THERMAL DEHYDRATION OF HYDRATED SALTS

Some of the earhest kinetic studies of solid state reactions were concerned with the dehydrations of ionic crystals and the progress made in determining the mechanisms of these reactions was an important stimulus towards the investigation of the reactions of a wider range of crystalline materials [2]. 

The decompositions of series of related solids, containing ligands other than water, e.g. ammoniates, alcoholates, etc., have been investigated and some show behaviour that is parallel to the reactions of hydrates. Some examples of such reactions are discussed in Chapter 17. 

The most suitable basis for the classification [5] of the structures of hydrates is by grouping together substances in which water molecules are in similar environments. One group of hydrates has water arranged in a three-dimensional lattice framework 

Alternatively, hydrates may be grouped on the basis of their cation water ratio even though this does not derive from any single chemical property. Wyckoff [6] emphasizes the arbitrary nature of considering hydrates as a separate class of compounds, because these substances should, in principle, be discussed with reference to other comparable molecular groupings. 

Early (1930 to 1940) kinetic studies of dehydrations contributed much to the development of the concept of the reaction interface as the important feature of nucleation and growth reactions [2]. Kinetic equations applicable to the decompositions of a vnde range of crystalline substances were developed. Large, well-formed crystals of hydrates could be prepared relatively easily and studies of these were particularly rewarding. The interpretation of kinetic data was supplemented by microscopic evidence concerning the formation and development of product nuclei. Recent work on dehydrations has included more precise determinations of the crystal structures of reactants, products and their interrelationships, including interface textures, in the attempt to resolve unanswered questions. 

---