introduction to cyclic v

7/27/2019 Introduction to Cyclic V

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An Introduction to Cyclic VoltammetryGary A. MabboilDepa rtment of C hemistry and Geology, Clem son University. Clem son. SC 29631

Cvclic voltammetrv has become a nonular tool in the last.fifteen years for stud;ingelectrochemwal reactions. Organicchemists have am lie d the techn iaue to the studv of hiusvn-th,,tic reaction pathways ( I ) and io s tudies of el& tro~h emi -callv eenerated free radicals (2). An increasing numher ofinoig&ic chemists have been using cyclic voltammetry toevaluate the effects of liaands on the oxidation/reductionpotential of the central metal ion in complexesand multinu-clear clusters 131.This type ui info rma t im plays an intrgrnl.part in many of the approac hes direcw d tuwan l snlar energyconvenio n 14) and in model studies of enz)m stic cata ly~ is 5).Knoaled ge o f the electrochemistry of a m etal complex can heuseful in the selection of th e proper oxidizing agent t o put th emetal complex in an interme diate oxidation s tate r f i L Elec-trochemical methoddogy has also heen exploited as a novelme ans of introducing f&tional groups and removing blockingagents (7).Th ere a re several good texts that deal with the theory andpractice of mode rn voltammetry in depth (8-10).Also, manyms tru me nta l analysis textbooks give aver y brief overview ofth e metho d. However, the needs of a researcher interested inapplying the technique for the first t ime are somewhere he-tween the se two extremes. Instructors of instrum enta l analysiswho wan t to teach m odern electrochemistry are faced with asimilar dilemma du e to the lack of suitahle background m a-terial for the stude nts to read. Th is article is intended to helpmee t those needs.The CV Experiment

T he voltage applied to th e "working"electrode is scannedlinearly from a n initial value,Ei, to a predetermined limit, Ehl,(known as th e switching potential) where th e direction of th escan is reversed (see Fig. l a) . Th e operator can halt the scananywhere or let th e instru me nt cycle between E h l and someother preselected value, Eh2. he c urre nt response is plottedas a function of the applied poten tial. Often there is very littledifference between the first cycle and successive scans.However, the changes th at do appear on repetitive cycles areimportant keys to unlocking information about reactionmechanisms (as will he shown later in this paper).Figure l b shows a current-vo ltage curve (or voltammogram)for Fe(C N)r3. As the potential is scanned in th e negative di-rection the curre nt rises to a peak and th en decays in a regularmanner. The current depends on two steps in the overallprocess, the mov emen t of electroactive material to the surfaceand the electron transfer reaction. Th e electron transfer ratecons tant for a reduction process is a function of potential andcan b e described theoretically.

an Fk r = k " enp (%(E - E'O) (1)k o s the stand ard heterogeneous electron-transfer rate con-stant. (Its value is a property of the reaction between theparticular com pound and t he electrode surface used.) T henumber of electrons transferred per molecule is n; is theFaraday; R is the universal gas constant; T is the Kelvintemperature; and En' s the formal reduction potential. (Th eprim e signifies th at th e effect on the free energy of the reac-ta nt s and pro ducts embodied in activity coefficients has beencombined with the thermodynamic reduction potential toform a ter m t hat is directly measurable b ut subject to solution

F l w 1. a)Applied potential program. EA,and Elpare switching potemials.b) Typical cyclic voltsm-am o 1mMKaFe(CN)a 1 a platinum elernode inBQUKWEI 0.1 M KC1 solution.The scan rate was 100mV/sec and the references l ~ c h d e as Ag/AgCI in 0.1 M KCi solutionconditions.) The term ii is known as the transfer coefficient.It arises becauseonly a fractionof th ee ne rm th at is put intothe sy stem (in the form of the app lied potentiall luwers theactivation energy harrier. Its value \,aries from zeru to unity(often -0.51 dep end ~n g n the shape of the i ree energy sur-faces for the reactants and products.'Th e exounmtial denendence oik ro n the annlied potentialaccounts for the steep rise in th e cu rrent. However, the elec-trolvsis of th e reac tant denletes i ts concentration near th esurlace. Since the experiment i i performed at a stationaryelectrode in an unstirred solution, diiiusion is the nrincipalmeans of moving th e rea ctant t o the surface. This relativelyslow mode of mass transport c annot maintain a stea dy-stateconcentration profile i n the region close to the elekrode.Therefore , the depletion zone grows. In a sense, the averagedistance tha t the ieac tant molewles mu st travel to reach thesurface increases. Consequently, the ra te of mass tra ns po rt,decreases. Th e dependence on mass transport, and the fa ctth at a finite rate for the reverse electron transfer process ispossible, prevent the current from increasing exponentially

