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ChemWiki:TheDynamicChemistryHypertext>AnalyticalChemistry>AnalyticalChemistry2.0>11:ElectrochemicalMethods>11A:OverviewofElectrochemistry

11A: Overview of ElectrochemistryThefocusofthischapterisonanalyticaltechniquesthatuseameasurementofpotential,charge,orcurrenttodetermineananalytesconcentrationortocharacterizeananalyteschemicalreactivity.Collectivelywecallthisareaofanalyticalchemistryelectrochemistrybecauseitsoriginatedfromthestudyofthemovementofelectronsinanoxidationreductionreaction.Despitethedifferenceininstrumentation,allelectrochemicaltechniquesshareseveralcommonfeatures.Beforeweconsiderindividualexamplesingreaterdetail, lets takeamoment toconsidersomeof thesesimilarities.Asyouwork through thechapter, thisoverviewwillhelpyoufocusonsimilaritiesbetweendifferentelectrochemicalmethodsofanalysis.Youwillfinditeasiertounderstandanewanalyticalmethodwhenyoucanseeitsrelationshiptoothersimilarmethods.

11A.1 Five Important ConceptsTounderstandelectrochemistryweneedtoappreciatefiveimportantandinterrelatedconcepts:1. theelectrodespotentialdeterminestheanalytesformattheelectrodessurface2. theconcentrationofanalyteattheelectrodessurfacemaynotbethesameasitsconcentrationinbulksolution3. inadditiontoanoxidationreductionreaction,theanalytemayparticipateinotherreactions4. currentisameasureoftherateoftheanalytesoxidationorreductionand5. wecannotsimultaneouslycontrolcurrentandpotential.

The Electrodes Potential Determines the Analytes FormInChapter6weintroduced the ladderdiagramasa tool forpredictinghowachange insolutionconditionsaffects thepositionofanequilibriumreaction.Foranoxidationreductionreaction,thepotentialdeterminesthereactionsposition.Figure11.1,forexample,showsaladderdiagramfortheFe3+/Fe2+andtheSn4+/Sn2+equilibria.IfweplaceanelectrodeinasolutionofFe3+andSn4+andadjustitspotentialto+0.500V,Fe3+reducestoFe2+,butSn4+remainsunchanged.

You may wish to review the earlier treatment of oxidationreduction reactions in Section 6D.4 and the development of ladder diagrams foroxidationreductionreactionsinSection6F.3.

Figure11.1RedoxladderdiagramforFe3+/Fe2+andforSn4+/Sn2+redoxcouples.Theareasinblueshowthepotentialrangewheretheoxidized

formsarethepredominatespeciesthereducedformsarethepredominatespeciesintheareasshowninpink.Notethatamorepositivepotentialfavorstheoxidizedforms.Atapotentialof+0.500V(greenarrow)Fe3+reducestoFe2+,butSn4+remainsunchanged.

Interfacial Concentrations May Not Equal Bulk ConcentrationsInChapter6weintroducedtheNernstequation,whichprovidesamathematicalrelationshipbetweentheelectrodespotentialandtheconcentrationsofananalytesoxidizedandreducedformsinsolution.Forexample,theNernstequationforFe3+andFe2+is

E=Eo(RT/nF)log([Fe2+]/[Fe3+])=Eo(0.05916/1)log([Fe2+]/

[Fe3+]11.1

whereEistheelectrodespotentialandEoisthestandardstatereductionpotentialforthereactionFe3+Fe2++e.Becauseitisthepotentialofthe electrode that determines the analytes form at the electrodes surface, the concentration terms in equation 11.1 are those at the electrode'ssurface,nottheconcentrationsinbulksolution.Thisdistinctionbetweensurfaceconcentrationsandbulkconcentrationsisimportant.SupposeweplaceanelectrodeinasolutionofFe3+andfixitspotential at 1.00 V. From the ladder diagram in Figure 11.1, we know that Fe3+ is stable at this potential and, as shown in Figure 11.2a, theconcentration of Fe3+ remains the same at all distances from the electrodes surface. If we change the electrodes potential to +0.500 V, theconcentrationofFe3+attheelectrodessurfacedecreasestoapproximatelyzero.AsshowninFigure11.2b,theconcentrationofFe3+increasesaswemove away from the electrodes surface until it equals the concentration of Fe3+ in bulk solution. The resulting concentration gradient causesadditionalFe3+fromthebulksolutiontodiffusetotheelectrodessurface.

WecallthesolutioncontainingthisconcentrationgradientinFe3+thediffusionlayer.WewillhavemoretosayaboutthisinSection11D.2.

Figure11.2ConcentrationofFe3+asafunctionofdistancefromtheelectrodessurfaceat(a)E=+1.00Vand(b)E=+0.500V.Theelectrodeisshowningrayandthesolutioninblue.

The Analyte May Participate in Other ReactionsFigure11.2showshowtheelectrodespotentialaffectstheconcentrationofFe3+,andhowtheconcentrationofFe3+variesasafunctionofdistancefrom the electrodes surface. The reduction of Fe3+ to Fe2+, which is governed by equation 11.1, may not be the only reaction affecting theconcentrationofFe3+inbulksolutionorattheelectrodessurface.TheadsorptionofFe3+at theelectrodessurfaceortheformationofametalligandcomplexinbulksolution,suchasFe(OH)2+,alsoaffectstheconcentrationofFe3+.

