Ion-Pair Extraction System for the Mutual Separation of Lanthanides Using Divalent Quadridentate Schiff Bases

For the selective extraction of trivalent lanthanides, the use of quadridentate divalent phenolic Schiff bases, such as N,N‘-bis(5-nitrosalicilidene)ethylenediamine (H2Nsalen) and N,N‘-bis(5-nitrosalicilidene)-o-phenylenediamine (H2Nsaloph), was investigated. Lanthanides made anionic 1:2 complexes with these ligands and could be extracted into nitrobenzene as ion-pairs with a suitable monovalent countercation in the aqueous phase, after which the ion-pair was dissociated almost perfectly. Although the order of the extractability of lanthanides between these ligands was H2Nsaloph > H2Nsalen, the mutual selectivity of them in the H2Nsalen system was higher than that in H2Nsaloph. Furthermore, with decreasing the size of the countercation, the mutual selectivity was enhanced although the extractability was lowered. A H2Nsalen−KCl extraction system, selected as an example, showed selectivity between lanthanides comparable with those of the better of other systems reported previously.