Differential titration of various functional groups in the drug molecule (N + Cl - and amino groups) is proposed to solve the problem of the determination of methylrosanilinium chloride content in the substance according to PhBs 4 (the results are higher than the theoretical possibility) by titrating 0.1 M HClO4 in acetic acid with potentiometric indication (glass and saturated calomel electrodes) 1/1 (as N + Cl - ). A mixture of 35 ml of chloroform with 15 ml of acetic acid anhydride was proposed and tested as a suitable medium for this titration; the weighed-out amount is about 0.1100 g, titration in the surroundings of end points after 0.30 ml of volumetric solution. The result of the determination is calculated from the differences of consumptions of the volumetric solutions for titrations of various functional groups (1 ml corresponds to 0.02040 g of the drug). Precision of the determination by this procedure is characterised by estimation of a standard deviation about 1.5% and the results are in a substantially better agreement with expectations.

        Key words: methylrosanilinium chloride – content determination – acidimetry – non-aqueous medium – differential titration