Indirect Spectrophotometric Quantification of Thioproperazine via Sulfoxide Formation

Abstract

An indirect spectrophotometric approach was developed for the determination of Thioproperazine (TPZ) dimesylate in pharmaceutical dosage forms. The method is based on the oxidation of the drug with potassium peroxymonosulfate (potassium caroate), resulting in the formation of a sulfoxide derivative exhibiting a molar absorption coefficient ε = 4.95 × 10³ L·mol^-1·cm^-1. Quantification was achieved by measuring the absorbance at 353 nm using a corresponding non-oxidized TPZ solution of equal concentration as the reference. The absorbance difference (ΔA) followed Beer's law and showed linearity over the concentration range of 1-20 μg/mL, with the calibration equation A = (47.3 ± 0.3) × 10^-3 × C, where C is expressed in μg/mL. The limit of quantification was 0.6 μg/mL. The proposed method was successfully applied to the quantitative analysis of TPZ in Majeptil® tablets, which are labeled to contain 10 mg of active substance. The assay exhibited high selectivity, as common excipients and degradation products did not affect the analytical signal. The method demonstrated good precision (Relative Standard Deviation (RSD) = 1.3%) and accuracy, with a relative bias of + 0.5% compared to the reference pharmacopoeial method.