Partition Coefficients

Specification Reference Physical Chemistry, Equilibria 25.2

Quick Notes

Partition coefficient (K_pc) is the ratio of concentrations of a solute between two immiscible solvents at equilibrium.
K_pc = [solute in solvent 1] / [solute in solvent 2]
Solute must be in the same physical state (e.g. both in liquid form).
Polarity matters: a solute will favour the solvent that has similar polarity.
K_pc is constant at a fixed temperature, but changes with temperature or solvent nature.

Full Notes

What Is a Partition Coefficient (K_pc)?

A partition coefficient (K_pc) describes how a solute is distributed between two immiscible solvents (liquids that don’t mix, like oil and water), when an equilibrium is established.

The solute must be present in the same physical state in both solvents (i.e., dissolved in both but not reacting or changing form).

CIE A-Level Chemistry diagram showing iodine partitioning between hexane and water as an example of Kpc.

It is often written as:

CIE A-Level Chemistry expression for partition coefficient Kpc = [X]organic / [X]aqueous.

K_pc is constant at a given temperature for a specific solute–solvent pair.

Calculating and Using Partition Coefficients

We can calculate a partition coefficient if we know the equilibrium concentrations of a solute in two layers.

Worked Example Calculation

Calculate K_pc if iodine is shaken with hexane and water, given the following concentrations at equilibrium:

[I₂] in hexane = 0.30 mol dm⁻³
[I₂] in water = 0.01 mol dm⁻³

K_pc = 0.30 / 0.01 = 30
This means iodine is 30 times more soluble in hexane than in water.

Factors Affecting Partition Coefficients

The main factor that affects K_pc is polarity (think of the ‘like dissolves like’ principle):

Polar solutes (like ethanol) dissolve better in polar solvents (like water).
Non-polar solutes (like iodine) dissolve better in non-polar solvents (like hexane or tetrachloromethane).

Example: Iodine and ethanol

Iodine (I₂) is non-polar and ethanol (CH₃CH₂OH) is polar:

CIE A-Level Chemistry diagram showing iodine solubility in hexane compared with water.

This makes iodine (I₂) more soluble in non-polar hexane and less soluble in polar water.
This gives a large K_pc value favouring the organic solvent.

Whereas ethanol is more soluble in water as it is polar, giving a lower K_pc value (favours aqueous solvent).

CIE A-Level Chemistry diagram showing ethanol partitioning more into the aqueous layer.

Other Factors affecting K_pc values

Temperature: changing temperature can shift equilibrium and affect solubility.
Hydrogen bonding or complex formation may alter solute behaviour between solvents.

Summary

Partition coefficient (K_pc) is the ratio of solute concentrations in two immiscible solvents.
K_pc = [solute in solvent 1] / [solute in solvent 2].
K_pc is constant for a given solute–solvent pair at a fixed temperature.
Polarity affects K_pc: solutes prefer solvents of similar polarity.
Non-polar solutes (e.g., iodine) dissolve better in non-polar solvents and polar solutes (e.g., ethanol) dissolve better in polar solvents.
Temperature and hydrogen bonding/complex formation can influence K_pc values.