# Acentric factor

The **acentric factor** ω 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 ω can be determined for any fluid from accurate experimental vapor pressure data. Preferably, these data should first be regressed against a vapor pressure equation, like ln(P) = A + B/T +C*ln(T) + D*T^6. (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.) Using the known critical temperature, Tc, vapor pressure at Tr=0.7 can then be used in the defining equation, above, to estimate acentric factor.

The definition of ω gives essentially zero for the noble gases argon, krypton, and xenon. is very close to zero for other spherical molecules.[2] Values of ω ≤ -1 correspond to vapor pressures above the critical pressure, and are non-physical.

By definition, a van der Waals fluid has a critical compressibility of 3/8 and an acentric factor of about −0.302024, indicating a small ultra-spherical molecule. A Redlich-Kwong fluid has a critical compressibility of 1/3 and an acentric factor of about 0.058280, close to nitrogen; without the temperature dependence of its attractive term, its acentric factor would be only -0.293572.

## Values of some common gases

Molecule |
Acentric Factor[3] |

Acetone | 0.304[4] |

Acetylene | 0.187 |

Ammonia | 0.253 |

Argon | 0.000 |

Carbon Dioxide | 0.228 |

Decane | 0.484 |

Ethanol | 0.644[4] |

Helium | -0.390 |

Hydrogen | -0.220 |

Krypton | 0.000 |

Methanol | 0.556[4] |

Neon | 0.000 |

Nitrogen | 0.040 |

Nitrous Oxide | 0.142 |

Oxygen | 0.022 |

Xenon | 0.000 |

## References

- Adewumi, Michael. "Acentric Factor and Corresponding States". Pennsylvania State University. Retrieved 2013-11-06.
- 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. - Yaws, Carl L. (2001).
*Matheson Gas Data Book*. McGraw-Hill. - Reid, R.C.; Prausnitz, J.M.; Poling, B.E.
*The Properties of Gases and Liquids*(4th ed.). McGraw-Hill. ISBN 0070517991.