Calorimetric method for rapid determination of critical water vapor pressure and kinetics of water sorption on hygroscopic compounds

Abstract

A rapid method for determining the equilibrium water vapor pressure over a hydrate or saturated solution has been developed. The vapor space over the sample in an isothermal, heat conduction calorimeter is titrated with water vapor generated by continuously scanning the temperature of a container of water outside the calorimeter. The rate of heat production in the sample, which is proportional to the rate of water sorption by the sample, remains very small until the critical water vapor pressure is reached. The heat rate (φ) increases markedly and is proportional to water vapor pressure above the critical water vapor pressure. The change in dφ/dp H2O signals the appearance of a new phase. The method was tested with NaBr, NaHSO 4, Na 2SO 4, NH 4Cl and (NH 4) 2SO 4 at 24, 34, 44 and 54°C. Critical water vapor pressures determined are all within 2 torr of those predicted from literature data. ΔH values for the reaction with H 2O(g), calculated from the temperature dependence of the critical water vapor pressure, are close to the enthalpy change for condensation of water and agree with literature data. In the regions of the titration curve before and after a phase change, the value of (dφ/dp H2O) is shown to be equal to the product of the rate constant for water sorption, a function of sample surface properties, and ΔH for the reaction. Values of (dφ/dp H2O), both before and after phase transitions, were determined at the four temperatures. The Arrhenius activation energy for water sorption is approximately zero for all the reactions studied.