Chromatographic Utilization of the Sorption Behaviour of Some Nitrophenols on Acid-Treated Alumina

Abstract

Alumina for adsorption chromatography has been subjected to chemical pretreatment with HNO₃ and CH₃COOH of various concentrations and samples of surface-phase pH 1.5–9.0 [Al₂O₃(n)] and 3.0–9.0 [Al₂O₃(a)] prepared. The surface area, NO⁻₃ and CH₃COO⁻ ion content and OH⁻ ion-exchange capacity of the samples were determined. Sorption/desorption studies of the nitrophenols, 2,4-dinitrophenol (DNP) and 2,4,6-trinitrophenol (picric acid, PA), from aqueous solution under a variety of experimental conditions, were carried out on the substrates. Sorption was found to depend on the pH of the substrate and the acid used for pretreatment. Mximum sorption was observed at pH 6.0 [Al₂O₃(n)] and pH 5.0 [Al₂O₃(a)]. The process was fast (10 min to 24 h) and exothermic (30–60°C), with low values for the isosteric heats of adsorption (−7 to −64 kJ/mol). The desorption efficiency and retarding influence of the anions on adsorption was in the order: PO^3–₄ > C₂O^2–₄ > SO^2–₄ > CH₃COO⁻ > NO⁻₃ Acid treatment, and hence the anion present (NO⁻₃ or CH₃COO⁻), appears to modify the sorption properties of alumina significantly. Anion-exchange sorption and weak chemical interactions (hydrogen bonding and π-electron interaction) appear to control the sorption of the solutes on pretreated alumina. The distribution coefficients for adsorption and desorption have been determined, and on the basis of these data the elution behaviour and column chromatography of the system examined.