Structural and Chemical Features of Extraction with Crown Ethers

Abstract

Unlike linear extracting agents, in the extraction of metal salts from aqueous solutions of inorganic acids with crown ethers, the inclusion compounds, whose composition depends on several external and internal factors, go to the organic phase. The study of the molecular structure of the formed complexes by X-ray diffraction analysis showed that adducts of crown ethers with inorganic acids are host–guest complexes in which the hydroxonium ion is in the polyether macrocycle cavity. When the aqueous phase contains metal ions capable of displacing the hydroxonium ions from the macrocycle (K+, Pb2+, Hg2+, Sr2+, NH4+), complexes containing metal cations as the “guest” in the macrocycle cavity, according to X-ray diffraction data, go to the organic phase. In addition, metals forming ionic associates (AuCl4-, FeCl4-, GaCl4-) in an aqueous solution are extracted with crown ethers in accordance with the anion-exchange mechanism. A system in which traces of metals in the 2 M HNO3 +5 M HCl mixture serve as the aqueous phase was proposed for estimation of the general extraction ability of crown ethers. Such a system can be used for metal extraction via any possible mechanism. The stereochemical peculiarities of the extraction ability of crown ethers (compared to linear molecules) can be used for selective extraction and separation of metals.