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Cyclic Voltammetry Experiment James J. Van Benschoten. Jane Y. Lewis, and William R. Heineman University o f Cincinnati. Cincinnati, OH 45221 Daryl A. Roston' and Peter T. Klsslnger Purdue University. W est Lafayene. IN 47907 Cyclic voltammetry (CV) is a very useful modern elec- troanalytical technique; its principles a nd theories have been discussed by Kissinger and Heineman I). Despit e the ad- vantages of the technique a nd its widespread use, there is a lack of CV experim ents in the literature for students. Thus, this experiment has been designed specifically for introducing CV to students who are at the advanced undergraduat e or earlv maduate level. There are three oarts to this exueriment, - each of which may be performed independently. Pa rt I uses the Fel1I(CN)c3-/Fe11(CN)s4- couule as an ex- ample of a well-behave b e~eitrochemica~i~ eversible system in aoueous solution. This exoeriment demonstrates deter- minition of the following: the formal reduction potential (E '): the number of electrons transferred in the redox process in ; he diffusion coefficient D); lectrochemical reveriibility; and the effects of varying concentration and scan rate. Part I1 investigates the electrochemical behavior of acet- aminophen, an ingredient in many over-the-counter phar- maceuticals (e.g., Tylenol*, Excedrin*, Nyquil*, etc.). This organic system is more complex, involving chemical as well as electrochemical reactions. Principles that are illustrated include: effect of a coupled chemical reaction on th e appear- ance of the CV; effect of pH on th e appearance of the CV and on t he mechanism of reaction; use of scan rate for elucidating mechanisti c information; and quantitative d eterminationof acetaminophen in a l'ylenolm tnhlet. P ar t 111-is designed to introduce the student to practical considerations in a cyclic voltammetry experiment. Instead of followine recommended conditions the student now is persuaded to investigate factors ~uch s: th e effect of 2 on the C Y: choosinr the aoorooriate electrode: ~srablishina O- .. . tential $can lim& and studying Pt electrode surface waves. It is honed that. uoon comoletion of this section, the stud ent can eff ktively design a n eiectrochemical experiment for an y particular chemical system of interest. Part I-Fundamentals of Cyclic Voltammetry T h e Fe111(CN)6 -/Fe (CN)$L- couple is kno wn t o be well- behaved bo th chemically and electrochemically (2). As such, it is often used a s a model system in electrochemical experi- ments. It can be used to determine electrode areas and to diagnose pro lems associated with new electrochemical cell designs. In this experiment, the couple is used to clearly demonstrate som e~important rinciples of cyclic voltam- metry. Experimental Reagents. All chemicals are reagent grade and are used assuch . A 10O-ml stock solutio n of apprn xim ateiy 10 mM K3F e(CN k is prepared in M KN03. From this, 25 ml each of approximately 2,4,6, and 8 m oluti ons in 1 M KN03 are made. In addition , a 25-ml K3Fe(C N)6 solution of approximately 4 mM is prepared in 1 M Na zS0 4. All so- lution concentrations must be accurately known. An unknown KsFe(CN)fi olution should be provided by the instructor. Apparatus. The cyclic voltammetry apparatus consists of a po- tentiastat with potential sean capability, a voltmeter (optional), a Current addr ess: Departm ent o f Chem istry. Northern Illinois Uni - ver sity . DeKalb. I L 60115. recorder, and a n elect rochemical cell (I) . The potentiostat used is a Bioanalyticsl Systems, Inc. CV-1R. Alternatively, any number of commercially available instruments, or a homebuilt model con- structed from conventional operational amplifiers can be used (3.4). The current versus potential curves are recorded on a Hewlett- Packard model 7015B XY recorder, As mentioned in reference 1 , the ootential scan varies linearlv with time. thu s it is feasible to use .