AnalyticalChemistryDeterminationofthehardnessofwaterbycomplexometrictitration1CaochenchenChendandanBai

huizhenCaijingjingChenjuan4235TheoryLabSuppliesProcedureResultsPurpose16DiscussAnalyticalChemistryAnalyticalChemistryPurposeMasterthetheoryandmethodforthedeterminationofthehardnessofwaterbycomplexometrictitration.3AnalyticalChemistryTheoryThetotalofCa2+

andMg2+

istitratedwithstandardEDTAusinganEriochromeBlackT(EBT)indicator.Duringtitrating,EDTAfirstlycomplexwithCa2+,andthenwithMg2+.Ifthesampledoesnotcontainmagnesium,Mg-EDTAisaddedtothetitrationflasktoprovideasharpendpointwithEBT,sincecalciumdoesnotformasufficientlystrongchelatewiththeindicatortogiveasharpendpoint.4AnalyticalChemistryThefollowingequationsrepresentthereactionofEDTAwithmetalsduringtitration.Titration:

Ca2++H2Y2-

→CaY2+2H+

Ca2++MgY2

→CaY2+Mg2+Endpoint:Mg2++HIn2-

→MgIn-+H+

MgIn-+H2Y2

→MgY2+HIn2-+H

(red)

(colorless)

(colorless)

(blue)Theory6LabSuppliesburet(50mL)pipette

theflask(500mL)conicalflask(250mL)thereagentbottle(500mL)dishsurfaceasmallbeaker(100mL)Apparatus

AnalyticalChemistryEthylenediaminetetraaceticaciddisodiumsalt(EDTAsolid),CaCO3(solid),triethanolamine,1:1NH3·H2O,1:1hydrochloricacid,magnesiumsolution(1gMgSO4·7H2Owasdissolvedinwateranddilutedto200mL),10%NaOHsolution,calciumindicator,Na2Ssolution,NH3-NH4Clbuffersolution(pH=10),chromeblackTindicatorReagents7LabSuppliesAnalyticalChemistryProcedurePreparationof0.01mol/LEDTAstandardsolutionStandardizationof0.01mol/LEDTAstandardsolutionusingcalciumcarbonateastheprimarystandardDeterminationofthehardnessoftapwaterAnalyticalChemistryWeigh1.12gNa2H2Y2H2Oina500-mLbeaker.AddthedeionizedwatertodissolvetheNa2H2Y2H2Ointhebeaker(slightlywarmupthesolution),andthenstorethe0.01mol/LEDTAstandardsolutioninasamplevial.300mL0.01mol/LEDTA9AnalyticalChemistryCoverthebeakerwithawatchglass.Dissolvecalciumcarbonatewithafewdropsof6mol/LHClandmakeuptoabout50.0mlofsolutionbytheadditionofwaterfollowedbyslightlyboilingtoremoveCO2.10AnalyticalChemistryPipette10.0mLcalciumcarbonatestandardsolution.Add20-mLwater,3-mLMg-EDTAsolution,5-mLpH10buffersolutionand4dropsofEBTintothe250mlErlenmeyerflask.Thecolorchangeisfromredtoblueattheendpointoftitration.11AnalyticalChemistryPipette100.0mlofthetapwaterprovidedintoa250mLErlenmeyerflask,towhichaddapproximately5-mLpH10buffersolutionand4dropsofEBTasindicator.Titratethetapwatersamplewiththe0.01mol/LEDTAsolution.12AnalyticalChemistryresultssampleⅠⅡⅢThevolumeofEDTAsolution(mL)10.4110.3110.37TheconcentrationofEDTA(mol/L)0.0096060.0096990.009643Theaverageconcentration(mol/L)0.009649Deviationofsingle

measurement(mol/L)-0.0000430.000050-0.000006Relativeaveragedeviation/%0.34[TheconcentrationofEDTA(mol/L)]13AnalyticalChemistryresultssampleⅠⅡⅢThevolumeofEDTAsolution(mL)8.888.828.92Thetotalhardness(mg/L)85.6885.1086.07Theaveragehardness(mg/L)85.62Deviationofsinglemeasurement(mg/L)-0.08-0.520.45Relativeaveragedeviation/%0.41[hardnessofwater(mg/L)]14AnalyticalChemistryDiscuss15Question:CanphenolphthaleinbeusedasindicatorfortheneutralizationoftheexcessiveHCl（hydrochloric

acid）incalciumcarbonatestandardsolutioninsteadofmethylred?？AnalyticalChemistryDiscussAnswer:No.ThetransitionpHrangeformethylredisfrom4.4to6.2,whilethatforphenolphthaleinisfrom8.2to10.0.AstheNH4Cl（ammonium

chloride）solutionisfaintlyacidic,thetitrationresultshouldrangeatacidicarea.Ifphenolphthaleinisusedasindicatorfortheneutralization,notonlyHCl（hydrochloric

acid）,butalsoNH4+

（Ammoniaions）

willbeneutralized.16AnalyticalChemistryDiscuss17Question:ApartfromEBT,