Distribution of isotopic water molecules, H 2O, HDO, and D 2O, in vapor and liquid phases in pure water and aqueous solution systems

Abstract

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