Methods for potentiostatic evaluation of heterogeneous kinetic parameters in the presence of an irreversible following chemical reaction

A theoretical study of Brdi?ka's adsorption waves and of the corresponding i-E curves in cyclic voltammetry and E-t curves in chronopotentiometry is presented under the following conditions: (a) the electrochemical reaction 0+ne?R is reversible; (b) both the oxidized and the reduced forms are strongly adsorbed; (c) the adsorption is rapid; (d) allowance is made in the isotherm for interactions between the molecules (Frumkin type isotherm). One mathematical function and its derivative are sufficient to describe the results in the three cases. The equations derived account in particular for the following experimental characteristics: (1) the linear variation of the half-wave potential of the adsorption wave (or of the chronopotentiogram) and of the voltammetric peak potential with the concentration c; (2) the changes in the slope of the wave when c varies and the discontinuous increase of current which is observed for high concentrations; (3) the changes in peak width and the hysteresis phenomenon observed in cyclic voltammetry; (4) the S-shaped experimental chronopotentiograms; (5) the appearance of prewaves (or postwaves) in cases where both O and R are equally adsorbed; (6) the fact that prewaves or postwaves usually appear for large molecules. The results show that the appearance of a Brdi?ka prewave or postwave is often caused by energy effects arising from the interactions between adsorbed molecules, rather than by energy effects caused by interactions with the electrode surface.