Interpretation of solute retention of some monovalent inorganic anions in anion-exchange chromatography using a dicarboxylic acid as an eluent

In single-column anion-exchange chromatography, the retention volume of some monovalent inorganic anions (Cl-, Br-,NO3-, NCS- and NO2-) were observed as a function of the pH of a mobile phase at a fixed concentration of 2-phenylmalonic acid or 1,4-benzenediacetic acid used as an eluent. The experimental retention, volume of such an anion was decreased with an increase in the pH of a mobile phase, and was able to be described by the following equation taking account of anion-exchange equilibria of a sample anion with a hydrogen dicarboxylate ion (HE-) and with a dicarboxylate ion (E2-): alpha(1s)/V-R'[HE-] = 1/m(T)wK(ex1) + (2K(a2)/m(T)w(2)K(ex2))(V-R'/alpha(1s)[H+]), where V-R', m(T), w, K-a2, Ke(x1) and K-ex2 are the adjusted retention volume of a given sample anion, the capacity for the anion-exchange of column packings and the weight of column packings packed into a separating column, the second acid-dissociation constant of the dicarboxylic acid used as an eluent, and equilibrium constants for the anion exchange of a sample anion with a monovalent hydrogen dicarboxylate ion and with a divalent dicarboxylate ion, respectively. The term alpha(1s), defined as K-as/([H+] + K-as), where K-as is the acid-dissociation constant of HX, is the mole fraction of a sample anion, X-, and is equal to 1 when using a strong acid anion as a sample anion.