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

Abstract

The formation constants ( K ML) in water of 1:1 complexes of benzo-18-crown-6 (B18C6) and 18-crown-6 (18C6) with Zn 2+ and Cd 2+, the sizes of which are much smaller than the ligand cavities, were determined at 25°C by conductometry. Compared with Cd 2+, the crown ethers form more stable complexes with Zn 2+ although the size of Zn 2+ 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 (M 2+) 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 M 2+ with B18C6. The extracted species were found to be MLA 2 and ML 2A 2, and the overall extraction constants ( K ex,1 and K ex,2, respectively) were obtained. The values of K ex,1 for these metal ions are almost the same, but the K ex,2 is larger for Zn 2+ than for Cd 2+. The extraction selectivity was interpreted quantitatively by the constituent equilibrium constants, i.e. K ML, the ion-pair extraction constant of ML 2+ with A −, and the adduct formation constant of MLA 2 with L in benzene.