Role of solvents in the synergistic extraction of metal complexes

The extraction equilibria of Co(II) and Zn(II) complexes with 4-benzoyl-2,4-dihydro-5-methyl-2-phenyl-3H-pyrazol-3-one in the presence of tributyl phosphate have been investigated in eight ‘inert’ solvents employing the radioisotopes cobalt-60 and zinc-65. The activity coefficients of different species in the organic phase and the solubility parameter of the adduct have been calculated with the help of regular solution theory. A regularity is observed in the correlation between the adduct formation constant and the activity coefficient of the species.