Question

The weak acid HA with a dissociation constant Ka is distributed between and organic solvent and water. If the only extractable species is the undissociated species HA with a partition coefficient K, derive an expression showing the dependence of the distribution ratio, D, on [H+] of the aqueous phase and draw conclusions from this expression.

Answer 1

The distribution ratio and partition coefficient are identical for a simple liquid - liquid extraction. As a result the distribution does not depend on the composition of the aqueous phase or the organic phase. But changing the pH of the aqueous phase will not affect the solution extraction efficiency.

Although the weak acid soluble in both phases, it conjugate weak base A- is soluble only in the aqueous phase. The Ka reaction which is called a secondary equilibrium reaction affects the extraction efficiency becasue it controls the relative abundance of HA in solution

If the solute participates in an additional equilibrium reaction within a phase, the distribution ratio and the partition coefficient will not be the same and will affect the extraction of weak acid HA by an organic phase in which ionic species are not soluble. So the partition coefficient and the distribution ratio are

Because the position of an acid–base equilibrium depends on the pH, the distribution ratio is pH-dependent. Equation for D that shows this dependency, we begin with the acid dissociation constant for HA.

factoring the [HAaq] from the denominator, replacing [HA org] / [HA aq] with Kd which gives a relationship between the distribution ratio D, and the pH of the aqueous solution