Metal binding by heterogeneous ligands: kinetic master curves from SSCP waves.
Stripping chronopotentiometry at scanned deposition potential, SSCP, is shown to be a powerful tool for determination of the distribution of metal dissociation rate constants, kd, for heterogeneous ligands. SSCP effectively scans a range of metal-to-ligand ratios from the foot to the plateau of the wave, thus allowing a portion of the distribution of k(d) values to be determined for a given bulk solution composition. In particular, the nature of the measurement renders accessible very low metal-to-ligand ratios which are otherwise difficult to attain by a bulk solution methodology, such as a potentiometric titration. In the presence of kinetic currents, the shape of the SSCP wave is modified as compared to the labile case. A data interpretation framework is developed which well-describes the SSCP wave for heterogeneous complexes in the kinetic current regime. The analysis utilizes the Freundlich binding isotherm together with the Koutecký-Koryta approximation, i.e., assuming a spatially discontinuous transition from labile to nonlabile behavior. For the case of Pb(II) and Cu(II) complexation by a peat fulvic acid, the existing body of data is drastically expanded to the low to very low metal-to-ligand ratio domain, thereby greatly improving the quality of the derived distribution parameters.
Town
Raewyn M
RM
Institute for Physics and Chemistry, University of Southern Denmark, Campusvej 55, DK-5230 Odense, Denmark. rmt@ifk.sdu.dk
D001578:Benzopyrans / Q000737:chemistry*; D002104:Cadmium / Q000737:chemistry*; D004785:Environmental Pollutants / Q000737:chemistry*; D007854:Lead / Q000737:chemistry*; D008024:Ligands; D011199:Potentiometry; D012987:Soil
D001578:Benzopyrans; D004785:Environmental Pollutants; D008024:Ligands; D012987:Soil; D002104:Cadmium; D007854:Lead; C005023:fulvic acid