Ion-pair reactions play a key role in the extraction of anionic metal ions –such as chlorinated, fluorinated, or sulfated ions, and oxonium anions– through the use of cationic extractants whose amino N atoms are protonated under acidic conditions. Focusing on these reactions, we investigated the extraction of Zr, Hf, Nb, and Ta anionic metal species present in sulfuric and hydrofluoric acids, and oxonium anions of group 6 and 7 elements, including Tc, Re, Cr, Mo, and W, under various conditions. The distribution ratio, D(M), for some anions exceeded 100. NTAamide (hexaalkyl-nitrilotriacetamide), having a tetradentate mode, exhibited the highest D values among all extractants tested. Monovalent oxonium anions, such as Tc and Re, showed higher D values than divalent anions, and these anions were efficiently extracted from acids containing affinitive anions, such as HF, HCl, and H2SO4, rather than from HNO3 and HClO4.
Sasaki et al. (Thu,) studied this question.