A comparison of the electrochemical properties of some azosalicylic acids at glassy carbon electrodes by cyclic and hydrodynamic voltammetry

The electrochemical properties of a number of azosalicylic acid derivatives have been compared at glassy carbon electrodes using cyclic and hydrodynamic voltammetry. The reduction of all studied compounds, except one, was found to be irreversible giving rise to disruption of the azo bridge and formation of the corresponding amines. It was found that the rate of cleavage of the azo bridge depended on the position of the hydroxy substituents on the aromatic rings of the azosalicylic compounds. A reoxidation wave, most likely due to the oxidation of a hydrate intermediate, was observed for compounds with either only one hydroxyl group, or hydroxyl groups in the meta-para or meta-meta positions relative to the azo bridge. The oxidation of the azosalicylic compounds with no, or only one, hydroxy group in the pura position was generally found to be irreversible yielding poorly defined oxidation waves. Azosalicylic acids with two hydroxy groups, either in the para position of both rings or in the ortho position of one ring and para position of the other, gave rise to well defined oxidation peaks at significantly less positive potentials. For the compounds with the hydroxyl groups in the pam position of both rings, rereduction waves were also seen on the return scan, indicating that the oxidation products were stable on the voltammetric time scale. (C) 1999 Elsevier Science Ltd. All rights reserved.