An improved apparatus based on the pressure-change method of Knobler is made for the purpose of obtaining interaction second virial coefficients with high accuracy. The apparatus is also used for the Burnett expansion method. Each interaction virial coefficient, B₁₂, is determined from the excess virial coefficient obtained by the Knobler method, and the pure quantities, B₁₂ and B₂₂, obtained by the Burnett method.
The second and third virial coefficients for carbon dioxide, methane and ethane were measured at 25°C by the Burnett method. The lowest pressure was about 25 kPa and the highest was about 3 MPa.
The excess second virial coefficients were obtained at 25°C by measuring pressure changes in mixing of two pure gases for the three binary systems consisting of the three gases. Mixtures produced in the process of the pressure-change method were further used in the Burnett method for obtaining B₁₂ in another way.
The interaction second virial coefficients Blz obtained for the systems of carbon dioxide - methane, carbon dioxide-ethane, and methane-ethane at 25°C are -64.8±0.4, -121.8±0.7, and -92.9±0.4 cm³•mol^-1, respectively.