分析化学-配位滴定

ChapterFive:

ComplexometryObjectivesandDemands：

1)

GraspstabilityofcomplexesofEDTAwithmetalions;2)Masterthevariationofmetalionconcentrationduringtitrationprocess.TeachingContentsandArrangements：1)ComplexesandFormationConstants—HowStableAreComplexes?2)Chelates:EDTA—TheUltimateTitratingAgentforMetals;3)Metal—EDTATitrationCurves;4)DetectionoftheEndPoint:Indicators—TheyAreChelatingAgents;5)OtherUsesofComplexes;6)FractionofDissociatingSpeciesinPolyligandComplexes:βValues—HowMuchofSpecies?7)CalculationofConditionalStabilityConstantsofComplexEquilibria.EmphasesandDifficulties：Emphases：

1)

UnderstandingCoefficientsCausedbyAcidity(lgαY(H)),Coexistingions(lgαY(N)),andComplexReaction(lgαM(L));EffectonComplexEquilibriawithSideReaction;2)CalculationofConditionalStabilityConstantsofComplexEquilibria.Difficulties：

SelectivityandAccuracyofMetalionsTitrationwhileCoexistingwithOtherIons.5.1.Generality

ofComplexometricTitrationComplexometrictitrationisatypeoftitrationbasedoncomplexformationbetweentheanalyteandtitrant.Manymetalionsformslightlydissociatedcomplexeswithvariousligands(complexingagents).

Theanalyticalchemistmakesjudicioususeofcomplexestomaskundesiredreactions.Theformationofcomplexescanalsoserveasthebasisofaccurateandconvenienttitrationsformetalionsinwhichthetitrantisacomplexingagent.

Complexometrictitrationsareusefulfordeterminingalargenumberofmetals.Selectivitycanbeachievedbyappropriateuseofmaskingagents(additionofothercomplexingagentsthatreactwithinterferingmetalions)andbypHcontrol,sincemostcomplexingagentsareweakacidsorweakbaseswhoseequilibriaareinfluencedbythepH.1)ComplexReactionComplexometrictitrationisatypeoftitrationbasedoncomplexformationbetweentheanalyteandtitrant.ThestabilityofcomplexcanbeexpressedasKstab,stabilityconstant,orKf,formationconstant.

2)TheoryBaseThemetalionisaLewisacid(electronpairacceptor),andthecomplexerisaLewisbase(electronpairdonor).Thenumberofmoleculesofthecomplexingagent,calledtheligand,willdependedonthecoordinationnumberofthemetalandonthenumberofcomplexinggroupsontheligandmolecules.Manycationswillformcomplexesinsolutionwithavarietyofsubstancesthathaveapairofunsharedelectrons(e.g.,onN,O,Satomsinthemolecule)capableofsatisfyingthecoordinationnumberofthemetal.ExampleofComplexingAgent–Ammonia(NH3)Ammonia(NH3)isasimplecomplexingagentwithonepairofunsharedelectronsthatwillcomplexcopperion:

Here,thecopperionactsasaLewisacid,andtheammoniaisaLewisbase.TheCu2+(hydrated)ionispaleblueinsolution,whiletheammonia(theammine)complexisdeepblue.Asimilarreactionoccurswiththegreenhydratednickeliontoformadeepblueamminecomplex.PaleBlueDeepBlue3)ComplexandComplexingAgentComplex(1)Simplecomplex:Itiscomposedofacenterionandmono-ligand,withoutcyclestructure(2)Chelate:…multi-ligand,withcycle…Complexingagent(1)Inorganiccomplexingagent:(2)Organiccomplexingagent:4)TypesofChelants“OO”TypeChelant“NN”TypeChelant“NO”TypeChelantSulfurcontainingChelantTrilonChelant(氨羧配合剂)4a)“OO”TypeChelantBondingmetalionwithtwooxygenatoms,e.g.Oxyacid(羟基酸),Polyproticacid,Polyhydricalcohol,Polyphenol,TartaricAcid(酒石酸)

