Extraction: Partition and Distribution Coefficients

Nernst's distribution law states that at constant temperature, a solute distributes between two immiscible solvents in a fixed ratio at equilibrium. This law governs solute partitioning and is mathematically expressed through the partition coefficient, which represents the ratio of solute concentration in the organic phase divided by that in the aqueous phase. This principle is fundamental to extraction techniques.

The partition coefficient (KD) is a thermodynamic equilibrium constant with a fixed value for a specific solute partitioning between two phases, regardless of the amount in each phase. The distribution coefficient (D) changes as the amount of solute in each phase changes. KD equals D only when the solute exists in one chemical form in both phases; they differ when the solute exists in multiple chemical forms.

The partition coefficient (KD) equals the distribution coefficient (D) when the solute exists in only one chemical form in both the aqueous and organic solvent phases. If the solute exists in two or more chemical forms in either phase, KD and D have different values because KD measures only the partitioning form while D accounts for all forms present.

The distribution coefficient (D) is pH-dependent because pH changes the relative amount of solute in different chemical forms within the solution. As pH shifts, the equilibrium between chemical forms changes, altering the concentration of the form that actually partitions between phases. This makes D variable, unlike the fixed partition coefficient (KD).

Partition and distribution coefficients determine how solutes separate in chromatography by controlling the movement of solutes between the mobile and stationary phases. Proper partitioning between two phases is required to separate components of interest from the rest of the mixture. These coefficients directly affect the efficiency and selectivity of chromatographic separation.

The partition coefficient (KD) remains constant regardless of the amount of solute in each phase because it is a thermodynamic equilibrium constant specific to a solute partitioning between two phases at a given temperature. This fixed value makes KD a reliable predictive tool for extraction efficiency, unlike the distribution coefficient which varies with solute concentration.

When a solute exists in multiple chemical forms, the partition coefficient (KD) is expressed as the concentration ratio of only the form that distributes between phases, not all forms present. In contrast, the distribution coefficient (D) includes the concentration of all chemical forms in the aqueous phase, making D larger than KD when multiple forms coexist.