A kinetic study of the dehydrations of the alums KCr(SO4)2. 12H2O and KAl(SO4)2. 12H2O

A kinetic study of the dehydrations of the alums KCr(SO4)2. 12H2O and KAl(SO4)2. 12H2O in water vapour atmospheres is reported. The product-yield-time data are described by the Avrami-Erofe’ev (n= 2) and contracting cube equations. The observed kinetic behaviour differed from expectation for this reaction, which is known to proceed through the growth of three-dimensional nuclei. The retention of the volatile product (water) in the vicinity of the reactant solid influences the course of interface generation and development during the early stages of dehydration. Later there is a ‘first order’ approach to the establishment of equilibrium between water vapour and the solid reactant/product mixture. This pattern of kinetic behaviour is confirmed by a parallel thermal analysis (DSC) study in dry nitrogen. Evidence was obtained that an appreciable quantity of water was retained by the solids throughout these reactions. This is consistent with the view that water, temporarily retained at the active reactant-product interface promotes the difficult recrystallization step. Photoacoustic studies on partly thermally dehydrated samples of KCr(SO4)2. 12H2O showed the formation of a temporary water depleted surface layer that readily rehydrated either by adsorption from the atmosphere or by facile intracrystalline water diffusion. All the present observations can be explained by a reaction mechanism based on two main features: (i) the dehydration reaction is initiated across the total surface but is limited in extent and cannot be considered topotactic; (ii) the rate determining step for the subsequent unlimited advance of dehydration into the solid bulk is product crystallization, possibly topotactic, and this confers apparent topotacticity to the whole processes.