大一学科任务-分析化学chapter3gravimetry

3AAreviewofChemicalStoichiometry3BPropertiesofprecipitatesandprecipitatingReagents3CAcritiqueofgravimetricmethod3DApplicationsofgravimetricMethodsCapter3GravimetricMethodsofAnalysis

SUMMARYp57Gravimetricmethodsofanalysisarebaseduponthemeasurementoftheweightoftheanalyteorofacompoundofknowncompositionthatcontainstheanalyte.TwoTypesofGravimetricMethods1.precipitationgravimetry:Agravimetricmethodinwhichthesignalistheweightofaprecipitate.Anelectrogravimetryisalsoagravimetricmethodinwhichthesignalisthemassofanelectrodepositonthecathodeoranodeinanelectrochemicalcell.E.g.suspendedsolidinsewage-treatment;Pb2+onPtelectrode(anode).2.volatilizationgravimetry:Agravimetricmethodinwhichthelossofavolatilespeciesgivesrisetothesignal.food’smoisturecontent

重量分析法概述

重量分析法的分类和特点：通过称量物质的质量进行分析。分类：根据被测组分与其他组分分离方法的不同，可分为，挥发法、沉淀法、提取法等。

1.挥发法：适用于挥发成分的测定。逸出、减轻。

2.沉淀法：主要方法。沉淀、过滤、洗涤、烘干（灼烧）、恒重。优缺点：不用基准物、相对误差±0.1%~0.2%；繁琐、费时、不适用于微量、痕量测定。3A-1EmpiricalFormulas,ChemicalformulasandtheMoleEmpiricalFormulas:simplestwhole-numberratioofatomsinachemicalcompound.CH2OChemicalformulas:specifiesthenumberofatomsinamolecule.CH2O,C2H4O2,C3H6O3,C6H12O6Molecularformula:providesstructuralinformation.C2H5OH,CH3OCH3theMole(fw,Millimole)p58Avogadro’s

number6.02×10233AAreviewofChemicalStoichiometry

p57Stoichiometryisdefinedastheweightrelationshipamongreactingchemicalspecies.3A-2StoichiometricRelationshipp59BalancedchemicalequationStoichiometryofthereactionExample3-3p59ChemicalFormula:

2AgNO3+Na2CO3==2NaNO3+Ag2CO3

↓reactingfw:2×169.9106.0weightofreactant:

WtAgNO3

2.33Here,7.47gofAgNO3

isneededtoconvert……3A-3CalculationofresultsfromGravimetricDataExample3-5p60(a)convertingrelationship:2Fe→Fe2O3

reactingfw:2×55.85159.7weightofreactant:WtFe

0.5394

Example3-7p62重量分析对沉淀的要求：

p633B引言

1沉淀形式和称量形式（二者可同可不同）沉淀形式：被测组分沉淀析出后的化学组成。称量形式：沉淀经处理后，称量时为固定组成不含水分的形式。p723B-6weighingform2对沉淀形式的要求：①溶解度要小；②易于过滤和干燥；③纯度要高。

3对称量形式的要求：①确定的化学组成；②放置稳定；③分子量大。A13+--Al2O3(C9H6NO)3Al3BPropertiesofprecipitatesandprecipitatingReagents

3B-1Particlesizeandfilterabilityofprecipitates

p63FactorthatdeterminetheparticlesizeofprecipitatesColloidalsuspensionsCrystallinesuspensionRS大colloidalprecipitate;

RS小crystalprecipitatep64

MechanismofprecipitateformationNucleation------ParticlegrowthExperimentalcontrolofparticlesizeMinimizesupersaturationleadtocrystallineprecipitates

p64Controlacidity------enhancetheparticlesize3BPropertiesofprecipitatesandprecipitatingReagents

