Spectrophotometric determination of some phenothiazine drugs using 3-methylbenzothiazolin-2-one hydrazone

Abstract

A rapid and sensitive method for the determination of thioridazine hydrochloride, methotrimeprazine, perazine, chlorpromazine hydrochloride and promazine hydrochloride, either in the pure form or in pharmaceutical formulations, is described. The method is based on the development of violet, blue or green products with 3-methylbenzothiazolin-2-one hydrazone in the presence of iron(III) salts. The reaction is thought to proceed via preliminary oxidation of the phenothiazine nucleus to a phenothiazinyl radical followed by coupling of the reagent in the 3-position. The wavelengths of maximum absorption range from 700 to 740 nm. Molar absorptivities range from 2.0 × 10⁴ to 4.0 × 10⁴ l mol^–1 cm^–1. A linear correlation was found between absorbance (at the λ_max.) and concentration. The resulting colours are well developed within 5 min at 60 °C and are stable for more than 24 h. Results of analyses of pure drugs and their dosage forms by the proposed method are in good agreement with those of the official BP 1980 and USP XXI procedures.