Determination of some antipsychotropic and anticholinergic phenothiazine drugs by vanadium (V) titration

Three simple, rapid and accurate titrimetric procedures using ammonium metavanadate have been developed for the determination of six phenothiazine drugs in pure form and in dosage forms. The procedures are based on the oxidation of phenothiazines in acid medium to colourless sulphoxides via orange or purple coloured intermediates. In the first method, phenothiazines are titrated directly in H2SO4-H3PO4 medium to a colourless end-point. In the second procedure, a known excess of oxidant is added and after a specified time, the residual oxidant is determined using standard ammonium ferrous sulphate titration. The third method employs electrometric end-point detection. The optimum reaction conditions and other analytical parameters are evaluated. The influence of the substrates commonly employed as excipients with phenothiazine drugs has been studied, Statistical comparison of the results with those of an official method shows excellent agreement and indicates no significant difference in precision.