Electrochemical studies on sulfonephthaleins. Part 2. Electrochemical reduction mechanism of catechol violet in aqueous solutions on a mercury electrode

Abstract

The electrochemical reduction mechanism of catechol violet at a hanging mercury electrode, HMDE, in aqueous buffer solutions of different pHs has been studied in detail by cyclic voltammetry, double potential step chronoamperometry, chronocoulometry and digital simulation. Catechol violet shows at lower pHs (< 7) a single diffusion-controlled two-electron wave while at higher pHs (> 8), two diffusion-controlled monoelectron waves are given. It is concluded that at lower pHs the reduction pathway follows an ECEC, first-order mechanism in which E represents a reversible electron transfer and C is an irreversible protonation reaction. The rate-determining step is the protonation of the monoanion formed after the second electron transfer. At higher pHs, the reduction proceeds via two steps, an irreversible electron transfer followed by an EC, first-order process. The homogeneous parameters were measured by comparing the experimental chronocoulometric data with the theoretical working curves. The heterogeneous parameters were obtained by comparing the experimental cyclic voltammetric responses with the digital simulated results.