The acentric factor ω is a conceptual number introduced by Kenneth Pitzer in 1955, proven to be very useful in the description of matter. It has become a standard for the phase characterization of single & pure components. The other state description parameters are molecular weight, critical temperature, critical pressure, and critical volume (or critical compressibility). The acentric factor is said to be a measure of the non-sphericity (centricity) of molecules. As it increases, the vapor curve is "pulled" down, resulting in higher boiling points.
It is defined as:
For many monatomic fluids
is close to 0.1, therefore . In many cases, lies above the boiling temperature of liquids at atmosphere pressure.
Values of ω can be determined for any fluid from accurate experimental vapor pressure data. Preferably, these data should first be regressed against a vapor pressure equation, like ln(P) = A + B/T +C*ln(T) + D*T^6. (In this regression, a careful check for erroneous vapor pressure measurements must be made, preferably using a log(P) vs. 1/T graph, and any obviously incorrect or dubious values should be discarded. The regression should then be re-run with the remaining good values until a good fit is obtained.) Using the known critical temperature, Tc, vapor pressure at Tr=0.7 can then be used in the defining equation, above, to estimate acentric factor.
The definition of ω gives essentially zero for the noble gases argon, krypton, and xenon. is very close to zero for other spherical molecules. Values of ω ≤ -1 correspond to vapor pressures above the critical pressure, and are non-physical.
By definition, a van der Waals fluid has a critical compressibility of 3/8 and an acentric factor of about −0.302024, indicating a small ultra-spherical molecule. A Redlich-Kwong fluid has a critical compressibility of 1/3 and an acentric factor of about 0.058280, close to nitrogen; without the temperature dependence of its attractive term, its acentric factor would be only -0.293572.
Values of some common gases
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