Some observations on the isothermal dehydration of uranyl(VI) nitrate hexahydrate above room temperature

Abstract

The dehydration of uranyl(VI) nitrate hexahydrate to the trihydrate has been studied gravimetrically between 313 and 343 K at varying water-vapour pressures in flowing air. With crushed crystals the dehydration rate is controlled by diffusion of water through the powder bed, even when using very shallow bed depths; with single crystals the kinetics of dehydration conform to a contracting-interface model. The activation energy is higher than that reported for vacuum decomposition. X-Ray powder data for the hexa- and tri-hydrates of uranyl(VI) nitrate have also been recorded. Single crystals of the former grew with a similar habit to that found by earlier workers, but with a different orientation of the crystal axes When partially dehydrated crystals were heated above the melting point of the hexahydrate (333 K) it was observed that the molten hexahydrate was encapsulated in a sheath of solid polycrystalline trihydrate. Under these conditions, the dehydration reaction became very slow but accelerated again on cooling below the hexahydrate melting point. This is a new observation, not previously reported for crystal hydrate decomposition.