~ a rrrip chart recorder, even tho&h 11 is not ns wnvenienr as an X) . f an <,rcill~mcope a nvailahlr, t can be used in place of the rerorder. he p~tr ntial utput i s fi~lhwed nd ep en de nt ly w w l t h n Keithley m odel 178 digital voltmeter. A three-electrode system is used for the CV experiment (I). The electrodes reauired are a Pt warkinc electrode area = 2.5 mm2), a Pt auxiliary electrode, and either a A~IA~CI r a saturated calomel electrode (SCEI reference electrode. Al l ootentials in th is exo eriment are referenced veraur. 'E Th e cell used la H~mnalytical ystems. In,. \ ( -2 ylaw wal with a titml top. The u,p ha iour hde s machmed to nerommudate the three rlectn~des nd a tu h for detrxygenating by bubbling w ith a stream of Ns. Procedure. Pretreatment of the platinum worki ng electrode surface may be require d. Simply polishing the surface wit h powdered alum ina and rinsing thoroughly with distilled water should suffice. Th e elec- trode can be sonicated in an ultrasonic bath if available. Th e cel l, as shown i n reference (I ) , s assembled and filled with 1 M KNOJ just until the ends of the electrodes are immersed (Note: avoid tilting the SC E; this may dis turb the HgIHgnCln and result in incorrect potentialsl. The system is deoxygenated by purging with Nz for approx imately 10 mi . Following this, N2 i s allowed to low over the solution to prevent 0 2 rom re-entering the eell for the rem ainder of the experiment. While the system is beingdeoxygenated, the scan parameters can be set. The working electrode should be disconnected or switched off during this procedure. The initial potential is set a t 0 . 80 V, and th e scan limits at 0.80 V and -0.12 V using the reco rder as a m onitor. All scans are initiated in the negative direction with a scan rate of 20 mV/s. These settin gs wil l be used u nless otherwise specified. When deox ygenation is complete, the w orking electrade is switched on. After allowing the cu rre nt to attainae onstant value (in about 10 s , the potential se n is initiated and a background CV of the sup- porting electrolyte solution is obtained. After turn ing off the working electrode, the eell is cleaned and re- filled with 4 mM KaFe(CNI6 in M KNO s. Following the same pro- cedure as above, a CV of the Fe'1L(CN)i /Fe1'(CN)8- couple is ob- tained. The effect of the sweep rate (u) on the voltamm oprams i s observed by using this same soiution and recording CV s a t the following rates: 20,50,75,100,125,150,115,and 200mV/s. Retweeneach scan, initial conditions at the electrode surface are restored by moving the working electrode gently up and down without actually removing it from so- lution. Care should be taken that no buhbles remain o n the elec- trodes. 'wwenrration likewiseaffrcts the magnrtudr of thr penkcurrent. This i seen by uhtainmg cans of 2 6 8 and I d mAf K ,l.'t.tC'Fi~fi ang nswwo rate u f 20 mV ,s A \r,ltarnm oeram of tne unkn<,wn K8nCN)a ~ solution should be ru n as well. Th e affect of the supporting electrolyte on the appearance of the CV isdemonstrated by recordingvoltammogramsof (1) 4 mM ferri- cyanide in 1 M KNOs and (2 ) mM ferricyanide in M Na2S01. Results and Discussion Using the data obta ined, a number of simple calculations c n he done that will yield considerable information about the ferri-ferrocyanide system. Anodic peak current cathodic peak current (iw), E ', and n can be determined as described 772 Journal o f Chemical Education
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Cyclic Voltammetry ExperimentJames J. V a n B e n s ch o t e n. J a n e Y. L e w i s , a n d W il li a m R . H e i n e m a nUniversity of Cincinnati. Cincinnati, OH 4 5 2 2 1D a r y l A. R o s t o n ' and P e t e r T. K l s s l n g e rPurd ue University. W est L afayen e. IN 47907