Current is a Measure of RateThereductionofFe3+toFe2+consumesanelectron,whichisdrawnfromtheelectrode.Theoxidationofanotherspecies,perhapsthesolvent,atasecond electrode serves as the source of this electron.The flow of electrons between the electrodes provides ameasurable current.Because thereductionofFe3+toFe2+consumesoneelectron,theflowofelectronsbetweentheelectrodesinotherwords,thecurrentisameasureoftherateof the reduction reaction. One important consequence of this observation is that the current is zerowhen the reaction Fe3+ Fe2+ + e is atequilibrium.

TherateofthereactionFe3+Fe2+e

isthechangeintheconcentrationofFe3+asafunctionoftime.

We Cannot Simultaneously Control Both Current and PotentialIfasolutionofFe3+andFe2+isatequilibrium,thecurrentiszeroandthepotentialisgivenbyequation11.1.Ifwechangethepotentialawayfromits equilibrium position, current flows as the system moves toward its new equilibrium position. Although the initial current is quite large, it

decreases over time reaching zero when the reaction reaches equilibrium. The current, therefore, changes in response to the applied potential.Alternatively,wecanpassafixedcurrentthroughtheelectrochemicalcell,forcingthereductionofFe3+toFe2+.BecausetheconcentrationsofFe3+

andFe2+areconstantlychanging,thepotential,asgivenbyequation11.1,alsochangesovertime.Inshort,ifwechoosetocontrolthepotential,thenwemustaccepttheresultingcurrent,andwemustaccepttheresultingpotentialifwechoosetocontrolthecurrent.

11A.2 Controlling and Measuring Current and PotentialElectrochemicalmeasurementsaremadeinanelectrochemicalcellconsistingoftwoormoreelectrodesandtheelectroniccircuitryforcontrollingandmeasuringthecurrentandthepotential.Inthissectionweintroducethebasiccomponentsofelectrochemicalinstrumentation.The simplest electrochemical celluses twoelectrodes.Thepotentialofoneelectrode is sensitive to theanalytesconcentration, and is called theworking electrode or the indicator electrode. The second electrode,whichwe call the counter electrode, completes the electrical circuit andprovidesareferencepotentialagainstwhichwemeasuretheworkingelectrodespotential.Ideallythecounterelectrodespotentialremainsconstantsothatwecanassigntotheworkingelectrodeanychangeintheoverallcellpotential.Ifthecounterelectrodespotentialisnotconstant,wereplaceitwithtwoelectrodes:areferenceelectrodewhosepotentialremainsconstantandanauxiliaryelectrodethatcompletestheelectricalcircuit.Becausewecannot simultaneouslycontrol thecurrent and thepotential, thereareonly threebasicexperimentaldesigns: (1)wecanmeasure thepotential when the current is zero, (2) we can measure the potential while controlling the current, and (3) we can measure the current whilecontrollingthepotential.EachoftheseexperimentaldesignsreliesonOhmslaw,whichstatesthatacurrent,i,passingthroughanelectricalcircuitofresistance,R,generatesapotential,E.

E=iREachoftheseexperimentaldesignsusesadifferenttypeofinstrument.Tohelpusunderstandhowwecancontrolandmeasurecurrentandpotential,wewill describe these instruments as if the analyst is operating themmanually. To do so the analyst observes a change in the current or thepotentialandmanuallyadjuststheinstrumentssettingstomaintainthedesiredexperimentalconditions.Itisimportanttounderstandthatmodernelectrochemicalinstrumentsprovideanautomated,electronicmeansforcontrollingandmeasuringcurrentandpotential,andthattheydosobyusingverydifferentelectroniccircuitry.

Forfurtherinformationaboutelectrochemicalinstrumentation,seethischaptersadditionalresources.

PotentiometersTomeasure the potential of an electrochemical cell under a condition of zero currentwe use apotentiometer. Figure 11.3 shows a schematicdiagram for amanual potentiometer, consisting of a power supply, an electrochemical cellwith aworking electrode and a counter electrode, anammeterformeasuringthecurrentpassingthroughtheelectrochemicalcell,anadjustable,slidewireresistor,andatapkeyforclosingthecircuitthroughtheelectrochemicalcell.UsingOhmslaw,thecurrentintheupperhalfofthecircuitis

iup=EPS/RabwhereEPSisthepowersupplyspotential,andRabistheresistancebetweenpointsaandboftheslidewireresistor.Inasimilarmanner,thecurrentinthelowerhalfofthecircuitis

ilow=Ecell/RcbwhereEcellisthepotentialdifferencebetweentheworkingelectrodeandthecounterelectrode,andRcbistheresistancebetweenthepointscandboftheslidewireresistor.Wheniup=ilow=0,nocurrentflowsthroughtheammeterandthepotentialoftheelectrochemicalcellis

