Electrochemical oxidation of niazid and isoniazid at mercury electrodes. Influence of the adsorption of the reaction product on the polarographic and voltammetric curves

Abstract

The electrochemical oxidation of niazid and isoniazid, at mercury electrodes, was studied by dc and differential pulse polarography and linear-sweep cyclic voltammetry in the pH range 6–13. At pH ⪢ 8.5, the positive scans show a prewave, in addition to the main oxidation wave, which can be suppressed by changing experimental variables such as the concentration, temperature and ethanol content in the medium. In the absence of the prewave, Tafel slopes and reaction orders were obtained at the potentials corresponding to the foot of the polarographic waves. On the basis of polarographic, voltammetric and kinetic results and taking into account the literature data, the oxidation processes were found to be of the ECE type, where the rate-determining step was the release of an H + ion from the intermediate formed after two reversible one-electron transfers. The results obtained for the prewave agree with those expected for a process in which the product is more strongly adsorbed than the reactant. It is also shown that the adsorption follows a Langmuir isotherm for which the Gibbs energy of adsorption is potential dependent.