定量分析12重量分析法Gravimetric-Methods-of-Analysis课件

QuantitativeAnalysis

定量分析

QuantitativeAnalysis

定量分析Chapter12

GravimetricMethodsofAnalysisChapter12

GravimetricMethods

Gravimetricmethodsofanalysisarebasedonthemeasurementofmass.Therearetwomajortypesofgravimetricmethods:

Precipitationmethods:inthismethodtheanalyteisconvertedtoasparinglysolubleprecipitate.Thisprecipitateisthenfiltered,washedfreeofimpurities,andconvertedtoaproductofknowncompositionbysuitableheattreatment,andtheproductisweighed.

Volatilizationmethods:inthistheanalyteoritsdecompositionproductsarevolatilizedatasuitabletemperature.Thevolatileproductisthencollectedandweighed,or,alternatively,themassoftheproductisdeterminedindirectlyfromthelossinmassofthesample. Gravimetricmethodsofanalys定量分析12重量分析法Gravimetric-Methods-of-Analysis课件PROPERTIESOFPRECIPITATESANDPRECIPITATINGREAGENTS Agravimetricprecipitatingagentshouldreactspecifically,andselectivelywiththeanalyte.TheidealprecipitatingreagentwouldreactwiththeanalytetogiveaproductthatisReadilyfilteredandwashedfreeofcontaminantsOfsufficientlylowsolubilitysothatnosignificantlossofthesolidoccursduringfiltrationandwashingUnreactivewithconstituentsoftheatmosphereOfknowncompositionafteritisdriedor,ifnecessary,ignitedPROPERTIESOFPRECIPITATESAND ParticleSizeandFilterabilityofPrecipitates Precipitatesmadeupoflargeparticlesaregenerallydesirableingravimetricworkbecauselargeparticlesareeasytofilterandwashfreeofimpurities.Inaddition,suchprecipitatesareusuallypurerthanareprecipitatesmadeupoffineparticles. ParticleSizeandFilterabiliWhatFactorsDetermineParticleSize? Theparticlesizeofsolidsformedbyprecipitationvariesenormously.Atoneextremearecolloidalsuspension,whosetinyparticlesareinvisibletothenakedeye(10-7to10-4cmindiameter).Colloidalparticlesshownotendencytosettlefromsolution,noraretheyeasilyfiltered.Attheotherextremeareparticleswithdimensionsontheorderoftenthsofmillimeterorgreater.Thetemporarydispersionofsuchparticlesintheliquidphaseiscalledacrystallinesuspension.Theparticlesofacrystallinesuspensiontendtosettlespontaneouslyandarereadilyfiltered.WhatFactorsDetermineParticl Theparticlesizeofaprecipitateisinfluencedbyexperimentalvariablesasprecipitatesolubility,temperature,reactantconcentrations,andtherateatwhichreactantsaremixed.Theparticlesizeisrelatedtoasinglepropertyofthesystemcalleditsrelativesupersaturation,where relativesupersaturation=(Q–S)/S Inthisequation,QistheconcentrationofthesoluteatanyinstantandSisitsequilibriumsolubility. When(Q–S)/Sislarge,theprecipitatetendstobecolloidal. when(Q–S)/Sissmall,acrystallinesolidismorelikely. TheparticlesizeofaprecipHowdoPrecipitatesForm?Precipitatesformintwoways,bynucleationandbyparticlegrowth.Theparticlesizeofafreshlyformedprecipitateisdeterminedbywhichwayisfaster. Innucleation,afewions,atoms,ormolecules(perhapsasfewasfourorfive)cometogethertoformastablesolid.Often,thesenucleiformonthesurfaceofsuspendedsolidcontaminants,suchasdustparticles.Furtherprecipitationtheninvolvesacompetitionbetweenadditionalnucleationandgrowthonexistingnuclei(particlegrowth).Ifnucleationpredominates,aprecipitatecontainingalargenumberofsmallparticlesresults;ifgrowthpredominates,asmallernumberoflargerparticlesisproduced.HowdoPrecipitatesForm?Prec ControllingParticleSize

