I I
Thomas P. DeAngelis and William R. Heineman University of Cincinnati Cincinnati, Ohio 45221
An Electrochemical Experiment Using an Optically Transparent Thin Layer Electrode
The electrochemical experiments in an undergraduate instrumental analvsis course typically involve the classical .. techniques of polarography, n,ulwnetry, and potentiomerry. The exwriment which i i described here was developed for the purpose of demonstrating some of the mnre recent electrorhem~calmethodology. An easily fahrirated optically transnarmt thin laver e l ~ r t r o d etOTTLEl is the hasis of the exr-periment (I). This simple electrode enables the techniques of thin laver electrochemistrv. -.cvclic voltammetrv. controlled potential coulometry, and spectroelectrochemistry to be introduced in one, unified experiment. The OTTLE also provides a unique means for illustrating the Nernst equation. As shown in Figure 1,the OTTLE consists of a transparent gold minigrid electrode sandwiched between two ordinary microscope slides which are separated 0.01 to 0.03 cm by strips of Teflon tape spacers. T o use the cell, the bottom edge is dipped into a small cup containing a few milliliters of the solution to be investigated. Reference and auxiliary electrodes are also immersed in this cup. Solution drawn into the OTTLE by application of suction a t the top corner maintains its level above the minimid bv capillary action. Electrochemical exon the thin layer of solution periments can &en be surrounding the minigrid. The gold minigrid is the basis of the OTTLE. It is an elert n h r m e d wire mesh which is available commercially in a variety of mesh sizes. The minigrid which we suggest for this experiment is 100 wireslin. and 82%transmittant to light. The wires are spaced close enough (see Fig. 1D for dimensions) that the minigrid behaves as a solid gold foil electrode in the thin laver cell. Aonroximatelv 45 OTTLE cells can be made from -,a 6 X &in. piece of 100 wirelin. minigrid costing -$80. An attractive feature of the thin laver techniaue is the speed with which coulometric results can be obtained on small auantities of solution ( 2 4 ) . The solution volume which undergoes elertrolysis is the rhin layer of solution hetween the micrnscooe ilides whirh is defined hv the area oi the minimid. The volime of this "cell" is onlf30-50 $1, and com&ete electrolvsis occurs in 30-60 s. Since rhe minigrirl electrode is trunsparent to light, optiral s ~ e r r r aofthe solurion in the thin cell volume surroundinz the minigrid can be recorded by passing light directly through the minierid as shown in Fieure 1.Spectra of electroactive species in dsferent oxidation slates can be obtained by placing the OTTLE in the samole comoartment of a spectrometer. Spectra are recorded after the-electroactive species has been converted to the desired oxidation state by applying an appropriate potential to the minigrid. In this experiment, the use of the OTTLE for determining formal redox potentials, n values, and spectra of redox couples is illustrated by measurements on solutions of ferricyanide and o-tolidine using a variety of electrochemical techniques
-
A.
Materials and Chemicals The OTTLE is constructed from ordinary microscope slides, 2-mil adhesive Teflon tape spacers (Fluorofilm DF1200 Teflon tape, Dilectrix Corp., Farmingdale, N.Y.), gold minigrid (100 wiresfin., 82% transmittance, Buckbee Mears Co., St. Paul, Minn.), and epoxy. Strips of Teflon tape -2 mm wide are cut and pressed (adhesive side to the glass) along the periphery of two precleaned micruscope slides as shown in Fidure 1. A jectwn of gold minigrid 1 X 3.5 cm is cut and poi~tionedwith tweezers within 3 mm of the bottom edgeof one 594 / Journal of Chemical Education
A
B
C
D
Figure 1. Optically transparent thin layer electrode. (A) Assembly of h e cell. ( 9 )Fmnt view. (0 Side view. (4Dimensions of 100 wireslin. gald minigrid. (a) Point of suctionapplication to change solution.(b) Teflon taw spacers.(c) mC croscope slides (1 X 3 in.). (4solution. (e) transparent gold minigrid elmode. (4optical path of spectrometer. (g)referenceand auxiliary electrcdes. (h)solution cup, and ( 0 epoxy holdingcell together.
