Protonation and ion exchange equilibria of weak base anion-exchange resins

Protonation and ion exchange equilibria of weak base anion-exchange resins, in which tertiary amine moieties were introduced as a functional group, were investigated by applying NMR spectroscopy to species adsorbed into the resins. ³¹P NMR signals of the phosphinate ion in the resin phases shifted to a lower field due to the influence of protonation of the tertiary amine groups of the resins in the pH range of 4-10. Protonation constants of the tertiary amine groups in styrene-divinylbenzene (DVB)-based resins were estimated to be K_H = 10^6.4 for Amberlite IRA96 and 10^6.5 for DIAION WA30 by the ³¹P NMR method using the phosphinate ion as a probe species. In addition to the low field shift caused by the protonation of the tertiary amine moieties, another low field shift was observed for the phosphinate ion in acrylic acid-DVB-based resins at a rather high pH. This shift should be due to an unexpected deprotonation in the acrylic resin: a tautomerism accompanying the proton release from the amide form to the imide one in the functional group, thus, the resin could exhibit a cation exchange property at the high pH. Protonation constants of the tertiary amine moieties in the acrylic resins were estimated to be 10^8.8 for DIAION WA10, 10^9.0 for Amberlite IRA67 and 10^9.3 for Bio-Rad AG 4-X4 on the basis of the Henderson-Hasselbalch equation using the resin phase pH estimated by the ¹³³Cs and ¹H NMR signal intensities.