Realization of the triple point of carbon dioxide evaluated by the ITS-90

We performed a precise temperature measurement of the triple point of carbon dioxide (CO2), which is one of the secondary reference points of the International Temperature Scale of 1990 (ITS-90), using an adiabatic calorimeter with five capsule-type standard platinum resistance thermometers (SPRTs). Impurity analyses showed that the purity of the CO2 sample was 99.999 37%. A thermal treatment process before the realization of the triple point reduced the width of the melting curve of the triple point of CO2 and increased the linearity of the melting curves with respect to the inverse of the melting fraction F. We determined the triple-point temperature of CO2 at the liquidus point by extrapolating the six melting curves obtained after the thermal treatment process. The triple-point temperature of CO2 measured in this work is 216.590 90 K 0.36 mK (k = 1). This result indicates that the triple point of CO2 can potentially be used as a fixed point based on the ITS-90 to replace the triple point of mercury.