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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 in the two phases.

(1.7)

Values of K are quite variable. The higher they are, the more solute is retained. At least for liquid chromatography, they depend on the intensity of three types of interactions: stationary phase/solute, mobile phase/solute, and mobile phase/stationary phase.

Like any equilibrium constant, it is a function of temperature. It is related to variations in free enthalpy, standard enthalpy, and entropy of the exchange reaction by means of the following relationships:

(1.8)

The experimental determination of K_T at two different temperatures enables us to calculate these variations, if we assume that the standard enthalpy and entropy variations remain virtually the same between these two temperatures. We can write:

(1.9)

If we know K_T at two temperatures, Eq. (1.9) helps us calculate the terms a and b.

(1.10)

and

(1.11)

In general, the standard enthalpy variation is negative. The same goes for the standard entropy variation, which corresponds to an increase in order when the solute is fixed on the stationary phase.