The electrode reactions of several Pb(II)-macrocyclic complexes and their adsorption on mercury electrodes

Voltocoulometric, chronocoulometric and cyclic voltammetric studies of the electrode reactions of the Pb(2,2,2)2+, Pb(2,2,1)2+ and Pb(2,2)2+ complexes were described ((2,2)-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane, (2,2,1)-4,7,13,16,21-pentaoxa-1,10-12 diazabicyclo[5.8.8]tricosane, (2,2,2)-4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacosane). It was found that such Pb(L)2+ complexes are adsorbed on the Hg electrode. The analysis of adsorption data for Pb(2,2,2)2+ in the presence of excess of ligand (2,2,2) shows that the complex is adsorbed according to the competitive adsorption mechanism. Electroreduction processes of the studied complexes proceed via two parallel pathways: from the solution and from the adsorbed state. The electrode kinetic parameters of lead(II) complexes free of adsorption influence were found by analysis of the normal pulse voltocoulometric curves recorded with a 1 ms delay after application of the short potential pulse. From the analysis of kinetic parameters it follows that the electroreduction of Pb(L)2+ is a two-step CE process with the rate determining step preceded by complex dissociation. The rate constants of the studied Pb(L)2+/Pb(Hg) systems are considerably lower than the rate constant of noncomplexed Pb2+ ion in aqueous solution. We have found that the reaction rate constants of the systems Pb(L)2+/Pb(Hg) decrease linearly with increasing stability of the complexes. This dependence is also obeyed by cadmium and thallium complexes with the same ligands.