Control of Water Activity During Reaction

One simple and convenient method is the addition directly to the reaction mixture of suitable pairs of solid salt hydrates. A given salt hydrate will give up its water at a characteristic water activity, transforming to a lower hydrate or an anhydrous form. If the pair are placed in a system of water activity below their characteristic transition value, the (higher) hydrate will tend to give up water to the rest of the system. Water release will continue until the whole system reaches the transition water activity (or 

Na2SO4.10H2O (crystals) Na2S04 (crystals) + IOH2O (1) 

Salt pair Equilibrium water activity Rate of water transfer Maximum temperature (°C)  

The pairs used are identified by a shorthand notation NaI.2/0 means a combination of NaI.2H20 and anhydrous Nal (i. e. OH2O). Equilibrium water activity values are for 25 °C. Fast water transfer indicates equilibration in a few minutes, slow that several hours may be needed. There is only limited information on the behavior of hydrate pairs giving lower water activities, though some indication that they generally tend to equilibrate slowly. From Zacharis et al.[23]. 

All of this describes just the thermodynamically favored directions of water transfer, for ideal crystalline solids. Many salt hydrate pairs seem to behave approximately ideally. However, if water activity is to be controlled close to the transition value, the rates of water release and uptake must be sufficient. Different salt pairs have very different rates of water exchange. It is difficult to give quantitative values, because the rates will depend on the size and shape of the crystals in each of the salt hydrate forms. This will depend on how they have been crystallized and handled subsequently. For example, cycling between hydrate forms, with gain and loss of water, will usually lead to a reduction in crystal size, and hence more rapid water exchange in future cycles. 

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