On the electroreduction mechanism of 2-pyridinecarboxylic (picolinic) acid on mercury electrodes

Abstract

This paper presents polarographic (direct current, dc, and differential pulse, DP) and voltammetric (linear-sweep cyclic voltammetry) studies on the electroreduction of picolinic acid (2-pyridinecarboxylic acid) on mercury electrodes. The studies were performed in the acidity range 2.25 M H 2SO 4 to pH 11. Above the last pH value no signals were obtained. Kinetic parameters such as Tafel slopes and electrochemical reaction orders have been determined at the potentials corresponding to the foot of the waves. From polarographic, voltammetric and kinetic results, reaction pathways are proposed. The reduction of the species protonated at the heterocyclic nitrogen occurs through a process in which the rate-determining step (r.d.s.), is a protonation placed after two reversible two-electron transfers. In this reaction, proton-donors other than the H + ion can be involved. At pH>1, the recombination of the carboxylate anion with the H + precedes the reduction process. The reduction of picolinate anion, protonated at the heterocyclic nitrogen, consisted of the uptake of one electron and two H + ions, followed by an irreversible one-electron transfer, which is the r.d.s. In both cases, the hydrated aldehyde obtained is reduced only partially, due to the slow rate of the dehydration reaction. Above pH 5.3 the protonation of the pyridine ring precedes the reduction.