Methods to Determine the Hydrate Composition

The work in Table 1.2 illustrates one of the early research difficulties that is still present—namely, the direct measurement of the water to gas ratio in hydrates (hydration number, n = water molecules per guest). Whereas many solids 

On a macroscopic basis, it is difficult to remove all excess water from the hydrate mass this causes a substantial decrease in the accuracy of hydrate composition measurements. Hydrate formations often occlude water within the solid in a metastable configuration, thereby invalidating the composition obtained upon dissociation. Mixed guest compositions of the hydrate are also confounded by the concentration of heavy components in the hydrate phase. Unless the associated gas reservoir is large, preferential hydration may result in variable gas consumption and perhaps an inhomogeneous hydrate phase as discussed in Chapter 6. 

Villard (1896) proposed an indirect macroscopic method to determine hydration number, which uses the heat of formation, both above and below the ice point. In his review, Schroeder (1927) indicates that after 1900, researchers abandoned direct measurement of hydrate phase composition, preferring Villard s method (see Section 4.6.2) that relies on easier measurements of pressure and temperature. Miller and Strong (1946) provided another thermodynamic method to determine hydration number, discussed in Section 4.6.2.2. 

Circone et al. (2005) obtained hydration numbers from direct macroscopic measurements of the amount of gas released during dissociation. Their results were in close agreement with those obtained by Galloway et al. (1970) from measurements of gas uptake during synthesis and release during decomposition, and by Handa (1986e) from calorimetric measurements. 

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