Thermometric Fixed Points of Hydrogen

Abstract

The vapour pressure versus temperature relationship for equilibrium H2 was obtained between its normal boiling and melting points. It was found that impurities other than Ne have no effect on the vapour pressure behaviour, presumably because the non-volatile impurities freeze out onto the cold walls of the apparatus and the only volatile impurity, He, appears to be insoluable in H2 and thus can easily be removed by pumping. The effect of catalysis on the rate of ortho-paraconversion was extensively investigated. It was found that the catalyst should be in contact with most of the sample at low temperatures, if stirring is not possible, in order that conversion takes place rapidly. Catalyst in contact with part of the condensed sample only does not work efficiently, probably because of the slow rate of ortho-para interdiffusion. The triple point pressures were found to be 52.763 mmHg (7034.5 N/m2) and 54.082 mmHg (7210.3 N/m2) for equilibrium and normal H2 respectively. The temperatures of the normal boiling and triple points of normal H2 were found to be 117 mK and 148 mK higher respectively than those of equilibrium H2.Boiling points (at 76 and 25 cmHg vapour pressures) and the triple point of equilibrium hydrogen are all defining fixed points on the International Practical Temperature Scale of 1968 (IPTS-68). This paper describes some experiments performed at the National Research Council of Canada aimed at realizing these thermometric fixed points. In addition vapour pressure versus temperature behaviour for liquid H2 samples of differing purities was also investigated.