# The Thermodynamics of Chromatography - Other Thermodynamic Methods that are Used for Studying Chromatographic Systems > Page 56

K_{AB} = b(K_{A - }K_{B}) +
K_{B }_{ }_{ } (28)

_{ }

where (K_{AB}) |
is the distribution coefficient of a solute between a mixture of solvents (A) and (B) and a stationary phase, |

(K_{A}) |
is the distribution coefficient of a solute between the pure solvent (A) and a stationary phase, |

(K_{B}) |
is the distribution coefficient of a solute between the pure solvent (B) and a stationary phase, |

and (b) | is the volume fraction of solvent (A) in the mixture. |

_{ }

The
distribution coefficients are referenced to the solvent mixture and not the
stationary phase and are thus are *the inverse of the
distribution coefficients employed in the chromatography elution equation*.

Now the
separation ratio (a_{(1)(2)}) of a pair of
solutes, both of which are ditributed between two phases (1) and (2), is
defined by,