Interpretation of Solute Retention of Some Monovalent Inorganic Anions in Anion-Exchange Chromatography Using a Dicarboxylic Acid as an Eluent

Abstract

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 (E^2-): α_1s/V_R′[HE^-] = 1/m_TwK_ex1 + (2K_a2/m_Tw²K_ex2)(V_R′/α_1s[H⁺]), where V_R′, m_T, w, K_a2, K_ex1 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 α_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.