Acentric factor

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The acentric factor \omega is a conceptual number introduced by Kenneth Pitzer in 1955, proven to be very useful in the description of matter.[1] 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 (or critical compressibility).The acentric factor is said to be a measure of the non-sphericity (centricity) of molecules.[2] As it increases, the vapor curve is "pulled" down, resulting in higher boiling points.

It is defined as:

\omega = - \log_{10} (p^{\rm{sat}}_r) - 1, {\rm \ at \ } T_r = 0.7.

where T_r = \frac{T}{T_c} is the reduced temperature, p^{\rm{sat}}_r = \frac{p^{\rm{sat}}}{p_c} is the reduced pressure saturation of vapors.

For many monatomic fluids

p_r^{\rm{sat}}{\rm \ at \ } T_r = 0.7,

is close to 0.1, therefore \omega \to 0. In many cases, T_r = 0.7 lies above the boiling temperature of gases at atmosphere pressure.

Values of \omega can be determined for any fluid from \{T_r, p_r\}, and a vapor measurement from T_r = 0.7, and for many liquid state matter is tabulated into many thermodynamical tables.

The definition of \omega gives zero-value for the noble gases argon, krypton, and xenon. \omega is almost exactly zero for other spherical molecules.[2] Experimental data yields compressibility factors for all fluids that are correlated by the same curves when Z (compressibility factor) is represented as a function of T_r and p_r. This is the basis premises of three-parameter theorem of corresponding states:

All fluids at any \omega-value, in \{T_r, p_r\}=const. conditions, have about the same Z-value, and same degree of convergence.[citation needed][dubious ]

Values of some common gases[edit]

Molecule Acentric Factor[3]
Acetylene 0.187
Ammonia 0.253
Argon 0.000
Carbon Dioxide 0.228
Decane 0.484
Helium -0.390
Hydrogen -0.220
Krypton 0.000
Neon 0.000
Nitrogen 0.040
Nitrous Oxide 0.142
Oxygen 0.022
Xenon 0.000

See also[edit]


  1. ^ Adewumi, Michael. "Acentric Factor and Corresponding States". Pennsylvania State University. Retrieved 2013-11-06. 
  2. ^ a b Saville, G. (2006). "ACENTRIC FACTOR". A-to-Z Guide to Thermodynamics, Heat and Mass Transfer, and Fluids Engineering. doi:10.1615/AtoZ.a.acentric_factor. 
  3. ^ Yaws, Carl L. (2001). Matheson Gas Data Book. McGraw-Hill.