4b)“NN”TypeChelantBondingmetalionwithtwonitrogenatoms,

e.g.Organicamine,Heterocycle(杂环)compoundlikephenanthroline

Complexofphenanthroline(邻二氮菲)andFe

4c)“NO”TypeChelantBondingmetalionwithoxygenandnitrogenatoms,

e.g.Trilon(氨羧配合剂)Chelant,hydroxyquinoline,andsomeazo-dyes,8-Hydroxyquinoline(8-羟基喹啉)

4d)SulfurcontainingChelant:

“SS”,“SO”,“SN”types“SS”Type

Tetratomicring:CopperReagent(二乙胺基二硫代甲酸钠)Thioglycolicacid(巯基乙酸)“SO”,“SN”Types

Penta-ring:4e)TrilonChelant(氨羧配合剂)Capableofbindingwithmanymetalions.Examplesarelistedbelow:

CyDTA(环己烷二胺四乙酸)

EGTA(乙二醇二乙醚二胺四乙酸)

EDTP(乙二胺四丙酸)

EDTA(乙二胺四乙酸)

Basicfunctionalgroup:5.2.ComplexesofEDTAwithMetalIons1)FeatureofEDTA2)DissociationequilibriumofH4Y3)MetalComplexeswithEDTA1)FeatureofEDTAEDTAor(ethylenediamminetetraaceticacid,乙二胺四乙酸orEDTA酸),otherwiseknownasedetate,versene,ordiaminoethanetetraaceticaciddisodiumsalt(Na2H2Y·2H2O=372.24).Ithasfourcarboxylgroupsandtwonitrogenatomsthatcanactaselectronpairdonors,thereforeitisalsoabidentate(有二齿的)ligandorLewisbase.ThischangesinanalkalisolutionwheretheEDTAmoleculeismorestable.ItcanbewrittenasH4Y.

Itisalsoahydrophiliccompoundanditsrecommendedstorageisinadryareawithadesiccator.UsesofEDTA1)Scavenging(v.净化,清除)metalions.----Inbiochemistryandmolecularbiology,thisisacommonmethodtoinactivateenzymes.2)Complexometrictitrations.3)Buffersolutions.4)Determinationofwaterhardness.

5)Useasawatersoftener.6)Usedinmedicineasatreatment(brandnameEndrate®,marketedbyHospira,Inc.;genericproductisalsoonthemarket)foracutehypercalcemia(n.[医]血钙过多)andleadpoisoning.UsesofEDTA7)Usedinmedicalandlaboratoryequipmentasananticoagulant(a.抗凝的;n.[药]抗凝血剂).8)Addedtosomeprocessedfoodsandespeciallycosmeticsasapreservativeandasequestrant(n.多价螯合剂).

9)Asomewhatcontroversialtherapy(n.疗法,治疗),calledchelationtherapyhasevolvedaroundmetalscavengerssuchasEDTA.10)Usedindentistryasarootcanal

irriganttoremovecompoundsoforganicandinorganicdebris(smearlayer)

11)Usedinphotographyasacomponentofbleach-fixusedtodissolveelementalsilverproducedduringdevelopment.12)Usedasasoilconditionertoallowcalcifuge(n.[植]避钙植物,嫌钙植物)plantstogrowinbaserichsoils.UseasAnticoagulant(n.[药]抗凝血剂)EDTAworksasananticoagulantbychelatingallthecalciumcontainedinblood.Calciumisneededforcoagulationtooccur;withoutcalciumbloodwillnotclot.Thecalciumlevelsbelowwhichclottingceasesarelowenoughtobelethal(a.致命的),soEDTAisonlyusedasananticoagulantoutsidethebody;forinstanceintubesofblood,andmedicalmachinery.EDTAiscontainedinpurple,lavenderandpinkVacutainer(tubesthatbloodistakenin),andcanbeintheformofapowder,orasmallamountofliquid,alreadyinthetube.

