Spectrophotometric determination of some phenothiazine drugs acting on the central nervous system using hexacyanoferrate(III)

A simple, rapid and sensitive spectrophotometric method for the determination of six phenothiazine drugs acting on the central nervous system is described. The method is based on the formation of a coloured product between hexacyanoferrate(III) (ferricyanide) and phenothiazines in a phosphoric acid medium The reaction involves oxidation of the phenothiazine nucleus into a semiquinonoid radical with maximum absorption at 500-525 nn for different phenothiazine drugs. A graph of absorbance versus concentration shows that Beer's Law is obeyed over the concentration range 2-40 mu gml(-1), with a molar absorptivity of 2.8x10(3)-10.9x10(3) 1 mol(-1) cm. The optimum reaction conditions and other analytical parameters have been evaluated. The influence of the substrates commonly employed as excipients with phenothiazine drugs has been studied. The method proposed has been successfully applied for the determination of phenothiazine drugs in various pharmaceutical formulations. The results obtained are in excellent agreement with those of an official method