THE GENERAL UTILITY OF THE NTH ORDER MODEL IN SOLID-STATE REACTION-KINETICS

Computerized kinetics analysis of simulated solid state reaction data has been used to demonstrate how the empirical nth order model may be employed as a diagnostic tool in thermoanalytical research. The characteristic shape of the curves of the extent and rate of reaction as a function of temperature, generated under non-isothermal conditions, together with the magnitude of the kinetics parameters when this data is subjected to nth order Arrhenius analysis, can be utilized to postulate a solid state reaction mechanism, and to calculate the pertinent activation energy and pre-exponential factor. This approach has been employed to analyze the non-isothermal thermogravimetric data characterizing the three stages in the thermal degradation of calcium oxalate monohydrate, postulated to conform to the D4, R3 and D4 mechanisms, respectively. Although the second stage appears to be essentially a single contracting-volume-controlled reaction, the variation of the activation energies and pre-exponential factors with heating rate for the first and third stages implies multiple diffusion-controlled reactions. The procedure has also been used to generate the kinetics parameters of the crystallization of an amorphous form of a candidate drug compound (A2 mechanism) from non-isothermally monitored differential scanning calorimetric data.