Captopril is an antihypertensive drug currently being administered in table
t 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 o
rder of reaction, one method is used to identify the reaction mechanism in
isothermal kinetics and two methods are used to identify the reaction mecha
nism in non isothermal kinetics. These methods use the equations establishe
d by Avrami-Erofeev. Arrhenius and Freeman and Carroll. However, a kinetic
analysis based on the "method of fit" using zero-order, first-order, and se
cond-order equations showed that a first-order process gave a good fit for
the Arrhenius plot at certain specific experimental conditions (i.e. very l
ow sample mass). Overall, a second-order process followed by a first-order
reaction for the main decomposition process of captopril showed an even bet
ter fit for the experiments. The possible reasons for this kinetic behavior
are presented. There was up to 2% carbon remaining at 500 degreesC. Therma
l 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.