Acentric factor

In thermodynamics, the acentric factor $omega$ 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

$omega\; =\; -\; log\_\{10\}\; (P^\{\; m\{sat\_r)\; -\; 1\; \{\; m\; at\; \}\; T\_r\; =\; 0\{.\}7$.

$T\_r$ stands for the reduced temperature $T\; over\; T\_c$.

For most simple fluids $P\_r^\{\; m\{sat$ 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 $omega$ can be determined for any fluid from $T\_c,\; P\_c$, and a single-vapor measurement made at $T\_r$ = 0.7. Values of $omega$ are tabulated in many thermodynamic tables.

The definition of $omega$ 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 $omega$, 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.

ee also

Equation of state