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

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.

T_r stands for the reduced temperature .

For most simple fluids at T_r is close to 0.1, therefore . 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 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.

See also

Equation of state