Amount concentration of Ar in O2 obtained by means of thermal analysis of certified mixtures at the α–β transition of O2, and preliminary confirmation of a new finding

For the first time, high-precision measurements were performed on the temperature of the α–β transition of oxygen, both pure and affected by different amount concentrations of Ar impurity up to an amount concentration x(Ar) ≈ 1·10−3 mol. The aim of this study was to find an accurate way to detect the amount concentration of argon in oxygen, which affects the triple point temperature of oxygen, previously without another possibility to determine it accurately (except with the 10-times less sensitive β–γ transition of oxygen). It was found that the influence on the α–β transition temperature is strong, with a maximum value at no supplied power of Ttr,0 = 23.894 85 K, with a standard uncertainty u(Ttr,0) = 60 μK, and a first derivative for nearly null amount concentration of 133.9 μK/10−6 mol Ar, with a standard uncertainty of 0.4 μK/10−6 mol Ar, allowing to detect an impurity of x(Ar) = 1·10−6 mol corresponding to an accuracy in temperature measurement of ≈100 μK. The correction equation is provided up to an amount concentration x(Ar) ≈ 1·10−3 mol, where the sensitivity to argon amount concentration is about half. In the course of this study, confirmation was found of previous findings of an unknown weak apparent transition occurring in oxygen (pure or containing up to x(Ar) ≈ 1·10−3 mol) just below the known α–β one by less than 0.1 K.