Solvent extraction of175,181Hf(IV) into solutions of HBPHA** in CHCl3, tetrachloro ethane (C2H2Cl4), CCl4 and benzene from mixtures of water with DMSO or DMFA at 2M HClO4 formal acidity was studied. With diluents containing acidic hydrogen (CHCl3, C2H2Cl4) the presence of DMSO or DMFA in the extraction system leads to an increased extractability of the Hf(IV) chelate. Possible
reasons for this synergism are discussed. Extractions into benzene and CCl4, as well as those into CHCl3 and C2H2Cl4 at high DMSO and DMFA concentrations, are antagonistic. This is caused by the decrease of the partition coefficient of HBPHA,
and changes in the solvation of Hf(IV) ion in the polar phase. The latter effect is very strong at high DMSO concentrations,
and with macroconcentrations of Hf(IV) finally leads to precipitation of crystalline adducts Hf(ClO4)4·xDMSO (x=7–8).