Kinetic parameters for electron transfer

Consideration of Eqs. (4.26) and (4.27) indicates that chronopotentiometry can be used to evaluate the kinetic parameters for electron-transfer reactions. By plotting the electrode potential E versus log [(r1/2 — rl/2)/r1/2], one obtains a linear curve whose slope permits evaluation of ana [or (1 — a)/ia]. Extrapolation of such a plot to the point where t — 0 causes the logarithmic terms of Eqs. (4.26) and (4.27) to go to zero and thereby permits kc and ka to be evaluated from the values of E,=0. Plots of Et=0 versus log Cb and log 1/i provide another means of evaluating ana. 

Thus if electron transfer occurs while an adsorbate layer remains on the-electrode surface, the oxidation or reduction must occur by the transfer of an electron over a far greater distance. Figure 1.6 showed the potential distribution on a molecular scale close to the electrode and it was noted that only the potential difference 02 0s ( ot 0M — 0s) was available to drive electron transfer the adsorbate layer will cause a substantial decrease in the 02 potential. In the limit, the presence of adsorbate may totally prevent electron transfer and then only a fraction (1 — 0) of the electrode is available for the 0/R couple. An alternative model would require displacement of adsorbate by 0 or R before electron transfer can occur and the extent of such a process will depend on AGads - Finally the adsorbate may also be able to act as a ligand for 0 and R and complex formation at the electrode surface may increase or decrease the kinetic parameters for electron transfer. 

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