Vapor pressure and related thermodynamic properties of Ar-36

The Ar-36-Ar-40 vapor pressure isotope effect in the liquid and solid phase s has been experimentally determined at about 100 temperatures between 83 a nd 97 K using a high-accuracy double-differential manometric technique. Dif ferences between the vapor pressure of a highly enriched sample of Ar-36 an d that of natural argon were measured simultaneously with the absolute vapo r pressure of natural argon. Absolute and differential vapor pressure measu rements show an estimated accuracy of 0.15% and 0.001%, respectively, with respect to the absolute vapor pressure of argon. Data were obtained in both the solid and liquid regions. The vapor pressure of Ar-36 is always higher than that of Ar-40 by about 0.5%. Triple-point temperatures and pressures were also measured for both isotopes. All data compare favorably with previ ous results reported in the literature and are nicely interpreted within th e framework of Bigeleisen's theory of isotope effects. Using a large temper ature extrapolation of our data, liquid-vapor isotope fractionation factors were successfully inferred from the vapor pressure measurements; the estim ated values agree with the experimental ones up to close to the critical po int. The isotopic difference in the molar enthalpies of vaporization and su blimation was also calculated from the vapor pressure data. Additionally, m olar volume and second virial coefficient isotope effects are estimated usi ng several different theoretical approaches.