THE ROLE OF AMORPHOUS INTERMEDIATE PRODUCTS IN THE DEHYDRATION OF CERTAIN HYDRATED SALTS

Following the dehydration of certain hydrated salts in high vacuum, an evolution of energy occurs. X-ray studies have shown that this energy liberation is due to an amorphous to crystalline transition in the products of dehydration. Measurements have been made of the integral heats of solution of the dehydration products of copper sulphate pentahydrate and zinc sulphate hexahydrate formed at a series of low dehydration pressures. From these measurements, the fractional amounts of relatively high energy amorphous products formed at a series of low dehydration pressures have been calculated. It has been found that as the water vapor pressure near the reaction interface is increased, the fractional amount of high energy product decreases to a minimum, then increases, passes through a maximum, followed by a slow decrease. These results are interpreted in terms of a possible dehydration mechanism, and an estimation made of the effect of water vapor pressure on the over-all reaction rate. In the course of this study, the integral heats of solution of the crystalline hydrates involved have been determined. The heats of transition of the amorphous to crystalline forms of copper and zinc sulphate monohydrates are reported.