4.11:
Extraction: Partition and Distribution Coefficients
According to Nernst's distribution law, at a constant temperature, solute S will distribute itself between two immiscible solvents as described by its partition coefficient KD.
KD of a solute is equal to its distribution coefficient, D, if the solute exists in one chemical form in both the aqueous and organic solvent phases.
Suppose the solute exists in two chemical forms, A and B, where only A distributes between the two phases then, KD is expressed as the concentration ratio of, A between the two solvent layers.
In contrast, D equals the concentration of A in the organic phase divided by the concentration of both A and B in the aqueous phase.
KD is a thermodynamic equilibrium constant for a specific solute partitioning between two solvents.
D is pH-dependent and changes according to the relative amount of solute in the solution.
Both KD and D are associated with the amount of solute moving from one solvent phase to another and play an important role in chromatography.
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 (KD) 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, KD 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.