Equation of State of Liquid-vapour Equilibrium with the Same Constants for Both Phases

Abstract

As is known, the general equation of state of van der Waals, published in the year 1873, as well as its numerous modifications, expresses the relation between pressure, volume and temperature of gases-vapours and liquids qualitatively well, but generally only with a rather rough approximation with the same constants for both phases, especially for the liquid phase. Nevertheless, in view of the average experiences with these equations, several of their results, relatively, could be considered as accurate or satisfactory ones. Recent comparative calculations of Shah and Thodos¹ about the most common fourteen equations of state, based on the critical data of argon and n-butane, clearly confirmed anew these circumstances. They have found the formula of Redlich and Kwong² as the relatively best one by stating that “… this simple equation possesses the remarkable ability to represent the gaseous and liquid regions rather accurately”; yet they give no detailed results for the liquid states. Ree³, somewhat earlier, reached a similar conclusion that “… the Berthelot equation of state⁴ gives satisfactory results of liquid and vapour volumes in the vapour–liquid equilibrium for Ar, Kr, Xe, N₂, O₂, CO and CH₄, …”, that is, they examined definitely only the volume values.