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 P^{c} is the critical pressure and P_{σ} is the vapor pressure at temperature T where T/T^{c} = 0.7 and T^{c} 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 P_{r} = P/P^{c}, T_{r} = T/T^{c}, Z_{0} is the compressibility factor of a substance comprising spherical molecules and Z_{1} is the correction for nonsphericity. Graphs of the functions Z_{1} and Z_{0}, and their equivalents for enthalpy and entropy, are given in many textbooks (see, for example, Bett, Rowlinson and Saville, 1975).

#### REFERENCES

Bett, Rowlinson and Saville (1975) Thermodynamics for Chemical Engineers, Athlone Press.