Anal. Chem. 1994,66, 497-502
Long Optical Path, Single Potential Step Chronoabsorptometric Determination of Heterogeneous Electron-Transfer Kinetic Parameters of Quasi-Reversible Processes Yu Zhangyu,’ Guo Tlande,t and Qln Mei Department of Chemistry, Qufu Normal University, Qufu Shandong 273165, P. R. China
A method for the determination of heterogeneous electrontransfer kinetic parameters of quasi-reversible electrode reactions is given. This method is based on chronoabsorptometry in the ultraviolet-visible region, with the aid of a long optical path thin-layer electrochemical cell (LOPTLC), in which the reactant is the only species absorbing at the monitored wavelength. It is an extension of the previously reported single potential step chronoabsorptometric method using optically transparent electrodes. The present method is assessed by using the electroreduction of ferricyanide in 1.0 M KCI at a graphite electrode. The fabrication of a simple cuvette LOPTLC is also described. Such electrode parametersas the formal reduction potential, electron-transfer number, formal heterogeneous electron-transferrate constant, electrochemicaltransfer coefficient, and diffusion coefficients of oxidized and reduced forms are calculated. Spectroelectrochemistry has been developed into an active interdisciplinary field since its inception in 1964.’ UV-visible spectroelectrochemical techniques have facilitated the study of electrochemical elucidated electrochemical reaction mechanisms,3v4and allowed determination of kinetics7 and thermodynamic8parameters of electron-transfer processes. It has been widely used for the study of electrode redox reactions in inorganic, organic, and biochemical systems and the study of electrode surface characteristics. The key part of the thin-layer spectroelectrochemical techniques is the thinlayer spectroelectrochemical cell. Before the 1980s, the most commonly used cells were optically transparent thin-layer electrochemical cells (OTTLEs), which employed optically transparent electrodes (OTES).~In these cells, the optical beam axis is perpendicular to the electrode; thus highsensitivity spectrophotometers must be employed for both short optical path and low solution absorbance. Another disadt Mathematics DepartmentofQufuNormal University,Qufu Shandong 273165, P. R. China. (1) Kuwana, T.; Darlington, R. K.; Leedy, D. W. Anal. Chem. 1964,36,20232025. (2) Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1976,9,241-248. (3) Li, H.; Yu, Z.; Gu, D. Chin. J. Appl. Chem. 1989,6(2). 92-94 (in Chinese). (4) Yu, Z.; Li, H.; Li, C.; Gu, D. Chin. J . Appl. Chem. 1989,6 (3), 59-61 (in Chinese). ( 5 ) Li, H.; Yu, Z.; Li, C.; Gu, D. Acta Phys.-Chim. Sin. 1990,6,735-738 (in Chinese). (6) Bancroft, E. E.; Blount, H. N.; Hawkridge, F. M. Anal. Chem. 1981,53, 1862-1866. (7) Albertson, D. E.; Blount, H. N.; Hawkridge, F. M. Anal. Chem. 1979,51,
556-560. (8) Sailasuta, N.; Anson, F. C.; Gray, H. B. J. Am. Chem. SOC.1979,101,455-
458.
