Kinetics of dehydration reactions-a comparison between isothermal and non-isothermal studies

Abstract

A comparison of results for the kinetics of dehydration of copper sulphate monohydrate from TG, DTA, DSC and isothermal weight-change measurements indicated that differences in impedance to diffusion of water vapour affected conclusions regarding the reaction mechanism. Isothermal studies indicated that the mechanism is in accordance with Jander's model for three-dimensional diffusion. Non-isothermal studies, on the other hand, suggested that Mampel's model for random nucleation and growth of nuclei without overlap was valid. The effect of impedance in the non-isothermal studies was an increase in the activation energy and the pre-exponential factor. When impedance to dehydration was low, non-isothermal measurements yielded activation energy values and pre-exponential factors in good agreement with those obtained from isothermal measurements, notwithstanding disagreement in the reaction mechanism. The distinction between 'crystalline' copper sulphate monohydrate and the species produced in situ by the dehydration of higher hydrates of copper sulphate, reported by Langfelderova et al., has been shown to be only apparent, and is due to the effect of water vapour from dehydration of CuSO₄·5H₂O and/or CuSO₄·3H₂O on the subsequent dehydration of CuSO₄·H₂O.