Experimentalvariablesthatminimizesupersaturationandthusleadtocrystallineprecipitatesincludeelevatedtemperaturestoincreasethesolubilityoftheprecipitate(SinEquation),dilutesolutions(tominimizeQ),andslowadditionoftheprecipitatingagentwithgoodstirring.Thelasttwomeasuresalsominimizetheconcentrationofthesolute(Q)atanygiveninstant. LargerparticlescanalsobeobtainedbypHcontrol,providedthesolubilityoftheprecipitatedependsonpH. ControllingParticleSizeColloidalPrecipitates CoagulationofColloids:Coagulationcanbehastenedbyheating,stirring,andaddinganelectrolytetothemedium. Colloidalsuspensionsarestablebecausealltheparticlespresentareeitherpositivelyornegativelycharged.Thischargeresultsfromcationsoranionsthatareboundtothesurfaceoftheparticles.Theprocessbywhichionsareretainedonthesurfaceofasolidisknownasadsorption.Wecanreadilydemonstratethatcolloidalparticlesarechargedbyobservingtheirmigrationwhenplacedinanelectricalfield.ColloidalPrecipitates定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件 PeptizationofColloids

Peptizationreferstotheprocessbywhichacoagulatedcolloidrevertstoitsoriginaldispersedstate.Whenacoagulatedcolloidiswashed,someoftheelectrolyteresponsibleforitscoagulationisleachedfromtheinternalliquidincontactwiththesolidparticles.Removalofthiselectrolytehastheeffectofincreasingthevolumeofthecounter-ionlayer.Therepulsiveforcesresponsiblefortheoriginalcolloidalstatearethenreestablished,andparticlesdetachthemselvesfromthecoagulatedmass.Thewashingsbecomecloudyasthefreshlydispersedparticlespassthroughthefilter. PeptizationofColloids CrystallinePrecipitates Crystallineprecipitatesaregenerallymoreeasilyfilteredandpurifiedthancoagulatedcolloids.Inaddition,thesizeofindividualcrystallineparticles,andthustheirfilterability,canbecontrolledtoadegree. TheparticlesizeofcrystallinesolidscanoftenbeimprovedsignificantlybyminimizingQ,maximizingS,orbothinEquation.MinimizationofQisgenerallyaccomplishedbyusingdilutesolutionandaddingtheprecipitatingfromhotsolutionorbyadjustingthepHoftheprecipitationmedium.

Digestionofcrystallineprecipitates(withoutstirring)forsometimeafterformationfrequentlyyieldsapurer,morefilterableproduct.Theimprovementinfilterabilityresultsfromthedissolutionandrecrystallization. CrystallinePrecipitates Coprecipitation Coprecipitationisthephenomenoninwhichsolublecompoundsareremovedfromsolutionduringprecipitateformation. Therearefourtypesofcoprecipitation: i)surfaceadsorption,ii)mixed-crystalformation,iii)occlusion,iv)mechanicalentrapment Surfaceadsorptionandmixedcrystalformationareequilibriumprocesses,whereasocclusionandmechanicalentrapmentarisefromthekineticsofcrystalgrowth. CoprecipitationSurfaceAdsorption Adsorptionisacommonsourceofcoprecipitationthatislikelytocausesignificantcontaminationofprecipitateswithlargespecificsurfaceareas,thatiscoagulatedcolloids. Coagulationofacolloiddoesnotsignificantlydecreasetheamountofadsorptionbecausethecoagulatedsolidstillcontainslargeinternalsurfaceareasthatremainexposedtothesolvent.Thecoprecipitatedcontaminantonthecoagulatedcolloidconsistsofthelatticeionoriginallyadsorbedonthesurfacebeforecoagulationandthecounterionofoppositechargeheldinthefilmofsolutionimmediatelyadjacenttotheparticle.Theneteffectofsurfaceadsorptionisthereforethecarryingdownofanotherwisesolublecompoundasasurfacecontaminant.SurfaceAdsorption MinimizingAdsorbedImpuritiesonColloids

Thepurityofmanycoagulatedcolloidsisimprovedbydigestion.Duringthisprocess,waterisexpelledfromthesolidtogiveadensermassthathasasmallerspecificsurfaceareaforadsorption.

Washingacoagulatecolloidwithasolutioncontainingavolatileelectrolytemayalsobehelpfulbecauseanynonvolatileelectrolyteaddedearliertocausecoagulationisdisplacebythevolatilespecies.Washinggenerallydoesnotremovemuchoftheprimarilyadsorbedionsbecausetheattractionbetweentheseionsandthesurfaceofthesolidistoostrong.Exchangeoccurs,howeverbetweenexistingcounterionsandionsinthewashliquid. MinimizingAdsorbedImpuritieReprecipitation

