Direct determination of equilibrium thermodynamic and kinetic parameters from isothermal heat conduction microcalorimetry

Calorimetry is recognized(1-8) as a noninvasive, nondestructive method for the determination of both the thermodynamic and the kinetic parameters asso ciated with chemical reactions. The most immediate applications of the tech nique have been found in the determination of long-term stability data part icularly for pharmaceuticals. However, the methods proposed required that c alorimetric data (thermal power, Phi, watts recorded as a function of time) be analyzed iteratively to obtain the order of the reaction n, the rate co nstant k, and the enthalpy change of reaction Delta H-R. A necessary assump tion in this process was that all of the sample placed into the calorimeter would react. This is obviously a severe constraint for the flexibility and application of the method. This paper reports a significant extension of t he procedure that allows direct calculation of all of the above parameters. Moreover, the equations that are developed permit the determination of the actual quantity of the sample placed into the calorimeter that will react. Indeed, for successful determination of the desired kinetic, thermodynamic , and equilibrium parameters, it is not necessary, in principle, to have an y knowledge about a sample other than its total mass. It is possible to det ermine, in addition to II, k, and Delta H-R, the equilibrium constant K for the reaction studied together with the associated values of the Gibbs func tion and entropy changes Delta (R)G and Delta S-R. Moreover, because the re action is to be studied over a temperature range, the activation energy E-a is also accessible.