Determination of promethazine by anodic differential-pulse voltammetry

A differential-pulse voltammetric method is described for the determination of promethazine hydrochloride. The method is based on the anodic oxidation of promethazine on a glassy carbon electrode at + 0.64 V vs. SCE in Prideaux buffer of pH 2.3. The reversibility of the oxidation was tested by cyclic voltammetry; the electrode process is quasi-reversible. From the results of microcoulometric experiments and a study of acid—base equilibria, a mechanism for the electrochemical oxidation is presented. The method is applied to determine promethazine hydrochloride in pharmaceutical formulations. Calibrations are linear over the 0.1–1 and 1–5 mg/50 cm³ ranges.