A simple oxidative coupling spectrophotometric method for the determination of promethazine hydrochloride in pharmaceutical tablets with preliminary biological activity evaluation

A simple, sensitive, and cost-effective spectrophotometric method was developed for the determination of promethazine hydrochloride in pharmaceutical tablet formulations. The proposed method is based on an oxidative coupling reaction in which promethazine hydrochloride is oxidized by ferric chloride and subsequently coupled with potassium ferricyanide to form a stable blue-violet colored complex. The absorbance of the formed complex was measured at 682 nm using distilled water as the solvent under mild experimental conditions. The analytical procedure was optimized with respect to reagent concentration, reaction time, temperature, and solvent to achieve maximum color intensity and stability. Under the optimized conditions, Beer’s law was obeyed over a concentration range of 1-40 µg/mL, with a high correlation coefficient, indicating good linearity. The method exhibited satisfactory sensitivity, precision, and accuracy, with low limits of detection and quantification. The proposed method was successfully applied to the determination of promethazine hydrochloride in commercial tablet formulations, and the results were in good agreement with the labeled amounts in addition, a preliminary biological activity study was conducted to investigate the antimicrobial behavior of promethazine hydrochloride and its oxidative coupling complex. The results revealed noticeable antifungal activity, particularly against Candida albicans, while moderate antibacterial activity was observed against selected bacterial strains. The observed differences in biological activity between the free drug and the formed complex suggest that chemical complexation may influence biological behavior. The proposed method offers a practical analytical alternative for routine quality control analysis of promethazine hydrochloride in pharmaceutical preparations and provides additional insight into the effect of oxidative coupling on biological activity.