Acentric factor

The acentric factor ω is a conceptual number introduced by 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 a-centric factor is said to be a measure of the non-sphericity (centricity) of molecules.

It is defined as:

$\omega = - \log_{10} (p^{\rm{sat}}_r) - 1, {\rm \ at \ } T_r = 0.7$.

where $T_r = \frac{T}{T_c}$ is the reduced temperature, $p^{\rm{sat}}_r = \frac{p^{\rm{sat}}}{p_c}$ is the reduced pressure saturation of vapors.

For many monoatomic, 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 gases at atmosphere pressure.

Values of ω can be determined for any fluid from {T_r,p_r}, and a vapor measurement from T_r = 0.7K, and for many liquid state matter is tabulated into many thermodynamical tables.

The definition of ω gives zero-value for the noble gases argon, krypton, and xenon. Experimental data yields compressibility factors for all fluids that are correlated by the same curves when Z (compressibility factor) is represented as a function of T_r and p_r. This is the basis premises of three-parameter theorem of corresponding states:

All fluids at any ω-value, in {T_r,p_r} = const. conditions, have about the same Z-value, and same degree of convergence.^{[citation needed]}

See also

• Equation of state
• Reduced pressure
• Reduced temperature