microscope slide. Location of the minigrid near the bottom minimizes iR drop in the cell. The second microsco~eslide is then laid on top to form a "sandwich" which is clamped into place. Eooxv andied alone the taoed edaes and allowed to cure at sbodfo; k hr holds the c e i together. The two pieces of minigrid extending from the cell edges are used for electrical contact. Since the minigrid is easily torn, these should be folded over onto the outside of the microscope slide and held in place by a dab of epoxy. After the epoxy dries, a piece of metal foil is folded over the minigrid and the edge of the microscope slides and clamped into place with an alligator clip connected to the potentiostat. The foil protects the minigrid from tearing while providing electricalcontact between~the minigrid and the alligator clip. Spertroelertrorhemiral experiments are performed by nwitimine the Ol"1'1.E in a sr~ectromererso that the liaht beam pass& directly through the minigrid. A spectrometer with a reasonably large sample compartment is desirable. A frame which conveniently suspends the OTTLE above the solution cup and fits into the spectrometer can readily be fabricated from black Lucite. (Plans are available from the author.) The ferricyanide solution for cell calibration is prepared by dissolving sufficient KaFe(CN)6 in 0.5 M KC1 supporting electrolyte to give a concentration of 4.00 X 10-3 M. The 1.00 X 10W3M o-tolidine solution is prepared by dissolving o-tolidine (Eastman Kodak) in a supporting electrolyte of 0.5 M acetic acid, 1.0 M HClOc The o-tolidine should first be dissolved in concentrated acetic acid and diluted almost to volume with distilled water before adding the perchloric acid to avoid solubility problems. Experimental Cyclic Voltammetry
Potential scan techniques such as cyclic voltammetry (2) are exceedingly useful in locating the redox potentials of electroactive species in a thin layer cell, just as they are in conventional electrochemical cell configurations such as are used in polarography. Figure 2 shows a cyclic voltammogram for ferricyanide in the OTTLE. The potential scan was ini-
E ~ *
-2001 05
I 0.4
I 0.3
0.2
E. V
Figure 2.Thin layer cyclic voltammagram of Scan rate 2 mV s-'.
0.1 YS
I
0
Figure 3. Thin layer controlled potential coulamehy. Charge-time curve for potential step 0.4 V-0.00 V-0.4 V versus SCE. 4.00 mM Fe(CNp-. 0.5 M KCI. I -Dl
SCE
4.00 mM Fe(CN)e3-.0.5
MKCI.
tiated a t +0.35 V in the negative direction. T h e onset of cathodic current indicates the one-electron reduction of ferricyanide to ferrocyanide. A rapid drop in current after the peak coincides with the complete electrolysis of ferricyanide in the thin solution layer. Switching the potential scan to the positive direction results in an anodic peak which signifies the reoxidation of ferrocvanide hack to ferricyanide. A formal redox potential for a r&ersihle couple can be determined from the average of the cathodic (EpJ and anodic (EpJ peak potentials.
The redox couple can he cycled indefinitely between the two redox forms by continuous switching of the scan direction. 1) Locate the redox potential for the Fe(cN)~~-/Fe(cN)e~couple by thin layer cyclic voltammetry on the standard Fe(CN)e3solution. Use a scan rate of 1-5 mV s-'. (Scan rates in the OTTLE should be slow to avoid distortion hv iR dron.) 21 Calculate EQ'for ferri-ferroevanide ~, from the ieak . wtentiak ~ . of the cyclic \.oltnmmoyramby eqn. (11.Convert this ralur from t h e S E to the SllF and mmpare with repuned valueiohulined under similar solution conditions (5). ~
~
~
~
~
where Q is charge, C; n is the numher of electrons transferred per molecule; F is Faraday's number, 96.487 C cq-I: I'is the solution volume of the thin layer cell, I; and C is the concentration of electroactive species, M. Figure 3 also shows the effect of stepping the potential back to the initial value of +0.40 V causing reoxidation of the ferrocyanide. Calibration of the cell volume, V, and thickness of the O'ITLE is achieved by potential step coulometry on a solution containing a species, such a s ferricyanide, of known C and n. 1) Determine QT by recording a Q-t curve during a potential step from +0.40-0.00 V versus SCE with the Fe(CN)s3- solution of
known concentration. 2) Determine QB by re~eatinathe ex~erimenton a solution containine ele&olvte. .. onlv~unbor~ne . .. 3) Calculate the volumeof t h thin ~ lavrrcell w i n g rqn. t2r From the area delincd hg the ininwid, ~ ~ i e u l nt the e thieknes d t he thin layer cell. Determination of l? ', n, and Spectra of o-Tolidine Once the cell volume has been calibrated with the standard solution of ferricyanide, the OTTLE can he used to characterize the electrochemistry of an organic compound. A good example is the oxidation of o-tolidine in acidic solution (I).