Cyc l i c vo l t ammet ry (CV) i s a ve ry us e fu l mode rn e lec -t roana ly t i ca l t e chn ique ; i t s p r inc ip le s a nd theor ie s have beend i s cus s ed by Kis s inge r and He ineman I) . D e s p i te t h e a d -van tages o f the t e ch n ique a nd i t s w ides pread us e , the re i s al ack of CV expe r im en t s in th e l i t e ra tu re fo r s tuden t s . T h us ,th i s expe r imen t has been de signed specifically for introducin gC V to s t u d e n t s w h o a r e a t t h e a d v a n ce d u n d e r g ra d u a te o rea r lv ma dua te l evel . T he re a re th ree oa r t s to t h i s exue r imen t ,-each of which ma y be pe rfo rmed ind epend en t ly .P a r t I u s es t h e Fel1I(CN)c3-/Fe11(CN)s4-c o u u l e a s a n e x -a m p l e o f a w e ll -b eh av eb e ~ e i t r o c h e m i c a ~ i ~eve rs ib le s ys temi n a o u e o u s s o l u ti o n. T h i s e x o e r i m e n t d e m o n s t r a t e s d e t e r -min i t ion o f the fo l lowing : the fo rma l reduc t ion po ten t i a l(E ' ): the nu mb er of e lec trons t ransferred in the redox processin ; he diffusion coefficient D);lectrochemical reveriibility;an d th e e f fec t s o f va ry ing concen t ra t ion an d s can ra t e .P a r t I1 i nves t iga te s the e l ec t rochemica l behav io r o f ace t -a m i n o p h e n , a n i n g r e d i e n t in m a n y o v e r - t h e- c o u n te r p h a r -ma ceut icals (e.g., Tylenol*, Excedrin*, Nyquil*, etc.). Th iso rgan ic s ys tem i s more complex , invo lv ing chemica l a s we l la s e l ec t rochemica l reac t ions . Pr inc ip le s th a t a re i l lu s t ra tedinc lude : e f fec t of a coup led chemica l reac t ion on th e appea r -a n c e o f t h e C V ; e ff e ct o f p H o n t h e a p p e a r a n c e of t h e C V a n don t he mechan i s m of react ion ; use o f s can ra t e fo r e luc ida t ingm e c h a n is t ic i n f o rm a t i o n ; a n d q u a n t i t a t iv e d e t e r m i n a t i o n o fa c e t a m i n o p h e n i n a l 'ylenolm tnh let.P a r t 1 11-is d e s i g n e d t o i n t r o d u c e t h e s t u d e n t t o p r a c t ic a lcons ide ra t ions in a cyc li c vo l t am met ry expe r imen t . Ins teadof fo l lowine recommended cond i t ions the s tuden t now i sp e r su a d e d t o i nv e st ig a te f a ct o r s ~ u c hs : th e effec t of 2 o nt h e C Y: c h o o s in r t h e a o o r o o r i a t e e l ec t ro d e: ~ s r a b l i s h i n a O-.. .t e n t i a l $ c an l i m & a n d s t u d y in g Pt elec trode su rface waves .It i s h o n e d t h a t . u o o n c o m o l e t i o n of t h i s s e c t io n , t h e s t u d e n tc a n e f f k t i v e l y d e s ig n a n e i e c tr o c h em i c a l e x p e r i m e n t f o r a n ypa r t i cu la r chem ica l s ys tem of in te re s t .Part I-Fundamenta ls of Cyclic Voltammetry
T h e Fe111(CN)6 -/Fe (CN)$L- cou ple is kno wn t o be well-behave d bo th chemica l ly and e lec trochemica l ly (2) . As s uch ,i t i s o f t en us ed a s a mode l s ys tem in e l ec t rochemica l expe r i -m e n ts . I t c a n b e u s ed t o d e t e r m i n e e le c t ro d e a r e a s a n d tod iagnos e p ro b lems a s s oc ia ted wi th new e lec t rochemica l ce l lde s igns . In th i s expe r imen t , th e coup le i s us ed t o c lea r lyd e m o n s t r a t e s o m e ~ i m p o r t a n t r i n c ip l e s of c y c li c v o l t am -me t ry .Experimental
Reagents. All chemicals are reagent grade and are used assuch . A10O-ml stock solutio n of apprn xim ateiy 10 mM K3F e(CN k ispreparedin M KN 03. From this, 25 ml each of approximately 2,4 ,6 , and 8m olutions in 1 M KN 03 are made. In addition , a 25-ml K3Fe(C N)6solution of approximately 4 mM is prepared in 1 M Na zS0 4.All so-lution concentrations must be accurately known. An unknownKsFe(CN)fi olution should be provided by the instructor.Apparatus. The cyclic voltammetry apparatus consists of a po-tentiastat with potential sean capability, a voltmeter (optional), a
Current addr ess: Dep artm ent of Chem istry. Northern Illinois Uni-versity . DeKalb. IL 60115.
