9.5 Place the stoppered centrifuge tube in the centrifuge,making sure thal 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. Nola.—The speed of the centrifuge must be increased slowly to insure complete coprecipitation. 9.0 After centrifuging remove the lube and withdraw and discard the supernate using the apparatus detailed in Figure 1.As the pesteur pipet is lowered into the tube the supernate is sucked over into the filtering flask.With care the supernate can be withdrawn to within ippruximately 0.1 ml above the precipitate. 9.7 To the remaining precipitate add 4.5 ml concentrated HNOa(7.4),100µl 30%H2O. 17.71 and 100µ1 calcium nitrate solution(7.3). Stopper the lube 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 vorification 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 mi aliquot of the pH-adjusted filtrate that has been spiked with Cr(Vl)(7.7). The amount of spike added should double the concentration found in the original aliquot. Under no circumstance should the increase he of less than 30µg Cr(Vf)/1.To verify the absence of an interference the spike recovery should be between 85%and 1.15%. 10.2 if the addition of the spike extends the concentration beyond the calibration curve the analysis solution should be diluted with blank solution.and the calculated results adjusted accordingly. 10.3 ]f the result of verification indicates a suppressive interference, the sample should be diluted and reanalyzed. 11.Analytical Notes 11.1 Nitrogen should nal be used as a purge gas because of passible CN band interference. 11.2 The use of pyrolytic graphite should be avoided when possible,since in some situal ions an enhanrement effect has been reported.