Selectivity in monocarboxylic acids extraction from their mixture solutions using an amine-based extractant: Effect of pH

Extraction selectivity and its dependence on pH were studied in binary mixtures of lactic, propionic, dichloroacetic, trichloroacetic, and hydrochloric acids using extractants containing Primene JMT, tris(2-ethylhexyl)amine, or tri-n-octylamine. The results are explained by extending the theory developed in previous publications of this series (Ind. Eng. Chem. Res. 2003, 42, 1285-1292 and 1293-1300). Three main cases were found: (1) In cases where the extractant is a weaker base than both anions of the extracted acids, the acids are extracted mainly by H-bonding or solvation interactions. The weaker and more hydrophobic acid is preferably extracted, and the selectivity increases with increasing pH. (2) In cases where the extractant is a stronger base than both anions of the extracted acids, both acids are extracted by ion-pair interactions. The strong acid is preferably extracted, and the selectivity increases with increasing pH. However, in some cases, the hydrophobicity of the anion is most important. (3) The third case is when the extractant is a stronger base than one acid's anion, but a weaker base than the other's. At elevated pH, the strongest acid in the system is preferably neutralized. That acid might be the undissociated one or the protonated amine bound to the stronger acid. Therefore, the selectivity can reverse direction with increasing pH.