recorder, and a n electrochemical cell ( I) .Th e potentiostat used is aBioanalyticsl Systems, Inc. CV-1R. Alternatively, any number ofcommercially available instruments, or a homebuilt model con-structed from conventional operational amplifiers can be used ( 3 . 4 )The current versus potential curves are recorded on a Hewlett-Packard model 7015B XY recorder, As mentioned in reference 1 ,the ootential scan varies linearlv with time. thu s it is feasible to use.~a rrrip chart recorder, even tho&h 11 is not ns wnvenienr as an X ) .f an
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POTMTIAL. V rr 5CEFigure 1. A) Cyclic voltammogramsof 4 mMKsFe(CN)sn 1 MKNOJ.Pi elec-bade. Scan rate ( 4= 20. 50. 75. 100. 125. 150. 175. end 200 mVls. 8)Plot01 iw YBTSUI v ~and 1 versus uU2 horn voltammogams in (A).
in reference I ) using the voltammogram of 4 m MK3Fe '(CN)6 in M KN03.Th e effect of the scan rate, v, on the appearance of the CVcan be seen in Figure 1A.As described by the Randles-Sevcikequation (I), i and ip increm as u ~. A plot of this equation(Fig. 1B) yields a straight line, the slope of which can be usedto determine th e diffusion coefficient D n cm2/s) of thesystem. Th e area of the working electrode will either have tobe provided by the instructor or experimentally deter-mined.Evidence indicating that the system is electrochemicallyreversible (discussed in reference (I ) ) can he seen in Figure1A as well. T he separation of the peak potentials AEJ sinde~enden tf v. This can he discussed bv the student .
The Kandlrs-Sevcik equation ( 1 )also indicates that i andi are d ir ed v oro~ urti onal o concentration. The effect of.increasing concentration on the appearance of the CV can beseen in Figure 2A. A calibration curve (Fig. 2B) can be con-structe d and used to determine the concentration of the un-known ferricvanide solution.A formal potential as reported in the lit erature is a workingnumber that is dependent on the conditions of t he experiment(5).Th e effect of th e particular supporting electrolyte usedis demonstrated by comparing CV's of K3Fen1(CN)6 n KN0 3and Na2SOa.Part ll-Effect of Coupled Chemlcal React ions
Acetaminophen N-acetyl-p-aminophenol, APAP), theactive ineredient in Tvlenol . is commonlv used as an a s ~ i r i nsubstitute. However, unlike aspirin, it is known to cause liverand kidnev damaee when administered in laree amounts. I tis suspectld tha t :metabolite of APAP is the actual hepato-toxic agent, thus APAP and its metabolites have been ex-tensively investigated (6).
Voltammetric studies in aqueous solution have revealedchemical as well as electrochemical steps 7). The APAPsystem therefore is useful in demons trating the mechanisticinformation th at can he obtained from CV's.Experimental
Reagents. All chemicals are reagent grade and are used as such. Thefallowing supporting electrolyte solutions are needed:500 ml of pH2.2 and 200 mlof DH6 Mcllvaine buffer with an ionicstrenethof 0.6M 8);200 ml of i.8M HSSO~.A stock solution of approximately 0.070MAPAP is prepared in0.05M perehlaric acid. This should be kept in the refrigerator when notin use Using the stock solution, 10-25 ml of an APAP solution isprepared in each of the three supporting electrolyte solutions. Theconcentration of these APAP solutions should be approximately 3mMFor the purpose of establishing a calibration curve, four additionalAPAP solutions in pH 2.2 buffer are needed. These four concentra-
0.8 0 4 0 0 ;-loL IWTENTIAL V vr SCE 2.0 4.0 6.0 6.0 100C O N C E N T R T I O N mM
F i a r e 2. IAl Cvcllc voltammmamof KI F~ C N) . n 1 MKNO*.Scan rate= 20mVls. P elechade. Concentration= 2. 4, 6. 8, and 10mM 8 )Plot of i, versusconcentration and i, vsrsus concentration horn voltammograms in (A).
