Dielectric behaviour of water adsorbed on partially dehydrated alums

Abstract

Partial dehydration of NH₄Fe, KCr, KAI, and NH₄Al sulphate alums in vacuo, and their subsequent exposure to water vapour, have been investigated at 19–23° using dielectric measurements at 4·0 Mc./sec. Initial values of ε′ and ε″ relate to equilibrium water vapour pressure data, and are higher for crushed samples. Marked decreases in ε′ and ε″ occur in the early stages of the dehydration of NH₄Fe and KCr alums, but not with KAl and NH₄Al alums, and the curve of ε″ against time for the dehydration of NH₄Fe alum shows a secondary maximum. Dehydrated NH₄Fe and KAl alums sorb water more readily than the dehydrated KCr and NH₄Al alums. Changes in ε″ during re-hydration are shown to reflect changes in adsorbed water and are independent of water of hydration. At constant re-hydration pressure, ε″ increases linearly with the equilibrium water vapour pressure of the original alum, indicating that adsorbed water is bound to the surface by a mechanism similar to that with which water of crystallisation is held inside the lattice. Changes in ε″ during re-hydration of dehydrated NH₄Fe alum differ from previous results obtained with Rochelle salt.