Some observations on oxidation-reduction indicators of the benzidine, naphthidine and diarylamine types

Abstract

A spectrophotometric investigation has been made of representative compounds of the benzidine class of indicators in an attempt to resolve fundamental problems concerning their oxidation mechanism in sulphuric acid media. In all instances the oxidation to the coloured compound is a single-step two-electron process. There is no detectable evidence for the formation of an intermediate, either of a benzidine from arylamines or of a semiquinone from a benzidine or naphthidine. Reduction or spontaneous decay of oxidised arylamines stops at the benzidine stage. All oxidised indicators are unstable both intrinsically and in the presence of excess of oxidant: unsubstituted benzidine and naphthidine are, additionally, photosensitive in the oxidised state. The spontaneous decomposition of the oxidised form regenerates the reduced form and is a disproportionation, probably of 2:1 stoicheiometry, but a kinetic study shows that the rate-controlling step is a unimolecular precursive reaction, which may be the relaxation of the triplet state of the coloured dication diradical oxidised form. Stability decreases on sulphonation and with increasing temperature, and increases with increasing sulphuric acid concentration and with progressive substitution of the amino hydrogen atoms.