Chromatographic utilization of the sorption behaviour of some nitrophenols on acid-treated alumina

Alumina for adsorption chromatography was subjected to chemical pretreatment with HClO₄ and HCl of various concentrations, and samples of surface-phase pH 1.0–9.0 were prepared. The surface area, CiO⁻₄ and Cl⁻ ion content and OH⁻ ion-exchange capacity of the samples were determired. A sorption—desorption study of the nitrophenols, 2,4-dinitrophenol and 2,4,6-trinitrophenol, from aqueous solutions, under a variety of experimental conditions was carried out on the substrates. The sorption was found to depend on the pH of the substrate and on the acid used for the pretreatment. Maximum sorption was observed in the pH range 4.5–5.5. The process was fast (10 min–24 h) and exothermic (30–60°C), with low values of isosteric heat of adsorption (−6 to −31 kJ mol⁻¹). The desorption efficiency and retarding influence of the anions on adsorption was in the order: PO³⁻₄ > C₂O²⁻₄ > SO²⁻₄ > CH₃COO⁻ > NO⁻₃. The a treatment, and hence, the anion present (ClO⁻₄ or Cl⁻), 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 were determined, and on the basis of these data, the elution behaviour and column chromatography were studied.