4.12 : Extraction: Effects of pH

Consider a neutral form of an amine, B, with a partition coefficient, K, in a liquid mixture containing organic and aqueous phases. The pH of the aqueous phase affects the charge on acidic and basic solutes, and the charged form is usually more soluble in the aqueous phase. Suppose the conjugate acid form of the amine is soluble only in the aqueous phase while the base form is soluble in both phases. Then the distribution coefficient, D, can be given as the ratio of amine concentration in the organic phase to the sum of concentrations of the amine and its conjugated acid in the aqueous phase. In this equation, the terms for partition coefficient and the acid dissociation constant, K_a, can be substituted with the concentrations, and the fraction of neutral amine in the aqueous phase can be determined by solving the equation. With ladder diagrams and a plot of log D versus pH, it can be concluded that a neutral base can be extracted into the aqueous phase at a pH low enough to convert it into its charged conjugate acid. Similarly, a charged acid can be extracted into the organic phase at a pH high enough to convert it to its neutral conjugate base.