ELECTROCHEMICAL REDUCTION OF ISATIC ACID AT MERCURY-ELECTRODES IN AQUEOUS-MEDIA

The electrochemical reduction of isatic acid at a mercury electrode ha s been studied in aqueous solutions over a wide pH range. Techniques u sed to study the mechanism for reduction of isatic acid include de and pulse polarography, linear sweep and cyclic voltammetry, electrocapil lary curves, chronocoulometry and bulk electrolysis with electrochemic al and UV spectroscopic monitoring of the reaction. Polarographic tech niques reveal the presence of three isatic acid reduction processes wh ich are assigned to the three forms of isatic acid in aqueous solution . The fact that these processes are predominant at close to the 100% l evel at pH 3, 5.75 and 12, respectively, suggests that the various for ms of isatic acid do not exchange rapidly on the time scale of the red uction and the exchange may be neglected at these pH values. Thus, the species of isatic acid present in solution can be studied individuall y by electrochemical methods under these experimental conditions. In a cidic solution an overall chemically irreversible, two electron-two pr oton 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 th e irreversible reduction process. At all pH values the reduction takes place at the cr-carbonyl group. In acidic media but not in weakly aci dic or basic media, reactant adsorption at the mercury electrodes was observed. The response of isatic acid was found to be the most sensiti ve at pH 3.0 with differential pulse polarography, and the detection l imit was 5.0x10(-8) M. (C) 1997 Elsevier Science B.V.