The

*Redlich-Kwong equation* (

Otto Redlich,

J. N. S. Kwong,
1949) is a cubic

equation of state loosely modeled after the

van der Waals
equation. However, there is no particular theoretical basis behind the
modifications that were adopted.

The *Redlich-Kwong equation* has been used extensively, and
has been the basis for several succesful modified forms (such as the
Soave equation). Nevertheless, it is currently considered that
modifying the *Redlich-Kwong equation* has reached the point of
diminishing returns, and alternative methods to improving equations of
state have to be developed.

The equation can **NOT** be used for liquid phase systems, or
vapor-liquid equilibria. The advantages of the *Redlich-Kwong
equation* are that it is a relatively simple equation, and that it
does not require much information about the substance or the components
of a mixture.

The basic form of the *Redlich-Kwong equation* is:

(P + a/ (T^{½} V (V + b)) (V - b) = RT

The constants *a* and *b* can be given in terms of
critical constants, T_{c} and P_{c}:

a = 0.42748 R^{2}T_{c}^{2.5} / P_{c}

a = 0.08664 RT_{c} / P_{c}

Otto Redlich, J. N. S. Kwong, *Chem. Rev.*, vol. 44, pp. 233-244, 1949.