Hydration of lanthanum tungstate (La/W = 5.6 and 5.3) studied by TG and simultaneous TG–DSC

Abstract

The hydration properties of lanthanum tungstate (La/W=5.6 and 5.3) have been studied by means of thermogravimetry in the temperature range 1100 to 300°C. The relative weight was independent of the oxygen partial pressure between oxidizing and reducing conditions over the entire temperature range showing that the oxygen stoichiometry is close to constant. The materials take up water upon decreasing temperature and increasing water vapor partial pressure, eventually reaching saturation. Moreover, there is a significant reversible change in the relative weight upon shifting from H2O to D2O containing atmospheres. The results are interpreted based on the most recent defect model for dissolution of protons in lanthanum tungstate yielding standard enthalpy and entropy changes of hydration of ΔHHydr0 ≈-90kJmol−1 and ΔSHydr0 ≈-115J (molK)−1. The hydration enthalpy was verified by direct measurements using TG–DSC, yielding -85kJmol−1. The thermodynamic parameters obtained herein are in general agreement with literature values from fitting conductivity data, and may be considered an improvement of these.