Goto3B-2

沉淀的类型：①晶形沉淀(>100nm)，BaSO4;②胶体(1～100nm)，Fe2O3·XH2O;p6310-6~10-4mm二者混合的是凝乳状沉淀，AgCl沉淀的形成：均相成核异相成核２～４个离子对晶核生成速度大于晶粒成长速度晶核生成速度小于晶粒成长速度冯韦曼经验公式沉淀的形成和纯度成核作用构晶离子晶核沉淀颗粒长大过程沉淀溶解度要小影响沉淀溶解度的因素：同离子效应SeeFig.盐效应酸效应SeeFig.配位效应SeeFig.其它：温度、溶剂（测K+，K2PtCl6沉淀在水中不可，在乙醇中可以）、沉淀颗粒大小(陈化）。s/mg.L-1T/℃BaSO4CaC2O4·H2OTheprecipitateistheproductofasimplemetathesis(置换)

reactionbetween

the

analyte

andthe

precipitant.Ag++Cl-AgClandequationsAgCl=[Ag+]=affectthesolubilityofAgCl.Butifthe[Cl-]isadded,thesolubilityofAgClwillincrease,astheinformationofAgCl2-,AgCl32-.Thesolidlinecanshowthissuperimposed.Alargeexcessofchloridemustbeavoided.

returnFig1.SolubilityofAgClasafunctionofpClThedashedlineshowsthepredictedsAgCl,assumingthatonlyreactionFig2.Ladderdiagramforphosphate;Fig.

SolubilitydiagramforCa3(PO4)2showingthepredominateformofphosphateforeachsegmentofthesolubilitycurve.return3B-2ColloidalprecipitatesStabilityofcolloids

p65Adsorbedions---primaryadsorptionlayer---counter-ionlayerslippingplane---zetaζpotentialCoagulationofcolloids---reducethezetapotentialp67HeatingthesuspensionforabriefperiodwithgoodstirringIncreasingtheoppositeelectrolyteconcentrationofthesolutionPeptizationofcolloids

p68PurewaterwashingorvolatileelectrolytewashingPracticaltreatmentofcolloidalprecipitates

p68Motherliquordigestion3BPropertiesofprecipitatesandprecipitatingReagents

3B-3Crystallineprecipitatesp68EasilyfilteredandpurifiedthanthecoagulatedcolloidMethodsofimprovingparticlesizeandfilterabilityMinimizeQ:diluting;addingreagentslowly;stirringor/andmaximizingS:heating;adjustingpHDigestion3BPropertiesofprecipitatesandprecipitatingReagents

晶形沉淀的条件：

*稀溶液中沉淀，降低均相成核作用

*热溶液中沉淀，溶解度增大、吸附减少

*搅拌下慢加沉淀剂，降低“局部过浓”现象

*陈化，使更大、更纯，加热、搅拌可缩短陈化时间Digestion陈化晶体↓RS↓Q↑S搅慢pH热稀ControllingParticleSize无定形沉淀的条件：

*无定形沉淀大都溶解度非常小，无法控制过饱和度

*热浓溶液中，加入沉淀剂速度不能慢。防胶体生成，防离子水化，易聚沉

*大量电解质（灼烧时易挥发，NH4Cl、NH4NO3）存在时沉淀，加速聚沉

*趁热过滤，不必陈化Fig.Schematicmodelofthesolid-solutioninterfaceataparticleofAgClinasolutioncontainingexcessAgNO33B-4Coprecipitationp69Atleastfoursoursesofcoprecipitation:Surfaceadsorption;Mixed-crystalformation;Occlusion;andmechanicalentrapmentSurfaceadsorptionp69Methodsforminimizingadsorbedimpuritiesoncolloids:Washing.ReprecipitationMixed-crystalformationp70OneoftheionsinthecrystallatticeisreplacedbyanotherOcclusionandmechanicalentrapmentp71CrystalgrowingrapidlyCrystallieclosetogetherduringgrowthCoprecipitationerrors↑+p713BPropertiesofprecipitatesandprecipitatingReagents

晶形沉淀的形成和纯度沉淀的纯度：

①共沉淀:沉淀生成过程中

＊表面吸附（静电选择吸附，与杂质浓度、杂质价态、温度、表面积有关，可洗涤减少）；p69

＊吸留与包夹(沉淀生长太快，可陈化或重结晶除去)；p71

＊生成混晶（杂质与构晶离子相似，BaSO4-PbSO4;AgCl-AgBr只能先除去杂质离子）p70

②后沉淀：沉淀放置过程中

CuS沉淀中含Zn+杂质离子，放置过程中ZnS析出。沉淀不能久放。3B-4CoprecipitationAvoidingImpuritiesFig.3SchematicmodelofAgClshowingdifferencebetweenbulkandsurfaceatoms.SilverandchlorideionsarenotshowntoscaleAvoidingImpurities