~
Coulometry Coulometrv in a thin laver cell is generallv. nerformed hv . applying a potential arross the cull which causes complete elenrolwis of the electroactive svecies. Electronic internation of the rkult ing current gi\.es theiota1 charge consumed hy the elerrrode prwess. This total charge, QT, contain* both t l l the Faradaic chargc, QF. due to the electrolvsis of the reartant and ( 2 ) a "hlank" chargc. . Qn, . which runtains contributlms from douhle layer charging and background reactions resulting from oxidationheduction of solvent and electrode (2). Figure 3 shows a charge-time curve in which ferricyanide is reduced to ferrocyanide hy a potential step from +0.4&0.00 V. The charge increases rapidly until all of the ferricyanide in the thin layer is reduced to ferrocyanide a t which time the curve levels off. QT is usually measured by extrapolating the linear portion of the curve to zero time as shown in Figure 3. Q... n is measured from the second charee-time curve which was obtained by repeating the experiment on supporting electrolyte only. The desired quantity, QF, which reflects the amount of reactant electrolyzed, can be calculated by suhtracting QB from QT. The Faradaic charge required for complete electrolysis in the thin layer cell is given by Faraday's law
The EO' of this reversible couvle is easilv determined bv. cvclic " voltammetry and n values very close to 2.0 are obtainable by coulometry. The extensive resonance of the oxidized form results in a bright yellow color which is easily seen in the vicinity of the minigrid. The color is sufficiently intense that excellent spectra (see Fig. 4, curve a) can he obtained even with the short path length of the OTTLE. 1) Assemble the thin layer apparatus in the cell compartment of s spectrometer. Mask the OTTLE with black tape so the light beam passes only through the center of the minigrid. 2) Locate the o-tolidine redox couple by cyclicvoltammetry, and calculate EO'for o-tolidine. Observe the color behavior in the vicinity of the minigrid during reduction and oxidation. 3) Determine QT by controlled potential coulometry. A potential step from +0.80 to +0.30 V versus SCE is suggested. 4) Record spectra of o-talidine in its completely reduced and
completely oxidized forms. Calculate the molar absorptivity, Volume 53.Number 9, September 1976 / 595
Fogwe 4 ihn layer s m a 010 97 mMMoldlne 0 5 Macstlcac a. 1 0 M W I 0 4 lor dMsrsnt va uss ol Ewd Cel thlckneos 0 017 cm (a)0 800. lb10 660. (cl 0 640. Id) 0 620. (ej 0 600. (00 580 lgl 0 400 V versus SCE
-10
0
1.0
log ltOI/IRl)
Figure 5. Plot of EW,,, of oxidized o-tolidineusing Beer's Law and the cell thickness which was measured coulometrically in the previous section. 5) Repeat the potential-stepcoulometry experiment on supporting electrolyte solution to determine QB. 6) Calculate n for a-tolidine by eqn. (2). 7) Postulate the electrode mechanism for o-tolidine oxidation. Nernst Equation: Spectroelectrochemlcal Study A unique spectroelectrochemical approach for examining the Nernst equation is possible with the OTTLE. In an electrochemical cell. the ratio of concentrations of oxidized to reduced forms of the electroactive couple a t the electrode surface is determined bv the ootential which is annlied .. to the electrode as defined bythe equation
versus log ([O]/[R]) from spectra in
Figure 4
r , for ,X,
Experimental Results F~[cN)~'Cyclic voltammetry. E" N = 161
F~IcN).';
o-toliaine
= 0.215
cyclic voltammetry. En' =
v versus SCE
~ s d=, 0.005.
0.608 V versus SCE lrd.
= 0.006.
