POLAROGRAPHIC STUDY OF THE INTERACTION BETWEEN HEAVY-METAL IONS AND SOME MACROCYCLIC LIGANDS IN BINARY ACETONITRILE + WATER MIXTURES

The complexation reactions between Tl+, Cd2+, Hg2+ and Pd2+ ions and 18-crown-6 (18C6), 1,10-diaza-18-crown-6 (C22) and cryptand C222 were studied in various CH3CN + H2O mixtures at 22-degrees-C using dc and differential pulse polarographic techniques. The stoichiometry and stability of the complexes were determined by monitoring the shift in half-wave or peak potential of the polarographic waves of metal ions against the ligand concentration. In the cases where irreversible polarograms were observed for M2+/M(Hg) systems, the reversible Hg2+/Hg amalgam couple was used as an electrochemical probe for the study of M2+ - ligand interactions. In all the solvent mixtures used, the stability of the resulting 1:1 complexes was found to vary in the order C222 > C22 > 18C6. The selectivity order for 18C6 complexes is Pb 2+ > Cd2+ > Hg2+ > Tl+, while for the C22 and C222 complexes it is Hg2+ much greater than Pb2+ > Cd2+ > Tl+. There is an inverse relationship between the complex formation constants and the amount of water in the mixed solvent. In all cases, a linear relation was observed between log K(f) and the mole fraction of acetonitrile.