Simple and controlled extrapolation of vapor pressures toward the triple point

Although experimental vapor pressures are abundant in the medium‐pressure range (p sat ≃ 5–100 kPa), their values are scarce and uncertain at lower pressures due to experimental difficulties in determining the data. Reliable values are, however, required by technology (separation processes dealing with mixtures containing high molar mass compounds) and environmental concerns (transport and fate of high‐boiling pollutants). The ability of frequently used vapor‐pressure equations to extrapolate the data at medium pressures toward the triple‐point temperature was tested on three groups of compounds (polar and nonpolar organic chemicals, n‐alkanes, and 1‐alkanols) for which reliable experimental data in the low‐pressure range were available. Results of a simple extrapolation were compared with those from a simultaneous correlation of vapor pressures in the medium‐pressure range and the differences between heat capacity of an ideal gas and that of the liquid available at lower temperatures. The latter is generally more reliable, as it is controlled by the exact thermodynamic constraints linking vapor pressures and the thermal data. The extent of improvement when using simultaneous correlation was examined, as well as the influence of the quality of input data on the results of extrapolation. Extrapolated values were compared with experimental vapor pressures and calorimetric enthalpies of vaporization that were selected from literature and free of any important systematic errors.