Anal. Chem. 1985, 57, 1501-1503
ample, values of d = 0.2 mm, a L 2.6 mm, and b L 1cm can be chosen. These values can ensure a limited edge effect with the electrode.
1501
(6) Porter, M. D.; Dong, Shaojung; Gui, Yu-Peng; Kuwana, T.Anal. Chem. 1984, 56, 2263-2265. (7) Lexa, D.; Saveant, J. M.; Zickier, J. J . A m . Chem. SOC. 1977, 99, 2786-2790. (8) Durllat, H.; Comtat, M. Anal. Chem. 1982, 54, 856-861. (9) Watanabe, T.; Honda, K. J . fhys. Chem. 1982, 86, 2617-2619. (10) Truxiiio, L. A,; Davis, D. G. Anal. Chem. 1975, 4 7 , 2260-2267. (11) Rohrbach, D. F.; Deutsch, E.; Heineman, W. R.; Pasternack, R. F. Inorg. Chem. 1977, 16, 2650-2652. (12) Hubbard, A. T.; Anson, F. C. Anal. Chem. 1966, 38,58-61. (13) Yildiz, A.; Kissinger, P. T.; Reiliey, C. N. Anal. Chem. 1968, 40, 1018-1 024.
Registry No. (TPP)Co", 14172-90-8;Pt, 7440-06-4;ferrocene, 102-54-5.
LITERATURE C I T E D (1) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. "Electroanalytical Chemistry"; Bard. A. J., Ed.; Marcel Dekker: New York, 1983; Vol.
13, pp 1-104. (2) Murray, R. W.; Heineman, W. R.; O'Dom, G. W. Anal. Chem. 1967, 39, 1666-1668. (3) Finkiea, H. 0.; Boggess, R. K.; Trogdon, J. W.; Schultz, F. A. Anal. Chem. 1983, 55, 1177-1179. (4) Norris, 6. J.; Meckstroth, M. L.; Heineman, W. R. Anal. Chem. 1976, 48,630-632. (5) Rhodes, R. K.; Kadish, K. M. Anal. Chem. 1981, 53, 1539-1541.
RECEIVED for review November 13, 1984. Accepted January 23, 1985. This work was supported by the National Science Foundation (Grant CHE-8215507), the National Institutes of Health (GM 25172), and the Robert A. Welch Foundation (Grant E-680).
Thin-Layer Spectroelectrochemical Cell D. A. Scherson,* Shantha Sarangapani, and F. L. Urbach* Case Center for Electrochemical Sciences and The Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 The use of an optically transparent thin-layer electrode (OTTLE) has become common practice in spectroscopic studies of electrogenerated species. Several OTTLE designs have been shown to yield satisfactory results in numerous applications in spectral regions ranging from the far-UV to the infrared regions (1, 2). One of the most widely used OTTLE systems ( 3 ) consists of a gold minigrid electrode sandwiched between two standard microscope slides separated by thin spacers of Teflon tape. This basic design is convenient for many studies but must be modified for spectroelectrochemical experiments involving oxygen-sensitive species or utilizing nonaqueous solvents which can attack the epoxy resin used in the cell construction. As part of a study in this laboratory involving the spectral characterization of oxygen-sensitive transition-metal complexes and their redox chemistry, a new optically transparent thin-layer electrochemical cell was designed and constructed. This cell, contained within a standard 20-mm quartz cuvette, allows use of the full spectral range (UV-vis-near-IR) and is impervious to attack by nonaqueous solvents. Furthermore, the cell is conveniently filled under strictly anaerobic conditions which are maintained throughout the spectroelectrochemical experiment. EXPERIMENTAL S E C T I O N Representations of the cell assembly including the delivery system are given in Figure 1. The optically transparent thin-layer electrode is generated within a standard 20 mm path quartz cuvette by a precision machined L-shaped Kel-F piece (C) which holds a quartz plate (H) against one end of the inside of the cuvette. Two strips of Teflon tape (Dilectrix Corp.), approximately 0.015 cm thick and 0.05 cm wide, are attached along the vertical edges of the quartz plate and serve as spacers to fix the effective optical path length. A gold minigrid (100 wires/in., Buckbee Mears Co.) is sandwiched between the quartz plate and the inner wall of the cuvette. A rectangular slot in the vertical arm of C provides an optical path through the 0 " L E . The L-shaped piece (C) and the quartz plate are held tightly against the inner cuvette wall by a second piece (G) (Teflon) on the outside of the cuvette. The piece G contains a slot for the optical path which matches that in C. The piece G is affixed to C in the correct position by means of two brass connector screws (E). Tension screws (D) are
