pH Dependence of Carboxylic and Mineral Acid Extraction by Amine-Based Extractants: Effects of pK_a, Amine Basicity, and Diluent Properties

Extraction mechanism strongly depends on the acid-base properties of the extractant as well as on those of the extracted acid. For amines of basicity similar to that of the anion of the extracted acid, or higher, extraction is governed by ion-pair formation. H-bonding of undissociated acid becomes important only for extraction of weak acids by weak bases. Extraction mechanism determines the effect of various parameters. Thus, the degree of extraction is most sensitive to pH at about the pK_a of the amine for extraction through ion-pair formation. Extraction by H-bonding, however, is most sensitive at about the pK_a of the acid. Polar and protic diluents enhance extraction through ion-pair formation, but may have no effect on H-bonding or even hinder it. Extraction through ion-pair formation prefers stronger acids at low pH while H-bonding extracts weaker acids more efficiently. Clarification of the extraction mechanism and its dependence on extracted acid properties and on the pH provide for designing extractants of high efficiency and selectivity.