Oxides in metal fixed points of the ITS-90

Abstract

In the range between 0 °C and 961 °C, the International Temperature Scale of 1990 (ITS-90) depends to a great extent on the freezing points of the pure metals gallium, indium, tin, zinc, aluminium and silver. An up-to-date realization of these fixed points is based on cells containing metals of ultra-high purity (6N or better) and should include a correction for the influence of relevant impurities. Still, chemical analyses of the fixed-point material can show large amounts of oxygen, which had to be neglected so far, because of the lack of detailed knowledge about it, presuming it could be removed from the cell by applying a vacuum (less than 1 Pa) for a few hours.