Effect of the hydrophobic anion of picrate on the extraction of europium(III) with diamide

The effect of the picrate (pic(-)) ion on Eu(III) extraction with a diamide (da) from aqueous nitric acid (0.001-0.1 mol dm(-3)) solutions into benzen e was clarified on the basis of the stoichiometry of ion-pair extractions b y analyzing the distribution data of Eu(III) and picric acid (Hpic). The ex traction of Eu(III) with da was extremely enhanced by adding a hydrophobic anion of pic(-) to the liquid-liquid system. From analyses of the distribut ion data of Eu(III) and Hpic, it was concluded that the effect of the hydro phobic anion was caused by the extraction of a single ion-pair of [Eu(da)(2 )](pic)(3). The nitrate ion had no relation with the cooperative effect of da and pic(-), i.e., the extraction of [Eu(da)(2)](NO3)(3), [Eu(da)(2)](pic )(NO3)(2), and [Eu(da)(2)](pic)(2)(NO3) was practically negligible as again st the [Eu(da)(2)](pic)(3) extraction. The cooperative effect was reduced b y an increase in the aqueous HNO3 concentration, which was induced by a dec rease in the pic(-) concentration in the aqueous phase; the dissociation of Hpic into pic(-) ion was suppressed by an increase in the H+ concentration , and a part of molecular Hpic was extracted into the organic phase. Furthe rmore, the extraction of Hpic was also enhanced by an increase in the da co ncentration is the organic phase, which also induced a decrease in the aque ous pic(-) concentration. This was found to be due to the formation of a 1 : 1 complex between da and Hpic. However, the extraction of the complexes b etween da and HNO3 was negligible in the acid concentration range in the pr esent study.