This Is a sirnplificatlon. For a well-written and much more detailedexplanation of the significance of Me transfer coefficientand symmebyfactors se e pp . 917-929 of ref.(27).Volume 60 Number 9 September 1983 697


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with putential . Eventually th r mass transpor t r t rp hecumrirate determining and thv current reaches ;I maximum. Sincethe nmcen t ra t ion y rad i rn t continues I, , decrease, th r rate oimass tramport wnrlnuep to de rr w ie causing the current I I Idwn y. Heymd the peak the current i, actuall!. ~ lrp en de nt nt i m em d i n d q ~ m d e n t i t h eap p l i ed l x ,t en ti ul . In t h i s d if f u -sion.limitrd region th r current is p rn lx ~ ti m al u 1 -. -.'An ndvnntagc uf the cyclic voltammetry exprriment is thefact that a s~gn ii isnnt mcrn trat lul l of pruduct t in thia casr .the r rduced iurm, has hren gen eratnl uear the electrode onth r forward scan. When th r >can direct ion is nwr . ;ed , thereduced form is oxidized back to the original starting materialand th e curre nt for the reverse process is recorded (see Fig.lh ) . Th e electron transfer rate constant for the reverse (oroxidation) process is similarly controlled by t he a pplied po-tential .(4 )

Determinallon of the Formal Reduction PotentialIt is common oract ice to reoor t th e averaee of the forward~ ~ ~and return pe akpotentials as th e formal rezuction potentialfor the redox couple. This is an approximation tha t is mostaccu rate when th e electron transfer process is reversible andthe diffusion coefficients for the oxidized an d reduced forms

are th e same. If the reaction is reversible, the n th e separationin the peak potentials, AEp,will be close to 58ln mV (at 25C).(Th is relat ionship can be used t o evaluate n.)By reversible, electrochemists mean th at th e reaction is fas tenough to maintain the concentrations of the oxidized andreduced forms in equilibrium with each othe r a t the electrodesurface. T he proper equil ibr ium rat io a t a given potential isdetermined by th e Nernst Equation:

where 0 is the oxidized form a nd R is the reduced form.How fast is fast enough? Many systems l w k reversible whenthe voltage is scanned slowly hut a t higher scan rates Upappe ars greater tha n 58ln mV. Reversihility is, then, a m atterof degree and depends on th e s tress that is applied to thesystem. Matsuda and Ayabe indicated th at for scanning vol-tammetry any deviations from reversible behavior will beimpercep tible if the value of k" (in cmlsec) is greater th an t henumerical value of 0 .3 1 ~ "~where u is the scan rate in V/sed3(11) . Voltammoerams recorded at scan rates UP to 10 Vlsec. "are common. Som e instruments are capable of scanning upto 1000 Vlsec. Therefore, electron transfer reactions with ra tecons tants greater t ha n 1 0 cmlsec will be reversible even in thevery fastest experiments.Redox couples whose peaks shif t far ther apar t with in-creasine scan rate are categorized as auasi-reversible. (Someauth ors merely say irreversible.) Th ere a re some cases in whichthe oeaks are so widelv separated ( k o s 2 X 10-"112 cmlsec)that 'no par ts of the t& peaks overlap on the potential axis a tall. The se ar e generally known as "totally irreversible" sys-tems. A subset of this class are those react ions th at yieldproducts th at c annot be recycled electrochemically to giveback the or iginal reactants ( for example, those tha t involveextensive bond breaking andlor loss of s ubst i tu ents to solu-tion). The se are "chemically irreversible" reactions, an d ma nyvield no return peak a t al l.Another cha r&teristic of reversible systems is th e depen -dence of the peak heigh t on the sq uare root of the scan rate.'At 25C the peak cu rrent is

