Square-wave voltammetry of quasi-reversible surface redox reactions

Abstract

Abstract 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 ⩽ K ox K red ⩽ 10 . By plotting the ratio of peak current and frequency as a function of frequency, the critical frequency fmax, for which this ratio is at the maximum, can be determined. Theoretically it is shown that the rate constant is related to fmax by the equation ks = κmaxfmax, where κmax is the critical kinetic parameter which depends on the transfer coefficient of the surface redox reaction. If α = 0.55 ± 0.5, κmax = 1.13, and if α is not known, the average value gk max = 1.18 ± 0.05 , which applied to 0.25 ⩽ α ⩽ 0.85, can be used. For other α values, the corresponding κmax parameters are given by the formula κmax = 1.13 + 1.13(α − 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 ks = (62 − 12pH)s−1 for pH ⩽ 4 and ks = 28 s−1 for pH 4.65.