Coulometric and spectroscopic investigations of the oxidation and reduction of some azosalicylic acids at glassy carbon electrodes

Constant potential coulometry in combination with cyclic voltammetry, UV–Vis, 1H and 13C NMR spectroscopy were used to identify the oxidation and reduction products for some structurally similar azosalicylic acids, including the azosalicylic drugs Sulfasalazine ® and Olsalazine ®. The experiments were carried out in aqueous solutions at pH 4.5 to 10.0. Voltammetric and UV–Vis spectroscopic comparisons involving standard substances confirm that all compounds studied, except 4,4′-azobis-[2-hydroxybenzoic acid], are reduced in an overall four-electron process to their corresponding amines. For 4,4′-azobis-[2-hydroxybenzoic acid], the reduction involves only two electrons and most likely gives rise to the corresponding hydrazo compound. The oxidations of 3,3′-azobis-[6-hydroxybenzoic acid] (Olsalazine ®), 3,3′-azoxybis-[6-hydroxybenzoic acid] and 2-hydroxy-5-[(3′-carboxy-2′-hydroxyphenyl)azo]benzoic acid were found to involve an initial two-electron step. Based on UV and NMR spectroscopic data, it is proposed that the oxidation of 3,3′-azobis-[6-hydroxybenzoic acid] gives rise to a 2,2′-dicarboxy-1,4-dibenzoquinone azine.