Thermal and photolytic degradation studies of promethazine hydrochloride: a stability-indicating assay

A stability-indicating GLC method for the analysis of promethazine hydrochloride in polyethylene glycol delivery systems is reported. This method is capable of distinguishing the intact drug from its thermal and photodegradation products. A linear relationship between peak height ratio (promethazine-promazine) and promethazine concentration is found up to a concentration of 600 micrograms/ml. Kinetic studies were performed to determine the photolytic and thermal degradation rates of promethazine hydrochloride as a function of pH. The activation energies at pH 2.98, 3.94, and 5.12 were obtained from linear Arrhenius plots and were found to be 6601, 5888, 5570 cal/mole, respectively. The first-order rate constant increased with increasing pH. The photolytic degradation of promethazine hydrochloride does not follow simple first-order kinetics.