0003-2700/94/0366-0497$04.50/0 Q 1994 Amerlcan Chemical Society
vantage is the limitation of the choice of OTE materials. Most of the OTEs are prepared by depositing a very thin transparent conductive film of metal or metal oxide on a transparent substrate (glass or quart^)^ or sandwiching a metal minigrid between two transparent substrates with Teflon tape spacers.l’)-12 An OTE formed by depositing a thin layer of carbon onto a glass or quartz substrate (C OTE)I3 or by sealing a reticulated vitreous carbon (RVC) slice between two microscope slides with epoxy14 has been reported. A mercurysurfaced OTE has also been prepared by electrodepositing a very thin mercury film onto a metal OTEI5-l8 or C OTE.l3 In recent years, OTTLEs are still used. Reported improvements in cell disigns described the fabrication of high surface area OTEs by depositing a thin gold film on RVC19 and reusable OTTLEs for use with oxygen-sensitivespeciesZoand organic solvents.21-23 In 1983,a novel long optical path thin-layer electrochemical cell (LOPTLC) was pr0posed2~that let the light traverse the cell parallel to the electrode surface. This kind of electrochemical cell greatly improves its optical sensitivity because of the increased optical path and allows the use of any electrode material. These advantages have made LOPTLCs more popular r e ~ e n t l y . ~ ’ -However, ~~ some of the reported LOPTLCs are large, which requires special cell holders, and (9) Hawkridge, F. M.; Kuwana, T. Anal. Chem. 1973,45, 1021-1027. (10) Murray, R. W.; Heineman, W. R.; O’Dom, G. W. Anal. Chem. 1967,39, 1666-1668. (11) Heineman, W. R.; Norris, B. J.; Goelz,J. F. Anal. Chem. 1975,47,79-84. (12) Piljac, L.; Tkalcec, M.; Grabaric. B. Anal. Chem. 1975,47, 1369-1372. (13) DeAngelis,T.P.;Hurst,R. W.;Yacynych,A. M.;Mark,H. B.,Jr.;Heineman, W. R.; Mattson, J. S. Anal. Chem. 1977,49,1395-1398. (14) Norvell, V. E.; Mamantov, G. Anal. Chem. 1977,49,1470-1472. (15) Heineman, W. R.; DeAngelis,T. P.; Goelz, J. F. Anal. Chem. 1975.47.13641369. (16) Meyer, M. L.; DeAngelis, T. P.; Heineman, W. R. Anal. Chem. 1977,49, 602-606. (17) Heineman, W. R.; Kuwana, T. Anal. Chem. 1971,43, 1075-1078. (18) Heineman, W. R.; Kuwana, T. Anal. Chem. 1972,44,1972-1978. (19) Zamponi, S.; Dimarino, M.; Marassi, R.; Czerwinski,A.J. Elecfroanal.Chem. 1988,248,341-348. (20) Pilkington, M. B. G.; Coles, B. A.; Compton, R. G. Anal. Chem. 1989,61, 1787-1789. (21) Nevin, W. A.; Lever, A. B. P. Anal. Chem. 1988,60,727-730. (22) Scherson, D. A.; Sarangapani, S.;Urbach, F. L.Anal. Chem. 1985,57,15011503. (23) Flowers, P. A., Nealy, G.Anal. Chem. 1990,62,2740-2742. (24) Zak, J.; Porter, M. D.; Kuwana, T. Anal. Chem. 1983,55, 2219-2222. (25) Porter, M. D.; Kuwana, T. Anal. Chem. 1984,56,529-534. (26) Gui, Y.; Porter, M. D.; Kuwana, T. Anal. Chem. 1985,57, 1474-1476. (27) Gui, Y.; Kuwana, T. Lmgmuir 1986,2,471-476. (28) Kusu, F.; Kuwana, T. Chem. Lett. 1988,92,5796-5800. (29) Zhang, C., Park, S . Anal. Chem. 1988.60,1639-1642. (30) Fosdick, L. E.; Anderson, J. L. Anal. Chem. 1988,60, 156-162, 163-168. (31) Lin,X.;Liu,D.; Wang,E.AcfaPh~~.-Chim.Sin. 1989,5,719-722(inChinesc). (32)Gui. Y.; Soper, S. A.; Kuwana, T. Anal. Chem. 1988,60, 1645-1648.
AnalytlcalChemistry, Vol. 66,No. 4, February 15, 1994
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some of them are difficult to assemble and use. The need to modify the sample compartment of the spectrophotometer can pose difficulties to other applications of the spectrophotometer. To solve these problems, new LOPTLCs called "cuvette" cells were designed.32 In this paper a long optical path, single potential step chronoabsorptometric (LOPSPS/ CA) method for the determination of heterogeneous electrontransfer kinetic parameters of quasi-reversible processes, along with the fabrication of a simple cuvette LOPTLC, is described and tested with experiments of ferricynide system.
