Experiment 5

DETERMINATIONOFSTANDARDREDOXPOTENTIALSFROM

POTENTIOMETRIC TITRATIONSPOTENTIOMETRICTITRATIONS

Formalredox potentialofFe2+/Fe3+

andofCe3+/Ce4+ systemin1MH2SO4

IISC,Bangalore

Oxidation andOxidationandReductionReactions

Oxidationistheprocessofthelossofelectronsfromanatomorion

Fe2+Fe3++e

Reductionistheprocessofthegainofelectronsfromanatomorion

3 2Fe3++eFe2+

Electrode PotentialElectrodePotential

Ifaplatinumwire(sourceofelectron)isdippedintoana p at u e (sou ce o e ect o ) s d pped to aaqueoussolutioncontaining,Fe3+andFe2+ions,withacounteranionpresenttopreserveelectroneutrality.Fe3+(aq)+e (metal) Fe2+(aq)

Iftheequilibriumliestotheleft,favouring theformationofFe3+(aq),inelectrodeaslightnegativechargeandinsolutionaslightpositivecharge,andviceversa(dynamic equilibrium) a charge separation and hence(dynamicequilibrium)achargeseparation,andhenceapotentialdifference,betweenthemetalandthesolutionwillbeobserved.

Nernst EquationNernstEquation

Fe3++ e Fe2+

An electrode potential is established on the metal

Fe +eFe

An electrode potential is established on the metalwire relative to the solution phase. Thecorrelation of an electrode potential and redoxsystem composition (in our case Fe3+/Fe2+ system)can be described by the Nernst equation:

3 2

2

3/

Fe Fe

RT FeE E lnnF Fe

nF Fe

Reference ElectrodeReferenceElectrode

It is not possible to measure the potential of aItisnotpossibletomeasurethepotentialofa

singleelectrodedirectly.

i h l b h ld h i lUsingtheexampleabove,howcouldthepotential

oftheplatinumwiredippingintothesolutionbe

measured?

Electrode potentials must be measured relative toElectrodepotentialsmustbemeasuredrelativeto

somereference.

ReferenceElectrodes

1 Standard Hydrogen Electrode1. StandardHydrogenElectrode

2. Calomel Reference Electrode2. CalomelReferenceElectrode

3. Silver/SilverChlorideReferenceElectrode

Standard Hydrogen ElectrodeStandardHydrogenElectrode

Arbitrarily,astandardhydrogenreferenceelectrode(SHE)y, y g ( )isusedasauniversalstandardandallelectrodepotentialsarequotedrelativetothis.ForexampleinthecaseoftheFe3+/Fe2+couple,

E0(Fe2+/Fe3+)=0.77Vimplyingthatthecell,Pt|H2(g)(p=1)|H+(a=1)||Fe2+(aq)(a=1),Fe3+(aq)( 1) | Pt(a=1)|Pthasanemf of0.77V,withtheplatinumelectrodedippinginto the Fe2+ / Fe3+ solution having a positive potentialintotheFe2+/Fe3+solutionhavingapositivepotential.

SHEH+ + e- = H2E = Eo 2.303RT/F log (PH2)1/2/aH+ Eo = 0 000 V If P = 1 atm E = 2 303RT/F pHEo = 0.000 V. If PH2 = 1 atm., E = -2.303RT/F pH

Hydrogenelectrode.Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

Practical Reference ElectrodesPracticalReferenceElectrodes

InpracticetheSHEisinconvenientforeverydayuse.p y y

Saturated calomel reference electrode (SCE) whichcomprises a mercury electrode coated in mercurychloride in contact with a saturated potassium chloridesolution. The solution contains potassium chloridesolution. The solution contains potassium chloridecrystals to ensure that saturation is maintained and thesystem is connected to the solution in question by afrit allowing ion exchange with this solution Thefrit, allowing ion exchange with this solution. Thepotential determining equilibrium is:

Hg2Cl2(s) + e 2Hg(l) + Cl(aq)Hg2Cl2(s) e 2Hg(l) Cl (aq)

SCEThe S.C.E. is a common reference electrode.