Th e cu rrent will be in amp eres when A is in cm2, Do is incm'lsec, u is in volts/sec, an d C;i is the bulk concen trat ion inmoleslcm? Th e peak cu rrent for a quasi-reversible system is

not proportiona l to u1l2except when th e pea ks are so widelyseparated th at th e system is more appropriately described astotally irreversible.Althoueh th e averaee of the oeak oo tentia ls can be a eood. .est imate of the E D ' ur a reversible redox react ion, one mustbe careful not to attribu te high accuracy to the determination.In Nicholson a nd Shain's classic pape r on the th eory of vol-tamm etrv a t s tat ionarv electrodes (12) . thev point o ut tha t.th e position of the return peak, even for a reversible systemcan shif t as much as 5 mV dep ending on the choice of theswitching potential, EA. Th e far ther Eh s from Ep, he morenearlv svmm etric about the reduction potential th e peaks will.he., .Alscl t h w work indicates that th cm dp #, in t hetween the,leaks is reitlly all es tim ate ot I.:, l .'* \ E l. 5 H term from pwiarography tha t was given to thepotent%l where the currentis half th e value of tha t on th e current plateau.) Fo rtuna tely,the dif fusion coeff icients have a small effect. Even if D ~ I D R= 2, the error introduced by assuming E1,2 = Eo' ould beonly -9mV for a single electron exchan ge process.Some workers have found th a t the b road po in t a t the topof the c urrent oeak mak es i t dif ficult to determine th e tru e~ ~~~~ ~ .position of the peak precisely. T hey prefer t o read th e po-tential where the cu rrent is half th e value of th e peak cu rrent.Th is half-peak po tential (fo r a reversible redox couple a t25C) is related to th e polarogra phic E1/2 value by t he fol-lowing equation :

28.0EP/2= Ei12f- V (71n(T he sign is positive for a reduction process.)Est imating formal reduction potentials f rom quasi- re-versihle voltammograms is less reliable the fa rthe r the peakseparation deviates from the reversible case. Theoretica l workshows tha t as th e scan rate is increased, slow electron tra nsferkinet ics can m ake the peak potentials shif t in such a m annerthat they are n u lu ng rr s\.m m rt ri i. h ~ u the b.1 . ur the rt.110~cuuple. Th e prublrm is wursr the fart her t hr rl r c tnm t rms ie rc~e i f i c i en t ,I , is from 0.5.A ruugh indlra ti~rll f the rn;~gniruded t h e errn r tha t may he in r ruducrd I I V w r r s g i n r t h e p ai r 01'cathodic and anodic peak potentials for a quasi-reversiblevoltammogram can be der ived f rom graphical dat a preparedby Matsuda and Ayabe ( 1 1 ) .For a n 0-value of 0 .3 and peakseparat ions of 108,312, and 592 mV, the m idpoint betweenth e peaks would be on the negative side of the a ctua l Eli2 valueby 2,4 4, and 104 mV, respectively. (For a n a-v alu e of 0.7 t heerror would have the same magnitude with the oppositesign.)

The m athematics that describe a scanning experiment are quitecom plicated . However, an application of F ick's First Law of Diffusiontells us that lhecurrent at any time is proportional to the concentrationgradient for the reactant:

A 1s the electrode area. 0, s the dnffusoon co ef f~ c~en tf the oxldlzedspecres, 1 is tame, and x s the dlstance from he electrode surface Anerperment in wh ch the voltage 8s stepped Instead of scanned out toa potential where the current is diffusion controlled is much easie r tomodel m athematically. (This is chronoamperometry.) n that case

(aco~ax).=,,,= c&=t (3)where Cb 1s the concentration of the oxld.zed materlal In the bulk so-lullon (on moles/cm'l It seem s reason able, Inen tnat scannmg to thesame voltage would also cause the current to fa 011 asRates of heterogeneous reactions are referenced to unit surf acearea and have units of moi sec-' ~ m - ~ .ince the rate is expressedas the product of kc , the rate constant, k, must have units of cm sec -'wh en concentration is aiven in mol cm-=.~~ ~~ ~' q-atoon (61assumes a planar e ~ec hode . or a spher cal elec tfle(such as a hang ng mercuw drop): I = I,,,,. + (0 52) nFADoColrowhere ro is the radius of the electrode.5E,,2 = P r RTlnF In

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SCAN RATEFigure 4. Ratio of anodic to cathodic (or reverse to forward) peak currents asa function o rate of voltage scan for various elechochemical mechanisms.(Adapted with permission from A M Chm . 36. 706-723 (1964))

