EXPERIMENT 8.docx
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EXPERIMENT 8.docx
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EXPERIMENT8
EXPERIMENT8
DeterminationoftheFormulaofaComplexIon
Introduction
Inthisexperimenttheformulaofthecomplexionformedinsolutionfromcopper(II)andEDTAisdeterminedusingspectrophotometricmeasurements.Themethodsemployedarewidelyusedininorganicandanalyticalchemistry.
ConsiderthecasewhereacationM2+reactswithaligandXtoformacoloredcomplexMXn2+:
M2++nXMXn2+
Anexampleisthereactionofcopper(II)ionswithmoleculesofammoniatoformspeciessuchasCu(NH3)42+.LetusassumethatM2+andXarecolorlessandthatthecomplexMXn2+iscolored.Theproblemistoevaluatentodeterminetheformulaofthecomplex.
Themethodofcontinuousvariationsinvolvesmeasuringtheabsorbancesofaseriesofsolutionsofvaryingcompositionatawavelengthatwhichthecomplexshowsitsmaximumabsorptivity.ThetotalnumberofmolesofM2+andXiskeptconstantwhilethemolefractionsofthereactantsarevaried.Theabsorbancesareplottedagainstmolefractionandthevalueofncanbedeterminedfromthemolefractionatwhichmaximumabsorbanceoccurs.InFig.8.1anillustrationisgiveninwhichthepeakoccursatamolefractionof0.75.ThisindicatesthattheformulaofthecomplexisMX32+.ThegraphiscurvednearthepointofmaximumabsorbanceasshowninFigure8.1.Thedeparturefromlinearityisgreaterthelessstablethecomplex.Thestraight-lineportionsofthegraphareextrapolateduntiltheycrosstodeterminethemolefractionatmaximalabsorbance.
Themole-ratiomethodemploysaseriesofsolutionsinwhichtheconcentrationofonereactant,saysM2+,isconstantandthatoftheother,sayX,isvaried.Theabsorbancesareplottedagainsttheratioofthemolesofligandtometal,asshowninFig.8.2.Theabsorbanceincreasestoamaximalvalueatamoleratioof3:
1andthenlevelsoff.Again,thegraphmaybecurvednearthepeakandthestraight-lineportionsmaybeextrapolateduntiltheyintersect.
Themole-ratiomethodandthemethodofcontinuousvariationsaremostreadilyappliedwhenonlyasinglecomplexisformedbetweenthereactingspecies.Copper(II)andEDTAreacttoformadeepblue1:
1complexwhichshowsaflatmaximuminabsorbanceintherange730to755nm.Thereactioninachloroacetatebuffer(chloroaceticacid=HX)canbewritten
CuX2+H2Y2-CuY2-+2HX
bluegreendeepblue
ThecopperispresentasthechloroacetatecomplexinthebufferandthechloroacetateligandisdisplacedbyEDTA,sincethelatterformsamuchmorestablecomplexwithcopperion.
Fig8.1Continuous-variationsplot.
Absorbance
MolefractionX-
Procedure
PreparationofSolutions
(a)Chloroacetatebuffer.Dissolve18.2gofchloroaceticacidin100mLofwaterandadjustthePHtoabout2using5MNaOH.Thendilutethesolutionto250mL.
(b)Cu2+,0.0200M.Weighaccuratelyasampleofcleancopperwireorfoilofabout0.32g(0.3175gforexactly0.0200M),andplaceitina250mLbeaker.Add5mLof1:
1nitricacidanddissolvethecopperbywarmingthesolutiononasteambathoroveralowflame.Add25mLofwaterandboilthesolutionforabout1min.Thenadd5mLofureasolution(1gin20mLofwater)andcontinueboilingforanotherminutetoremoveoxidesofnitrogen.Coolthesolutionintapwaterandneutralizetheacidwith1:
3ammonia,addingtheammoniacarefullyuntilapaleblueprecipitateofcopper(II)hydroxideisobtained.Nowaddglacialaceticaciddropbydropuntiltheprecipitateredissolves.Transferthesolutiontoa250mLvolumetricflask,dilutetothemark,andmixbyinvertingtheflask.
(c)EDTA,0.0200M.Dissolve1.861gofdisodiumdihydrogenEDTAdihydrateinabout100mLofwaterina250mLbeaker.Transferthesolutiontoa250mLvolumetricflaskanddilutetothemark.Mixthesolutionthoroughly.
Absorbance
Ratiomolesligandtomolesmetalion
Fig.8.2Mole-ratioplot
MethodofContinuousVariations.Usinga10mLmeasuringpipet,preparethefollowingmixturesfromtheprecedingsolutions.
Solution
Buffer,mL
Cu2+,mL
EDTA,mL
1
10.0
10.0
0.0
2
10.0
9.0
1.0
3
10.0
7.0
3.0
4
10.0
5.0
5.0
5
10.0
3.0
7.0
6
10.0
1.0
9.0
7
10.0
0.0
10.0
Stirthesolutionsandmeasuretheabsorbanceofeachat730nmusingdistilledwaterasthereference.Forthemeasurementsat730nmtheinfrared-sensitivephototubeandaredfiltershouldbeused.
Sincetheabsorbanceofthechloroacetatecomplexisappreciable(asshownbysolution1),theabsorbancesmeasuredmustbecorrectedtogivetheabsorbanceduetothecopper-EDTAcomplexalone.Thiscorrectionshouldbemadeforsolutions1,2,and3asfollows:
Ac=Am-×A1
whereAmistheabsorbancemeasured,A1istheabsorbanceofsolution1,andAcisthecorrectedabsorbance.NocorrectionneedbemadeforsolutionsinwhichsufficientEDTAhas
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