4.11 : Extraction: Partition and Distribution Coefficients

The distribution law or Nernst's distribution law is the law that governs the distribution of a solute between two immiscible solvents. This law, also known as the partition law, states that if a solute is added to the mixture of two immiscible solvents at a constant temperature, the solute is distributed between the two solvents in such a way that the ratio of solute concentrations in the solvents remains constant at equilibrium.

For extracting a solute from an aqueous phase into an organic phase, the Nernst distribution law can be mathematically expressed as an equation between the partition coefficient (K_D) and the ratio of the solute concentration in the organic phase divided by that in the aqueous phase. The partition coefficient equals the distribution ratio (D) if the solute exists only in one chemical form in each solvent phase. However, K_D and D have different values if the solute exists in more than one chemical form in either of the solvent phases.

Since the partition coefficient is a thermodynamic equilibrium constant, it has a fixed value for a specific solute partitioning between two phases, regardless of the amount in each phase. In contrast, the distribution value, D, changes as the amount of solute in each phase changes. The partition and distribution coefficients influence the separation of solutes in techniques such as chromatography, where proper partitioning between two phases is required to separate the component of interest from the rest of the mixture.