I I0.01 0.1 1.0 10 100S C A N RATE

Figwe 5.Rateo shin of potmliil as a functiono scan rate tavariws elemodemechanisms. (Adapted with permission from Anal. Chem.. 38, 706-723

4 shows the general trend tha t the peak curre nt ratio followsas a fun ction of scan rate for each of th e cases involving re-versible electron tran sfer listed in th e table. Also of some di-agnostic value is the rate a t which Epl?shifts as a function ofscan rate. These trends ar e presented m Figure 5. (Reference(12) gives a deta iled developm ent of each if these cases. Pol-cyn a nd Shain also described multistep charge-transfer re-actions where an intermediate oxidation state is formed (16).Treatment of a multi-electron reaction with an interveningchemical step has also been given (17,1 8). Data from othertypes of electrochemical experiments is often needed toevalu ate rate c onsta nts for reactions with coupled chemicalsteps.)Measurem ent of the peak curren t is fairly simple for theforward scan. T h e proper baseline can be obtained by re-cordine the hackmound current for a scan without th e anal*"under the same conditions (same electrolyte, surface pre-trea tme nt. e tc.). T he reverse scan is more comp licated sincethe electrolysis for the forward pro res ~s til luntributes tuthetotal current until the scan ha3 passed the foot of the (forward)wave again. The g enerally accepted approac h is to assume thatthe contribution of the forward process to the total currentcontinues to decrease with the square root of the time duringthe reverse scan. Th e baseline curve for the retu rn scan canbe obtained by stopping th e forward scan at the switchingpotential with the recorder sweeping along the x-axis as afunc tion of time. T his is shown in Figure 6a.A second approach is to stop th e scan at a convenient sp ot(a t least 35mV past E , for the forward scan) and hold thepotential un til the cu rrent is relatively constant. The ap pro-pria te baseline is shown in Figure 6h (13).700 Journal o f Chemical Education

Figure 6.Methods for determining the proper baseline for measurementofthepeak current for the reverse scan. (a)RecMding the signal versus time andstopping lhe first scan at h. b)Using an X- Y recorder and stopping the scanst EL until iapproachesa steady state befwe scanning back. (c ) Parametersused lor calculationof lhe current ratio using Nicholson's method, eqn. (10).

Th ere ar e occasions when neithe r of these two methods isconvenient. Nicholson (19) has indicated that the propercurrent ratio can also be calculated using the followingequation.i - 0.485(ih- +- + 0.086 (8)Lpr 'pr 'PCIn eqn. (8), i is the peak current for the forward process; (id 0is the ab solute curre nt a t the switching potential; and (i,.)~is the uncorrected retu rn peak c urrent measured from thecu rren t axis. (See Fig. 6c.)The re are some system s in which a coupled chemical reac-ti on ~ i e l d slectroactive by-products. In these cases multiplescans can be beneficial. For example, the pro duc t of electrontransfer in the oxidation of aniline is thought to he a freeradical that very rapidly dim erizes (13).

T he p-arninodipheny lamine (111) th at is formed is muchmore easily oxidized at th e applied potential an d a fu rther twoelectron oxidation occurs.


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trodes see references (25) and (261.)A three-electrode system

F gwe 7 C V o f an l ln e at a g l a s s y Carbon e l e m m e m 0 1 M K C I and 0 0 5 M~ O ~ S S I U ~yxogen p h m l a l e 50 1R 10 n w h 4 ) Scan rates 50 m v l s e c . AgIAgC8n 0 1 M K CI reference Donea lone noocates second scan

This last reaction is a reversible process and the reduction stepis observed (Fig. 7) at 0.2 V on the return scan. The oxidationof thep-aminidiphenylamine appears as a new peak on thesecond scan (dotted line). The appearance of the new peak wasan important clue to the identity of the product of the oxi-dation reaction.The Calculation of Formation Constants

The formation constants for complexes in both the oxidizedand reduced forms are useful in evaluating the a-donor anda-acceptor properties of the ligands (20,21). The shif t in theformal reduction potential for the complex from the reductionpotential for the aquo-complex is dependent on the ratio ofthe formation constants for th e reduced and oxidized forms.For example, consider th e reactions below.