THEORY For the heterogeneous electron-transfer process
The concentration of R in the transform plane is given33e by X
")Doll2
)] (4)
where
Hence by
kr
O+ne+R
erfc( Qt1/2+ 2(Dot)1/2
CR(X--,t)
(1)
kb
where kf and kb are the heterogeneous electron-transfer rate constant for the forward and backward reactions, respectively. If an single potential step chronoabsorptometric (SPS/ CA) method is used to determine heterogeneous electron-transfer kinetic parameters in OTTLEs and LOPTLCs, the basic principles will be different. This is because the direction of the concentration gradient is parallel to the light path in OTTLEs, whereas the concentration gradient is perpendicular to the light path in LOPTLCs. Blount and co-workers6q7 proposed the theory for the SPS/CA determination of heterogeneous electron-transfer kinetic parameters with OTTLEs, which has been successfully applied in our l a b ~ r a t o r y . ~ The theory for the LOPSPS/CA determination of heterogeneous electron-transfer kinetic parameters using LOPTLCs is discussed below. We divide the solution thin layer in the cell into m equal parts running parallel to the electrode suface. If m is large enough, the concentration in every part (the ith part) can be seen as a constant. It is assumed that under study there is only the oxidized form (0)of species existing in solution before the potential step in the electron-transfer process eq 1. If the reactant 0 is the only absorbent in the system at wavelength A, the absorbance of the ith part is then
where b is the sample path length, h is the thickness of the solution thin layer in the cell, to(A) is the molar absorptivity of 0 at A, Cb is the bulk molar concentration of 0, and CR((ih/m),t) is the molar concen tration of electron reaction product R at a distance ih/m from the electrode surface at time t after the potential step. The total absorbance of the system is then
=0
(6)
one can rewrite eq 3 as
exp(Q2t) erfc(Qt'/2) - 1 1 (7) If the magnitude of the potential step applied to the cell is sufficiently large to cause the forward reaction in eq 1 to proceed at a diffusion-controlled rate, then eq 7 becomes
Equation 8 shows that, under the condition of diffusion control, the absorbance has a linear relation with t1I2and DO (the diffusion coefficient of 0)can be deduced from its slope. In comparison with the well-known t h e ~ r i e sthe , ~ normal ~~ absorbance is defined as (9)
Equations 7 and 8 are substituted into eq 9. Hence, by defining = kft1/2/Do112,we have
r
where
From Butler-Volmer equations,33b
and
(3) 490
Analytical Chemistty, Vol. 66,No. 4, February 15, 1994
(33) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980; (a) Chapter 5; (b) Chapter 3; (c) Chapter 6; (d) Chapter 10; (e) Chapter 4.
FRONT VIEW
SIDE VIEW
Flgure 1. Schematic illustrationof the simple thin-layer electrode: (a) graphite plate; (b) Teflon tape spacers; (c) glass plate; (d) Teflon plat.
one can obtain 0. 2
0. 4
0. 0
POTENTIAL,V vs SCE where 9 is the overpotential (7 = E a g p 4 - E O ' ) , a is the electrochemical transfer coefficient, k, is the formal heterogeneous electron-transfer rate constant, and DR is the diffusion coefficient of R. From the above discussion, it is clear that, by substituting in the definition of normalized absorbance, our equation can be transformed into the same AN(h,t) expression suggested by Blount et al. Therefore, no matter which cell is used, either LOPTLC or OTTLE, the method for the determination of kr and a is the same. If (Do/DR)'12is already known, one can get kf by measuring a series of absorbance points AN(h,t) at different overpotentials, and the plot of log kf with respect to 7 should be a straight line, with its intercept being logk:' and its slope being -anF/ 2.303RT, from which kif and a can be obtained. To calculate the value of (Do/DR)'/', the system is kept at a reduction potential where all of 0 are reduced into R, Le., let C i = C;, and then, vice versa, R is converted into 0 at a diffusion-controlled rate. The change in the absorbance Ai(h,t) with t can be described by the following relation
Flgure 2. Cyclic voltammogram of 5.0 mM K3Fe(CN)e In 1.0 M KCI at a graphite thin-layer electrode. Sweep rate, 2 mV/s; temperature, 20 OC.
