DETERMINATION OF THE DENSITY FROM SIMULTANEOUS MEASUREMENTS OF THE REFRACTIVE-INDEX AND THE DIELECTRIC-CONSTANT OF GASEOUS CH4, SF6 AND CO2

Simultaneous measurements of the refractive index (n) and the dielectr ic constant (epsilon) have been carried out for some non polar gases [ CH4, CO2 and SF6] to study the comparative advantage of each method fo r the determination of the density with the use of our own experimenta l values of the first refractivity and the first dielectric virial coe fficients as well as known values of the higher refractivity and diele ctric virial coefficients. Measurements were carried out at 289.15, 29 8.15 and 323.15 K up to 23 MPa for CH4, at 323.15, 333.15 and 348.15 K up to 17 MPa for SF6 and at 308.15, 313.15 and 323.15 K up to 23 MPa for CO2. Comparison with the IUPAC EOS values of the density shows goo d agreement (less than 1.0%) with the density values obtained both fro m the dielectric constant as well as the refractive index for CH4. For SF6, the density values obtained from the refractive index and the di electric constant were compared with the equation of state proposed by de Reuck et al.(1991). For CO2, the agreement with IUPAC EOS is good (of the order of 1.0%) both for the dielectric constant as well as the refractive index up to the critical density. Above the critical densi ty one observes a large deviation (of the order of 10%) between the de nsity values obtained from the dielectric constant with those determin ed from the refractive index. The latter agrees with the EOS within 1. 0%. Measurements for CO2 and SF6, were in the supercritical region whe reas those for CH4 were far from the critical point.