Some misconceptions in the kinetic analyses of pharmaceutical systems

Some problems encountered during the formulation of reaction mechanisms, by the interpretation of kinetic and other experimental observations, in stud ies of reactions that occur on heating reactants that are initially solid, are critically reviewed. The representative chemical changes selected for i nclusion here appear to be relatively simple. It has been found, however, t hat the provision of acceptable representational models is less straightfor ward than might have been anticipated. Accordingly, it is concluded that, w hen interpreting kinetic data, it is essential to consider most carefully t he significance of all experimental measurements in the context of the over all change. Interpretational difficulties, recognised in the literature, that identify inconsistencies that may complicate kinetic analysis are illustrated throug h consideration of the following systems (and other examples): characterisa tion of the changes in reaction stoichiometry that result from the presence of a gaseous atmosphere during NiSO4.6H(2)O dehydration; the sensitivity o f kinetic behaviour to reaction conditions during the dehydration of NiC2O4 .2H(2)O and in the dissociation of CaCO3; the formation of intermediates an d the possibility of melting during the decompositions of KMnO4 and of copp er(II) malonate. It is concluded that the formulation of reaction models through the interpr etation of kinetic (with other) data for solid-state decomposition is more difficult than is sometimes acknowledged. It is not known how widely proble ms of the types described in this short review apply. These are mentioned h ere as types of behaviour that require more general consideration in kineti c and mechanistic studies of the changes that occur on heating reactants th at are originally solid.