REFRACTIVE-INDEX AND DENSITY ISOTHERMS FOR METHANE FROM 273-K TO 373-K AND AT PRESSURES UP TO 34-MPA

Comprehensive refractive index measurements n(P,T) were carried out on pure methane in the temperature range from 273.15 to 373.15 K and at pressures up to 34 MPa. Two coupled laser interferometers were used for the experiments, one containing methane, the other the reference gas nitrogen maintained at the same pressure and a fixed temperature T0, serving as a manometer. The uncertainty of the refractive index was estimated to be less than +/-5 X 10(-7) at 1 MPa and +/-5 X 10(-6) at 34 MPa. To measure the higher-order refractivity virial coefficients B(R), C(R), ... of the Lorentz-Lorenz expansion, overflow experiments were made on the 323.15 K isotherm at pressures up to 35 MPa. In the investigated range of pressure B(R) and C(R) are the only significant refractivity virial coefficients. A comparison with literature values shows that B(R) and C(R) are independent of temperature in the range from 220 to 373 K. On the basis of n(P,T) measurements, the density rho(P,T) was calculated by including B(R) and C(R). The uncertainty of the density was estimated to be not greater than +/-0.05%.