DISTRIBUTION OF ISOTOPIC WATER MOLECULES, H2O, HDO, AND D2O, IN VAPOR AND LIQUID PHASES IN PURE WATER AND AQUEOUS SOLUTION SYSTEMS

Abstract

The distribution of isotopic water molecules, H2O, HDO, and D2O, in vapor and liquid phases in pure water and aqueous solution systems is related to the D/H fractionation factor between liquid and vapor phases, αL-V, and the vapor pressure ratio between pure H2O and D2O, PH2O/PD2O. As pure HDO does not exist, it is complicated to compare the experimental data of the vapor pressure ratio of pure H2O and D2O with those of a D/H fractionation factor between liquid and vapor phases directly.In pure water and aqueous solution systems, the γ value, defined as γ = ln(PH2O/PD2O)/ln(PH2O/PHDO) = ln(PH2O/PD2O)/lnαL-V, is calculated with the assumption that the experimental data of the D/H fractionation factor αL-V is equal to the vapor pressure ratio between pure H2O and hypothetical pure HDO, PH2O/PHDO. The γ0 values in a pure water system decreased from 1.92 to 1.88 in the temperature range from 0 to 100°C, and the γm values in aqueous solutions for alkali halide salts and urea at 25°C were similarly evaluated using the value of γ0 = 1.92. Using the γ values, the equilibrium constant of the liquid phase for the reaction, H2O + D2O = 2HDO, in pure water and aqueous sodium chloride solution systems, can be estimated from the equilibrium constant for the vapor phase.