<P> Consider the case of nickel (II) chloride hexahydrate . This species has the formula NiCl (H O). Crystallographic analysis reveals that the solid consists of (trans - NiCl (H O)) subunits that are hydrogen bonded to each other as well as two additional molecules of H O. Thus 1 / 3 of the water molecules in the crystal are not directly bonded to Ni, and these might be termed "water of crystallization". </P> <P> The water content of most compounds can be determined with a knowledge of its formula . An unknown sample can be determined through thermogravimetric analysis (TGA) where the sample is heated strongly, and the accurate weight of a sample is plotted against the temperature . The amount of water driven off is then divided by the molar mass of water to obtain the number of molecules of water bound to the salt . </P> <P> Water is particularly common solvent to be found in crystals because it is small and polar . But all solvents can be found in some host crystals . Water is noteworthy because it is reactive, whereas other solvents such as benzene are considered to be chemically innocuous . Occasionally more than one solvent is found in a crystal, and often the stoichiometry is variable, reflected in the crystallographic concept of "partial occupancy ." It is common and conventional for a chemist to "dry" a sample with a combination of vacuum and heat "to constant weight ." </P> <P> For other solvents of crystallization, analysis is conveniently accomplished by dissolving the sample in a deuterated solvent and analyzing the sample for solvent signals by NMR spectroscopy . Single crystal X-ray crystallography is often able to detect the presence of these solvents of crystallization as well . Other methods may be currently available . </P>

One of most common solvent used for crystallization is