# Principles and Practice of Chromatography - Factors Controlling Retention > The Thermodynamic Explanation of Retention > Page 16

* i.e.*,
_{}

where (DH^{o}) is the Standard
Enthalpy Change,

** ** and
(DS^{o}) is the Standard
Entropy Change.

_{} (2)

_{
} (3)

It is seen
that if the *standard entropy change* and *standard enthalpy change*
for the distribution could be calculated then the distribution coefficient (K)
and, consequently, the retention volume could also be predicted. Unfortunately,
these properties are difficult, if not impossible, to isolate and estimate and
so the magnitude of the overall distribution coefficient can not be estimated
in this way. Nevertheless, once the phase system has been identified, with
sufficient experimental data being available, empirical equations can be
developed to optimize a given distribution system for a specific separation.
Computer programs, based on this rationale, are available for LC to carry out
such optimization procedures for solvent mixtures having three or more
components. Nevertheless, the appropriate stationary phase is still usually
identified from the types of interactions that need to be exploited to effect
the required separation. By measuring the retention volume of a solute over a
range of temperatures equation (2) can also be used to identify the type of
retention mechanism that is operative in a particular separation .