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., pH(hn) < 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 approximate to pK(a1). (2) For systems in which pK(a1) < pH(hn) < 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 approximate to 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) < pH(hn), the divalent ion pair R3NH+...A(2-)...+HNR3 is formed. Thus, the extraction curve shows an additional drop-off at pH approximate to pK(a2).