Selectivity in Stripping of Alkali-Metal Cations from Crown Ether Carboxylate Complexes

The potential of crown ethers (macrocyclic polyethers) as the next generation of specific extracting agents for metal ions was markedly enhanced by the introduction of crown ethers which bear pendent proton-ionizable groups (1–5). It has been shown that metal ion extraction does not require concomitant transfer of an aqueous phase anion into the organic medium (6). For potential practical applications of these proton-ionizable crown ethers in which hard aqueous phase anions (chloride, nitrate, sulfate) would be involved, this factor is of immense importance. In addition, the combination of ion binding cavities possessing fixed dimensions with protonionizable groups creates novel bifunctional chelating agents. A final factor is the case with which extracted metal ions may be stripped from the organic phase by shaking with aqueous mineral acid.