Acentric factor

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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

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

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

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

Acentric factor accounts for characteristic of molecular structure. Values of $ω$ can be determined for any fluid from $T c, P c$ , and a single-vapor measurement measurment made at $T r$ = 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