ThesodiumorpotassiumsaltsofEDTA(K2EDTA,K3EDTA,Na2EDTA)areusedinVacutainertubes.2)DissociationequilibriumofH4YpH=1pH>12

HoweverinhighpHEDTAformsaY4-asthealkalipHacceptstheH+ionsfromthecarboxylgroups.Thisisnowahexadentate

ligandandcanformaoctahedralcomplexwithmostmetal2+ions(M2+),hencethenamecomplexometerictitration.2)DissociationequilibriumofH4YpH=1pH>12

HoweverinhighpHEDTAformsaY4-asthealkalipHacceptstheH+ionsfromthecarboxylgroups.Thisisnowahexadentate

ligandandcanformaoctahedralcomplexwithmostmetal2+ions(M2+),hencethenamecomplexometerictitration.

ThecomplexformedwiththeionMY2-willbeina1:1ratioandisareasonwhyEDTAissowidelyusedtodetermineamountsofmetalionssuchasmanganeseandcalciuminwaterortohelptreatleadpoisoninginhumansastheEDTAbindswiththeheavy,toxicmetal.

EDTAEquilibria

pH=1pH>123)ComplexesofEDTAwithMetalIonsEDTAoritsdisodiumsaltisachelatingagent,formingcoordinationcompoundswithmostdivalent(ortrivalent)metal

ions,suchascalcium(Ca2+)andmagnesium(Mg2+)orcopper(Cu2+).FormationConstant,orStabilityConstant:3)ComplexesofEDTAwithMetalIonsStructureDiagramofMetal-EDTAChelate

ConcludedFeaturesforEDTAEDTAstandsforethylenediamintetraaceticacid.EDTAformsstrong1:1hexadentalcomplexeswithmostmetalions(=six-coordinategeometry).ThemetaliscoordinatedtooxygenandnitrogenofEDTA.EDTAhassixdeprotonationconstants((K1-K6),withK1-K4arereferringtothedeprotonationofthecarboxylicacidsandK5&K6refertotheamins.TheneutralcompoundisH4Y,themostacidiconeisH6Y2+andthemostbasiconesisY4-.

ThecommonlyusedcompoundisthedisodiumsaltNa2H2Y·2H2O.Ametal-EDTAcomplexbecomeslessstableatlowerpH.

K1K2K3K4K5K65.3.EffectonStabilityofEDTAComplexeswithMetalionsHydroxideEffectAuxiliaryEffectAcidityEffectInterferingEffectMixedEffectMainSideReaction1)AcidityEffectandItsCoefficientMainreaction:

Acidityeffect:TheabilityofY4-inmainreactionisloweredbytheinteractionofY4-andH+.Acidityeffectcoefficient:

where,[Y’]representstheanalyticalconcentrationofEDTA,[Y4-]representstheconcentrationofcomponentY4-,whichcanformacomplexwithmetalion.Then,SideReactionAcidityeffectcoefficientAsβ1,β2,…,β6arecumulativestabilityconstants,thenαY(H)iscorrelatedwithacidityofsolution.IfthereisnosecondaryreactionbetweenH+andY4-,thenαY(H)=1.Example5.1Example5.2

Example5.1：

PleasecalculatetheacidityeffectcoefficientofcyanidewhileatpH=5.00?Solution:

lgαCN(H)=4.21

Example5.2:

PleasecalculatetheacidityeffectcoefficientofEDTAwhileatpH=2.00?Solution:

pKa1=-0.9,pKa2=-1.6,pKa3=-2.0,pKa4=-2.67,pKa5=-6.16,pKa6=-10.26

Example5.2:

PleasecalculatetheacidityeffectcoefficientofEDTAwhileatpH=2.00?Solution:

pKa1=-0.9,pKa2=-1.6,pKa3=-2.0,pKa4=-2.67,pKa5=-6.16,pKa6=-10.26

lgαY(H)=13.512)EffectofMetalIonHydroxideeffectorelectrolysiseffect:

Auxiliaryeffect:

EffectofMetalIonThetotalcoefficientofsecondaryreactionformetalioncanbeexpressedas:Where,[M’]=[M]+[MOH]+[M(OH)2]+…+[M(OH)n]+[ML]+[ML2]+…+[MLn]Thus,αM=αM(L)+αM(OH)–1

Ifexistingmulti-complexingagent,thenForexample,Example5.3Example5.4

Example5.3:In0.10mol/LofAlF63-solution,theconcentrationoffreeF-is0.010mol/L,PleasecalculatetheconcentrationofAl3+?

