1556
ANALYTICAL CHEMISTRY
mixtures. The phyaical methods-e.g., thermal conductivit L , refractometry, et?.-ordinarily applied to the analysis of these fractions do not distinguish the possible presence of a third constituent and the analytical values may contain large and completely unsuspected errors when such a contaminant orcurs. Even when it does not allon total analysis of the mixture involved, the effusiometric examination can still shed some light on the degree of complexity of the sample. Other Applications. If a specimen of a completely unknown g x i is a t hand, it is advisable to determine the proportion of effusion a t a minimum of two time intervals. Using Equation 2 iii the interpretation of these data, we secure two or more independent cvsluations of the apparent molecular weight of the original specimen. If the vwlues 10 obtained are all concordant, the molecular weight ot the g:ts, and thence a valuable clue to its identity, is derived. (Thi?, of course, does not exclude the possihility that the sample is a mixture of two gases of nearly identical molecular weight.) On the other hand, a systematic variation in the value9 of the apparent molecular weight of the gas provides a definitive indication of the complexity of the sample. The divergence of the bracketed results in the fourth column of Tiihle I1 suggests the magnitude of the corresponding variation in thc apparent molecular weights calculated by the application of Equation 2 to a mixture containing molecular species of divcme we i ghtm . When incomplete analytical data are available from some other hource(s) there ensues a noteworthy simplification, and siniultaneous increase in power, of the effusiometric methods.
-4s a concrete example, consider the problem of identifying a minor component that is not absorbable in volumetric gas analytical procedures. From such procedures the complete quantitative make-up of the sample, and the qualitative character of all but the unabsorbed component, would be known. The determination of the molecular weight of the unknown constituent can then be based on a single measurement of the effusion rate. The "identification" of the argon in air provides an exemplification of this possibility. Assume that it has been found that air consists of the following components in the indicated proportions: ovygen 21.0; nitrogen 78.0; component X 0.9. A single effusion trial made with air that had been passed over Ascarite gave PI!Pc a q 0.5811 after 5 minutes For a ternary mivture n e may write:
of the molecular weights of the various components is not very large. COlVCLUSION
The method that has been described brings reasonably accurate analysis of binary mixtures within the compass of effusiometric work. Rather more important, it is one of very few purely physical methods (other than those based on spectrometry and spectrophotometry) that permits of even approximate quantitative analysis of polycomponent mixtures and is also capable of furnishing some indication of the qualitative make-up of such mixtures. The proposed method of analysis should be particularly valuable for the examination of any group of gases that are not readily separated from one another and that show suhstantial gradations in their niolecular weights. Two such groups are the paraffin hydrocarbons and the inert gases. This exploratory investigation has been conducted with relatively crude equipment, and the experimental techniques could be cwnsiderably refined. Thus, x n increase in the size of the effusion vessel, the size of the sample, and the duration of the experiment should greatly improve the accuracy of all the measurements, with a corresponding improvement in the quality of the analytical resultq. Furthermore, i t appears possible to replaw the present method of measuring the proportion of effusion, in terms of the residual quantity of gas in the effusion chamber, by an alternative technique that would not require a separate trial for the determination of each individual proportion. Thus, one might use an indirect hut semicontinuous method of assaying the proportion of effusion, measuring the quantities of gas that pass through to the low-prcssure side of the orifice. The authors hope to investigate the effect of these improvements. However, the effusiometer described will do practically everything (except handle condensable vapors) that can he done by any previously described effusiometer, and it is capable of many other determinations that no previous effusiomrter has brought even remotely within the range of possibility ,iCKYOWLEI)C;MIE\IT
The authors are indebted to E. H. deButts and It. E. Lundin for a number of helpful suggestions. LITEK4TURE CITED
ill1 the mole fractions, and two of the three molecular weights, are known. Solution for the third molecular weight then gives
from which the molecular weight of component X is calculated as 39.3. The closeness of this approximation to 40, the molecular weight of argon, is rather striking when it is recalled that less than 1 of the unknown constituent was present, and that the spread
(1) Benson, S. W., and C'aswell, S.,J . Phys. Colloid.
