The dehydration of copper sulphate pentahydrate

Abstract

In a previous paper were described the results of an investigation of the rate of dehydration of a hemihydrate, where the reaction presumably proceeds to completion without the occurrence of intermediate hydrates. In the course of the work it became evident that further information might be gained from the study of a dehydration reaction which could take place in several stages. For this purpose copper sulphate is eminently suitable since a penta-, a tri-and a monohydrate exist. It was hoped that by a choice of suitable conditions, the several stages of the reaction might be observed. The dehydration of copper sulphate pentahydrate has been extensively investigated, notably by Rae, Crowther and Coutts, and recently by Garner and Tanner. The most direct method of following the course of such a reaction is clearly the measurement of the rate of change of mass of a number of small crystals of uniform size. Then, if the reaction is assumed to occur at the interface separating the solid phases, values for the mean rate of linear propagation can be derived. These values will have real significance only if the liberated water molecules are immediately removed from the mixture of reactant and resultant. The experiments of Rae and of Crowther and Coutts, made at atmospheric pressure are invalidated since the free diffusion of the water vapour was restricted by the air pressure. The results of Garner and Tanner obtained in high vacua will be considered in detail later.