Electrochemical reduction of isatic acid at mercury electrodes in aqueous media

The electrochemical reduction of isatic acid at a mercury electrode has been studied in aqueous solutions over a wide pH range. Techniques used to study the mechanism for reduction of isatic acid include dc and pulse polarography, linear sweep and cyclic voltammetry, electrocapillary curves, chronocoulometry and bulk electrolysis with electrochemical and UV spectroscopic monitoring of the reaction. Polarographic techniques reveal the presence of three isatic acid reduction processes which are assigned to the three forms of isatic acid in aqueous solution. The fact that these processes are predominant at close to the 100% level at pH 3, 5.75 and 12, respectively, suggests that the various forms of isatic acid do not exchange rapidly on the time scale of the reduction and the exchange may be neglected at these pH values. Thus, the species of isatic acid present in solution can be studied individually by electrochemical methods under these experimental conditions. In acidic solution an overall chemically irreversible, two electron–two proton reduction process is observed, whereas in weakly acidic solution the process is an irreversible two electron–one proton reduction step, and in alkaline media, two electrons but no proton are involved in the irreversible reduction process. At all pH values the reduction takes place at the α-carbonyl group. In acidic media but not in weakly acidic or basic media, reactant adsorption at the mercury electrodes was observed. The response of isatic acid was found to be the most sensitive at pH 3.0 with differential pulse polarography, and the detection limit was 5.0×10−8 M.