Effect of substituted phenolates as a counterion on the ion-pair extraction of alkali metal ions using crown ether

Abstract

The effect of a counterion was investigated in the solvent extraction of alkali metal ions using crown ether. Nitrobenzene was used as the solvent, 18-crown-6 was used as the extracting reagent, rubidium ion, which suits the cavity size of 18-crown-6, was selected as the model of alkali metal ions. In order to vary the distribution of the electric charge and the hydrophobicity of the counterion, various substituted phenols were used as the counterion. The maximum extraction ratio of rubidium ion was increased with non-localization of the distribution of the electric charge and with its hydrophobicity higher. The maximum extraction ratio was increased by introducing chloro substituent to the counterion, but it was not more effective than introducing nitro substituent. The pH where the extraction curve rose depended upon the pK_a of the counterion. In this extraction system, the value of the pH controlled only the condition of the dissociation of the counterion.