Vapour Pressure of H218O Ice (—50 to 0°C) and H218O Water (0 to 170°C)

Abstract

The vapour pressure difference between H216O and highly enriched H218O samples was measured directly with a capacitance type differential monometer between –50 and 80°C and with a U-type mercury manometer between 80 and 170°C. Data for the vapour pressure isotope effect in the ice range gave a good fit to the equation ln([graphic omitted])= 3.710/T+ 0.00132 (223 < T < 273), and those in the liquid range to ln ([graphic omitted])= 5.869/T– 0.0108 (293 < T < 443).

From the temperature dependence of the vapour pressure ratios the isotope effects on the heat of sublimation and evaporation were calculated. At the melting point Δ(ΔHsub)= 31 J mol–1 while Δ(ΔHevap)= 77 J mol–1 were found. The results are compared with earlier measurements and with data for the HOH/DOD system, where the isotope effect on the heat of fusion is much larger and of opposite sign.