A Predictive Procedure for Vapor Pressure

A vapor pressure equation containing only three parameters has been proposed to calculate vapor pressures from the triple point to the normal boiling point of both organic and inorganic liquids. One of the three parameters is the normal boiling point. The other two parameters are obtained from only two pieces of experimental data, namely, the enthalpy of vaporization at the normal boiling point, and the enthalpy of vaporization at 298.15 K. There are several empirical procedures for the calculation of the boiling points of most liquids. The enthalpy of vaporization at the boiling point is obtainable from the Trouton’s constant. The enthalpy of vaporization at 298.15 K is obtainable from group contributions. Thus one obtains a simple predictive procedure for the vapor pressure. We call our vapor pressure equation the quadratic equation. By the addition of one more constant we may make the quadratic equation a cubic equation. Comparison of the quadratic equation with the Antoine equation has shown that the new equation is much more accurate than the latter in the prediction of vapor pressures and enthalpies of vaporization, partircularly when large extrapolations to low temperatures are necessary. The Cox equation, which employs four parameters, is closely related to the quadratic equation. It is, however, inferior to the quadratic equation when it comes to compounds of high molecular weight, which usually have very low pressures at 298.15 K. The cubic equation, which contains one parameter more than the quadratic equation, is equally defective when it comes to compounds of high molecular weight. The attractive features of the three-constant quadratic equation are that (1) its constants have some simple physical significance, (2) its constants can be determined from the molecular structure, and (3) it is the most accurate equation for the representation of vapor pressure data from the triple point to the normal boiling point.