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# Acentric factor

In thermodynamics, the acentric factor ω is a factor originally used by K.S. Pitzer and coworkers as an expression in an equation for the compressibility factor. It is defined as

$\omega = - \log_{10} P^{\rm{sat}}_r - 1 {\rm \ at \ } T_r = 0{.}7$.

Tr stands for the reduced temperature $T \over T_c$.

For most simple fluids $P_r^{\rm{sat}}$ at Tr is close to 0.1, therefore $\omega \approx 0$. In many cases, Tr = 0.7 lies near the normal boiling point.

Acentric factor accounts for characteristic of molecular structure. Values of ω can be determined for any fluid from Tc,Pc, and a single-vapor measurement measurment made at Tr = 0.7. Values of ω are tabulated in many thermodynamic tables.

The definition of ω makes its value zero for argon, krypton, and xenon, and experimental data yield compressibility factors for all three fluids that are correlated by the same curves when Z (compressibility factor is represented as a function of Tr and Pr. This is the basic premise of the following three-parameter theorem of corresponding states:

All fluids having the same value of ω, when compared at the same Tr and Pr, have about the same value of Z, and all deviate from idea-gas behavior to about the same degree.