Electrochemical reactions with protonations at equilibrium: Part XV. The 2e −, 2H + bi-cubic scheme

Abstract

A theoretical study of the 2e −, 2H + reaction is presented for when the molecule possesses an electroinactive protonation site other than the electroactive site. The results are valid for a surface reaction (adsorbed redox system, Langmuir isotherm) or for a heterogeneous electrochemical reaction (semi-infinite linear diffusion) with protonations taking place in solution near the electrode. As in the other papers of this series, it is assumed that the protonations are fast (at equilibrium), that the symmetry factors of the electrochemical reactions are equal to 0.5, and that the disproportionation and dimerization reactions can be neglected. The reaction can be described by using a bi-cubic scheme, consisting of two cubes linked by one of their edges. In a certain pH range, the reaction appears as a direct 2e −, 2H + reaction; the system behaves as a simple reaction with two successive electron exchanges. Two apparent electrochemical rate constants, which depend on the individual electrochemical rate constants, on the pH, and on the difference between the redox potentials can be defined. The graphs of the variations of the logarithm of the apparent rate constants are made up of successive rectilinear segments with slopes of 0, ± 1 4 , ± 1 2 , etc. The reaction path (order of addition of protons and electrons) can be very complex, and the analysis requires a careful examination of all the factors involved. The case where a further protonation takes place has been examined. The changes produced are analysed.