The refractive index and Lorenz–Lorentz function of fluid methane

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Abstract
The refractive index of gaseous and liquid methane was measured between 95 and 300 K and to pressures of 225 bar (1 bar = 105 Pa). The measurements were performed at the 198Hg vapor green line, λ=546.2 nm, with a Fabry–Perot interferometer referred to vacuum. The refractive index data were combined with the previously measured densities of methane to calculate the Lorenz–Lorentz (LL) function. Refractometric virial coefficients were obtained from analysis of the small (∼0.5%) maximum exhibited by the (LL) function with increasing density. BR, the second refractometric virial coefficient, is estimated to be ∼6.0 (cm3/mol)2 and is almost independent of temperature between 220 and 300 K. The critical point refractive index, nc=1.10333, was extrapolated from a rectilinear diameter treatment of the saturated liquid and vapor results. The critical point refractive index was combined with an estimate of the critical point (LL) function to yield a critical density of methane, ρc= 10.16±0.01 mol/l.