Sorption—desorption studies of cationic dyes on silica gel pretreated with alkalis in relation to chromatography

Abstract

Silica gel for adsorption chromatography was subjected to chemical pretreatment with NaOH and KOH of various concentrations, and surface-phase samples (pH 7.0–11.0) were prepared. The surface area, Na + and K + ion content and H + ion-exchange capacity of the samples were determined. Sorption-desorption behaviour of two cationic dyes (safranine and brilliant cresyl blue) with change in pH and under varying experimental conditions, on these substrates was studied. The sorption was found to vary with the surface pH of the substrate and alkali used for pretreatment. Thus, adsorption was found to increase in the pH range 6.5–10.5, and then decrease at pH 11.0. Maximum sorption was shown at pH 10.5. Variation of adsorption with time (10 min–24 h), temperature (30–60°C) and regeneration of the substrates with aqueous electrolytes was also investigated. The influence of the cations in retarding the adsorption, and their desorption efficacy was found to be in the order: Th 4+ > Al 3+ > Mg 2+ > Na +. The alkali treatment, and hence the cation present (Na + or K +), appears to modify the sorption properties of silica gel significantly. Sorption by slow ion exchange of the dye cations (single or as micelles) with the cations on the surface of silica gel (Na + or K +) as well as chemical interaction to some extent (at higher pH values of the substrates) is proposed. The distribution coefficients for adsorption ( K d) and desorption ( K D) were determined, and on the basis of these studies, the elution behaviour and column chromatography was studied.