Acentric factor

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The acentric factor 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:


where is the reduced temperature, is the reduced saturation vapor pressure.

For many monatomic fluids


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

Values of can be determined for any fluid from accurate experimental vapor pressure data. Preferably, these data should first be regressed against a reliable vapor pressure equation such as the following:

ln(P) = A + B/T +C*ln(T) + D*T^6

(This equation fits vapor pressure over a very wide range of temperature for most components, but is by no means the only one that should be considered.) In this regression, a careful check for erroneous vapor pressure measurements must be made, preferably using a log(P) vs. 1/T graph, and any obviously incorrect or dubious values should be discarded. The regression should then be re-run with the remaining good values until a good fit is obtained. The vapor pressure at Tr=0.7 can then be used in the defining equation, above, to estimate acentric factor.

Then, using the known critical temperature, Tc, find the temperature at Tr = 0.7. At this temperature, calculate the vapor pressure, Psat, from the regressed equation.

The definition of gives a zero-value for the noble gases argon, krypton, and xenon. is very close to zero for other spherical molecules.[2]

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.