Se(IV) Electrochemistry on silver: a combined EQCM and cyclic voltammetry investigation

The electrochemical behavior of Se(IV) on silver was investigated by cyclic voltammetry and electrochemical quartz-crystal microbalance (EQCM) measurements. As already reported in the literature, Se(IV) electrochemistry is always complex, and on silver even more, due to the formation of a compound. Our results confirm that the reduction process of Se(IV) occurs through two reaction paths, Se(IV) → Se(0) and Se(IV) → Se(-II); the product Se(-II) then reacts with Se(IV) through a comproportionation reaction. The latter step leads to red Se that, according to the literature, is the only electroactive form of Se(0). The presence of the electroactive red Se is evident both in the negative range of potentials, through the reduction Se(0) → Se(-II), and in the less negative range of potentials, through the oxidation Se(0) → Se(IV). Moreover, our measurements pointed to the formation of a deposit that never redissolves. This deposit seems to be the electroinactive gray Se. The electrochemical behavior of Se(IV) was investigated in the whole potential range accessible on silver. Our results confirm the occurrence of competitive processes whose predominance depends on the scan rate, as well as on the potential limits of voltammetry. A detailed table with the processes occurring in different potential ranges was drawn up.