Substoichiometric hydrates

In this category of models, the water molecules are localized. They may either be in their normal position (in which case we would have substoichiometric hydrates of the limited hydrate S,( + p) H2O), or in an interstitial position, in which case we are dealing with an overstoichiometric hydrate of the inferior hydrate S,... 

The calculation tells us that this view of things also yields expression [4.24] for the isotherm. In fact, this means that a zone of divariance with localized water molecules can be viewed either as a substoichiometric hydrate in relation to a superior hydrate, or as an overstoichiometric hydrate in relation to an inferior hydrate (or an anhydrous salt). The two models are absolutely equivalent. 

A reproducible coprecipitation or adsorption reaction of a constant amount of the element of interest can be used in substoichiometry. An interesting example is the substoichiometric radioactivation analysis for oxygen, based on the reproducible isolation of fluorine or fluorosilicate with a substoichiometric amount of hydrated tin dioxide It has been applied to the determination of oxygen in silicon crystal. 

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