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.
