A Semi-Empirical Equation for the Vapor Pressure of Liquids as a Function of Temperature

The slight reverse curvature in the plot of logP versus 1/T (P, vapor pressure; T, absolute temperature) may be explained on the basis of the nonideal behavior of the vapor together with the change in heat of vaporization with temperature. If it is assumed that ΔH is linear with T and that the van der Waals a/V² term is a first approximation to the deviation from the ideal, the following equation may be obtained by integration of the Clapeyron equation: $$\log \mathrm{P=A+}\frac{B}{T}\mathrm{+C\hspace{0.17em}}\log \mathrm{T+D}\frac{P}{T^2}.$$ The last term is the nonideal gas correction with constant D=a/2.303R². With constants A, B, and C determined empirically, the equation reproduces experimental vapor pressures from the triple point to the critical point with an average deviation of 0.3 percent.