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

The triple points of carbon dioxide (CO 2 TP) and sulfur hexafluoride (SF 6 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 CO 2 TP and SF 6 TP need to be improved. Compounds of CO 2 and SF 6 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 CO 2 TP and SF 6 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 CO 2 TP. This narrow variation implies that the refining process causes only an insignificant change compared with the natural abundances of the minority isotopes 13 C and 18 O in the source gas. Consequently, the small change will probably cause a minor inconsistency for measurements of the CO 2 TP. By contrast, for measurements of the SF 6 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 CO 2 and the TP of SF 6 are desirable, particularly for the latter molecule.