Advanced Topic 507 of 1102 Kinetics & Equilibrium
Partition Coefficient
The ratio of a solute's concentration in two immiscible solvents at equilibrium.
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In-depth explanation of Partition Coefficient
The partition coefficient K_D is the ratio of the concentration of a solute in one solvent to its concentration in a second, immiscible solvent when equilibrium has been established between them. For a solute that has the same molecular form in both solvents, K_D = C1/C2 is constant at a given temperature. Partition coefficients are the basis of liquid-liquid extraction, which removes solutes from one solvent into another, and of many chromatographic separations. In pharmacology and environmental science the octanol-water partition coefficient, expressed as log P, estimates the lipophilicity of a drug or pollutant and hence its ability to cross biological membranes.
Going deeper: Partition Coefficient
Key ideas: For extraction: with volume V1 of solvent containing amount x, after one extraction with V2 of the second solvent the fraction left is V1 / (V1 + K V2) when K = C2/C1. For n extractions with V2/n the fraction left is (V1 / (V1 + K V2/n))^n. Log P above 5 suggests bioaccumulation, and in drug design values of 0 to 3 often permit membrane crossing.
Common mistakes and connections: Students apply the constant ratio when the solute associates or dissociates in one phase, for example a carboxylic acid dimerizing in benzene or ionizing in water, where the simple ratio fails. Another error is to invert the ratio convention. State which phase is in the numerator. Also temperature affects K. Ties to distribution law, chromatography and solubility.
Visualization of Partition Coefficient
Octanol-water log P values: higher numbers mean more lipophilic compounds that partition into the organic layer and cross membranes more easily. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Iodine partitions strongly into hexane rather than water, which is why it can be extracted from aqueous solution into an organic solvent.
Frequently asked questions about Partition Coefficient
What is Partition Coefficient?
The ratio of a solute's concentration in two immiscible solvents at equilibrium.
Is Partition Coefficient a beginner, intermediate or advanced topic?
Partition Coefficient is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Partition Coefficient?
Study these earlier topics in Kinetics & Equilibrium first: Equilibrium in Biology, Buffer Equilibria, Heterogeneous Equilibrium Vapor.
What comes after Partition Coefficient?
The next topics in Kinetics & Equilibrium are Distribution Law and Equilibrium Constant of Water.
Can I practise Partition Coefficient with quizzes?
Yes. The interactive lesson includes a self-check question on Partition Coefficient, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Partition Coefficient fits in Kinetics & Equilibrium
Partition Coefficient is topic 88 of 91 in Kinetics & Equilibrium.