Nernst partition coefficient K

The retention time of the solute on the column can be sub-divided into two terms (hold-up time), which cumulates the times during which it is 

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 Cy and Q vary in the column, but their ratio is constant. 

Chromatography and thermodynamics. Thermodynamic relationships can be applied to the distribution equilibria defined above. K, (Cs/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 AG° for this transformation can be deduced  

In gas chromatography, where K can be easily determined at two different temperatures, it is possible to obtain the variations in standard enthalpy AH° and entropy AS° (if it is accepted that the entropy and the enthalpy have not changed)  

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