Fig.4Exampleofcoprecipitation:(a)

Mixed-crystalformation,whereCandArepresentthecation-anionpaircomprisingtheanalyteoftheprecipitantand

M

istheimpurity.p70(b)Schematicofanocclusionbyentrapmentofsupernatantsolution.p71(c)

SurfaceadsorptionofexcessC.p693B-5Precipitationfromhomogenoussolutionp713BPropertiesofprecipitatesandprecipitatingReagents均匀沉淀法：p73Table3-1

*通过缓慢的化学反应过程，逐步地、均匀地在溶液中产生沉淀剂，使沉淀在整个溶液中均匀地缓慢地形成，因而生成的沉淀颗粒较大。

*例：CaC2O4生成

在酸性含Ca2+试液中加入过量草酸和尿素，加热，尿素逐渐水解生成NH3和CO2，NH3增加C2O42-量，CO2起搅拌作用，可得到大而纯的CaC2O4颗粒。沉淀粒度的控制3B-6Dryingandignitionofprecipitatesp723BPropertiesofprecipitatesandprecipitatingReagents

WeighingformDryingthePrecipitate

烘干：烘箱，干燥器里恒重。灼烧：麻佛炉，800℃Weight:

分析天平称重。Equipment:Combustion:detectresidues.burner,oven,mufflefurnace,analyticalbalanceConstant-temperature:detectgases.absorbenttraps(closedcontainerpurge-gas)TG:Instrument,electromagneticbalance,electricfurnace.3C-1Timerequiredforagravimetricanalysisp743C-2Sensitivityandaccuracyofgravimetricmethodsp743C-3specificityofgravimetricmethodsp753C-4Equipmentforgravimetricmethodsp75相对误差±0.1%~0.2%；繁琐、费时、不适用于微量、痕量测定。3CAcritiqueofgravimetricmethod

p74typicalgravimetricprocedure:

①quantitativelyprecipitatetheanalytefromsolution；②collecttheprecipitatebyfilteringandwashittoremoveimpurities；③drythesolidinanoventoremovesolvent；④weighthesolidonananalyticalbalance；⑤calculatetheanalyteconcentrationintheoriginalsolutionbasedontheweightoftheprecipitate3D-1Inorganicprecipitatingagents

p76Table3-23D-2Reducingreagentsp77Table3-33D-3Organicprecipitatingagents

8-hydroxyquinolinep77

coordinationconpoundsdimethylglyoximechelatesSodiumtetraphenylboronchelatingagents3D-4Gravimetricorganic-functional-groupanalysisp79Table3-43D-5Volatilizationproceduresp783DApplicationsofgravimetricMethods

VolatilizationGravimetry:Thermallyorchemicallyposeasolidsample.Thevolatileproductscanprovidequantitativeinformation.Thecompleteresidueremainingmaybeweighted.Thermogravimetry:Aformofvolatilizationgravimetryinwhichthechangeinasample’smassismonitoredwhileitisheated.Theproductsofathermalpositioncanbededucedbymonitoringthesample’smassasafunctionofappliedtemperature.Thelossofavolatilegasonthermalpositionisindicatedbyastepinthethermogram.3DApplicationsofgravimetricMethods

225℃lossofH2O;residueisCaC2O4450℃lossofCO;residueisCaCO3700-850℃lossofCO2;

residueisCaOFig.9ThermogramforCaC2O4•H2OThermogram:Agraphshowingchangeinmassasafunctionofappliedtemperature.p73ExamplesofGravimetricLabProceduresGravimetricDeterminationofIron1.

Determineconstantweightofthecrucibles(坩埚）

2.Oxidationofironsample3.Precipitationofironhydroxideandfiltering4.Ignitionofironhydroxidetoironoxide5.Determineconstantweightofthecruciblesplusironoxide6.CalculationofironinthesampleFilteringthePrecipitateFig.6Properprocedureforfilteringsolidsusingfilterpaper.Thefilterpapercirclein(a)isfoldedinhalf(b),andfoldedinhalfagain(c).Thefilterpaperisparted(d),andasmallcorneristornoff(e).The