Precise determination of the compressibility factor of methane, nitrogen, and their mixtures from refractive index measurements

Abstract

We show that the absolute determination of the refractive index, when combined with an expansion technique for obtaining the higher-order coefficients of the Lorentz-Lorenz expansion, leads to precise values of density. A grating interferometer has been developed for the refractive index measurements as a function of pressure. The advantage of a grating interferometer is that it performs a reversible counting and generates a DC compensated signal from the interference fringes. The pressure is also measured with an interferometer, previously calibrated with an oil-type precision piston gauge. For the precise determination of the compressibility factor, the absolute measurement of the refractive index is combined with the differential technique to determine the refractivity virial coefficients of the Lorentz-Lorenz expansion. The compressibility factors of methane, nitrogen, and their mixtures have been determined at 323.15 K for pressures up to 335 bar. The optical method for the determination of the compressibility factor not only is shown to be precise, but also has the ability to produce numerous experimental points in a short time as compared to other methods.