N=81
coulometry. n = 2.0 ( s d 0 . 1 3 . N = 4) Nernst equation. E" = 00.61 v weerrus SCE a Reported value, reference 5 En' = 0.4581 V verrur SHE Tor 4 X 1 0 ~ 4 M F e ( C N ) 6 3 ? F e ( C N ) ~, 0.5 " M KC1 = 0.216 V versus SCE.
erns st
E,,lid
= Em
[O] + 0.059 -log n PI
(3)
In the case of the OTTLE, the potential applied to the minigrid will determine this concentration ratio in the entire solution comprising the thin layer cell (6).Upon application of a potential, electrolysis rapidly adjusts the ratio [O]/[R] to the value required to satisfy eqn. (3). Figure 4 shows spectra of o-tolidine in an OTTLE for a series of applied potentials. Curve a was recorded after application of +0.800 V, which caused complete oxidation of otolidine ([O]/[R] > 1000). Curve g was recorded after application of +0.400 V, causing complete reduction ([O]/[R] < 0.001). The intermediate spectra correspond to intermediate values of Eapplied. Since the absorbance at 438 nm reflects the amount of o-tolidine in the oxidized form via Beer's Law, the ratio [O]/[R] which corresponds to each value of Eapplied can he calculated from the spectra by eqn. (4). A"-
A.
As shown for curve d in Figure 4, As is the absorbance when o-tolidine is completely oxidized; AI, the absorbance when entirely reduced; and Az, the absorbance for the mixture of oxidized and reduced forms. Figure 5 shows a plot of Eappfinlversus log ([O]/[R]) for the data in Figure 4. The plot is linear as predicted by the Nernst equation. The slope of the plot is 30.8 mV which corresponds to an n value of 1.92, and the intercept is 0.612 Vversus SCE which corresponds to an Eo' of 0.854 V versus SHE. 596 / Journal of Chemical Education
1) Assemhle the thin layer apparatus in the cell compartment of a spectrometer. 2) Apply a potential of +0.800 V versus SCE, and record the spectrum from 325-550 nm after electrolysis has ceased. Repeat this procedure for applied potentials of +0.660, +0.640, +0.620, +0.600, +0.580, and +0.400 V. 3) Plot Eappli,a versus log ([O]/[R]) and determine n and Eo' for the o-tolidine couple. Results and Discussion Variations of this experiment have been used by the author for two vears a t the summer course in "Advanced Instrumental ~ e t h o d in s Electrode Kinetics" a t Ohio State University and in the undergraduate Instrumental Measurements course a t the University of Cincinnati. The experiment has been organized so that the ferri-ferrocvanide c o u ~ l is e used mainly for learning the experimentaitechniques of cyclic voltammetry and controlled potential coulometry and calibrating the OTTLE. The student then characterizes the electrochemical oxidation of o-tolidine hv obtainine EO'. n. and spectra of the oxidized and reduced forms. ~ ~ ~res& i c z which have been obtained are shown in the table. From the coulometrically determined n value of two, students can ~lsuallydeduce the structure of the oxidized form and correlate the intense yellow color with the extended conjugation. The experiment enat~lescomparison of En'and n valum determined t ~ ythe dynamic techniquesof cyclic voltammetry and coulometry with those measured by an equilibrium method involving the Nernst equation. The intense yellow of the oxidized form of o-tolidine illustrates several points. I t visually confirms to the student that
electrolysis in the thin layer cell is indeed confinedto the thin solution layer surrounding the minigrid. The effect of uncompensated iR drop is easily seen since, during a n oxidation, the yellow color initially appears a t the hottom of the grid (closest to the reference probe for minimum solution resistance) and moves to the top. The arrival of the color at the top of the *id coincides with the drop in current on the cyclic voltammogram, signifying complete electrolysis. Probably the most significant aspect of this experiment is techniques and concepts which the variety of are illustrated with the rather simple cell. An added benefit
is the ahsence of mercury as an electrode material with its attendant problems.
Literature Clted (11 MU^, R. ~ ~ i nW.eR.,~and~O ..D O ~G. . w.. ilnol. them., 39. ,666 (19671. (21 Huhbard.A.T..andAnson.~.c.."~heTheoryand~~sctirrof~lectrachorniatrywiUl Thin h y u Cells." in "Eleetroanalytied Cherni8try.l. Val. 4, IEdifor: Bard, A. J.), ~ a r ~de k k e rNEW , ~ o r l r1970. , (31 Hubbsrd, A. T.,CRC critical R ~ SAnal. . Chom., 3. MI (1973). MI " " 1 ~ C. N.. Re". Pule end A P P ~Ch=m.. 18.137 (1968). (51 Kolfhoff, I. M.,and Tomsicek, W. J., J. Phys. Chem., 39.945 (1935). (61 ~eineman.W.a,.N O ~ ~ B.S~ ,. . s n dG O ~ I Z . J . F., A".(. c h m , il7.79 (19751.
w..
Volume 53, Number 9, September 1976 / 597