The cell half-reaction is: Hg2Cl2 + e- = Hg + Cl-.

E = Eo 0.0592 log 1/aCl- = 0.242 V for saturated KCl.

G Ch i ti

Commercial saturated calomel electrode.

Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

SCESCE

It can be seen that the potential of theIt can be seen that the potential of thecalomel electrode is dependent only on thechloride ion activity. This is constant as thesolution is saturated with chloride ions. Thepotential of a calomel electrode relative to a

d d h d l d i 0 242V ifstandard hydrogen electrode is +0.242V, so ifmeasurements are made using a calomelelectrode rather than a hydrogen electrodeelectrode rather than a hydrogen electrode,this value must be added to the measuredvoltagevoltage.

/Ag/AgCl Ref.Electrode

A A Cl AgAgCl (satd),KCl (satd)

( ) ( ) ( )AgCl(s)+eAg(s)+Cl(aq)E =0.199V

ElectrodesandPotentiometryPotentiometryUseofElectrodestoMeasureVoltagesthatProvideChemicali f tiinformation

- Variouselectrodeshavebeendesignedtorespondselectivelytospecificanalytes

IndicatorElectrode:electrodethatrespondstoanalyte and

/donates/acceptselectrons

ReferenceElectrode:secondcellataconstantpotential

Cell voltage is difference Cellvoltageisdifferencebetweentheindicatorandreferenceelectrode

ComponentsofaPotentiometric Cell

1. Referenceelectrode

2 Salt bridge2. Saltbridge

3. Analyte

4. Indicatorelectrode

Ecell=EindicatorEreferenceFormostelectrochemicalmeasurementsjunctionpotentialissmallenoughtoignore

Potentiometric Analysis

Reference electrode A halfcell with an accurately knownelectrode potential, Eref, that is independent of theconcentration of the analyte or any other ions in the solution Always treated as the lefthand electrode

Indicator electrode which is immersed in a solution of the Indicator electrode which is immersed in a solution of theanalyte, develops a potential, Eind, that depends on theactivity of the analyte. Is selective in its response

Salt bridge Preventing components of the analyte solutionfrom mixing with those of the reference electrode Apotential develops across the liquid junctions at each end ofpotential develops across the liquid junctions at each end ofthe salt bridge. Potassium chloride is a nearly idealelectrolyte for the salt bridge because the mobilities of theK i d th Cl i lK+ion and the Cl ion are nearly

ElectrodesandPotentiometryO iOverview Potentialchangeonlydependentononecell

concentrations Referenceelectrode isfixedorsaturated doesnt

change!

]Fe[. 05916022200591607710 2 ]Cllog[..]Fe[]Fe[log..Ecell

05916022201

0591607710 3

Referenceelectrode,[Cl]isconstant

Potentialofthecellonlydependson[Fe2+]&

[Fe3+]

Ptwireisindicator

Unknownsolutionof[Fe2+]&[Fe3+]

electrodewhosepotentialrespondsto[Fe2+]/[Fe3+]

Potentiometric Titrations

A potentiometric titration involves measurement ofthe potential of a suitable indicator electrode as afunction of titrant volume.

The measurement is base on the titrant volume The measurement is base on the titrant volumethat causes a rapid change in potential near theequivalence point.

d d h Potentiometric titrations provide data that aremore reliable than data from titrations that usechemical indicators. They are particularly usefuly p ywith colored or turbid solutions and for detectingthe presence of unsuspected species.

Potentiometric Titration Fe2+ is titrated against Ce4+. The potential developed ona platinum electrode immersed in the solution( d i t th RE) i l tt d f ti f(measured against the RE) is plotted as a function ofthe volume of Ce4+ added

The half equations for this redox reaction are as follows:The half equations for this redox reaction are as follows: Fe3+ (aq)+e Fe2+(aq) Ce4+ (aq)+e Ce3+ (aq)( q) ( q)The redox potential of the Ce4+ / Ce3+ couple is muchmore positive than that of the Fe3+ / Fe2+ couple, so ifCe4+ is added to a solution containing Fe2+ effectivelyCe4+ is added to a solution containing Fe2+, effectivelyall of the Ce4+ reacts to oxidize Fe2+ to Fe3+ and theoverall reaction is as follows.

