分析化学英文课件06沉淀滴定法-Precipitation-Titration

AnalyticalChemistry分析化学Chapter6PrecipitationTitration§6.1TitrationsBasedinPrecipitationReactions§6.2TitrationCurve§6.3DetectionoftheEndPointinArgentometry§6.1TitrationsBasedonPrecipitationReactionsAprecipitationreactionmustsatisfiedthefollowingrequirementsbeforeitcanbeused:(1)Thesolubilityoftheproducedprecipitateshouldbeextremelylow(2)Thereactionshouldberapidandproceedquantitatively(3)SomemethodsmustbeavailabletodetecttheendpointAgCl(s)=Ag++Cl-KspAgCl=[Ag+][Cl-]§6.2TitrationCurveForexample:0.1mol·L-1KClwith0.1mol·L-1AgNO3(1)AtthebeginningofthetitrationpCl=1(2)AttheequivalencepointKsp(AgCl)=[Ag+][Cl-];[Ag+]=[Cl-][Cl-]=(Ksp(AgCl))1/2=1.25×10-5mol·L-1SopCl=4.90(3)Aftertheequivalencepoint[Cl-]=Ksp(AgCl)/[Ag+]Factorsinfluencingthetitration:(1)c:whenKspisconstant,thelargertheconcentration,thelargerthebreakofthecurve(2)Ksp:whentheconcentrationisconstant,thesmallertheKsp,thelargerthebreakofthecurve§6.3DetectionofTheEndPointinArgenometry§6.3.1MohrMethod—usingpotassiumchromateasindicator(1)Thismethodcanbeusedtodeterminethechloride,andthestandardsolutionisAgNO3.Reactions:Ag＋+Cl－=AgClAttheequivalencepoint：［Ag＋］=(ＫspAgCl)1/2=1.25×10－５CrO42－(yellow)+2Ag＋=Ag2CrO4（red）SotheconcentrationoftheCrO42-is：［CrO42－］=ＫspAg2CrO4/[Ag＋]2＝5.8×10－2mol/L§6.3.2VolhardMethod—usingferricalumasindicatorTheVolhardmethodisaprocedurefortitratingsilver(Ⅰ)withstandardpotassiumthiocyanine.EquationsfortheVolhardtitrationofsilveraregiven:Ag＋+SCN－=AgSCN↓(titrationreaction)Fe3＋+SCN－=FeSCN2＋(end-pointreaction)(1)Thetitrationiscarriedoutinacidsolution.(2)Thismethodcanbeusedtodeterminechlorideandotheranions.titrationreaction:Ag++X-1=AgX(s)+excessAg+back-titrationreaction:SCN－+excessAg+=AgSCN↓end-pointreaction:Fe3++SCN－=FeSCN2+§6.3.3FajansMethod—usingadsorptionindicatorIntheadsorptionindicatormethodforhalideions,theendpointreactionsoccursonthesurfaceofthesilverhalideprecipitate.Theadsorptionindicatorsaresomecoloredorganiccompounds.Fluoresceinisakindofadsorptionindicator.Itcanbeadsorbedonthesurfaceofthecolloidalparticlesandchangecolor.