Solution:

ThecumulativestabilityconstantsofAlF63-areβ1=1.4×10^6,β2=1.4×10^11,β3=1.0×10^15,β4=5.6×10^17,β5=2.3×10^19,β6=6.9×10^19;[F-]=0.010mol/L,cAlF63-=0.10mol/L

AlF3AlF4-AlF52-maincomplexesExample5.4:In0.010mol/Lofzinc-ammoniasolution,theconcentrationoffreeNH3is0.10mol/LwhilepH=10,Pleasecalculatethetotalcoefficientofsecondaryreactionforzincion?

Solution:

ThecumulativestabilityconstantsofZn(NH3)42+arelogβ1—logβ4:2.27,4.61,7.01,9.06;WhilepH=10,αZn(OH)=10^2.4;3)ConditionalStability/Formation

ConstantHydroxideEffectAuxiliaryEffectAcidityEffectInterferingEffectMixedEffectMainSideReactionAccordingtomainreaction,theformationconstantorstabilityconstantisexpressedas:3.1)ConsideringacidityeffectofEDTA:

TheconditionalstabilityconstantvalueholdsforonlyaspecifiedpH.

Where,[Y’]representstheanalyticalconcentrationofEDTAincludingalltheexistingforms.[Y4-]or[Y]representstheconcentrationofonlycomponentY4-,whichcanformacomplexwithmetalion.

RelationshipbetweenpHandK’MY:

pH↑

logαY(H)↓K’MY↑Complexreactioncompletely↑

Benefitforcomplexometrictitration

3.2)Consideringeffectofacidityandmetalion:

3.3)Effectoncomplexometrictitration:

a)Formetalion:hydroxideeffect(MOH)andauxiliaryeffect(ML)fromother

complexingagents;b)ForEDTA:acidityeffectandinterferingeffectfromothercoexistingmetalions;c)ForMY:mixedeffect(MHYandMOHY)ItisveryimportanttocontrolpHvalueinsolutionforcomplexometrictitration.4)AppropriatepHConditionThecontrolofappropriatepHconditionisdependedontheacidityeffectofEDTAandthehydroxideeffectofmetalion.End-pointerrorpermitted:ΔpM≤±(0.2—0.5),RE≤±0.1%Then,Thisistheconditionfordeterminationofsinglecomponentmetalionwithcomplexometrictitration.Example5.5Example5.5WhatisthepermittedlowestpHvaluewhentitrating0.01mol/LCa2+solutionwithEDTA?(logKCaY=10.69)Solution:CheckinTable5-2,pHmin>7.6

RingbomCurve:Curveofacidityeffect

RingbomCurve:Curveofacidityeffect5.4.TitrationCurve1)Similartoacid-basetitration2)Transitionrangeatstoichiometricpoint,

pM=-lg[Mn+]3)Consideringsidereactions4)MetalindicatorisdecidedbytheexperimentExample5.6:Titrationwith0.01000mol/LEDTAinto20.00mL0.01000mol/LCa2+solution(1)Beforetitration:

OnlyCa2+existinginsolution,thepCaisdependedontheoriginalconcentration.Then,pCa=2.00

Example5.6:Titrationwith0.01000mol/LEDTAinto20.00mL0.01000mol/LCa2+solution(2)Startingtitrationtillstoichiometricpoint:

