GC016 Distribution Constant

 

The distribution constant, Kc, is a parameter that defines how fast a solute moves down the column. For a solute A that moves between the mobile phase and the stationary phase, in an equilibrium reaction, we can write the equilibrium constant Kc as the ratio between the concentration of A in the stationary phase and its concentration in the mobile phase. Larger values of this coefficient mean the solute is retained for longer periods because its sorbs more readily in the stationary phase.

Even though an equilibrium constant is used, that does not imply the process is in equilibrium, because the mobile gas phase is continuously moving solute molecules down the column. However, the partition coefficient is adequate as a descriptor when the kinetics of mass transfer are fast, because the system will operate near to equilibrium conditions.

Another assumption in the development of the theory is that the solutes do not interact with one another. Thus, the formation of azeotropes and interaction of very similar compounds are neglected. Since the concentrations of the solutes in the column is low and the solutes are increasingly separated during the process, the assumption is reasonable most of the time.

When interactions do occur, however, the theory and the chromatographic results will deviate from each other; peak shapes and retention times may be different.