Effects of Isotopes on the Triple Points of Carbon Dioxide and Sulfur Hexafluoride

Abstract

The triple points of carbon dioxide (CO₂ TP) and sulfur hexafluoride (SF₆ TP) are candidate substitutes for the triple point of mercury from the set of defining points of the international temperature scale of 1990. For the replacement to be successful, the measurement performances of CO₂ TP and SF₆ TP need to be improved. Compounds of CO₂ and SF₆ contain different isotopes of carbon, oxygen and sulfur_. They fractionate in the solid and liquid phase of the compounds and shift the phase transition temperatures. Thus, the isotopic effect is a probable cause for inconsistency of measurements of CO₂ TP and SF₆ TP, which has motivated us to investigate it. According to the reported fractionation factors and abundances of isotopes, we predict the temperature corrections to vary from − 0.023 mK to 0.051 mK for measurements of the CO₂ TP. This narrow variation implies that the refining process causes only an insignificant change compared with the natural abundances of the minority isotopes ¹³C and ¹⁸O in the source gas. Consequently, the small change will probably cause a minor inconsistency for measurements of the CO₂ TP. By contrast, for measurements of the SF₆ TP the predicted temperature corrections range from − 0.022 mK to 1.223 mK. Our predictions suggest that further investigations of the isotopic effect on measurements of the TP of CO₂ and the TP of SF₆ are desirable, particularly for the latter molecule.