Equilibria between stoichiometric hydrates

In the equilibrium reaction [4R.6], the two hydrates constitute two solid phases, the law of mass action applied to that equilibrium is written as  

This is expressed by the fact that at a given temperature, the equilibrium water vapor is fixed. 

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The equilibrium thermodynamics of stoichiometric hydrates has been described by several authors. The overview presented here is intended both to review the basic thermodynamics of crystalline hydrate formation/stability and to highlight the intrinsic differences between polymorphic systems and hydrate systems (a discussion of the kinetics of dehydration/hydration will be given in Section IV). The following description is a hybrid based on the work of Grant and Higuchi [7] and that of Carstensen [8]. 

Figure 4.5. Equilibrium isotherm between water vapor and a) a stoichimetric hydrate and b) a non-stoichiometric hydrate...

Figure 4.7. Equilibrium isotherms between water vapor and a non-stoichiometric hydrate with non-localized water molecules...

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