DroppingEDTAandformCaYcomplextillcompleted,thepCaisdependedontheresidualCa2+.Ifadding18.00mLEDTA,theresidualCa2+of2.00mLhasnotformedcomplexwithEDTA.Then,pCa=3.28Example5.6:Titrationwith0.01000mol/LEDTAinto20.00mL0.01000mol/LCa2+solution(3)Atstoichiometricpoint:Whenadding20.00mLEDTAsolution,Ca2+iscompletelyformedcomplexwithEDTA.Inthesolution,[Ca2+]=[Y],[CaY]=0.5CCa,thenSo,pCa=6.50Example5.6:Titrationwith0.01000mol/LEDTAinto20.00mL0.01000mol/LCa2+solution(4)Afterstoichiometricpoint:WhenaddingexcessEDTA,thesolutioncontainsCaYandresidualY.Ifadding20.02mLEDTA,theresidualEDTAis0.02mL.Then,pCa=7.69Table5.1Titrationwith0.01000mol/LEDTAinto20.00mL0.01000mol/LCa2+solutionAddingEDTApCaWhilepH=12.50pCaWhilepH=8.00pHvariationmL%0.0000.002.0012.518.0090.03.283.2819.8099.04.304.3019.9899.95.305.1220.00100.06.505.3611.820.02100.17.695.60lgαY(H)0.01

20.20101.08.696.42lgαCa(OH):small

22.00110.09.697.4240.00200.010.698.42lgKCaY=10.69Titrationwith0.01000mol/LEDTAinto0.01000mol/LmetalionsolutionCa2+Ni2+[NH3]+[NH4+]=0.1mol/LEDTAadded/%5.5.MetalIndicatorsandEnd-Point1)MetalindicatorsRed(Ca2+,Mg2+,Zn2+,Cd2+)BluepH=8-11a.Conditionfortitration:

KM(CBT)<<KM(EDTA)b.ChoseappropriatepHrange:

RedpH<6BluepH8-11

OrangepH>12

5.5.MetalIndicatorsandEnd-Point2)Rulesasmetalindicators:a.Bothindicatorandcomplexcolorsaremuchdifferent;b.Coloringreactionissensitive,fastandreversible;c.Coloringcomplexisstable;d.Complexiswatersoluble;e.Metalindicatorisstable.3)CommonmetalindicatorsChromeBlackT,Cu-PAN4)Otherend-pointindicatingmethods5.6.TitrationofMetalIonsinMixtrue

1)TitrationseparatelybypHcontrol(1)Forsinglemetalion,theconditionoftitrationis:End-pointerrorpermitted:ΔpM≤±(0.2—0.5),RE≤±0.1%5.6.TitrationofMetalIonsinMixtrue

1)TitrationseparatelybypHcontrol(2)Twocomponents:MandinterferingionNWhenKMY>KNY,andαY(N)>>αY(H),

5.6.TitrationofMetalIonsinMixtrue

1)TitrationseparatelybypHcontrol(2)Twocomponents:MandinterferingionNΔpM≈±0.3,RE≤±0.5%

WhenKMY>KNY,andαY(N)>>αY(H),

αY(N)=KNY·CNNeglectinglgαM,

OrifCM=CN,ΔlgK≥55.6.TitrationofMetalIonsinMixtrue

1)TitrationseparatelybypHcontrol(1)Forsinglemetalion,theconditionoftitrationis:End-pointerrorpermitted:ΔpM≤±(0.2—0.5),RE≤±0.1%

(2)Twocomponents:MandinterferingionN(3)Metalcomponents≥3ΔpM≈±0.3,RE≤±0.5%

Summarization：Example5.7WhentheconcentrationsofBi3+andPb3+allare0.01mol/L,PleasemakeajudgmentwhetherBi3+canbetitratedwithEDTAornot?Solution:Knowndata:lgKBiY=27.94,lgKPbY=18.04,CBi=CPb=0.01mol/LCBi=CPb=0.01mol/L,ΔlgK=lgKBiY-lgKPbY=27.94–18.04=9.90>5So,Bi3+canbetitratedwithEDTAselectively.2)MaskingandDe-maskingComplexmaskingPrecipitatemaskingRedoxmaskingDe-masking3)Pre-separation4)Usingothercomplexingagents5.7.ApplicationofComplexometryDirectTitrationBackTitrationReplacementTitrationIndirectTitrationExample5.8InthepH=10.0ammonium-buffersolutioncontaining0.02mol/Lzincion(Zn2+)and0.02mol/Lmagnesiumion(Mg2+),canZn2+be