Density and Virial Coefficients of Gaseous Butane from 265 to 450 K at Pressures to 3.3 MPa

The Burnett-isochoric (B-I) method has been used to measure gas densities and virial coefficients for butane from 265 to 450 K (IPTS, 1990; Preston-Thomas, 1990). Two independent B-I runs were performed but both with a base isotherm of 450 K, which is well above the critical temperature of 425 K. Significant physical adsorption of butane molecules onto the highly-polished, stainless steel cell walls was found below 375 K in agreement with conclusions reached by Ewing and associates in comparing their sonic-velocity-based density virial coefficients with those from the P − V − T literature. Our data below 375 K were then corrected for adsorption errors by previously published procedures developed by our laboratory for highly polar gases. Using statistical weighting of our two B-I runs, recommended density second virial coefficients B ( T ) are reported from 265 to 450 K whereas third virial coefficients C ( T ) are reported only from 325 to 450 K as values below 325 K are too uncertain due to the vapor pressure dropping under 300 kPa. However, then the virial equation of state truncated after B ( T ) is sufficient to represent the gas densities. At the lower temperatures of this investigation, our B ( T ) values lie between those from sonic velocities and the more negative values from the P − V − T literature, which are uncorrected for adsorption errors; we are closer to the values from sonic velocities and about 1 / 4 of the way between the two sets. At the higher temperatures, we agree very closely with the better P − V − T measurements whereas the sonic-velocity-based values become increasingly more negative.