Measurements of Gaseous Diffusion Coefficients

Accurate knowledge of molecular diffusion coeffi· cients and their temperature dependence is important in obtaining fundamental information on forces be· tween unlike molecules. Until recently most diffusion measurements were conducted over short temperature intervals because the conventional methods such as the Loschmidt or the Stefan, require the thermostating of large, static systems. Walker and Westenberg [lP have developed a method which involves measuring concentration profiles downstream of a point source of tracer gas in a uniform laminar jet of carrier gas . Diffusion coefficient for permanent gas pairs over a temperature interval 300 to 1100 K have been measured using this technique. Determining diffusion coefficients using gas cro· matographic techniques is relatively new. Taylor [21 derived an expression for the radial diffusion term for a liquid dispersion flowing in a tube. The same expres· sion was derived by Giddings [3] and Golay [4] as a special case of a general theory of gas chromatography. Giddings and Seager [5] have measured diffusion coefficients using this method. The method reported in this paper is also based on gas chromatography techniques. In brief, a trace amount of one gas was injected into a carrier gas stream of different composition flowing through a column at a constant flow rate. The concentration profile of the gas dispersion was determined at two points downstream from the point of sample injection by thermal conductivity detectors. The binary diffu· sion coefficients were calculated from the dimensions of the recorded peaks and time required [or the peak