Solvent extraction of microamounts of europium and cerium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B−) in the presence of polyethylene glycol PEG 600 (L) has been investigated. The equilibrium data have been explained assuming
that the complexes HL+, H2L2+ and ML3+ (M3+ = Eu3+, Ce3+; L = PEG 600) are extracted into the organic phase. The values of extraction and stability constants of the species in nitrobenzene
saturated with water have been determined. It was found that the stability constants of the EuL3+ and CeL3+ cationic complex species (L = PEG 600) in water-saturated nitrobenzene are the same.