The influence of oxalic acid on rates of strong chelate exchange

Chelate exchange reactions often control the bioavailability of trace metal ions in soils. Soils contain many possible ligands, such as oxalate, in addition to the primary reactants -- strong chelating agents (e.g. siderophores and synthetic chelating agents). Here we explore the influence of oxalate on the pathway and rate of ligand exchange between nickel(II) nitrilotriacetic acid (NiNTA) and 1,2-cyclohexylenedinitrilotetraacetic acid (CDTA). Capillary electrophoresis was used to monitor the changes in free chelating agent and nickel-chelating agent concentration over time at different pHs and reactant concentrations. Below pH 8, the presence of oxalate moderately catalyzes the reaction, whereas above pH 8, the reaction is moderately inhibited. The increase or decrease in rates at each pH increases in magnitude with increasing oxalate concentration. Kinetic modeling is used to support a proposed reaction pathway for this chelate exchange reaction in the presence of oxalate.