partition coefficient
/ par-TISH-un koh-uh-FISH-unt /
Shake a little iodine with a jar holding both oil and water, then let them settle into two layers. The iodine does not all go one way; it splits between the layers in a fixed proportion, lingering more in whichever layer it 'likes' better. That settled ratio is the partition coefficient — a number for how a substance shares itself between two phases.
Formally, the partition coefficient is the ratio of a solute's concentration in one phase to its concentration in another, once the two phases sit in equilibrium. In chromatography it captures how a compound divides itself between the stationary and mobile phases: a larger value means the compound prefers the stationary phase and therefore travels more slowly.
It matters because separation is built on differences in partition coefficients — if two compounds share themselves between the phases in different ratios, they will move at different speeds and pull apart. The honest caveat is that the value holds only at true equilibrium and for a given temperature and pair of phases; change the solvent, the temperature, or overload the system, and the tidy ratio drifts.
A compound with a partition coefficient of 4 between a stationary liquid film and the mobile phase sits, on average, four times as concentrated in the film as in the flow, so it spends most of its time held back and emerges late.
A higher ratio means more time hiding in the stationary phase, hence slower travel.
The partition coefficient and the distribution constant are closely related; in practice partition coefficient often refers to one chemical species, while distribution constant can lump together all forms of a solute that may exist.