Oxidimetric titration of some phenothiazine neuroleptics and antiallergics with potassium dichromate

Three simple, rapid and accurate titrimetric procedures using potassium dichromate as titrant are described for the determination of six phenothiazine drugs in a pure form and in dosage forms. The procedures are based on the oxidation of phenothiazines in an acid medium to colorless sulfoxides via orange- or purple-colored intermediates. In the first method, phenothiazines are titrated directly in a H2SO4 medium to a colorless end-point. In the second procedure, a known excess of oxidant is added, and after a specified time the residual oxidant is determined iodometrically. The third method employs potentiometric end-point detection. The optimum reaction conditions and other analytical parameters are evaluated and the molar-ratio of the reaction is also calculated. The influence of the substrates commonly employed as excipients with phenothiazine drugs has been studied. A statistical comparison of the results obtained for pure drugs with those of an official method shows excellent agreement and indicates no significant difference in precision.