A Loschmidt cell combined with holographic interferometry for binary diffusion experiments in gas mixtures including first measurements on the argon–neon system

Abstract

A new variant of the Loschmidt technique has been developed for measuring binary diffusion coefficients in gas mixtures in a temperature range from 10 to 80 °C and for pressures between 0.1 and 1 MPa. The two half cells of the thermostatted diffusion cell have a rectangular cross section and are fixed one upon the other. They can be connected and separated by means of a sliding plate provided with a pneumatically operated seal. The concentration in both half cells is determined simultaneously during the diffusion process using an optical system for holographic interferometry for each. The change in the refractive index results in an interference pattern which is recorded as a function of time. The concentrations of the diffusing components are derived by means of the Lorentz–Lorenz equation. The binary diffusion coefficients are calculated via the integrated ideal diffusion equation for the complete mole fraction range performing only a unique diffusion experiment. The performance of the apparatus is demonstrated on first measurements on the argon–neon system at 293.15 K. Separate refractive index measurements are carried out leading to values for the first refractivity virial coefficient of the pure gases with an estimated uncertainty of ±0.1%. This low uncertainty is required for the aimed uncertainty of ±0.5…1% for the diffusion measurements to determine the concentration and density dependences of the binary diffusion coefficient.