Discrepancies in Solvent Extraction Slope Analysis Caused by Aqueous Buffer Complexation

Abstract

ABSTRACT The interaction between an aqueous phthalate buffer and the rare earth metal europium(III) was investigated to demonstrate the effect of buffer complexation of Eu on solvent extraction slope analysis. Some literature has suggested the formation of aqueous rare earth-hydroxide complexes in solvent extraction systems due to depressed slopes in log–log plots of extraction data. These claims were made despite the experiments being conducted under acidic conditions where extraction of rare earth-hydroxide complexes cannot affect the acid dependence of the extraction, and despite neglecting to account for aqueous buffer ligand complexation. We investigated the extractant concentration and acid dependencies of the Eu distribution ratio for two bis(2-ethylhexyl)phosphoric acid/o-xylene extraction systems with varying aqueous phase compositions. The first system contained no buffer and produced log–log plots with slopes close to 3, while the second system containing aqueous phthalate buffer resulted in greatly diminished distribution ratio values and acid dependencies with variable slopes. These experiments, in conjunction with fluorescence spectroscopy of the aqueous phases, suggest that the rare earth elements complex with aqueous buffer rather than hydroxide at pH values less than 6.