Solvent extraction of microamounts of europium and cerium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B−) in the presence of polyethylene glycol PEG 1000 (L) has been investigated. The equilibrium data have been explained assuming
that the species H2L2+ and ML3+ (M3+ = Eu3+, Ce3+) are extracted into the organic phase. The values of extraction and stability constants of the complexes in nitrobenzene
saturated with water have been determined. It was found that the stability constants of the cationic complex species EuL3+ and CeL3+ in water-saturated nitrobenzene are practically the same.