Partition equilibrium

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The most common chemical equilibrium systems involve reactants and products in the same phase - either all gases or all solutions. However, it is also possible to get equilibria between substances in different phases, such as two liquids that do not mix (are immiscible).

For example, ammonia is soluble in both water and the organic solvent trichloromethane - two immiscible solvents. If ammonia is first dissolved in water, and then an equal volume of trichloromethane is added, and the two liquids shaken together, the following equilibrium is established: K_c = [NH_{3 (CHCl3)}]/[NH_{3 (aq)}]

The equilibrium concentrations of ammonia in each layer can be established by titration with standard acid solution. It can thus be determined that K_c remains constant, with a value of 0.4 in this case.

This particular kind of equilibrium constant measures how a substance distributes or partitions itself between two immiscible solvents. It is called the partition coefficient or distribution coefficient.

Substances that are ionic or polar are more soluble in water than in non-polar organic solvents and vice-versa.

Partition equilibrium chromatography is a type of chromatography that is typically used in GC. The stationary phase is a high boiling liquid bonded to solid surface and the mobile phase is a gas.