EC(EE) process in the reduction of the herbicide clopyralid on mercury electrodes

This paper presents polarographic (direct current, DC and differential pulse, DP) and voltammetric (linear-sweep cyclic voltammetry) studies on the electroreduction of clopyralid (3,6-dichloro-2-pyridine carboxylic acid) on mercury electrodes. In order to obtain the dissociation constants of clopyralid, UV–vis spectra were recorded as a function of the pH; pK values of 1.4 ± 0.1 (–COOH group) and 4.4 ± 0.1 (pyridinic nitrogen) were obtained. The electrochemical studies were performed in the acidity range 6.5 M H2SO4 to pH 7. 0. Above the last pH value no signals were obtained. In DC polarography, clopyralid presents two reduction waves in the acidity range 2.25 M H2SO4 to pH 3.5. These waves merge into one unique “combined” wave above pH 3.5, disappearing at pH 7.0. DP polarograms showed two peaks in the entire pH range studied, appearing at more negative potentials as the pH was increased. Kinetic parameters such as Tafel slopes and electrochemical reaction orders have been determined at potentials corresponding to the foot of the waves. From these results, together with those obtained by cyclic voltammetry, reaction pathways have been proposed. The reduction of the species protonated at the heterocyclic nitrogen corresponds to an EC(EE) scheme (first peak), that is, a chemical reaction in equilibrium placed after a reversible electron transfer and followed by the reductions of both species appearing in the equilibrium. The acidic components of the buffer are involved in the rate-determining step (r.d.s.), as occurs in the reduction of other pyridine derivatives. At pH > 1.5, the recombination of the carboxylate anion with H+ precedes the reduction process. For peak 2 the process must consist of a reversible one-electron transfer to yield a neutral radical, followed by the reaction with an H+ ion to yield a radical cation and a water molecule, this reaction being the r.d.s. The end product is then formed after the transfer of an electron and a H+ ion.