The dehydration of crystals of chrome alum

Abstract

The loss of water from a salt hydrate can occur with or without changes in the lattice structure. Thus, the water of crystallization of zeolites can be removed without the collapse of the original lattice; whereas in many other hydrates, such as the alums or copper sulphate pentahydrate, the removal of water is associated with changes in the structure of the solid and the production of a new solid phase. In the latter an interface is formed separating the two solid phases, and it is at this interface that the dehydration occurs. The water liberated at the interface passes through a superposed layer of product before it reaches open space. The rate of liberation of water may be dependent on the thickness of the superposed layer of product, as is seen in the case of the dehydration in vacuum of copper sulphate pentahydrate (Garner and Tanner 1930; Smith and Topley 1931). This effect was called “impedance” by Smith and Topley, and they showed that the “impedance” practically vanishes if the dehydration is studied in water-vapour pressures lower than the dis­sociation pressure of the salt hydrate. Since water vapour accelerates the rate of crystallization of the product (Colvin and Hume 1938) it is clear that it is the degree of crystallization of the product that determines whether impedance will occur or not. The two processes, (1) the loss of water from the interface and (2) the subsequent reorganization of the lattice, do not appear to be very closely linked, since the solid product in many examples undergoes considerable shrinkage subsequent to the loss of water (see plate 18 A, for chrome alum). The following model of the dehydration process serves to indicate the possibilities. I represents the solid reactant, and II is a transition layer derived from I by the loss of water either without lattice change or by a collapse of the original lattice. Ill is the product undergoing slow crystallization, and IV is the open space which may be a hard vacuum or may contain water vapour.