The effect of ten pure diluents and different mixtures of nitrobenzene and toluene on the synergic extraction of Co2+ by the thenoyltrifluoroacetone (HTTA)-tribenzylamine (TBA) mixture is studied. The logarithm of the extraction constant of
Co(TTA)2 increases with increasing dielectric constant of the diluent, whereas the logarithm of the formation constant of the extracted
adduct Co(TTA)2TBA decreases with increasing dielectric constant of the diluent. This behaviour is explained by changes in the hydrophobic
character of the extracted species. The theory of regular solutions is successfully applied to the extracted adduct. Consideration
of the self-association of the amine salt is necessary in the treatment of the equilibria involved.