Equilibrium studies on complexation in water and solvent extraction of zinc(II) and cadmium(II) with benzo-18-crown-6

The formation constants (KML) in water of 1:1 complexes of benzo-18-crown-6 (B18C6) and 18-crown-6 (18C6) with Zn2+ and Cd2+, the sizes of which are much smaller than the ligand cavities, were determined at 25°C by conductometry. Compared with Cd2+, the crown ethers form more stable complexes with Zn2+ although the size of Zn2+ is less suited for the cavities. B18C6 forms a more stable complex with each metal ion than 18C6. Moreover, the extraction equilibria of these metal ions (M2+) with B18C6 (L) for the benzene/water system in the presence of picric acid (HA) were investigated at 25°C. The association between L and HA in benzene was examined for evaluating the intrinsic extraction equilibria of M2+ with B18C6. The extracted species were found to be MLA2 and ML2A2, and the overall extraction constants (Kex,1 and Kex,2, respectively) were obtained. The values of Kex,1 for these metal ions are almost the same, but the Kex,2 is larger for Zn2+ than for Cd2+. The extraction selectivity was interpreted quantitatively by the constituent equilibrium constants, i.e. KML, the ion-pair extraction constant of ML2+ with A−, and the adduct formation constant of MLA2 with L in benzene.