Kirkendall Effect in Gaseous Diffusion

An experiment is described which is the analog in gases of the well-known Kirkendall effect in solids. The experiment can also be thought of as a measurement of the pressure differences arising in a diffusing gas mixture. Seven gas mixtures were measured at 30°C and over a maximum pressure range of about 0.2 to 1.7 atm as follows: H2–D2, CO2–C3H8, He–Ar, He–CO2, H2–CO2, D2–CO2, and H2–(H2 + CO2 mixture). The results are discussed in terms of the usual phenomenological theory of the Kirkendall effect and the rigorous kinetic theory of gases. The phenomenological theory is shown to be incorrect, but the kinetic theory is capable of accounting for all the effects observed, usually quantitatively. The results can be used to determine gaseous diffusion coefficients from the observed marker motion, and the diffusion coefficients obtained are in reasonable agreement with those obtained by other methods.