Molar Refraction of Carbon Dioxide and Its Dependence on Density When Stored in Steel Containers

Abstract

Precise measurements of molar refraction of carbon dioxide as a function of density from zero to 0.06 gram/cc, extend the work previously reported. Repeated runs, involving several hundred observations with the displacement interferometer, indicated an unmistakable but unaccountable negative variation of molar refraction as a function of density, until it was discovered that the age of the gas affected the results. Several hundred additional readings made on samples of gas of various ages, with and without a drying tube of magnesium perchlorate in the line, show that the molar refraction of fresh CO₂ is independent of density, as would be expected of a nonpolar substance. It is suggested that as ``bone dry'' CO₂ ages under pressure in steel cylinders for periods of a year or more, small amounts of CO are formed by chemical reaction. Supporting evidence for this view has been obtained by measuring the decrease in mass of a carefully weighed sample of CuO in a glass tube, as a measured volume of the CO₂ gas was passed through it at a temperature of 250°C. Because CO is strongly polar, traces of it would account for a variation of molar refraction with density such as has been observed in increasing proportions for samples of CO₂ ranging in age from one to several years.