Acentric factor

Template:Multiple issues The acentric factor Template:Math 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:

$\omega =-\log _{10}(p_{r}^{\rm {sat}})-1,{\rm {\ at\ }}T_{r}=0.7$.

where $T_{r}={\frac {T}{T_{c}}}$ is the reduced temperature, $p_{r}^{\rm {sat}}={\frac {p^{\rm {sat}}}{p_{c}}}$ is the reduced saturation vapor pressure.

For many monatomic fluids

$p_{r}^{\rm {sat}}{\rm {\ at\ }}T_{r}=0.7$,

is close to 0.1, therefore $\omega \to 0$. In many cases, $T_{r}=0.7$ lies above the boiling temperature of liquids at atmosphere pressure.

Values of Template:Math 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 Template:Math. (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 Template:Math gives essentially zero for the noble gases argon, krypton, and xenon. $\omega$ is very close to zero for other spherical molecules. Values of Template:Math 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

Molecule	Acentric Factor
Acetone	0.304
Acetylene	0.187
Ammonia	0.253
Argon	0.000
Carbon Dioxide	0.228
Decane	0.484
Ethanol	0.644
Helium	-0.390
Hydrogen	-0.220
Krypton	0.000
Methanol	0.556
Neon	0.000
Nitrogen	0.040
Nitrous Oxide	0.142
Oxygen	0.022
Xenon	0.000

See also

• Equation of state
• Reduced pressure
• Reduced temperature