The speed of sound was measured in gaseous nitrogen trifluoride, ethylene oxide, and trimethyl gallium using a highly precise acoustic resonance technique. The measurements span the temperature range 200 K to 425 K and reach pressures up to the lessor of 1500 kPa or 80 % of the sample s vapor pressure. The speed-of-sound measurements have a relative standard uncertainty of less than 0.01 %. The data were analyzed to obtain the constant-pressure ideal-gas heat capacity C as a function of the temperature with a relative standard uncertainty of 0.1 %. The values of C are in agreement with those determined from spectroscopic data. The speed-of-sound data were fitted by virial equations of state to obtain the temperature-dependent density virial coefficients. Two virial coefficient models were employed, one based on square-well intermolecular potentials, and the second based on a hard-core Lennard-Jones intermolecular potential. The resulting virial equations reproduced the sound speed data to within 0.02 %, and may be used to calculate vapor densities with relative standard uncertainties of 0.1 % or less.
Citation: International Journal of Thermophysics
Pub Type: Journals
C2H4O, equation-of-state, ethylene oxide, Ga(CH3)3, ideal-gas heat-capacity, intermolecular potential, NF3, nitrogen trifluoride, speed-of-sound