Spectrophotometric method for the determination of phenothiazines and its application to phenothiazine drugs

Abstract

A method for the spectrophotometric determination of four phenothiazines (promazine hydrochloride, mepazine hydrochloride, chlorpromazine hydrochloride and prochlorperazine maleate), based on the coloured compounds formed between the phenothiazines and molybdoarsenic acid, is described. The infrared and electron spin resonance spectra of these coloured compounds showed that the molybdoarsenic acid oxidises phenothiazines to a radical cation with which it subsequently forms the coloured compound. The method is simple and rapid. The influence of the substrates commonly employed as excipients with phenothiazine drugs was studied. The proposed method has been applied to the analysis of commercial phenothiazine-containing preparations, the results of which are in good agreement with those obtained by the official method of the British Pharmacopoeia.