Adsorption, autoinhibition and autocatalysis in polarography and in linear potential sweep voltammetry

Abstract

The effects of a strong adsorption of the depolarizer and/or of the product of the electrochemical reaction on the currents obtained in polarography and in linear potential sweep voltammetry (l.p.s.v.) are considered. (a) A study of the surface electrochemical reaction (reaction of molecules in the adsorbed state) is presented. Mathematical expressions are derived for the current when the reaction is reversible (Brdicka prewaves or postwaves) for polarography and l.p.s.v. The case where the product of the reaction undergoes an irreversible chemical reaction of the first or second order in l.p.s.v. has been treated. Experimental examples are given: the theory accounts for the characteristics of the Brdicka prewaves and postwaves and of the adsorption peaks obtained in l.p.s.v. (b) The electrochemical process is modified by a film of adsorbed molecules (autoinhibition or autocatalysis). Three types of autoinhibition are considered: (I) The inhibiting film is constituted of the molecules of the depolarizer itself. (II) The molecules of the film result from a chemical reaction which occurs parallel with the inhibited electrochemical reaction. (III) The film is made of the final product of the inhibited electrochemical reaction. Mathematical expressions for the i−t curves and for the polarograms have been derived and experimental examples are given. In case III, autoinhibition waves similar in shape to Brdicka prewaves are often observed (pseudo-prewaves); criteria permitting a distinction to be made between prewaves and pseudo-waves are discussed. The case of an autocatalysis by the product of the reaction has been treated; an experimental example is given. The equations of the i−t curves (polarography) and i−E curves (l.p.s.v.) have also been calculated when the electrochemical reaction is irreversible. These equations are obeyed for small coverages. When the film of the depolarizer is compact anomalies appear, which are due to interactions between the molecules.