Role of hydrophobic counteranions in the ion pair extraction of lanthanides(III) with an electrically neutral extractant

The effect of a hydrophobic and weakly coordinate bonding anion, tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (TFPB-), on the extraction of lanthanides(III) with an electrically neutral extractant, tetraoctyldiglycolamide (TODGA), has been examined. By adding H+TFPB-, the extraction of these metal ions is greatly enhanced; besides, the selectivity is also much improved. The metal extraction with TFPB- working as a counteranion is, formally, in a category of ion pair extraction, involving ion exchange of H+ paired with TFPB- for the metal ion. However, with such a hydrophobic anion, a peculiar phenomenon, which has never been seen in traditional ion pair extraction, occurs, i.e., the distribution ratio of a lanthanide(III) is only slightly dependent on the concentration of TODGA over a wide range, at least, from 5 x 10(-4) to 0.4 M, or rather, it is somewhat decreased at high TODGA concentrations. This is, seemingly, against the general rule of solvent extraction, but can be explained coherently from the viewpoint of chemical equilibrium. Two ion pair extraction equilibria, one of which has the opposite sign to the other in the dependence of the distribution ratio of the metal on the concentration of TODGA, are postulated. The slight change in the metal distribution ratio over the wide range of the TODGA concentration is understood as a "buffer zone" occurring between the two different ion pair extraction equilibria with TFPB-.