Study of the electrochemical reduction of nicotinic acid at a mercury electrode

Abstract

A more comprehensive study than those reported previously in the literature has been carried out on the electrochemical reduction of nicotinic acid in aqueous solutions over a wide pH range. The influence of the pH, reactant concentration and drop time on the different polarographic and kinetic parameters in the two electroreduction processes involved is studied. Experimental DP curves and the theoretical profiles corresponding to different mechanisms are compared. The experimental results indicate that the process corresponding to the wave observed in very strong acid media involves two electron transfers followed by a protonation step yielding the corresponding nicotinic aldehyde. Two waves can be observed in the 0.5–5 pH range. The first corresponds to the dimerization of radicals produced in the first electron transfer via a radical-radical coupling in the reaction layer. The second originates from the reduction of these radicals, which competes with the dimerization step. The only wave observed in neutral and basic media corresponds to a dimerization analogous to that yielding the first wave observed in the pH range 0.5–5. The voltammetric results indicate that the dimer oxidizes at potentials very far from those at which it is formed. In any case, the presence of Triton X-100 inhibits the dimerization process. The C- E curves indicate that nicotinic acid is not appreciably adsorbed on the mercury surface at the working concentration.