Dehydration and rehydration behaviour of zinc and magnesium sulphate heptahydrates

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Abstract
The kinetics of the isothermal dehydration of zinc and magnesium sulphate heptahydrates were studied both in vacuo and in the presence of air. Decomposition in vacuo followed a first‐order mechanism, while that in the presence of air followed an interfacial mechanism obeying the Avrami–Eroféev equation. In the case of zinc sulphate decomposing in air, the fusion of the salt in its water of crystallisation interfered with the observed phenomena. The energy of activation in both cases was calculated to be comparable to the latent heat of vaporisation of water, and it is suggested that this would measure the heat of desorption of water from the surface of the dehydration product. When decomposition was carried out in vacuo at comparatively elevated temperatures, the process changed to an activated diffusion or surface diffusion. Dehydration in vacuo yielded an amorphous intermediate, as well as final products, whereas dehydration in the presence of air yielded crystalline products. Differential thermal analysis proved the stepwise nature of the dehydration process.The rehydration from the vapour phase was also studied on the dehydration products of both zinc and magnesium sulphate heptahydrates, and it was shown that the rehydration isotherms could be significantly affected by both the extent and the nature of the surface of the dehydration product. The specific surface areas were measured by low temperature adsorption of nitrogen.