The fundamental physico-chemical parameter of chromatography is the equilibrium constant K, termed the partition coefficient, quantifying the ratio of the concentrations of each compound within the two phases.
Values of K are very variable since they can be large (e.g. 1000), when the mobile phase is a gas or small (e.g. 2) when the two phases are in the condensed state. Each compound occupies only a limited space on the column, with a variable concentration in each place, therefore the true values of CM
and C
vary in the column, but their ratio is constant.
Chromatography and thermodynamics. Thermodynamic relationships can be applied to the distribution equilibria defined above. K, C
, the equilibrium constant relative to the concentrations C of the compound in the mobile phase (M) and stationary phase (S) can be calculated from chromatography experiments. Thus, knowing the temperature of the experiment, the variation of the standard free energy G
for this transformation can be deduced:
The values of these three parameters are all negative, indicating a spontaneous transformation. It is to be expected that the entropy is decreased when the compound moves from the mobile phase to the stationary phase where it is fixed. In the same way the Van’t Hoff equation can be used in a fairly rigorous way to predict the effect of temperature on the retention time of a compound. From this it is clear that for detailed studies in chromatography, classic thermodynamics are applicable.