tightened against C to increase the clamping action between the pieces C and G. A brass screw (F) provides electrical connection with the gold minigrid by pressing a small piece of brass into contact with the top of the minigrid which extends above the cuvette. A groove is milled in G to serve as a guiding channel for the brass plate. The horizontal arm of C provides inlets to the side of the optical path for the delivery system (B), the auxiliary and reference electrodes, and a vent for the inert purge gas. A platinum wire serves as the auxiliary electrode and a miniature sodium chloride saturated calomel electrode (SSCE) or potassium chloride saturated calomel electrode (SCE) acts as the reference electrode. The cell is assembled conveniently by holding the L-shaped piece (C) such that the optical path arm is horizontal and placing the quartz plate (H) and the gold minigrid in their appropriate positions. The cuvette is then slid over these assembled components and the piece (G) is connected by the screws (E) and (D). Insertion of the remaining electrodes and inlet tubing completes the assembly. The electrolyte delivery system consists of a cylindrical glass container with two holes drilled in the Teflon cap to accommodate capillary tubing which serves as the inlet (A) for inert purge gas and the outlet (B) for transfer of solution to the cell. When oxygen-sensitive solutions are to be examined, the container is filled within an inert atmosphere box and the capillary tubing is inserted. The filled delivery system is brought out of the inert atmosphere box and connected to the cell and a purge stream of inert gas is passed through the container (with (B) raised) and through the cell. After the entire system is purged, 1-2 mL of solution is transferred to the cell by lowering (B). A slight tipping of the cuvette is sufficient t o cause the optically transparent thin-layer electrode to be completely filled by capillary action. The continuous passage of solvent-saturated inert gas over the solution in the cell is accomplished by simply lowering (A) and raising (B). Spectral measurements were made with a Cary 14 recording spectrophotometer. The electrochemical cell was reproducibly positioned in the sample compartment by means of a specially designed Lucite holder. Electrochemical experiments were performed with a conventional potentiostat (RDE 3, Pine Instruments, Grove City, PA), a PAR Model 175 universal programmer, and a Houston Instruments Model 2000 X-Y recorder. For comparisons of potential values obtained vs. a sodium chloride saturated calomel electrode (SSCE)with literature values reported
0003-2700/85/0357-15OI$Ol.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985 DELIVERY
L-SHAPED HOLDER/COVER (C)
ON
SUPPORT
\
AUXiLiARV” ELECTRODE
REFE/RmcE ELECTRODE SALT BRIDGE
QUARTZ PLATE (H)
-
s 750
500
250
Potential ( m V vs.
0
-250
SSCE)
Figure 2. Cyclic voltammograms of 4 mM K,Fe(CN), in 0.5 M KCI obtained in the optically transparent thin-layer electrochemical cell at scan rates of 1 mV.s-‘ (A) and 2 mV.s-’ (B).
b.
0.7
v0.6 -
Flgure 1. (a) Schematic diagram of the optically transparent thin-layer electrochemical cell and delivery system. (b) Exploded view of the major components of the cell. G1
vs. a potassium chloride saturated calomel electrode (SCE),the following values were used SSCE, 0.236 V vs. NHE; SCE, 0.241 V vs. NHE (4).
