Effect of pH on Dicarboxylic Acids Extraction by Amine-Based Extractants

The work presented here studies the effect of pH on the extraction of malic, maleic, and glutaric acids by Primene JMT, tris(2-ethylhexyl)amine , and tri-n-octylamine and the IR and NMR spectra of the formed organic phases. Three categories were found: (1) In the first case, the extractant is a weaker base than both anions of the acid, i.e., pHhn < pK a1 < pK a2. The undissociated acid is the dominant species in the organic phase. Extraction is mainly due to H-bonding or solvation interactions and shows a drop-off at pH ≈ pK a1. (2) For systems in which pK a1 < pHhn < pK a2, at acid-to-amine molar ratios greater than 1, undissociated acid is above-stoichiometrically extracted, R3NH+···AH-···HAH, in addition to ion-paired acid. As the pH is increased, the undissociated acid is neutralized first. Thus, the extraction curve shows a drop-off at pH ≈ pK a1. At stoichiometric loading, the monovalent ion pair, R3NH+···AH-, becomes the dominant species. Upon further pH elevation, the protonated amine R3NH+ is the neutralized acid. (3) In systems where pK a1 < pK a2 < pHhn, the divalent ion pair R3NH+···A2-···+HNR3 is formed. Thus, the extraction curve shows an additional drop-off at pH ≈ pK a2.