Effect of deuteration on the thermal dehydration of lithium sulfate monohydrate

Abstract

Effects of deuteration on the thermal stability, enthalpy change and kinetic parameters for the thermal dehydration of Li2SO 4 9 H20 were examined by means of TG and DSC. The enthalpy change for the monohydrate was smaller by 1.63 -}-0.75 k J/tool than that for the monodeuterate. The rate constant for the monohydrate was a little larger than that for the monodeuterate. However, the thermal stability, activation energy and frequency factor were practically unaffected by deuteration of the hydrate. It is suggested that the activation entropy as well as the activation energy should be considered in explaining the kinetic deuterium isotope effect in the thermal dehydration of Li2SOa " "H20.Recently, we have determined effects of deuteration on the thermal dehydration processes of CAC204 9 H20 [1], H3BO3 [2,3] and CuSO4 9 5 H20 [4,5] by means of TG and DSC. It was found from these studies that the size or direction of such an isotope effect varies from sample to sample. In addition, these results proved to be useful for the better understanding of the dehydration processes.It thus seems of interest to study the dehydration process of various hydrates extensively by means of TG and DSC. The present paper deals with both thermodynamic and kinetic isotope effects of deuterium in the thermal dehydration of Li2SO4 9 H20 by means of TG-DSC recorded simultaneously, the kinetics [6,7] and thermodynamics [8] having been examined earlier. ExperimentalReagent grade Li2SO 4 9 H20 was ground and sieved to the size fraction of 170-200 mesh. The monohydrate was dehydrated thermally and the anhydrous product was rehydrated with water vapour in a small sealed bottle to give Li2SO 4 9 9 H20. The deuterium analog was prepared with 99.8 % deuterium oxide vapour under identical conditions. The identification was made by means of IR and TG. About 1.56 9 10 -4 tool of the sample was weighed into a platinum crucible 5 mm in diameter and 2.5 mm in depth, a-Alumina was used as a reference material. TG-DSC traces were recorded simultaneously at a heating rate of 1.04 K/rain