269 .I. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Electroanal. Chem., 324 (1992) 269-289 Elsevier Sequoia S.A., Lausanne JEC 01887 Study of the electrochemical reduction of isonicotinic acid at a mercury electrode R. Rodriguez-Amaro, R. Perez, L. Camacho and J.J. Ruiz l Departamento de Quimica FiXca y Termodin~mica Aplicada, Facultad de Ciencias, Universidad de CMoba, San Albert0 Magna s/n, E-14004 Grdoba &ainJ (Received 30 July 1991; in revised form 16 October 1991) Abstract A moie comprehensive study than those previously reported in the literature was carried out on the electrochemical reduction of isonicotinic acid (INA) in aqueous solutions over a wide pH range. The experimental results indicate that, in acid media, the reduction of INA is a first-order process involving two reversible electron transfers followed by a protonation step that yields the corresponding isonicotinic hydrated aldehyde, even though the reduction can continue to yield the alcohol after a prior dehydration of the aldehyde. In neutral and basic media the electroreduction process is preceded by a slow chemical protonation reaction. Moreover, the contribution of a second-order step, probably a radical-radical coupling to yield the dimer, in addition to the first-order process must also be considered. A general reduction scheme based on a CECE mechanism complicated with different acid-base and hydration-dehydration equilibria is proposed, and from this, a quantitative explanation for the variation of I, and Et,, with the pH is provided. INTRODUCTION There are several papers in the chemical literature dealing with the electro- chemical reduction of isonicotinic acid (INA) at a mercury electrode which report rather divergent results [l-93. The earliest polarographic studies [l-4] were limited to descriptions of the variation of IL and Z& with pH. It was concluded that in the pH regions where I, is constant, the waves are governed by diffusion, while whenever I, decreases with pH, a prior chemical recombination with H+ ions endows the waves with a l To whom all correspondence should be addressed. 0022-0728/92/%05.00 0 1992 - Elsevier Sequoia S.A. Ah rights reserved