Dielectric behaviour of adsorbed water during dehydration of potassium chrome alum

Abstract

Dielectric measurements at 4 MHz have been used to investigate the behaviour of adsorbed water during dehydration of potassium chrome alum at –23° to 70 °C, in vacuo and in air. The dielectric absorption is generally independent of changes in surface area during dehydration, and anomalous behaviour at temperatures below ca. 30 °C is attributed to formation of a partly amorphous product. The rate of dehydration is controlled by diffusion of water vapour, according to the equation, α=k₁(t^½–t_i^½), where α is the fractional decomposition and t_i is the apparent induction period. Activation energies derived from the variations of k₁ and t_i with temperature are 11·5 ± 0·2 and 15 ± 2 kcal mol^–1 respectively.