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The anomalous thermodynamic behavior of fluids near the critical point can be described in terms of sealing laws. In this paper we consider two critical region equations of state, to be referred to as the NBS equation and the Linear Model parametric equation, that satisfy the scaling laws. A complete formulation of the thermodynamic properties in terms of the two equations is given. The statistical methods used for fitting these equations to experimental data are described. Each of the equations is fitted to experimental equation of state data for six fluids, namely He3, He4, Xe, CO2, O2, and H2O. An evaluation of the recorded experimental material is included. We find that the two equations represent the experimental data in the range ‖T−Tc‖/Tc0.03 and ‖p−pc‖/pc 0.25 equally well and that the exponents and amplitudes of the power laws deduced from the two equations agree closely. The optimum critical exponents appear to vary little from substance to substance. Moreover, a restricted version of the Linear Model with only two freely adjustable constants, in addition to the critical point parameters and the critical exponents, fits the data well in most cases, in agreement with expectations based on universality of critical behavior. The principle of universality is discussed and applied to predict critical region parameters for nine additional fluids, including several for which only limited experimental information is available. These additional fluids are Ar, Kr, N2, H2, CH4, C2H4, SF6, NH3 and D2O. We thus conclude with a single universal equation for the critical region of all fifteen fluids considered in this paper.
Sengers et al. (Thu,) studied this question.