I
. 350
400
450
500
WAVELENGTH, nm
-
The slope of the Ai(X,t) t112 straight line can be used to get DR, which can then be used to get the value of (Dol DR)l/2.
EXPER I MENTAL SECT1ON Cell Fabrication. A simple cuvette LOPTLC was designed in accordance with the size of an ordinary cuvette with a 0.5-cm optical path length. The working electrode, as shown in Figure 1, is a pice of optically pure graphite plate (Shanghai Graphite Factory). Its height, width, and thickness are about 6,0.5, and 0.4 cm, respectively. The graphite plate was placed in metled paraffin at 100 O C for 4 h to impregnate34and then was polished on filter paper to remove the paraffin from the surface. The whole plate was covered by a thin layer of epoxy resin (Type KD-504A, Shanghai Tiandong Adhesives Factory) except for those areas which are used as the electrode surface and the connecting surface, and it was dried naturally at room temperature for 24 h. T h e electrode surface was (34) SzuCs, A.; Hitchens, G.D.;Bockris, J.O M .J . Electroanal. Chem. 1989,275,
133-148.
Figure 3. Spectra of 5.0 mM KaFe(CN)e in 1.O M KCIat several applied potentials: (a) 0.55, 0.60; (b) 0.24 (c) 0.22; (d) 0.20; (e) 0.18; (f) 0.16; (9) 0.00 -0.10 V vs SCE.
polished first with successively fine polishing paper and then with a piece of soft polishing cloth. A glass plate (-0.5 X 4 cm) cut from a microscope slide was glued to the electrode surface with a Teflon tape spacer (-0.2 X 0.1 X 0.015 cm; Jiangshan Hongxing Electric Device Factory, Ningbo) in each of the four angles between them. The thickness and the volume of the thin layer are 0.15 mm and -30 pL, respectively. A Teflon plate of -0.1-cm thickness was fixed onto the bottom of the graphite plate. An electrode made like this can be tightly inserted into the cuvette. A solution volume of 1 mL is poured into the cuvette, and then the thin-layer electrode and the Pt plate auxiliary electrode are inserted against two sides of the cuvette, respectively. The saturated calomel reference electrode (SCE, Type 232, Shanghai Dianguang Device Factory) is inserted between them. The side of the cuvette to the incident light was covered with a black mask containing a rectangular window of a width equal to or slightly larger than the thickness of the thin layer. The optical beam is thus confined to the
-
AnalflicaIChemistry, Vol. 66, No. 4, February 15, 1994
499
50
40
30
20
10
0
TIME, sec Flgure 6. Plot of A: vs t for LOPSPSICA measurement: A: = 0.069 AU; = 420 nm; 1 = 450 mV.
6
2
4
-2.2-
0
-
T I M E , sec
Flgure 4. Absorbance-time plot for LOPSPSXA measurement. Solution, 5.0 mM K3Fe(CM)BIn 1.0 M KCI, h = 420 nm; 1,(a) -20, (b) =-30, (c) -40, (d) -50, (e) -60, (f) -80, (9) -100, and (h) -450 mV.
4 w -2.60
3
-3.0I
, - 100
I
I
I
- 60
-20
7/mV Flgure 7. Dependence of log k, on overpotential.
2
3
4
5
(TIME ,sec)
Flgure 5. Plot of A: vs tl/* for K3Fe(CN)Bin 1.0 M KCI: A i = 0.069 AU; h = 420 nm; 9 = -450 mV.
