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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. |
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