分析化学英文课件07配位滴定法Complexometric-Titrations

AnalyticalChemistry分析化学Chapter7ComplexometricTitrations§7.1ComplexFormationReactions§7.2EDTAChelates§7.3EffectofpHonEDTAEquilibria§7.4TitrationCurves§7.5DetectionoftheEndPoint§7.6SeparateTitrationsofMetalIons§7.1ComplexformationreactionsLigand:Themoleculeorionthatdonatesanunsharedelectronpairtothemetaliontoformametal-lignadbond.Suchasammoniaandwater.Coordonationnumberofametalion:thenumberofmetal-ligandbondsitforms.Formationconstant,Kf:Anequilibrium-constantexpressioninthecomplexformation.Alsocalledstabilityconstant,Ks.Dissociationconstant,Kd:Anequilibrium-constantexpressioninthecomplexdissociation.SimplythereciprocaloftheKf.Forexample:Cd2++4I-=CdI42-,Kf=[CdI42-]/[Cd2+][I-]4Kd=1/Kf=[Cd2+][I-]4/[CdI42-]§7.2EDTAChelatesChelatingagent:Anorganicagentthathastwoormoregroupscapableofcomplexingwithametalion.Chelate:thecomplexformediscalledachelate.Ligand:thechelatingagentChelometrictitration:atypeofcomplexometrictitration,thetitrationwithachelatingagent.Complexinggroups:thegroupswhichcontainapairoffreeelectronscapableofcomplexingwithametalion.EDTA:6complexinggroup.Five-orsix-memberedringisrelativefreeofstrainandismorestableandisoftenformedinchelates.ＥＤＴＡethylenediaminetetraaceticacidNi+Y=NiYmovingTableKMYforsomemetal-EDTAcomplexesa.Foralkalinemetals，lgKMY3,thechelatesarelessstableb.Foralkalineearthmetal,lgKMY=8~11c.Fortransitionmetals,rareearthmetalandAl3+,lgKMY=15~19d.Fortrivalent,quaternarymetalsandHg2+,lgKMY20§7.3EffectofpHonEDTAEquilibria(1)pH12,Y4-ispredominate;(2)Y4-iseffectivespeciestochelatewithmetalion;FractionsofeachEDTAspeciesasafunctionofpHmovingEffectofpHandαY(H)αY(H)=[Y']/[Y]theratiooftotalconcentrationofallformsoftheuncomplexedEDTA[Y']totheconcentrationofthereactiveEDTAspecies[Y].(αY(H)isthereciprocalofthefractionofY4-())lgαY(H)atdifferentpHｃ．Commonly:αY(H)1,[Y'][Y]When:αY(H)=1,[Y']=[Y]ｄ．αY(H)=1/δBymeansofαY（H）,aconditionalconstantmaybecalculatedformetal-EDTAcomplexatanypH.ａ．αY(H）decreasewiththeincreaseofpH；ｂ．thebiggertheαY(H),thehighertheeffectofhydrogenion；ConditionalformationconstantTitration：Mn++Y4-=MYKMY=[MY]/（[Mn+][Y4-]）αY(H)=[Y']/[Y4-]So:[MY]/([M][Y'])=KMY/αY（H）=KMY'lgK'MY=lgKMY-lgαY（H）Insomecases,othersidereactioneffectshavetobeconsidered.Example:CalculatelgK'ZnYatpH=2.0andpH=5.0Solution：FromTableofK：lgKZnY=16.5atpH=2.0,lgαY(H)=13.5atpH=5.0,lgαY(H)=6.6lgK'MY=lgKMY-lgαY(H)so,atpH=2.0,lgK'ZnY=16.5-13.5=3.0atpH=5.0,lgK'ZnY=16.5-6.6=9.9MinimumpHforeffectivetitration1.pHeffectonthetitration(1)ThehigherthepHis，thelowertheisandthehigherKMY'is,i.e.beneficialtothecomplexometrictitration；(2)IfpHistoohigh，metalionmayhydrolyze,forminghydroxide.2.ThereisminimumpHvaluesatwhichK’ishighenoughandmetaliondoesnothydrolyze.3.TheminimumpHvalueforagivenmetaliondependsonthetolerableerrorandtherequiredpMchangeattheendpoint：4.K'MY=[MY]/([M][Y'])=c/(c0.1%c0.1%)=1/(c10-6)lgcK'MY≥6lgαY(H)≤lgKMY+lgc-6Thetolerableerroris±0.1%,pMforthevisualendpointdetectionis±0.2,sothecriterionforeffectivetitrationofasinglemetalis:5.CalculationofminimumpHforagivenmetalion.1/2MYpHpH)(1010TE'Kc等6.lgαY(H)—minimumpHfordifferentmetal§7.4TitrationCurvespM~mililitersofEDTAorpercenttitration.Theconditionalconstantisusedinthecalculation.Example:20mL0.01000mol/LCa2+solutionistitratedwith0.01000mol/LEDTA.1．AtpH12αY(H)=0；KMY’=KMY(1)Beforetitration，theconcentrationofCa2+is：[Ca2+]=0.01mol/LpCa=-lg[Ca2+]=-lg0.01=2.00(2)As19.98mLofEDTAisadded[Ca2+]=0.010000.02/(20.00+19.98)=510-6mol/LpCa=5.3(3)Attheequivalencepoint[CaY]=0.01/2=0.005mol/L[Ca2+]=[Y]KMY=1010.69Fromtheconditionconstant:0.005/X2=1010.69[Ca2+]=3.210-7mol/LpCa=6.49(4)If0.02mLofexcessEDTAisaddedbeyondtheequivalencepoint2．AtpH=9lgαY(H)=1.29lgKMY’=lgKMY-αY(H)=109.40[Y]=0.010000.02/(20.00+20.02)=510-6mol/LpCa=7.69EffectofKsandpHonthemagnitudeofbreak(1)EffectofKsonthemagnitudeofbreakThehigherK'MYis,thebiggerthebreakis.ThehigherpHis,thebiggerthebreakis.(2)EffectofpHonthemagnitudeofbreak§7.5DetectionofTheEndPointIndicators:Complexagentwhichformsacomplexofadifferentcolorwiththemetalionbeingtitrated.1.IndicatorsForexample：Beforetitration,EBTisaddedintoMg2+solution(pH8~10)，colorofthesolutionisred：EBT(blue)+Mg2+=Mg2+-EBT(red)Attheequivalencepoint，EDTAbreakupMg2+-EBT,andthecolorofthesolutionchangestoblue：Mg2+-EBT(red)+EDTA=EBT+Mg2+-EDTA(blue)TheindicatorshouldbeusedatsuitablepHH2In-HIn-2In-3pH68-1112moving2.Requirementsformetallochromicindicators(1)AtthepHoftitration,thereiscolordifferencebetweenthecomplexandthedissociatedindicators.(2)Ksofthecomplexbetweentheindicatorandthemetalshouldbesuitable.a．Shouldnotbetoolarge,canbedisplacedbytheEDTA.moving1b．Shouldnotbetooweak,orEDTAwillreplaceitbeforetheequivalentpoint.c.Themetalindicatorcomplexshouldbe10~100timeslessstablethanthemetal-titrantcomplex.moving23.Commonlyusedmetallochromicindicators(1)EriochromeblackT(EBT)a.SuitablepHvalueis9~10b.UsedinthetitrationofZn2+,Mg2+,Cd2+c.Colorchangefromredtoblued.Theindicatorshouldbekeptasdiluteaspossibleandstillgiveagoodcolor(2)Calcon-carboxylicacida.AtpH=7,purple；AtpH=12-13,blue.b.Usedintitra