仪器分析英文原版教案(10)

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1、Voltammetry(Chapter25)Electrochemistrytechniquesbasedoncurrent(i)measurementasfunctionofvoltage(Eappl)VoltageSupply-+VariableResistormaxminCellIWorkingCounterElectrodeElectrodeVReferenceElectrodeWorkingelectrode(microelectrode)placewhereredoxoccurssurfaceareafewmm2to

2、limitcurrentflowReferenceelectrodeconstantpotentialreference(SCE)Counterelectrodeinertmaterial(Hg,Pt)playsnopartinredoxbutcompletescircuitSupportingelectrolytealkalimetalsaltdoesnotreactwithelectrodesbuthasconductivityCEM333page12.1Whynotuse2electrodes?OKinpotentiome

3、try-verysmallcurrents.Now,wanttomeasurecurrent(larger=better)but¥potentialdropswhencurrentistakenfromelectrode(IRdrop)¥mustminimizecurrentwithdrawnfromreferenceelectrodesurfacePotentiostat(voltagesource)drivescell¥supplieswhatevervoltageneededbetweenworkingandcounter

4、electrodestomaintainspecificvoltagebetweenworkingandreferenceelectrodeNOTE:¥Almostallcurrentcarriedbetweenworkingandcounterelectrodes¥Voltagemeasuredbetweenworkingandreferenceelectrodes¥Analytedissolvedincellnotatelectrodesurface!CEM333page12.2Excitationsignals(Fig25

5、-2)CEM333page12.3MicroelectrodesC,Au,Pt,Hgeachusefulincertainsolutions/voltagerangesFig25-4At-velimit,oxidationofwater+-2H2O®4H+O2(g)+4eAt+velimit,reductionofwater--2H2O+2e®H2+2OHCEM333page12.4Varieswithmaterial/solutionduetodifferentoverpotentialsOverpotentialhalway

6、sreducestheoreticalcellpotentialwhencurrentisflowingh=Ecurrent-EequilibriumOverpotential(overvoltage)developsasaresultofelectrodepolarization:¥concentrationpolarization-masstransportlimited¥adsorption/desorptionpolarization-rateofsurfaceattach/detachment¥charge-trans

7、ferpolarization-rateofredoxreaction¥reactionpolarization-rateofredoxreactionofintermediateinredoxreactionOverpotentialmeansmustapplygreaterpotentialbeforeredoxchemistryoccursCEM333page12.5Hgparticularlyuseful(i)highoverpotentialat-velimit(ii)easytopreparecleansurface

8、HgMicroelectrodes:(Fig25-3)CEM333page12.6Voltammograms(voltammetricwaves)aregraphsofcurrent(i)vs.appliedvoltage(Eappl)Fig25-5Hgmicr