The effect of water vapour pressure on the kinetics of the thermal dehydration of erbium formate dihydrate

Abstract

The kinetics of the thermal dehydration of erbium formate dihydrate has been studied by means of isothermal gravimetry under various water vapour pressures from 5 × 10 −4 to 8 Torr. The kinetics of the dehydration, on the whole, can be described by the contracting volume model, R 3. The rate of dehydration k exhibited an unusual dependence upon the water vapour pressure: with increasing water vapour pressure, the rates increased at first, passed through a maximum, and then decreased gradually to a constant value. These phenomena were analogous to the Smith-Topley effect, and seemed to be correlated to the crystallinity of the dehydrated products. At low water vapour pressure (≈ 10 −4 Torr), the amorphous dehydrated products cover the surface of the reaction particles and interfere with the escape of the dissociated water molecules. Therefore, the rate of dehydration is slow. The presence of a few water molecules seems to promote recrystallization of the dehydrated products. The recrystallization results in the formation of wide channels between the dehydrated particles, and the dissociated water molecules can easily escape through these channels. Therefore, the dehydration rate seems to increase. At higher vapour pressure, the rate decreased gradually because of the reverse reaction caused by atmospheric water molecules.