Interpretation of covolume in an extension of the Redlich–Kwong equation of state for real gases

This work discusses an extension of the Redlich–Kwong (RK) equation of state for real gases that modifies its attractive part by introducing a third constant, fitted to the nearly-universal value of the compressibility factor at the critical point, Z c ≈ 0.27. From this, the model predicts a covolume b=V_c/(6.33 ± 0.62) for 90