Often the formal potential can he evaluated by cyclic vol-tanmetry when one of the complexes is toounstable t o permita direct determination of En' by potentiometric measurementof an equimolar solution of the two forms. If either Kru orK I Ican be evaluated independently (22), then the o ther can becalculated from En' data.Practical Conslderatlons

Most modern equipment uses a three-electrode cell in whicha counter or auxiliary electrode provides the current that isneeded a t the working electrode (23,241. Therefore, virtuallyno current flows through the reference electrode and its po-tential remains constant. (For discussion of reference elec-

close to the working electrode surface. The voltage representedby the product of the current and uncompensated resistance(mainly the solution resistance between the reference andworking electrodes) is wasted and does not appear across theelectrode/solution interface. Since the peak current increaseswith the square root of the scan rate, the voltage error in-creases at the same rate. In cases where the error is of the orderof millivolts, the peaks from even a totally reversible redoxcouple will appear to separate as though the system werequasi-reversible. Therefore the experimenter should be cau-tious about drawing conclusions concerning electron transferkinetics when either the current or uncompensated resistanceis large. Some manufacturers incorporate a positive feedbackcircuit in their equipment to compensate for iR loss.Another source of error in the applied voltage arises fromthe variation in the rates a t which ions diffuse across th e ionbridge separating the reference electrode from the samplesolution. The net difference in the movement of cationscompared to anions results in a charge separation and,therefore an electrochemical potential difference. Thesejunction potentials can be minimized by chwsing supportingelectrolyte ions with similar diffusion coefficients (such as K+and C1-).Mercury working electrodes are limited to the negativepotential range. Platinum and various carbon electrodes arepopular for performing oxidations. However, solid electrodesare susceptible to adsorption, or surface fouling, and surfaceoxidation. For example, platinum forms an oxide film thatshows a large reduction peak near +0.8 V in 0.5 M HzSOaversus the normal hydrogen electrode. Fortunately, a surfacecurrent can be identified easily as such since the peak heightis directly proportional to the scan rate. Often, judicious choiceof solution conditions and electrode pretreatment can mini-mize this problem.In summary, cyclic voltammetry is a convenient tool forol-iainine oualitative information about electron transfersprocesses. It is also a rapid method for obtaining good esti-mates of formal reduction potentials, formation constants and,sometimes, the number of electrons transferred per reactantmolecule and rate constants. if the user is aware of its limita-tions.Acknowledgment

The author gratefully acknowledges the helpful commentsand encouragement of Dr. James C. Fanning and Dr. John D.Petersen in the preparation of this articleLiterature CHed

(11 B0hbit t .J. M.,and Will s ,Joh nP..r l O r # Chrm.45.1978 (19801.(2) Nelaen. Stephen F. , Kessel, Carl R., Bzien, David J., and Weinhold. Frank, J. Org.Chom.,45.2116(198(1) .(3 ) Powers,Michael J.. and Meyer, Thomas J. ,J.Ampr Chem So c , 102,1289 (19W).(4 Kalvanssundaram. K..Kiw i J.Gratre1. M ..Hdu. Chim. Aeto.61.272U (1978) ..isi ~ i i ,. A , an d spenm,J. T., jnorg. c h m , S, 2 ~ 5e n ) .(61 Headridre. J. R.. Eledrachomicsl Techniques for l n o ~ a n ie hemist*." Academic

.....ILL! \lal. uaa. )l aud A v s k . Y . / E k r r , h rm . 5 .194 .J'so.'t U I . ~ ,~ n . H an d 5b n . t , .1 .)no1 ( n r m .x.: nt l 'm ,t l l . A . ln m. . l u p h h . F l a r . r l , ~ m ~ h t ntihd Fl*t rdn ." M n r d Uckker. hr YI rk.!SSP.( 1 4 ) G i l m a n . S . , E l ~ r f r m ~ I .h m . , 2.111 (1967) .(15) Parsons. R.. J. Elsclrmnol. Chem., 21.35 (19691.( 16) Polcyn . D. . s n d S h ain , I . .An al . Ch ~m .38 .37 0 .3 76 1966) .( 17) Nich olmn . R.S . , s n d S h ain , l . .Aw l . Ch em. ,37, 178 (1965).(18) Ssvssnt,J. M.. Elarfrmh im. Ad o, 12 ,753 (1967).(19) Nicholson. R. S..Aw l. Cham., 38.1406 (1966) .(201 K a ch n .C . L a n d T a u h e .C . G . .J. Arn e, . Ch em S m .98 .68 9 ( 1976)

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introduction to cyclic v

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