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

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 C-13 and O-18 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.

Automated summary

This study investigates the isotopic effects of carbon dioxide and sulfur hexafluoride on their triple point measurements. The temperature corrections for CO2 TP show a narrow variation, resulting in minor inconsistencies, while for SF6 TP, the predicted temperature corrections range widely, indicating a need for further investigation.