solution between the graphite and the glass plates of the thinlayer electrode. Instrumentation and Reagents. Cyclic voltammetry was performed with a PAR Model 363 potentiostat/galvanostat, a Hokuto Dento HB- 104 function generator, and a Yokogawa YEW 3086 x-y recorder. Spectroelectrochemical measurements were made with a Fujian Shanming HDV-7C potentiostat and a Hitachi Model 330 or Beijing WFZ-900D4 spectrophotometer and was connected to an AST 286 computer for the control and data manipulation. All chemicals were of analytical grade. The supporting electrode was 1.O M KCl. Doubly distilled water was used for all preparations. All the potentials reported here are referenced to the SCE. All spectroelectrochemical measurements werecarried out at room temperature, 25(fl) OC. RESULTS AND DISCUSSION The CV current-potential curves were obtained for a solution of 5.0mM Fe(CN)63- in 1.OM KCI at a conventional 500
Analytical Chemistry, Vol. 66, No. 4, February 15, 1994
graphite disk electrode. An increase of scan rate led to an increase of the difference between the oxidation peaks and reduction peaks. This phenomenon preliminarily showed that the electron-transfer process is q u a ~ i - r e v e r s i b l e .Figure ~~~~~~ 2 shows the cyclic voltammogram of this quasi-reversible system at a scan rate of 2 mV/s for the graphite LOPTLC. From this CV curve, it can be determined what potential should be used in the experiments to get some electrode parameters,6 such as the formal electrode potential, E O ’ , electron-transfer number, n, and DO,DR,kit, a,etc. Spectra obtained for the system during a sequence of applied potentials are shown in Figure 3. The wavelength of the maximum absorbance in the spectra is chosen as the characteristic wavelength, i.e., h = 420 nm. At this wavelength, the plot of Eapp1idvs log( [O]/ [R]) is a straight line. From the slope and intercept of the straight line, it can be calculated that n = 0.97, Eo’ = 0.205V vs SCE (SD = i0.002 V, N = 5), which agrees with the one previously reported.’ * Before each potential step, the system was kept at a more positive potential, for example, at 0.55 V vs SCE, to let the thin-layer solution only contain ferricynide. The changes of the absorbance with time at various potential steps are shown in Figure 4, in which curve h is obtained under the condition of diffusion control. The relation betwee Ag(420nm~)and t l / * should be a straight line according to eq 8, but bending is observed at both ends, as shown in Figure 5. This phenomenon was also observed in our previous investigations ( 3 5 ) Anson, F. Electrochemistry and Electroanalytical Chemistry; Beijing University Publishing House: Beijing, 1983; Chapter 1 (in Chinese).
Table 1. Heterogeneous Electron-Transfer Rate Constants for the Single Potentlal Step Reductlon of Ferricyanide at a Graphlte Electrode Evaluated by the LOPSPWCA Technique
kf x 103 (cm/s)O
0
time (s)
-100
-80
-60
-50
-40
-30
-20
1 2 3 4 5 6 mean
5.38 5.24 5.85 5.74 5.71 5.63 5.59
3.79 3.33 3.37 3.32 3.22 3.31 3.39
2.98 2.69 2.53 2.39 2.24 2.17 2.50
2.76 2.35 2.05 1.94 1.78 1.67 2.09
2.12 1.80 1.56 1.46 1.33 1.23 1.58
1.77 1.44 1.23 1.10 1.01 1.00 1.26
1.14 1.08 0.92 0.84 0.76 0.75 0.92
For 7 of -100 to -20 mV.
