METHOD FOR DETERMINING THE DENSITY OF PURE AND MODIFIED SUPERCRITICAL FLUIDS

Densities of three pure supercritical fluids (CO2 N2O, and CHClF2) and one modified fluid (CO2 modified with 5 volume % methanol) were determined over a wide range of pressures (60-600 atm) and temperatures (40-150-degrees-C) using a simple device constructed from a conventional supercritical fluid extraction cell. Triplicate measurements showed excellent reproducibilities, with percent relative standard deviations typically ranging from 0. 1 to 0.7 %. The calculated densities of the pure fluids based on modified equations of state were generally in good agreement with experimental measurements (<4 % deviation), with the largest errors occurring near the critical point. Calculated values for CO2/5 % methanol (v/v) also showed good agreement with experimental values except at the lowest pressure, 100 atm, where deviations were as high as 9.8 %. The technique is fast (a typical measurement for a pure fluid takes ca. 5-1 0 min), accurate, and reproducible, and the apparatus can easily be assembled in most analytical laboratories that use supercritical fluids.