Abstract
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 whch incorporate calorimetrically
accessible data (a, the power, and q, the heat output) and which
also include the rate constant, k, the change in enthalpy of the
reaction, deltaH, 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, Le., 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.
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