The dehydration of ammonium, potassium and some mixed alums

Abstract

The shapes of the dehydration nuclei and their rates of growth have been investigated in hard vacuum and in the presence of water vapour. The activation energies and ^-factors for the interface reaction in hard vacuum are those normally expected for solid reactions occurring at room temperature. The thermodynamic heat of dissociation is within ±0.5 kcal. of the heat of activation, indicating a very close coupling between the process of evaporation of water and the growth of the new solid phases. The main process occurs layer by layer along the 100 planes of the hydrate. There is evidence of the production of a lattice defect of water in the hydrates before nuclei are formed. Water vapour slightly accelerates the rate of reaction at very low water pressures, but the main effect at higher pressures is a retardation which is probably due to the blocking of active areas in the interface by adsorbed molecules of water. In mixed potassium and chromium alums, the layer growth is inhibited and dehydration occurs ahead of the normal interface, as if the hydrates were behaving as zeolites. The reaction can occur in several stages and very complex phenomena are observed. Explanations: are given of the abnormal reactions occurring with some hydrates, in which there is a breakdown of the close coupling between the dehydration process and the formation of the new solid phases.