SECOND ACOUSTIC VIRIAL-COEFFICIENTS OF NITROGEN BETWEEN 80-K AND 373-K

The speed of sound in nitrogen has been measured in the temperature range 80 to 373 K and in the density range 10 to 200 mol/m3 using a spherical resonator. Values of the second acoustic virial coefficient have been determined from the results with an imprecision of no worse than 0.11 cm3/mol. This estimation of imprecision includes all known sources of systematic and random errors at the level of one standard deviation. The results are compared with values calculated from two recently proposed intermolecular potential-energy functions for this system both of which were based partially on second virial coefficient data. Although neither of these functions proved able to predict the acoustic virial coefficients to within the high precision of the present measurements, the potential of Ling and Rigby (Mol. Phys. 51 (1984) 855) gives results that deviate by less than 1 cm3/mol. We have also obtained very precise values of the dynamic perfect-gas heat capacity of nitrogen from the zero-density limit of the sound speed measured in the frequency range 3-22 kHz. These resolve the effects of centrifugal distortion and are in good agreement with statistical mechanical calculations which include that effect. We report new calibration measurements in argon over the temperature range 90 to 373 K from which the mean radius of the resonator was determined as a function of temperature. In addition, these results provide values of the second acoustic virial coefficient for argon which deviate from those calculated from the interatomic potential energy function for this system by only 0.8 cm3/mol at 90 K and by less than 0.1 cm3/mol above 300 K. Ordinary (p, V, T) second virial coefficients have been calculated from the acoustic results for both gases. These are believed to be of superior accuracy to directly measured values at low temperatures.