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Estimating the Triple Point Isotope Effect and the Corresponding Uncertainties for Cryogenic Fixed Points



Weston L. Tew


The estimation of sensitivities of melting temperatures due to isotopic variations in monatomic and diatomic atmospheric gases is developed from both theoretical and semi-empirical standpoints. The state of what is currently known regarding vapor pressure isotope effects (VPIE) and triple point isotope effects (TPIE) is briefly summarized for the noble gases (except He), and for selected diatomic molecules including oxygen. An approximate expression is derived for the relative shift in the melting temperature with isotopic substitution. In general, the magnitude of the effects diminish with increasing molecular mass and increasing temperature. Knowledge of the VPIE, molar volumes, and heat of fusion are sufficient to estimate the temperature shift or isotopic sensitivity coefficient via the derived expression. The usefulness of this approach is demonstrated in the estimation of isotopic sensitivities and uncertainties for triple points of xenon and molecular oxygen for which very few or otherwise no documented estimates were previously available. The calculated sensitivities are considerably higher than previous estimates for Xe, and lower than other estimates in the case of oxygen. In both these cases the predicted sensitivities are small and the corresponding uncertainties are
International Journal of Thermophysics


atmospheric gases, isotopes, noble gases, oxygen, temperature, triple point, vapor pressure isotope effect, Xenon


Tew, W. (2008), Estimating the Triple Point Isotope Effect and the Corresponding Uncertainties for Cryogenic Fixed Points, International Journal of Thermophysics, [online], (Accessed April 14, 2024)
Created February 1, 2008, Updated February 19, 2017