The acentric factor is a conceptual number introduced by Kenneth Pitzer in 1955, proven to be useful in the description of fluids. It has become a standard for the phase characterization of single and pure components, along with other state description parameters such as molecular weight, critical temperature, critical pressure, and critical volume (or critical compressibility). The acentric factor is also said to be a measure of the non-sphericity (centricity) of molecules.
Pitzer defined from the relationship
: <math>\omega = -\log_{10}(p^\text{sat}_\text{r}) - 1 \text{ at } T_\text{r} = 0.7,</math>
where
<math>p^\text{sat}_\text{r} = p^\text{sat} / p_c</math> is the reduced saturation vapor pressure, and
<math>T_\text{r} = T / T_c</math> is the reduced temperature.
Pitzer developed this factor by studying the vapor-pressure curves of various pure substances. Thermodynamically, the vapor-pressure curve for pure components can be mathematically described using the Clausius–Clapeyron equation.
The integrated form of equation is mainly used for obtaining vapor-pressure data mathematically. This integrated version shows that the relationship between the logarithm of vapor pressure and the reciprocal of absolute temperature is approximately linear.
Values of some common gases
{| class="wikitable"
! Molecule
! Acentric factor
|-
| Acetone || 0.304
|-
| Acetylene || 0.187
|-
| Ammonia || 0.253
|-
| Argon || 0.000
|-
| Carbon dioxide || 0.228
|-
| Decane || 0.484
|-
| Ethanol || 0.644