Acentric Factor

The acentric factor 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:

.

where is the reduced temperature, is the reduced pressure saturation of vapors.

For many monoatomic, fluids

,

is close to 0.1, therefore . In many cases, lies above the boiling temperature of gases at atmosphere pressure.

Values of can be determined for any fluid from, and a vapor measurement from, and for many liquid state matter is tabulated into many thermodynamical tables.

The definition of 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 (compressibility factor) is represented as a function of and . This is the basis premises of three-parameter theorem of corresponding states:

All fluids at any -value, in conditions, have about the same -value, and same degree of convergence.