Determination Of Thermodynamic And Kinetic-Parameters From Isothermal Heat-Conduction Microcalorimetry - Applications To Long-Term-Reaction Studies

The application of heat conduction isothermal microcalorimetry has been proposed for some time as a rapid and general technique for the determination of both thermodynamic and kinetic parameters of chemical reactions. These applications have been suggested as being of particular relevance to solid-state reactions and, industrially important, to the prediction of long-term stability and of compatibility data for pharmaceutical materials. However, there has yet to be the development of a general procedure that does not require additional noncalorimetric data and that is free of assumptions, which can be used to determine the thermodynamic and kinetic parameters for a reaction, from calorimetric data. It is the purpose of this paper to describe such a general approach which does not depend upon knowledge of initial concentrations (quantity), enthalpy, or any predetermined reaction order. Equations have been developed which incorporate calorimetrically accessible data (Phi, the power, and q, the heat output) and which also include the rate constant, k, the change in enthalpy of the reaction, Delta H, and the order of reaction. A second procedure is also described which depends only on the analysis of the calorimetric signal and which involves no formal chemical kinetic based equations. The methods described allow estimation of, for example, the annual extent of degradation of a solid compound. The methods developed have been tested through examination of both calculated and experimental data. The experimental work examined very slow reactions (lifetime of years) of known order (there are little reliable enthalpy data available for slow reactions) and involved calorimetric observation of these reactions for up to 50 h. In all cases, the method yielded the appropriate, i.e., conforms to literature data, rate constant, reaction order, and, where available, reaction enthalpy. Some situations in which this microcalorimetric approach and subsequent data analysis will be of utility are discussed.