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:
ω = − log 10 ( p r s a t ) − 1 , a t T r = 0.7 .
where T r = T T c is the reduced temperature, p r s a t = p s a t p c is the reduced saturation vapor pressure.
For many monatomic fluids
p r s a t a t T r = 0.7 ,
is close to 0.1, therefore ω → 0 . In many cases, T r = 0.7 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 reliable vapor pressure equation such as the following:
ln(P) = A + B/T +C*ln(T) + D*T^6
(This equation fits vapor pressure over a very wide range of temperature for most components, but is by no means the only one that should be considered.) 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. The vapor pressure at Tr=0.7 can then be used in the defining equation, above, to estimate acentric factor.
Then, using the known critical temperature, Tc, find the temperature at Tr = 0.7. At this temperature, calculate the vapor pressure, Psat, from the regressed equation.
The definition of ω gives a zero-value for the noble gases argon, krypton, and xenon. ω is very close to zero for other spherical molecules.