Adrasticbuteffectivewaytominimizetheeffectsofadsorptionisreprecipitation,ordoubleprecipitation.Here,thefilteredsolidisredissolvedandreprecipitated.Thefirstprecipitateordinarilycarriesdownonlyafractionofthecontaminantpresentintheoriginalsolvent.Thus,thesolutioncontainingtheredissolvedprecipitatehasasignificantlylowercontaminantconcentrationthantheoriginal,andevenlessadsorptionoccursduringthesecondprecipitation.Reprecipitationaddssubstantiallytothetimerequiredforananalysis.Reprecipitation

Mixed-CrystalFormation Inmixed-crystalformation,oneoftheionsinthecrystallatticeofasolidisreplacedbyanionofanotherelement.Forthisexchangetooccur,itisnecessarythatthetwoionshavethesamechargeandthattheirsizesdifferbynomorethanabout5%.Furthermore,thetwosaltsmustbelongtothesamecrystalclass.Forexample,MgKPO4,inMgNH4PO4,SrSO4inBaSO4,andMnSinCdS. Theextentofmixed-crystalcontaminationincreasesastheratioofcontaminanttoanalyteconcentrationincreases.Mixed-crystalformationistroublesomebecauselittlecanbedoneaboutit.Separationoftheinterferingionmayhavetobecarriedoutbeforethefinalprecipitationstep.Alternatively,adifferentprecipitatingreagentmaybeused. Mixed-CrystalFormation

OcclusionandMechanicalEntrapment Whenacrystalisgrowingrapidlyduringprecipitateformation,foreignionsinthecounter-ionlayermaybecometrapped,oroccluded,withinthegrowingcrystal. Mechanicalentrapmentoccurswhencrystalslieclosetogetherduringgrowth.Here,severalcrystalsgrowtogetherandinsodoingtrapaportionofthesolutioninatinypocket. Bothocclusionandmechanicalentrapmentareataminimumwhentherateofprecipitateformationislow,thatis,underconditionsoflowsupersaturation.Digestionisoftenremarkablyhelpfulinreducingthesetypesofcopreipitation.Therapidsolutionandreprecipitationthatgoesonattheelevatedtemperatureofdigestionopensupthepocketsandallowstheimpuritiestoescapeintothesolution. OcclusionandMechanicalEntr PrecipitationfromHomogeneousSolution Precipitationfromhomogeneoussolutionisatechniqueinwhichaprecipitatingagentisgeneratedinasolutionoftheanalytebyaslowchemicalreaction.Localreagentexcessesdonotoccurbecausetheprecipitatingagentappearsgraduallyandhomogeneouslythroughoutthesolutionandreactsimmediatelywiththeanalyte.Asaresult,therelativesupersaturationiskeptlowduringtheentireprecipitation.Ingeneral,homogeneouslyformedprecipitates,bothcolloidalandcrystalline,arebettersuitedforanalysisthanasolidformedbydirectadditionofaprecipitatingreagent. PrecipitationfromHomogeneou定量分析12重量分析法Gravimetric-Methods-of-Analysis课件DRYINGANDIGNITIONOFPRECIPITATES Afterfiltration,agravimetricprecipitateisheateduntilitsmassbecomesconstant.Heatingremovesthesolventandanyvolatilespeciescarrieddownwiththeprecipitate.Someprecipitatesarealsoignitedtodecomposethesolidandformacompoundofknowncomposition.Thisnewcompoundisoftencalledtheweighingform. Thetemperaturerequiredtoproduceasuitableweighingformvariesfromprecipitatetoprecipitate.DRYINGANDIGNITIONOFPRECI定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件

QuantitativeAnalysis

定量分析

QuantitativeAnalysis

定量分析Chapter12

GravimetricMethodsofAnalysisChapter12

GravimetricMethods

Gravimetricmethodsofanalysisarebasedonthemeasurementofmass.Therearetwomajortypesofgravimetricmethods:

Precipitationmethods:inthismethodtheanalyteisconvertedtoasparinglysolubleprecipitate.Thisprecipitateisthenfiltered,washedfreeofimpurities,andconvertedtoaproductofknowncompositionbysuitableheattreatment,andtheproductisweighed.

