Water of crystallization, aquo complexes and solid hydrates

Water of crystallization, aquo complexes and solid hydrates 

Many salts crystallize from aqueous solution not as the anhydrous compound but as a well-defined hydrate. Still other solid phases have variable quantities of water associated with them, and there is an almost continuous gradation in the degree of association or bonding between the molecules of water and the other components of the crystal. It is convenient to recognise five limiting types of interaction though the boundaries between them are vague 

3 and 5H2O, and NaC1.2H20 is also known. The space-filling role of water molecules is even more evident with very large anions such as those of the hctcropoly acids (p. 1013), e.g. H3[PWi3O40]-29H2O. 

Berfcz and M. Balla-Achs, Gos Ihdrates. Elsevier. Amsterdam. 1983, 343 pp. 

Such reactions are discussed at appropriate points throughout the book as each individual compound is being considered. A particularly important set of reactions in this category is the synthesis of element hydrides by hydrolysis of certain sulfides (to give H2S), nitrides (to give NH3), phosphides (PH3), carbides (C Hm), borides (B Hm), etc. Useful reviews are available on hydrometallurgy (the recovery of metals by use of aqueous solutions at relatively low temperatures), hydrothermal syntheses and the use of supercritical water as a reaction medium for chemistry. 

Crystal suucture of HPFs.fiHjO showing the cavity formed by 24 H2O molecules disposed with their O atoms at the vertices of a truncated octahe dron. The PF( octahedra occupy centre and comers of the cubic unit cell, i.c. one PFf, at the centre of each cavity.  

Another important reaction (between H2O, I2 and SO2) forms the basis of the quantitative determination of water when present in small amounts. The reaction, originally investigated by 

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