Waring and Riedel Functions for the Liquid–Vapor Coexistence Curve

There exists a minimum in the Waring function, ψ­(T) = −d­(ln p)/d­(1/T), and in the Riedel function, α­(T) = d­(ln p)/d­(ln T), in the liquid–vapor coexistence curve for most fluids. By analyzing National Institute of Standards and Technology data for the molar enthalpy of vaporization and the compressibility variation at the liquid–vapor phase change of 105 fluids, we find that the temperatures of these minima are linearly correlated with the critical temperature, Tc. Using reduced coordinates, we also demonstrate that the minima are well-correlated with the acentric factor. These correlations are used for testing four well-known vapor pressure equations in the Pitzer corresponding states scheme.