Abstract
Lacey's theory, concerning a gas-solid reaction rate is applied to a stability study of a pharmaceutical solid. An accelerated air oxidation of ascorbic acid is analysed to predict its shelf life at room temperature as an example. The temperature dependence of the diffusion constant is found to be 48 kcal/mole, and that of the chemical reaction rate, 26 kcal/mole. The diffusion constant, and 2nd order reaction rate constant at 25° are estimated to be 1.6×10-12cm2/hr, and 4.3×10-4 liter/mole-hr, respectively. The 1% degradation life of ascorbic acid in a disk of mannitol conglomerate containing 10% ascorbic acid is estimated to be about 200 years in the absence of moisture.
