4d estimation of detection limits：检出限4d估计

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1、10EA–ACWinterChemistryLecture6February12,2003TofinishupChapter13,let’sspendafewminuteslookingatindicatorsforEDTAtitrations.Thereareover200organiccompoundsthathavebeenusedasindicatorsformetalionsinEDTAtitrations.Ingeneraltheseindicatorsareorganicdyesthatformcolored

2、complexeswithmetalionsinapMrangethatischaracteristicoftheparticularcationanddye.Thecomplexesareoftenintenselycoloredandarediscernibletotheeyeatconcentrationsintherangeof106to10-7M.XXXXXXEriochromeBlackTisatypicalmetalionindicatorthatisusedinthetitrationofseveral

3、commoncations.Asshowninthetransparency,thiscompoundcontainsasulfonicacidgroup,whichiscompletelydissociatedinwater,andtwophenolicgroupsthatonlypartiallydissociate.XXXXXItsbehaviorasaweakacidisdescribedbytheequationsH2In<==>HIn2+H+K1=5x10-7redblueHIn2<==>In3-+H+

4、K2=2.8x1012blueorangeNotethattheacidsandtheirconjugatebaseshavedifferentcolors.Thus,EriochromeBlackTbehavesasanacid/baseindicatoraswellasametalionindicator.ThemetalcomplexesofEriochromeBlackTaregenerallyred.SoisH2Inaswecansee.SoatransitionfromMIn-toH2Indoesnotc

5、hangecolor.HoweverifweadjustthepHto7orabovesothattheindicatorisintheblueformoftheHIn2,thenwecandetectanendpointwhichisredtoblue.Untiltheendpointinatitration,theindicatorcomplexestheexcessmetalion,sothesolutionisred.EDTAhasmuchhighermetalcomplexformationconstantst

6、hantheindicators,soasEDTAisaddedtothesolutionitbindstheavailablemetalionscomplexes.JustbeforetheequilvalencepointEDTAasboundallthefreemetalionsandtakensionsawayfromtheindicatorcomplexes.ThuswhenEDTAhascomplexedallthemetalions,thesolutionturnsblueastheindicatorisco

7、nvertedfromMIn-toH2In.ComplexometrictitrationswithEDTAhavebeenappliedtothedeterminationofvirtuallyeverymetalcationwiththeexceptionofthealkalimetalions.XXXXXX10Let’sreturnforamomenttoanEDTA-calciumtitration.Thistitrationcurvewascalculatedfortheconditionsoftheprobl

8、emwesolvedlastweek.TheequivalencepointisatpCa=6.36.AtitrationcurveformagnesiumisalsoshownunderthesamepH10conditions.Noticeseveralthings.Thecalciumtitrat