RESULTS AND DISCUSSION The reproducibilityof the OTTLE geometry upon repeated assembly was examined by calibration tests to determine the optical path length and volume of the OTTLE. The optical path length of the OTTLE was determined from the absorbance at 373 nm ( E = 4813 M-l cm-l) of a standard chromate solution (5) prepared by dissolving 0.3000 g of K,CrzO, (Baker analyzed reagent) in 1L of 0.05 N KOH to give [Cr042-] = 2.04 X M. Five calibration runs were made and between each run the cell was completely disassembled and the gold minigrid and other components were thoroughly washed and dried with acetone. The average path length determined was 0.0145 cm with an average deviation of =t0.0005cm. The volume of the OTTLE was determined by measuring the charge passed in the controlled potential reduction at 0.0 V vs. SSCE of a solution of 4.00 mM K3Fe(CN), in 0.5 M KC1. The area under the current-time curve was measured after subtraction of the small background current. The average volume of the OTTLE from five runs (with complete disassembly of the cell between runs) was 32 MLwith an average deviation of f2 pL. This degree of reproducibility in OTTLE path length and volume is typical for repeated assembly by the same worker. The reproducibility of these parameters between different workers is not this good and calibrations must be carried out by each worker. Calibration tests indicated that a reproducible OTTLE geometry could be achieved upon repeated assembly and the cell was evaluated for futher electrochemical and spectroelectrochemical measurements. Cyclic voltammograms of the Fe(CN)t-/Fe(CN),4- couple in the thin-layer cell (Figure 2) exhibit some effect from the uncompensated resistance due to the restricted ionic path but the distortion of the voltammograms is not large at these slow scan rates. The areas under the cathodic and anodic waves are nearly identical and the value of Eo’= 0.213 V vs. SSCE is in agreement with literature values (3,6). Similarly, a well-shaped cyclic voltammogram
u c O n
&
0.50.4
-
v, n
a
0.30.20.1
400
360
440 Xhm)
400
520
Flgure 3. Thin-layer spectra of 1.05 mM o-tolidine in 0.5 M acetic a c i d l l . 0 M HClO4, obtained at different values of applied potential. Applied potentials (V vs. SCE) are 0.800 (A), 0.650 (B), 0.640 (C), 0.630 (D), 0.620 (E), 0.610 (F), 0.600 (G),0.590 (H),and 0.440 (I).
of o-tolidine (1.05 mM in 0.5 M acetic acid, 1.0 M HC104)gave a value of Eo’= 0.609 vs. SCE in agreement with a previous report (3). The oxidation of o-tolidine was monitored spectroelectrochemically at nine potentials (Figure 3). The values of the maximum absorbance at 438 nm were used to calculate the ratios of the oxidized and reduced species by the equation log
101 A2 - A , - = log tR1 A3 - A2
where Az is the observed absorbance, A, is the absorbance of the oxidized form, and A , is the absorbance of the reduced yielded a form. A Nernstian plot of log [O]/[R] vs. Eapplied slope of 29.6 mV (n = 1.99) and a value of Eo’= 0.616 V vs. SCE in excellent agreement with literature values (3, 6). The capability of the new thin-layer spectroelectrochemical cell to maintain anaerobic conditions was tested by transferring a 9.0 mM solution of bis(2,2’-bipyridine)copper(II) perchlorate in dimethylformamide into the cell and carrying out a controlled potential reduction to the copper(1) form at -0.300 V vs. SSCE. The cell was returned to open-circuit
1503
Anal. Chern. 1985, 57,1503-1504
condition and the absorbance of the bis(Z,Z'-bipyridine)copper(1) species at 440 nm (t = 4800 M-l cm-l) (7)was monitored with time. The decrease in absorbance at 440 nm was less than 1%after 1 h.
ACKNOWLEDGMENT The authors express their appreciation to Ernest Yeager for fruitful discussions and suggestions, to Keith Schultz of CWRU for his advice and skillful machining during the course of the design and construction, and to Michael R. McDevitt and Suzanne Feke for their assistance in testing the cell.
LITERATURE CITED (1) Rhodes, R.; Kadish, K. M. Anal. Chem. 1981, 53, 1539. (2) Heineman, W. R.; Anderson, C. W.; Halsail, H. 6.;Hurst, M. M.; John-
son, J. M.; Kreishman, G. P.; Norris, B. J.; Simone, M. J.; Su, C.-H. I n "Electrochemical and Spectrochemical Studies of Biological Redox Components"; Kadish, K. M., Ed.; American Chemical Society: Washington, DC, 1982; ACS Advances in Chemistry Series No. 201, Chapter 1. (3) DeAngelis, T. P.; Heineman, W. R. J . Chem. Educ. 1976, 53, 594-597. (4) Ives, D. J. G.; Janz, G. J. "Reference Electrodes"; Academic Press: New York, 1961. (5) Haupt, G. W. J . Res. Natl. Bur. Stand. (US'.) 1952, 48, 414. (6) Koithoff, I. M.; Tomiscek, W. J. J . Phys. Chem. 1935, 39, 945-954. (7) Kitagawa, S.;Munakata, M. Inorg. Chem. 1981, 20,2261-2267.
