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

The formation constants (K-ML) 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 degrees C by c onductometry. Compared with Cd2+, the crown ethers form more stable complex es with Zn2+ although the size of Zn2+ is less suited for the cavities. B18 C6 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 ben zene/water system in the presence of picric acid (HA) were investigated at 25 degrees C. The association between L and HA in benzene was examined for evaluating the intrinsic extraction equilibria of M2+ With B18C6. The extra cted species were found to be MLA(2) and ML(2)A(2), and the overall extract ion constants (K-ex,K-1 and K-ex,K-2, respectively) were obtained. The valu es of K-ex,K-1, for these metal ions are almost the same, but the K-ex,K-2, is larger for Zn2+ than for Cd2+. The extraction selectivity was interpret ed quantitatively by the constituent equilibrium constants, i.e. K-ML, the ion-pair extraction constant of ML2+ with A(-), and the adduct formation co nstant of MLA(2) with L in benzene. (C) 2000 Elsevier Science B.V. All righ ts reserved.