Competitive potentiometric study of the thermodynamics of complexation of some transition and heavy metal ions with dibenzopyridino-18-crown-6 in methanol using Ag+ ion as a probe

The complexation of dibenzopyridino-18-crown-6 with some transition and heavy metal ions in methanol solution at various temperatures was studied by a competitive potentiometric method using a Ag+/Ag electrode system. The stoichiometry and stability of the resulting complexes were computed by the MINIQUAD program. The stability of the resulting complexes varied in the order Ag+ > Pb2+ > Tl+ > Cu2+ > Cd2+ > Zn2+. The enthalpy and entropy of the resulting 1:1 complexes were evaluated from the temperature dependence of the stability constants. The complexes of all cations were enthalpy-stabilized but entropy-destabilized, except for Ag+ and Pb2+ ions, which were also entropy- stabilized.