Vapor Pressures of Tritium Oxide and Deuterium Oxide. Interpretation of the Isotope Effects

Measurements are reported on the difference in vapor pressures of H2O and T2O ( − 9°-114°C) and of H2O and D2O (4°-114°C). The relative vapor pressures of the pure liquids are represented by ln (pH2O / pT2O) = ( − 103.87 ± 0.57) / T + (46 480 ± 210) / T2 and ln (pH2O / pD2O) = ( − 70.87 ± 0.43) / T + (33 630 ± 160) / T2. The normal boiling points of T2O and D2O are 101.51° and 101.42°C. Isotopic differences in heats of vaporization are given from 10° to 110°C. Triple-point temperatures and pressures are summarized. Vapor-pressure crossover temperatures are: D2O–T2O, 116.2° ± 0.5°C; H2O–T2O, 190 ± 10°C (estimated); H2O–D2O, 220.7° ± 0.1°C (literature). The data were also fitted to extensions of the above equations which included correction terms based on Bigeleisen's formulation of the vapor-pressure isotope effect and a uniformist harmonic-oscillator model of water. Estimates of the coefficients of 1 / T and 1 / T2 for H2O–D2O from Raman spectra are in reasonable agreement with coefficients from this data fit. Ratios of corresponding coefficients for H2O–D2O and H2O–T2O agree with theory.