Critical review of information relevant to the correction of the effect of chemical impurities in gases used for the realization of ITS-90 fixed points

According to the Guide to the Expression of Uncertainty in Measurement (GUM) on which the expression of uncertainty in the field of metrology is based, since 1995, 'it is assumed that the results of a measurement have been corrected for all recognised significant systematic effects'. Since the International Temperature Scale of 1990 specifies that the substances used for the realization of the 'fixed points' be 'ideally pure', to fully implement the intent of the GUM corrections should be applied for any chemical impurities that affect the value of the measurand. In general, thermometrists' opinion is that significant laboratory research and more literature search are still needed for further progress towards reliable corrections. This paper, reviewing the available literature data, shows that the situation is more favourable in the case of the substances used for the realization of the Scale reference points in the range 13.8 K to 273.16 K based on the use of hydrogen, neon, oxygen and argon. The appendix reports a similar review also for nitrogen. Then the paper discusses the other conditions, physical-chemical and thermal, of the substances inside the thermometric cells, concurring with the chemical impurity effects to the overall state of knowledge of the realized triple point temperature relevant to the solution of the problem of performing the corrections.