Table 2. Heterogeneous Electron-Transfer Kinetic Data for the Reductlon of Ferrlcyanlde
electrode graphite carbon film
boron carbide glassy carbon
technique
solution conditions
long optical path spectroelectrochemistry rotated disk electrode voltammetry pulsed rotation voltammetry
5.0 mM K3Fe(CN)e in 1.0 M KC1
6.5(&0.4)X 10-4
0.56(&0.06)
a
0.1 mM KaFe(CN)e in 0.1 M phosphate, pH 7.5 20 MMeach of &Fe(CN)e and Kze(CN)6 in 0.1 M KC1 0.1 mM K3Fe(CN)6in 0.1 M phosphate, pH 7.5 10 pM &Fe(CN)e in 0.1 M phosphate, pH 7.5 2.0 rM each of &Fe(CN)e and K.$'e(CN)e in 0.1 M KCl 0.1 mM K$e(CN)e in 0.1 M DhosDhate. Dh 7.5 O.imM*eachb'f KsFe(CN)e and &Fe(CN)e in 0.1 M KCl 2.0 mM &Fe(CN)6 and 0.5 mM &Fe(CN)e in 1.0 M KN03 0.1 mM K3Fe(CN)e in 0.1 M phosphate, pH 7.5 0.1 mM &Fe(CN)e in 0.1 M KF
4.9(f0.2)
0.201(f0.04)
43
0.465(*0.08)
44bvc
8.2(&0.3)X 10-4
0.296(*0.003)
43
4.7(10.7) X 103
0.23(f0.03)
45
1.22(f0.11) x 10-2
0.685(&0.04)
44c
6.5(&1.5)X 103
0.38(f0.03)
43
0.378(&0.07)
44
0.49
46
0.49(f0.02)
43
0.45(*0.03)
43
rotated disk electrode voltammetry pulsed rotation voltammetry pulsed rotation voltammetry turbulent tubular electrode voltammetry pulsed rotation voltammetry
platinum
faradaic rectification turbulent tubular electrode voltammetry turbulent tubular electrode voltammetry
gold
a
k: (cm/s) X
1.58(f0.29)
2.63(f0.24)
a
10-4 X
X
109
le3
10.5 X 7(f1)
X
103
7.0(&0.6) X 103
ref
*
This work. Pyrolytic carbon film. Electrochemical pretreatment of f 4 V at 70 Hz, 10-15 s.
for the reductions of [Fe(SCN)3(H20)3]( 18-C-6) in nonaqueous solvent^.^ It can be obtained from the slope of the AE(420nm,t) t1/2linear relation that diffusion coefficient DO = 7.3(f0.5) X 10" cm2/s, which is in agreement with value36 of 7.65 X 10" cm2/s. If, at constant applied potential -0.25 V vs SCE, ferricynide in the solution thin-layer is reduced completely to ferrocynide, i.e., the absorbance at 420 mm reaches a steady value, then the reverse reation is conducted under the condition of diffusion control; the absorbance-time behavior can be obtained as shown in Figure 6. If Ai(420nm,t) is plotted against ?'I2, the slope of its linear portion gives the diffusion coefficient DR = 6.4(f0.5) X 10" cm2/s according to eq 15, which is also in agreement with the value36 of 6.50 X 10" cm2/s. The values of the heterogeneous electron-transfer kinetic parameters were calculated with a program written in this laboratory. The program, which includes two parts-working curves and heterogeneous electron-transfer kinetic parameters, is available and was used in our previous work.5 For data analysis, working curves were used which are analogous to those shown in Figure 1 of ref 6 but constructed for the experimental overpotential employed from our program of the working curves. From Figure 4 and eq 9, the values of
-
(36) Von stackelberg, M.; Pilgram, M.; Too-2. 350.
Elekrrochem. 1953,57,342-
A~(420nm,t)are obtained at intervals of 1 s in a time range from 1 to 6 s at various overpotentials. The values of kr calculated with our program of the heterogeneous electrontransfer kinetic parameters are shown in Table 1 . Using the average number, the plot of log kfagainst is a straight line shown in Figure 7. Data analyses of experiments done five times afford the values of a and ki' tabulated in Table 2. A number of workers have investigated the ferri-/ferrocyanide oxidation-reduction system.6p7*374 Their results indicate that the heterogeneous electron-transfer kinetics depend on electrode material, pretreatment of electrode surface, and solution condition (cf. Table 2). Representative results about the oxidation of ferrocyanide have been summari~ed,~ and some about the reduction of ferricyanide are included in Table 2. The CV curve shown in Figure 2 is similar to those previously reportedl3,l4~*9.22in some ways. A half-wave (37) Elving, P. J.; Smith, D. L. Anal. Chem. 1960, 32, 1849-1854. (38) Thornton, D. C.; Corby, K. T.; Spendel, V. A,; Jordan, J.; Robbat, A., Jr.; Rutstrom, D. J.; Gross, M.; Ritzler, G. Anal. Chem. 1985, 57, 150-155. (39) Beilby, A. L.; Brooks, W., Jr.; Lawrence, G. L. Anal. Chem. 1964,36,22-26. (40) Hanania, G. I. H.; Imine, D. H.; Eaton, W. A,; George, P. J . Phys. Chem. 1976, 71,2022-2030. (41) Galus, B. Z.;Adams, R. N. J . Phys. Chem. 1963, 67, 866-871. (42) Bindra, P.; Gerischer, H.; Peter, L. M. J . Electroanul. Chem. 1974.57.435438. (43) Blaedel, W. J.; Schieffer, G. W. J . Electrounal. Chem. 1977, 80, 259-271. (44) Blaedel, W. J.; Mabbott, G. A. Anal. Chem. 1978, 50, 933-936. (45) Blaedel, W. J.; Engstrom, R. C. Anal. Chem. 1978.50, 476-479. (46) Agarwal, H. P.; Qureshi, S. J . Electroanal. Chem. 1977, 75, 697-703.
