SQUARE-WAVE VOLTAMMETRY OF QUASI-REVERSIBLE SURFACE REDOX REACTIONS

A ''quasi-reversible maximum'' in square-wave voltammetry can be used for the determination of the standard reaction rate constant of a redox reaction between an adsorbed reactant and a product if their adsorption constants satisfy the condition 0.1 less than or equal to K-OX/K-red less than or equal to 10. By plotting the ratio of peak current and frequency as a function of frequency, the critical frequency f(max) for which this ratio is at the maximum, can be determined. Theoretically it is shown that the rate constant is related to f(max) by the equation k(s) = kappa(max)f(max), where kappa(max) is the critical kinetic parameter which depends on the transfer coefficient of the surface redox reaction. If alpha = 0.55 +/- 0.5, kappa(max) = 1.13, and if alpha is not known, the average value ($) over bar kappa(max) = 1.18 +/- 0.05, which applied to 0.25 less than or equal to alpha less than or equal to 0.85, can be used. For other alpha values, the corresponding kappa(max) parameters are given by the formula kappa(max) = 1.13 + 1.13(alpha - 0.55)(2). The method was applied to the redox reaction of adsorbed azobenzene in perchlorate and acetate electrolytes of various pH. The rate constants are k(s) = (62 - 12pH) s(-1) for pH less than or equal to 4 and k(s) = 28 s(-1) for pH 4.65.