Critical Isotherm and the Equation of State of Liquid-Vapor Systems

Abstract

Pressure‐volume‐temperature data on xenon, carbon dioxide, and hydrogen are analyzed. Special attention is paid to the regions fairly close to, but not in the immediate neighborhood of, the critical point. It is shown that in each case the critical isotherm is of the fourth degree, one degree higher than that of the coexistence curve, as is required by the theorem of Rice. However, the relation between the critical isotherm and the coexistence curve is determined by the almost constant value of ∂²P/∂T∂_ρ in the PVT region considered, and this is different from its value at the critical point. The critical isobar is also considered and it is shown that it is of the fourth degree near the critical point, but of lower degree further away. A general equation of state is deduced for the PVT region under discussion, and the implications of this equation of state with respect to the coexistence curve and the critical isotherm are considered.