RECEIVED for review September 20,1984.Accepted January 22,1985.Diamond Shamrock Corp. provided partial support for this work with a Vittorio de Nora Diamond Shamrock Postdoctoral Fellowship to D.S.
Simple Relationship for Calculating Backward to Forward Peak-Current Ratios in Cyclic Voltammetry Gin0 Bontempelli" and Franco Magno
Istituto di Chimica Analitica, Universitci di Padoua, via Marzolo 1, 35100 Padoua, Italy Salvatore Daniele
Dipartimento di Spettroscopia, Elettrochimica e Chimica Fisica, Universitd di Venezia, Dorsoduro 2137, 30123 Venezia, Italy The peak-current ratio (ipb/ipf; b, backward; f, forward) is probably the most significant quantity provided by cyclic voltammetry in that it is in general strongly affected by the nature of the process involved as well as by the values of the parameters characterizing this process. However, its measurement is made difficult by the need to individuate the appropriate base line for ipb(1). To overcome this difficulty, semiempirical relationships have been proposed for the significant but particular cases of reversible electrode processes followed by both first-order (2) and second-order (3) irreversible chemical reactions. In all other cases, the evaluation of the peak current relative to the backward process requires the troublesome estimation of the contribution given at Epb by the forward electrode process. This contribution can be experimentally determined by extending the forward scan beyond the switching potential ( I ) , but the use of this method is precluded when further electrode processes are able to interfere. Otherwise, some mathematical procedures have been proposed for extrapolating the descending branch of the forward wave to the back peak potential, all based on the consideration that the current becomes purely diffusion controlled a t a potential sufficiently past the peak. In particular, a semiempirical method has been presented by Polcyn and Shain ( 4 ) as a part of the multistep charge transfer analysis. I t is however restricted to totally reversible or irreversible electrode processes and, in addition, in the latter case its application requires the knowledge of the charge transfer coefficient a. Subsequently, rather different results have been obtained (5)for these two types of processes as well as for reversible charge transfers followed by irreversible chemical reactions, but the relevant equations have been criticized later (6). Furthermore, the increasing availability of digital circuits allows the extension of the forward voltammetric response to be performed through on-line data treatment by computers (7, 8). The need to study a rather complicated two-step electrode process for which the change of the peak-current ratio with
the scan rate appeared to be, within itself, a diagnostic criterion (9),prompted us to draw a simple and profitable operative equation which makes possible, for any type of electrochemical process involved, the easy and rapid calculation from a single cyclic voltammogram, without of the ratio ipb/ipf the explicit extrapolation of the forward wave into the diffusion controlled region. The purpose of this paper is to report the procedure adopted to derive such equation. It is based on the same approach used also in ref 4 which takes advantage of the fact that the current exhibits typical diffusion controlled decay independent of potential, beginning from a definite potential sufficiently past the peak. This statement implies that when any further potential shift does not change the electrolysis conditions, the current-voltage curve obeys effectively the simple Cottrell equation
i ( t - t?'Iz = const
(1)
where t ' is the hypothetical origin for the current-time curve (in the linear sweep experiment scale) which matches the diffusion part of the stationary electrode voltammogram. In particular, with reference to Figure 1, this equation will be obeyed by the current iA,recorded at the switching potential E,, as well as by the current i, relative to the potential E , chosen in such a way that
E,
- EA =
EA - Epb
(2)
This last current represents the actual contribution of the forward process at E p b and hence it must be added to (ipb)' for a correct computation of the peak-current ratio ipb/ipf
= [(ipb)'
+ ixl/ipf
(3)
Thus, we can write
i x ( t x- t')l12= i,(t, - tq1Iz
(4)
Since experimental curves are recorded in current-voltage coordinates, it is more convenient to replace times with PO-
0003-2700/85/0357-1503$01.50/0@ 1985 American Chemical Society