tions should span the range of 0.10 to 5.0 m M ; 10-25 ml of each areneeded.An unknown solution is prepared by dissolving a Tylenolm tabletin 250 ml of pH 2.2 buffer. A workable concentration is obtained bydiluting a 5-ml aliquot of this solution to 50 ml.All APAP solution concentrations need to be accurately known.Apparatus The apparatus used in this experiment is identical tothat in Part I with the exception of the carbon paste workii electrode(area = 9.1 mm2).Procedure. The carbon oaste electrode is ~rena reds described in~ ~the Hiuanalytirnl Sysrems Inc. manual. Thw elrrtrc,dt must hecarefully parked tu avoid erroneous resulu and rare ;lhould ce mkennot to gauge it once it is ready for use.Using the 3 m M APAP solution in pH 2.2 buffer, scan limits areestablished at 1.0 V and -0.2 V. Scans are initiated in the positivedirection at 0.0 V. Cvclic voltammomams of the 3mM APAP solutionin each of the 3 buffers are then obcined at a scan rate of40 mV/s and250 mV1s. The solution should be stirred between each run.As was demonstrated in Part I, CV's can be used for quantitativeanalysis. A standard curve can be constructed using the series of 5APAP solutions in pH 2.2 buffer. From this, the concentration ofAPAP in the Tylenol solution is determined.Results and Discussion
Th e oxidation mechanism of APAP i s as follows
OHI
APAP11
NAPQI
APAP is electrochemically oxidized in a pH-dependent, 2-electron. 2-proton Drocess to N-acetvl-D-~uinoneimine. .(NAPQI') (step 1). ~ i eccurrence of follow-up chemical re-actions involving NAPQl is pH dependent. Bv varving the pH. . .of th e media and the scan rate o ft he cyclic voltammetry ex-periment, chemical reactions involving NAPQI can bemapped-out. At p H values 26, NAPQI exists in the stableunprotonated form 11).Cyclic voltammograms recorded forN A P at pH 6 are shown in Figure 3.Reasonably well-definedanodic and cathodic waves are evident. The anodic currentrepresents step n the mechanism detailed above while the
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Figure 3 Cyclic voltammogramr of 3 6 m M APAP in pH 6 Mcllvsine buffer.Carbon paste electrodecathodic current represents the reverse of this step. Thesimilarity in appearance of the p H 6 cyclic vo l tamm ~gram sobserved with 40 and 250 mV/s sca n rates indicates th at th einvolved species ar e stable in th e tim e dom ain of the cyclicvoltammetrv exper iment. T he large separat ion between theanodic an dc ath od ic peak currents th e pH cyclic vol-tamm ogram s is a ma nifestation of sluggish electron tran sferkinetics.Under more acidic conditions, NAPQI is immediatelyprotonated (st ep 21, yieldmg a less ita bl eb ut elrctn,cherni-calls a rtivr species (1111 ah ir h rapidlv yields ( ste p ;I) a hy-clrnted fir m IYr that is eler t roch~~mical lvnactiw at the ex-~ ~ - ~ ~ ~ ~amin ed potentials. Cyclic voltammograms shown in Figure4 are consistent with this mechanism. The DH of the mediawas 2.2. A small cathodic wave due t o the reduction of pro-ton ate d NAPQ I (111) is evide nt when t he scan r ate of 250 mV/s. .is employed. This wave is even more pronounced when fasterscan rates a re emnloved: however. faster scan rates reauire th e.use of an oscilloscope to record thk current-potential rksponse.With a slower scan rate of 40 mV/s, a cathodic wave for thereduction of proton ated NAPQI is not observed. All of theproton ated NA PQI (111) is converted to the inactive hyd rate dform (IV) before negative enough potentials are reacheddurinz the reverse scan of the cvclic voltammetrv exoeri-
