Density Dependence of Lorentz–Lorenz Coefficient for Sulfur Hexafluoride

The Lorentz–Lorenz coefficient for sulfur hexafluoride has been measured over the density range 0.1 g/cm3 to 1.3 g/cm3. The critical density has been measured and is 0.736 ± 0.001 g/cm3. Refractive index and density are both measured in the same experiment yielding values of the Lorentz–Lorenz coefficient accurate to 0.05% for densities near critical. The method utilizes a prism-shaped high pressure cell which can be removed from a temperature controlled holder and weighed on a precision balance. The cell is equipped with a needle valve which allows the high pressure gas to be bled out in steps. Refractive index is measured as a function of weight and hence density. Studies of sulfur hexafluoride indicate a small variation of less than 0.5% over the density range covered. A knowledge of the density dependence is necessary for interpretation and verification of the validity of recent experiments near the critical points of pure fluids.