On the Electrochemical Reduction of the Herbicide Picloram on Mercury Electrodes

• Published in: Journal of the Electrochemical Society 2006, Vol.153 (2), p.E33-E37
• Main Authors: Corredor, M. C., Mellado, J. M. Rodríguez, Montoya, M. Ruiz
• Format: Article
• Language: English
• ISSN: 0013-4651
• DOI: 10.1149/1.2147372

Polarographic and voltammetric studies on the electroreduction of picloram (4-amino-3,5,6-trichloro2-pyridinecarboxylic acid) on mercury electrodes are performed. The dissociation constant of the -COOH group was obtained from the UV-visible absorption spectra (pK = 2.2 - 0.03). The electrochemical studies were performed in the acidity range 2.25 M H2SO4 to pH 7. In dc polarography picloram presents one or two waves of reduction depending on the medium pH. Degree of polymerization (DP) polarograms showed two or three peaks. The overall reduction process involves the uptake of four electrons. Kinetics parameters such as Tafel slopes and electrochemical reaction orders have been determined at the potentials corresponding to the foot of the waves. From the experimental reaction pathways are proposed. The reduction of the species protonated at the heterocyclic nitrogen reactions correspond to a chemical reaction in equilibrium placed after a reversible electron transfer and followed by the reductions of both species appearing in the equilibrium EC(EE) scheme. The acidic components of the buffer are involved in the rate-determining step. At pH > 2 the recombination of the carboxylate anion with the H' ion precedes the reduction process. Peak 3 must correspond to the reduction of the protonated picloram carboxylate.