Ecell=(Rcb/Rab)EPS 11.2

TodetermineEcellwemomentarilypress the tapkeyandobserve thecurrentat theammeter. If thecurrent isnotzero,weadjust the slidewireresistorandremeasurethecurrent,continuingthisprocessuntilthecurrentiszero.Whenthecurrentiszero,weuseequation11.2tocalculateEcell.Usingthetapkeytomomentarilyclosethecircuitcontainingtheelectrochemicalcell,minimizesthecurrentpassingthroughthecellandlimitsthechangeinthecompositionoftheelectrochemicalcell.Forexample,passingacurrentof109Athroughtheelectrochemicalcellfor1schangestheconcentrationsofspecies in thecellbyapproximately1014moles.Modernpotentiometersuseoperationalamplifiers tocreateahighimpedancevoltmetercapableofmeasuringthepotentialwhiledrawingacurrentoflessthan109A.

Figure11.3Schematicdiagramofamanualpotentiometer:CisthecounterelectrodeWistheworkingelectrodeSWisaslidewireresistorTisatapkeyandiisanammeterformeasuringcurrent.

GalvanostatsAgalvanostatallowsus tocontrol thecurrent flowing throughanelectrochemicalcell.Aschematicdiagramofaconstantcurrentgalvanostat isshowninFigure11.4.Thecurrentflowingfromthepowersupplythroughtheworkingelectrodeis

i=EPS/(R+Rcell)whereEPSisthepotentialofthepowersupply,Ristheresistanceoftheresistor,andRcellistheresistanceoftheelectrochemicalcell.IfR>>Rcell,thenthecurrentbetweentheauxiliaryandworkingelectrodesis

i=EPS/RconstantTomonitorthepotentialoftheworkingelectrode,whichchangesasthecompositionoftheelectrochemicalcellchanges,wecanincludeanoptionalreferenceelectrodeandahighimpedancepotentiometer.

Figure11.4Schematicdiagramofagalvanostat:AistheauxiliaryelectrodeWistheworkingelectrodeRisanoptionalreferenceelectrode,Eisahighimpedancepotentiometer,andiisanammeter.Theworkingelectrodeandtheoptionalreferenceelectrodeareconnectedtoaground.

PotentiostatsApotentiostatallowsustocontrol thepotentialof theworkingelectrode.Figure11.5showsaschematicdiagramforamanualpotentiostat.Thepotentialoftheworkingelectrodeismeasuredrelativetoaconstantpotentialreferenceelectrodethatisconnectedtotheworkingelectrodethroughahighimpedance potentiometer. To set the working electrodes potential we adjust the slide wire resistor, which is connected to the auxiliaryelectrode. If theworkingelectrodespotential begins todrift,wecanadjust the slidewire resistor to return thepotential to its initial value.The

current flowingbetween theauxiliaryelectrodeand theworkingelectrode ismeasuredwithanammeter.Modernpotentiostats includewaveformgeneratorsthatallowustoapplyatimedependentpotentialprofile,suchasaseriesofpotentialpulses,totheworkingelectrode.

Figure11.5Schematicdiagramforamanualpotentiostat:WistheworkingelectrodeAistheauxiliaryelectrodeRisthereferenceelectrodeSWisaslidewireresistor,Eisahighimpendancepotentiometerandiisanammeter.

11A.3 Interfacial Electrochemical TechniquesBecausethischapterfocusesoninterfacialelectrochemicaltechniques,letsclassifythemintoseveralcategories.Figure11.6providesoneversionofa family tree highlighting the experimental conditions, the analytical signal, and the corresponding electrochemical techniques. Among theexperimentalconditionsunderourcontrolarethepotentialorthecurrent,andwhetherwestirtheanalytessolution.At the first level,wedivide interfacialelectrochemical techniques into static techniquesanddynamic techniques. Ina static techniquewedonotallow current to pass through the analytes solution. Potentiometry, in which we measure the potential of an electrochemical cell under staticconditions,isoneofthemostimportantquantitativeelectrochemicalmethods,andisdiscussedindetailinsection11B.Dynamic techniques, inwhichwe allow current to flow through the analytes solution, comprise the largest groupof interfacial electrochemicaltechniques.Coulometry,inwhichwemeasurecurrentasafunctionoftime,iscoveredinSection11C.Amperometryandvoltammetry,inwhichwemeasurecurrentasafunctionofafixedorvariablepotential,isthesubjectofSection11D.

Figure11.6Familytreehighlightinganumberofinterfacialelectrochemicaltechniques.Thespecifictechniquesareshowninred,theexperimentalconditionsareshowninblue,andtheanalyticalsignalsareshowningreen.

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Thematerial in this sectionparticularly the five importantconceptsdrawsuponavision forunderstandingelectrochemistryoutlinedbyLarryFaulknerinthearticleUnderstandingElectrochemistry:SomeDistinctiveConcepts,J.Chem.Educ.1983,60,262264.Seealso,Kissinger,P.T.Bott,A.W.ElectrochemistryfortheNonElectrochemist,CurrentSeparations,2002,20:2,5153.11BPotentiometricMethods

ContributorsDavidHarvey(DePauwUniversity)

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