A critical test for the purity of gases

This paper reports a method by which the purity of gases (and liquids having normal boiling points below room temperatures) may be critically tested. The method involves one approximately isothermal distillation, whereby a small initial distillate, a middle cut, and a small final residue are obtained. The pressures of these three fractions are intercompared by means of a simple differential manometer and form the basis for indicating impurities of higher and lower boiling points than that of the approximately pure substance. The method eliminates the exacting temperature requirements of the widely used criterion of constancy of pressure during an isothermal condensation or vaporization. At the same time, it offers more information than the isothermal condensation, and is suitable for testing mixtures containing more than two components. In many cases the order of magnitude of the impurities may be calculated. Like the isothermal condensation, it is not suitable for studying azeotropic mixtures. The preparation of a pure gas is not an easy matter, and satisfactory tests for purity are often difficult to formulate and usually difficult to perform. In many cases the method of preparation is about as good a guarantee of purity as the method of test subsequently applied, and sometimes it offers even more reliable assurance of purity. These complications are largely responsible for the fact that reported investigations deal abundantly with gases of doubtful purity. The ordinary methods of so-called " exact" volumetric gas analysis, which are often employed in the effort to establish the purity of a gas, are subject to errors of such magnitude as to be unfit for this purpose. It is sometimes possible to design very special chemical methods which may offer satisfactory evidence of purity. In general, however, the measurement of one or more of the physical properties of a gas affords the basis for the determination of its purity. One of the most frequently employed criteria for purity is the constancy of pressure during isothermal condensation or vaporization. The method is very useful, but has limitations which are not generally recognized. Washburn * has discussed the physical limitations of this method. In this connection, Washburn suggested a more sensitive and convenient method for determining purity in case the substance to be examined is nearly pure. This suggestion is given on page 595 (paragraph 3) of his paper, and forms the basis of the method described in the present discussion. The apparatus and detailed technique, which have been found suitable for the application of this method, are reported in order to make the method easily available. At the same time, the original suggestion has been extended in order to permit the collection of additional information. 1 E. W. Washburn, Constancy of pressure during isothermal condensation or vaporization as a criterion of purity, Zs. physik. Chemie (Cohen-Festband), p. 592, 1927.