Catalytic polarographic wave in the vanadium–cupferron system

• Electroreduction of vanadium–cupferron complex has been examined.
• A catalytic wave is observed at − 0.6 ÷ − 0.7 V (SCE).
• Complexes of V(II) with cupferron are supposed to be a catalyst.
• A rate constant of heterogeneous reaction in this system has been evaluated.

Surface catalytic polarograpfic wave in vanadium(V)–cupferron system has been studied in the buffer media with pH 5 ÷ 7.5. The catalytic wave is observed at − 0.6 ÷ − 0.7 V (SCE), and it could be enhanced by addition of quaternary ammonium salts. This fact implies that catalytically active vanadium species are anionic. As the height of this wave is proportional to vanadium concentration, it may be useful as an analytical signal. Adsorbed anionic vanadium(II) complex with cupferron is supposed to be a catalyst in this process. A peak shaped wave at more negative potentials is assumed to be a result of catalytic action of vanadium(II) hydroxide. Some kinetic parameters of the catalytic wave in vanadium–cupferron system are presented.The mechanism of catalytic reaction has been considered, and the rate constant of the surface chemical reaction between adsorbed vanadium(II) and cupferron has been evaluated: k = (3 ± 1)·106 cm3·mol− 1·s− 1. pH dependence of k has not been observed; it implies that no prior protonation is encountered. The catalytic reduction of cupferron and its direct electrochemical reduction differ in some important aspects such as number of electrons transferred or presence of prior protonation reaction.