Analytical Chemistry, Vol. 66, No. 4, February 15, 1994
501
potential of 0.219 V vs SCE and a peak separation of 58 mV are calculated from Figure 2. The half-wave potential, E1/2, is approximately equal to the formal electrode potential for reversible and some quasi-reversible processes. Ell2 or Eo’ is a function of the temperature. For the ferri-/ferrocyanide couple, the temperature coefficient, dEo’/dT, is -2.43 mV/K determined at a glassy carbon electrode by the optical fiber spectroelectrochemical technique4’ and -2.69(f0.04) mV/K determined at a platin’um electrode by the emf method.40These show that Eo’ is close to E1/2 in this paper. The cyclic voltammetric behavior (Figure 2) indicates poor thin-layer electrolysis in this ce11.24J2J3dThis is because there is bounded diffusion in the cell. However, it profits the determination of the diffusion coefficients, because a time window is used, in which the relation of Ag(X,t) or A i ( X , t ) tl/* appears as a straight line, which accords with eq 8 or 15 derived by assuming the condition of the semi-infinite diffusion. An equation33e provides a handy rule of thumb for estimating the thickness of a diffusion layer. A value of DR (Do)is 6.3 X 10“ (7.3 X lod) cm2/s in this paper, so that a diffusion layer thickness of 0.15 mm is built up in 18 (1 5) s. In fact, the time windows (Figures 5 and 6) are wider. This makes the straight line more easily affirmed. A disadvantage of the cell is that the time for electrochemical equilibration is longer than other^.'^,^^ The equilibration time between each change in Eapplid was ca. 5 min in Figure 3.
-
-
CONCLUSIONS The long optical path spectroelectrochemical method for calculating some electrode parameters, EO’, n, DO,DR,kit,a, (47) Zhu,
502
Y.;Xie, Q. Chem. J. Chin. (Iniu. 1991, 12, 151C-1512 (in Chinese).
AnalyticalChemistry, Vol. 66, No. 4, February 15, 1994
etc., is feasible. In the calculation, the accurate values of the molar absorptivity and bulk molar concentration of a substance need not be known. It is important to study the electrontransfer reactions of biological molecules because biological sample preparations usually contain unknown impurities, which are difficult to isolate and remove. The experimental values obtained above indicate that our electrochemical cell is practically usable. The cuvette LOPTLC allows room in the sample compartment for accessories as temperature-controlled cell holders. During a sequence of controlled temperatures, the formal electrode potentials are determined by the spectropotentiostatic technique; thus the thermodynamic parameters of electron-transfer reactions are calculated.* The spectroelectrochemistry only employes a small amount of solution for obtaining electrochemical parameters and properties of the redox processes. This will benefit electrochemical researches involving precious species, such as some biochemicals, crown ether complexes, and anitcancer pharmaceuticals.
ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of Shandong Province, China (9028). Theauthors are very grateful to Z. Lu, J. Li, and D. Gu for their kind help and fruitful discussions. Received for review May 19, 1993. Accepted October 18, 1993.” a
Abstract published in Aduance ACS Absrracrs, December 15, 1993.
