Dehydration of calcium hydrogen phosphate dihydrate

Abstract

The dehydration of CaHPO4,2H2O has been studied isothermally between 409 and 537 K and over a range of partial pressures of water vapour between 1·3 cN m–2 and 6·0 kN m–2 at a total pressure of 1 atm. The rate of dehydration is controlled by diffusion of water, and below 478 K the activation energy is 190 kJ mol–1 and this value is not affected by the partial pressure of water vapour. However, above 478 K the activation energy drops markedly and is then affected by partial pressure of water vapour. Some evidence has been obtained for the formation of a monohydrate as an intermediate and, while this is not conclusive, this postulate helps to clarify the results of previous work.