Acentric factor


The acentric factor 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. The acentric factor is said to be a measure of the non-sphericity of molecules. As it increases, the vapor curve is "pulled" down, resulting in higher boiling points.
It is defined as:
where
is the reduced temperature,
is the reduced saturation vapor pressure.
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 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. 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 gives essentially zero for the noble gases argon, krypton, and xenon. is very close to zero for other spherical molecules. Values of 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

MoleculeAcentric Factor
Acetone0.304
Acetylene0.187
Ammonia0.253
Argon0.000
Carbon Dioxide0.228
Decane0.484
Ethanol0.644
Helium-0.390
Hydrogen-0.220
Krypton0.000
Methanol0.556
Neon0.000
Nitrogen0.040
Nitrous Oxide0.142
Oxygen0.022
Xenon0.000