Hvdrated NAPQI ( IV) conver ts (s tep 4) to benzoquinone;however, the media has to he extreme acidic for thk rate ofthe process to be sienificant enough th at reduction of benzo-qui ne is observed during the cyclic voltammetry experi-ment. The media for the cyclic voltammograms detailed inFigure 5 was 1 8 HzS 04. A poorly defined cathodic wave forth e reduction of benzoquinone (V) was observed when a scanrat e of 250 mV/s was employed. The reduction wave is broadbecause th e formation of benzoquinone (V) from hydratedNAPQ I (IV ) occurs during the reverse scan. When the scanrate is 40 mV/s, th e increased length of time require d to reachnegative enough pote ntials during th e reverse scan allows forthe accumulation of benzoquinone (V). Consequently, awell-defined reduction wave was observed for henzoouinoneV ) when the slower scan raw wnsernployrd.The se rm d scanin thr nositive direction virld3 u ~nodic wave in addition t oth at oE APA P which coriesponds to t he oxidation of hydro-quinone, the reduction prod uct of benzoquinone.Part Ill-Dissolved Oxygen, Potential Limlts, Su rfa ceElfects
When investigating a chemical system for the first tim e withcyclic voltammetry, there are several experimental conditionsthat need to be established. Among these are choosing theappro priate electrode, solvent, and supporting electrolyte; and
E,VOLTS vr epclr o e // 70 4 r 0 . z
Figure4 Cyclic voltammcqramsof 3 6 mMAPAP in pH 2 2 Mcllvainebuffer.Carbon paste electrode
Figwe5. Cyclic wli am wm ms of 3 6 mM APAP in 1 8M W mbm pssteelectrode
determining w hether 2 s an interference. Generally, this isaccomplished by some am oun t of trial an d error. This effortis justified, however, because a s shown in pa rts I an d 11, con-siderable information c n be obtained from a relatively simpleexperiment. Th is section dem onstrate s how these selectionsare ma de, thus giving the stude nt insight on how todesign aneffirirnt V experimrnt for almost anv chemical systtm ofinterest.Experimental
Reagents All chemicals are reagent grade and require no furtherpurification. Solutions of approximately 5 ml1 0M H2SO nd 25ml 1.0 M KNOuare required.A~ o a r o t u s he electrochemical aooa ratus is described in Dart I
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O1 REMOVED
0.4 0.0 -0.4 -0.8 1 . 2POTENTIAL, V vs SCE
Figure 6 Cyclic volternmqlrams ol 1.0MKWsshowingeffectof OZ.Oxygenrernovd by deaxygenatingwilh N2 n 10min. Au electrode. Scan rate 50rnV/s.
.1.6 00 0.0 -08 -1.6
POTPITIAL Y vr SCEF r g ~ e Cyclic voltarnmDgramso 0
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is observed. These w aves have been studied and can be used 14) Kimingcr, P T. n LabcrstoryT ~ h n i q u e an E l e c tr 4y f i c a l ChemhV/. K Ysbge r.P. T nd Heineman. W. K. IEditomI Marrel Dakkar, New York, in pma. Chapt.to determine accurately electrode area 9, 0 . As demon- E.strated in Figure 8 if the potential range is suffici ently imite d, 51 ~ d t h o f f . ~ . . ~ n m a i e c k ,. J . J ~ h y shem . 39.945 1935).a very clean workable background can be obtained. fil Miner. I . .I.. Rice. J. H Rimin. R. M.. Kiasin~or.P. T.. Aml. Chsm.. 53. 2258119811.171 Preddy, C. A . Miner. D. .I. Meinsma. D. A . Kiasinpr, P. T. Anal. Chem., sub--; .-ALiterature Cited
11) Kissinwr. P. T..Heineman, W. R.. 3. CHEM. DUC..EO, 702 119831.21 De An~ e 1 i s .T . ., Heineman, W. R., J CHBM. BDVC..53.94 11976l.(3) Schwan. W . M.,Shain, I. A n d Chem..35.177011963).
..... .8) Riving. P. J., Markoulitz. J M..Rorenthal. .. Ami . Chm~. 28.1179 1956).91 sayer .T..~nlxrta.X..I. ~ . . - ~ ~ ~ ~ i ~ ~ ~ r e ll ~ t m h t ~ i s t ~orchemista. widey.N ow York. 1914. pg. -9.I101 Bsuer. H. H.. El e c tr di e : Modern Ideas Concerninr El rd rd o Reactions. GwrgThismr l 'uhl~sherr ,StutOa~.Gsrmany.972. ~ p .W103.
776 Journal of C hemical Education
