Determination of some phenothiazine drugs based on colour reaction with potassium periodate

A simple, rapid and sensitive method for the determination of six phenothiazine drugs, either in pure form or in pharmaceutical formulations, is described. The method is based on the formation of stable phenothiazine free radical by the use of potassium periodate as the chromogenic reagent in sulphuric acid medium. The reaction is suggested to proceed via oxidation of the phenothiazine nucleus into a semiquinonoid radical. The wavelengths of maximum absorption range from 500 to 525mm. Molar absorptivities range from 2.05x10(3) to 11.48x10(3) 1mol(-1)cm(-1). A linear correlation was found between absorbance at lambda max and concentration. The resulting colours are well developed within 0-10 min and are stable for at least 45 min. Results of the analysis of pure drugs and their dosage forms are in good agreement with those of the official BP 1993 procedures.