Volatilizationmethods:inthistheanalyteoritsdecompositionproductsarevolatilizedatasuitabletemperature.Thevolatileproductisthencollectedandweighed,or,alternatively,themassoftheproductisdeterminedindirectlyfromthelossinmassofthesample. Gravimetricmethodsofanalys定量分析12重量分析法Gravimetric-Methods-of-Analysis课件PROPERTIESOFPRECIPITATESANDPRECIPITATINGREAGENTS Agravimetricprecipitatingagentshouldreactspecifically,andselectivelywiththeanalyte.TheidealprecipitatingreagentwouldreactwiththeanalytetogiveaproductthatisReadilyfilteredandwashedfreeofcontaminantsOfsufficientlylowsolubilitysothatnosignificantlossofthesolidoccursduringfiltrationandwashingUnreactivewithconstituentsoftheatmosphereOfknowncompositionafteritisdriedor,ifnecessary,ignitedPROPERTIESOFPRECIPITATESAND ParticleSizeandFilterabilityofPrecipitates Precipitatesmadeupoflargeparticlesaregenerallydesirableingravimetricworkbecauselargeparticlesareeasytofilterandwashfreeofimpurities.Inaddition,suchprecipitatesareusuallypurerthanareprecipitatesmadeupoffineparticles. ParticleSizeandFilterabiliWhatFactorsDetermineParticleSize? Theparticlesizeofsolidsformedbyprecipitationvariesenormously.Atoneextremearecolloidalsuspension,whosetinyparticlesareinvisibletothenakedeye(10-7to10-4cmindiameter).Colloidalparticlesshownotendencytosettlefromsolution,noraretheyeasilyfiltered.Attheotherextremeareparticleswithdimensionsontheorderoftenthsofmillimeterorgreater.Thetemporarydispersionofsuchparticlesintheliquidphaseiscalledacrystallinesuspension.Theparticlesofacrystallinesuspensiontendtosettlespontaneouslyandarereadilyfiltered.WhatFactorsDetermineParticl Theparticlesizeofaprecipitateisinfluencedbyexperimentalvariablesasprecipitatesolubility,temperature,reactantconcentrations,andtherateatwhichreactantsaremixed.Theparticlesizeisrelatedtoasinglepropertyofthesystemcalleditsrelativesupersaturation,where relativesupersaturation=(Q–S)/S Inthisequation,QistheconcentrationofthesoluteatanyinstantandSisitsequilibriumsolubility. When(Q–S)/Sislarge,theprecipitatetendstobecolloidal. when(Q–S)/Sissmall,acrystallinesolidismorelikely. TheparticlesizeofaprecipHowdoPrecipitatesForm?Precipitatesformintwoways,bynucleationandbyparticlegrowth.Theparticlesizeofafreshlyformedprecipitateisdeterminedbywhichwayisfaster. Innucleation,afewions,atoms,ormolecules(perhapsasfewasfourorfive)cometogethertoformastablesolid.Often,thesenucleiformonthesurfaceofsuspendedsolidcontaminants,suchasdustparticles.Furtherprecipitationtheninvolvesacompetitionbetweenadditionalnucleationandgrowthonexistingnuclei(particlegrowth).Ifnucleationpredominates,aprecipitatecontainingalargenumberofsmallparticlesresults;ifgrowthpredominates,asmallernumberoflargerparticlesisproduced.HowdoPrecipitatesForm?Prec ControllingParticleSize

Experimentalvariablesthatminimizesupersaturationandthusleadtocrystallineprecipitatesincludeelevatedtemperaturestoincreasethesolubilityoftheprecipitate(SinEquation),dilutesolutions(tominimizeQ),andslowadditionoftheprecipitatingagentwithgoodstirring.Thelasttwomeasuresalsominimizetheconcentrationofthesolute(Q)atanygiveninstant. LargerparticlescanalsobeobtainedbypHcontrol,providedthesolubilityoftheprecipitatedependsonpH. ControllingParticleSizeColloidalPrecipitates CoagulationofColloids:Coagulationcanbehastenedbyheating,stirring,andaddinganelectrolytetothemedium. Colloidalsuspensionsarestablebecausealltheparticlespresentareeitherpositivelyornegativelycharged.Thischargeresultsfromcationsoranionsthatareboundtothesurfaceoftheparticles.Theprocessbywhichionsareretainedonthesurfaceofasolidisknownasadsorption.Wecanreadilydemonstratethatcolloidalparticlesarechargedbyobservingtheirmigrationwhenplacedinanelectricalfield.ColloidalPrecipitates定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件定量分析12重量分析法Gravimetric-Methods-of-Analysis课件 PeptizationofColloids

