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1、〈231〉HEAVYMETALSThistestisprovidedtodemonstratethatthecontentofmetallicimpuritiesthatarecoloredbysulfideion,underthespecifiedtestconditions,doesnotexceedtheHeavymetalslimitspecifiedintheindividualmonographinpercentage(byweight)ofleadinthetestsubstance,asdeterminedbyconcomitantvisualcomparison(see
2、VisualComparisoninthesectionProcedureunderSpectroDhotometryandLight-Scattering〈851〉)withacontrolpreparedfromaStandardLeadSolution.[NOTE一Substancesthattypicallywillrespondtothistestarelead,mercury,bismuth,arsenic,antimony,tin,cadmium,silver,copper,andmolybdenum.]DeterminetheamountofheavymetalsbyMe
3、thodI,unlessotherwisespecifiedintheindividualmonograph・MethodIisusedforsubstancesthatyieldclear,colorlesspreparationsunderthespecifiedtestconditions・Method//isusedforsubstancesthatdonotyieldclear,colorlesspreparationsunderthetestconditionsspecifiedforMethod/,orforsubstancesthat,byvirtueoftheircom
4、plexnature,interferewiththeprecipitationofmetalsbysulfideion,orforfixedandvolatileoils.MethodIII,awet-digestionmethod,isusedonlyinthosecaseswhereneitherMethod/norMethod//canbeused.SpecialReagentsLeadNitrateStockSolution—Dissolve159.8mgofleadnitratein100mLofwatertowhichhasbeenadded1mLofnitricacid,
5、thendilutewithwaterto1000mL.Prepareandstorethissolutioninglasscontainersfreefromsolubleleadsalts.StandardLeadSolution一Onthedayofuse,dilute10.0mLofLeadNitrateStockSolutionwithwaterto100.0mL・EachmLofStandardLeadSolutioncontainstheequivalentof10pgoflead・Acomparisonsolutionpreparedonthebasisof100pLof
6、StandardLeadSolutionpergofsubstaneebeingtestedcontainstheequivalentof1partofleadpermillionpartsofsubstaneebeingtested・METHODIpH3.5AcetateBuffer一Dissolve25.0gofammoniumacetatein25mLofwater,andadd38.0mLof6Nhydrochloricacid.Adjust,ifnecessary,with6Nammoniumhydroxideor6NhydrochloricacidtoapHof3.5,dil
7、utewithwaterto100mL,andmix.StandardPreparation—Intoa50-mLcolor-comparisontubepipet2mLofStandardLeadSolution(20pgofPb),anddilutewithwaterto25mL.UsingapHmeterorshort-rangepHindicatorpaperasexternalindicator,adjustwith1Na