Fe2+ (aq)+Ce4+ (aq)Ce3+(aq)+Fe3+ (aq)

Potentiometric TitrationPotentiometric Titration

In redox titrations the concentration of theInredox titrations,theconcentrationofthesubstancesorionsinvolvedinthereactioncontinuously keeps changing in the course ofcontinuouslykeepschanginginthecourseofthetitration.Hence,theredox potentialofthesolution must also change Measurements ofsolutionmustalsochange.Measurementsofthepotentialofthecellasafunctionofthevolume of Ce4+ added during a titration of Ce4+volumeofCe addedduringatitrationofCeagainstFe2+ willthenleadtoaplotwiththreedistinct sectionsdistinctsections.


Region I Before equivalence pointRegionI Beforeequivalencepoint Inthefirst,alltheCe4+ addedoxidizesFe2+ toFe3+ so no Ce4+ is present in the cell inFe3+ sonoCe4+ ispresentinthecellinsignificantquantity,whilebothFe3+ andFe2+

are present Thus measurements of the emfarepresent.Thus,measurementsoftheemfaremadeontheFe3+/Fe2+ couple,basedonthe cell: Ag AgCl | KCl || Fe3+ Fe2+ | Ptthecell:Ag,AgCl |KCl ||Fe3+,Fe2+ |Pt.

2RT FeE E E l 3 2 3/ refFe FeE E E lnnF Fe


At half neutralization pointAthalfneutralizationpointAthalfneutralizationpoint,[Fe2+]=[Fe3+];then

Henceonecancalculatetheformalpotentialfromthehalfneutralizationpointofthetitrationcurve,ifEref0 isknown

3 2/ refF FE E E

3 2/ refFe Fe


Region II At equivalence pointRegionII Atequivalencepoint thepointatwhichalltheFe2+ hasbeenoxidized to Fe3+ by Ce4+ By definition theoxidizedtoFe3+ byCe4+.Bydefinitiontheequivalencepointoccurswhen:[Fe3+]=[Ce3+]and [Fe2+] = [Ce4+] The potential of the systemand[Fe2+]=[Ce4+].Thepotentialofthesystemcanbegivenbyboth:

4 3

3

4/

2

logeq Ce Ce refRT CeE EnF Ce

E

3 2

2

3/F logeq Fe refe

RT FeE EnF

EFe


Adding and rearranging these two expressionsAddingandrearrangingthesetwoexpressionsandsubstituting[Fe3+]=[Ce3+]and[Fe2+]=[Ce4+] we get:[Ce ],weget:

E E 3 2 4 3/F / 2

Fe e C C refe eeqE

EE

E


Region III After equivalence pointRegionIII AfterequivalencepointInthethirdsection,noFe2+ remainstobeoxidized by the Ce4+ so measurements areoxidizedbytheCe4+,someasurementsaremadeontheCe4+/Ce3+ couplebasedonthecell Ag AgCl | KCl || Ce4+ Ce3+ | PtcellAg,AgCl |KCl ||Ce4+,Ce3+ |Pt.

TheNernstequationforthiscellis:

4 3

3

4/logeq reC C f

RT CE E eE

4 3 4/ geq reCe Ce f enF C


By plotting the redox potential correspondingByplottingtheredox potentialcorrespondingtodifferentpointsinthetitration,atitrationcurve similar to the curve obtained in an acidcurvesimilartothecurveobtainedinanacidbasemethodisobtainedandthisgraphcanbe used to detect different stages of titrationbeusedtodetectdifferentstagesoftitrationandfinallytheformalredox potentialsofFe2+/Fe3+ and of Ce3+/Ce4+ systemFe /Fe andofCe /Ce system.

Experiment 5

Documents

Transcript of Experiment 5