The Influence of Calcium on Rates of Ligand Exchange Between Strong Chelating Agents by Capillary Electrophoresis

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Chelating agents are widely used in plants and human industries to capture and control metal ions. Constituent ions in soils (e.g. Ca2+, Mg2+) influence the exchange of a metal ion between two chelating agents. Predicting changes in reaction pathways and the magnitude of kinetic effects is fundamental to understanding dynamic metal speciation where strong chelating ligands control metal ion speciation. This research focuses on the influence of calcium ions on the exchange rate of Ni between nitrilotriacetic acid (NTA) and 1,2-cyclohexylenedinitrilotetraacetic acid (CDTA). Capillary electrophoresis was used to monitor the changes in free chelating agent and nickel-chelating agent complex concentration with time at different pHs and reactant concentrations. The presence of calcium ions reduces overall exchange rates and alters reaction order under the conditions studied. Kinetic modeling is used to support a proposed mechanism for the influence of calcium on this exchange reaction.