The thermal analysis study of the drug captopril

Captopril is an antihypertensive drug currently being administered in tablet form. The thermal analysis study was carried out using a simultaneous TG-DTA unit. Both the isothermal and non-isothermal experiments are preformed to investigate the thermal degradation process of captopril in its natural state as a solid. The runs were performed in a flowing nitrogen atmosphere. Captopril melted at 106 degreesC followed by decomposition. Based on the order of reaction, one method is used to identify the reaction mechanism in isothermal kinetics and two methods are used to identify the reaction mechanism in non isothermal kinetics. These methods use the equations established by Avrami-Erofeev. Arrhenius and Freeman and Carroll. However, a kinetic analysis based on the "method of fit" using zero-order, first-order, and second-order equations showed that a first-order process gave a good fit for the Arrhenius plot at certain specific experimental conditions (i.e. very low sample mass). Overall, a second-order process followed by a first-order reaction for the main decomposition process of captopril showed an even better fit for the experiments. The possible reasons for this kinetic behavior are presented. There was up to 2% carbon remaining at 500 degreesC. Thermal analysis was supplemented using Fourier Transform infrared spectroscopy (FTIR), X-ray diffraction and scanning electron microscopy (SEM) methods to identify the captopril with any degradation products which may have formed. (C) 2001 Elsevier Science B.V. All rights reserved.