Distribution coefficient (partition coefficient)

The distribution coefficient describes the relative concentrations of a chemical compound in two immiscible solvents.

When a solute, X, with different solubilities in two immiscible solvents, e.g. water and hexane, is shaken in a separating funnel containing both solvents and left to settle, a dynamic equilibrium is established such that the rate of the solute moving from the aqueous layer to the organic layer is the same as the rate of the solute from the organic layer to the aqueous layer.

X(aq)\rightleftharpoons X(org)

The equilibrium constant, called the distribution coefficient (or partition coefficient) is given by:




10.00 g of benzoic acid that is dissolved in 100 ml of water is shaken with 50 ml of chloroform and left to settle. 20 ml is then extracted from the aqueous layer and titrated with 6.40 ml of 0.1000 M of NaOH. Calculate the distribution coefficient for benzoic acid in the two solvents.


C_6H_5COOH(l)+NaOH(aq)\rightarrow C_6H_5COO^-Na^+(aq)+H_2O(l)

K_d=\frac{\frac{\frac{10.00}{122.05}-(0.1000\times 0.00064\times 5)}{0.05}}{\frac{0.1000\times 0.0064\times 5}{0.1}}=49.2


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