The Acentric Factor was first proposed by Pitzer as a measure of the amount by which the thermodynamic properties of a particular substance differ from those predicted by the Principle of Corresponding States This principle strictly applies only to a fluid (liquid or gas) comprised of spherical molecules. Fluids containing nonspherical molecules, or those with polar groups, show systematic deviations in their thermodynamic properties from their spherical counterparts. It is these deviations which are correlated with the acentric factor.
The acentric factor is defined as:
where Pc is the critical pressure and Pσ is the vapor pressure at temperature T where T/Tc = 0.7 and Tc is the critical temperature.
For spherical molecules, ω is almost exactly zero. Nonspherical molecules have values above zero, but only the most severely nonspherical have values which approach unity.
If ω is small (say, less than 0.2), the departures from corresponding states are approximately linear in ω and, for example, the Compressibility Factor Z can be written as:
where Pr = P/Pc, Tr = T/Tc, Z0 is the compressibility factor of a substance comprising spherical molecules and Z1 is the correction for nonsphericity. Graphs of the functions Z1 and Z0, and their equivalents for enthalpy and entropy, are given in many textbooks (see, for example, Bett, Rowlinson and Saville, 1975).
Bett, Rowlinson and Saville (1975) Thermodynamics for Chemical Engineers, Athlone Press.