CALORIMETRIC METHOD FOR RAPID-DETERMINATION OF CRITICAL WATER-VAPOR PRESSURE AND KINETICS OF WATER SORPTION ON HYGROSCOPIC COMPOUNDS

A rapid method for determining the equilibrium water vapor pressure ov er a hydrate or saturated solution has been developed. The vapor space over the sample in an isothermal, heat conduction calorimeter is titr ated with water vapor generated by continuously scanning the temperatu re of a container of water outside the calorimeter. The rate of heat p roduction in the sample, which is proportional to the late of water so rption by the sample, remains very small until the critical water vapo r pressure is reached. The heat rate (phi) increases markedly and is p roportional to water vapor pressure above the critical water vapor pre ssure, The change in d phi/dp(H2O) signals the appearance of a new pha se. The method was tested with NaBr, NaHSO4, Na2SO4, NH4Cl and (NH4)(2 )SO4 at 24, 34, 44 and 54 degrees C. Critical water vapor pressures de termined are all within 2 torr of those predicted from literature data . Delta H values for the reaction with H2O(g), calculated from the tem perature dependence of the critical water vapor pressure, are close to the enthalpy change for condensation of water and agree with literatu re data. In the regions of the titration curve before and after a phas e change, the value of (d phi/dp(H2O)) is shown to be equal to the pro duct of the rate constant for water sorption, a function of sample sur face properties, and Delta H for the reaction. Values of (d phi/dp(H2O )), both before and after phase transitions, were determined at the fo ur temperatures. The Arrhenius activation energy for water sorption is approximately zero for all the reactions studied.