Equilibrium reactions in non-stoichiometric hydrates

We will now model divariant equilibria by non-stoichiometry of the hydrates. Various models are envisaged, and we interpret different examples. 

As is the case with the dififerent solid compounds found in this volume, we will model non-stoichiometry of the hydrates using quasi-chemistry of stmcture elements. This approach, however, presents a munber of difficulties because these hydrates are relatively-complex solids, with at least three main components the anion (which itself is usually complex), the cation and water. In cases where the salt can accept several successive limited hydrates, even the water molecules are not all equivalent in terms of the sites they occupy and the energy of their bond to the lattice. In order to simplify the model, we will use a pseudo-binary approach, considering the hydrated salt to be formed of two main components - one of which is the water involved in the equilibrium in question (p molecules per salt molecule) and the other is the skeletal structure of the salt, involving the anhydrous part, and possibly the n water molecules of inferior hydrates not involved in the equilibrium. 

For example, we will look at isothermal curves. Thus, we can disregard the point defects in the skeleton, which will remain more or less constant in nature and in number. Hence, we are only interested in the structure elements relating to water, and we distinguish  

We will make the hypothesis of the predominant defect in a given hydrate, supposing that water molecules are never to be found on a site normally occupied by the skeleton. 

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