Degradation Rates of Captopril in Aqueous Medium Through Buffer-Catalysis Oxidation

Initial degradation rates of captopril (0.1 mg/mL) in acetate, citric, and phosphate buffer solutions with different buffer concentrations at 80° (µ=0.5) were studied at pH 6.0. All degradation reactions of captopril solutions fitted an apparent first-order plot. The degradation rates of captopril rose with increasing buffer concentrations. A mechanism involving the buffer-catalysis oxidation reaction of captopril was proposed in this study. The low apparent first-order degradation rates of captopril in citric buffer solutions might have been due to the chelating effect of the citric buffer, which reduced the metal-catalysis oxidation reaction of captopril. Therefore, using low concentrations of citric buffer to improve solution stability seems to be an appropriate approach in a liquid formulation development of captopril. To select citric buffer at a low concentration for the dissolution medium might be the right choice for a sustained-release formulation dissolution study of captopril.