Some misconceptions in the kinetic analyses of pharmaceutical systems

Abstract

Some problems encountered during the formulation of reaction mechanisms, by the interpretation of kinetic and other experimental observations, in studies of reactions that occur on heating reactants that are initially solid, are critically reviewed. The representative chemical changes selected for inclusion here appear to be relatively simple. It has been found, however, that the provision of acceptable representational models is less straight-forward than might have been anticipated. Accordingly, it is concluded that, when interpreting kinetic data, it is essential to consider most carefully the significance of all experimental measurements in the context of the overall change. Interpretational difficulties, recognised in the literature, that identify inconsistencies that may complicate kinetic analysis are illustrated through consideration of the following systems (and other examples): characterisation of the changes in reaction stoichiometry that result from the presence of a gaseous atmosphere during NiSO4.6H2O dehydration; the sensitivity of kinetic behaviour to reaction conditions during the dehydration of NiC2O4.2H2O and in the dissociation of CaCO3; the formation of intermediates and the possibility of melting during the decompositions of KMnO4 and of copper(II) malonate. It is concluded that the formulation of reaction models through the interpretation of kinetic (with other) data for solid-state decomposition is more difficult than is sometimes acknowledged. It is not known how widely problems of the types described in this short review apply. These are mentioned here as types of behaviour that require more general consideration in kinetic and mechanistic studies of the changes that occur on heating reactants that are originally solid.