Peptizationreferstotheprocessbywhichacoagulatedcolloidrevertstoitsoriginaldispersedstate.Whenacoagulatedcolloidiswashed,someoftheelectrolyteresponsibleforitscoagulationisleachedfromtheinternalliquidincontactwiththesolidparticles.Removalofthiselectrolytehastheeffectofincreasingthevolumeofthecounter-ionlayer.Therepulsiveforcesresponsiblefortheoriginalcolloidalstatearethenreestablished,andparticlesdetachthemselvesfromthecoagulatedmass.Thewashingsbecomecloudyasthefreshlydispersedparticlespassthroughthefilter. PeptizationofColloids CrystallinePrecipitates Crystallineprecipitatesaregenerallymoreeasilyfilteredandpurifiedthancoagulatedcolloids.Inaddition,thesizeofindividualcrystallineparticles,andthustheirfilterability,canbecontrolledtoadegree. TheparticlesizeofcrystallinesolidscanoftenbeimprovedsignificantlybyminimizingQ,maximizingS,orbothinEquation.MinimizationofQisgenerallyaccomplishedbyusingdilutesolutionandaddingtheprecipitatingfromhotsolutionorbyadjustingthepHoftheprecipitationmedium.

Digestionofcrystallineprecipitates(withoutstirring)forsometimeafterformationfrequentlyyieldsapurer,morefilterableproduct.Theimprovementinfilterabilityresultsfromthedissolutionandrecrystallization. CrystallinePrecipitates Coprecipitation Coprecipitationisthephenomenoninwhichsolublecompoundsareremovedfromsolutionduringprecipitateformation. Therearefourtypesofcoprecipitation: i)surfaceadsorption,ii)mixed-crystalformation,iii)occlusion,iv)mechanicalentrapment Surfaceadsorptionandmixedcrystalformationareequilibriumprocesses,whereasocclusionandmechanicalentrapmentarisefromthekineticsofcrystalgrowth. CoprecipitationSurfaceAdsorption Adsorptionisacommonsourceofcoprecipitationthatislikelytocausesignificantcontaminationofprecipitateswithlargespecificsurfaceareas,thatiscoagulatedcolloids. Coagulationofacolloiddoesnotsignificantlydecreasetheamountofadsorptionbecausethecoagulatedsolidstillcontainslargeinternalsurfaceareasthatremainexposedtothesolvent.Thecoprecipitatedcontaminantonthecoagulatedcolloidconsistsofthelatticeionoriginallyadsorbedonthesurfacebeforecoagulationandthecounterionofoppositechargeheldinthefilmofsolutionimmediatelyadjacenttotheparticle.Theneteffectofsurfaceadsorptionisthereforethecarryingdownofanotherwisesolublecompoundasasurfacecontaminant.SurfaceAdsorption MinimizingAdsorbedImpuritiesonColloids

Thepurityofmanycoagulatedcolloidsisimprovedbydigestion.Duringthisprocess,waterisexpelledfromthesolidtogiveadensermassthathasasmallerspecificsurfaceareaforadsorption.

Washingacoagulatecolloidwithasolutioncontainingavolatileelectrolytemayalsobehelpfulbecauseanynonvolatileelectrolyteaddedearliertocausecoagulationisdisplacebythevolatilespecies.Washinggenerallydoesnotremovemuchoftheprimarilyadsorbedionsbecausetheattractionbetweentheseionsandthesurfaceofthesolidistoostrong.Exchangeoccurs,howeverbetweenexistingcounterionsandionsinthewashliquid. MinimizingAdsorbedImpuritieReprecipitation

Adrasticbuteffectivewaytominimizetheeffectsofadsorptionisreprecipitation,ordoubleprecipitation.Here,thefilteredsolidisredissolvedandreprecipitated.Thefirstprecipitateordinarilycarriesdownonlyafractionofthecontaminantpresentintheoriginalsolvent.Thus,thesolutioncontainingtheredissolvedprecipitatehasasignificantlylowercontaminantconcentrationthantheoriginal,andevenlessadsorptionoccursduringthesecondprecipitation.Reprecipitationaddssubstantiallytothetimerequiredforananalysis.Reprecipitation

Mixed-CrystalFormation Inmixed-crystalformation,oneoftheionsinthecrystallatticeofasolidisreplacedbyanionofanotherelement.Forthisexchangetooccur,itisnecessarythatthetwoionshavethesamechargeandthattheirsizesdifferbynomorethanabout5%.Furthermore,thetwosaltsmustbelongtothesamecrystalclass.Forexample,MgKPO4,inMgNH4PO4,SrSO4inBaSO4,andMnSinCdS. Theextentofmixed-crystalcontaminationincreasesastheratioofcontaminanttoanalyteconcentrationincreases.Mixed-crystalformationistroublesomebecauselittlecanbedoneaboutit.Separationoftheinterferingionmayhavetobecarriedoutbeforethefinalprecipitationstep.Alternatively,adifferentpreci