KINETICS OF TRACE-METAL COMPLEXATION - ROLE OF ALKALINE-EARTH METALS

The rate of reaction of inorganic copper with the model ligand ethylenediaminetetraacetic acid (EDTA) is significantly retarded in the presence of calcium at seawater concentrations. The (pseudo-first-order) half-life for inorganic copper reacting with EDTA in seawater is about 2 h at 10-7 M EDTA. This kinetic hindrance to the formation of the thermodynamically favored CuEDTA species results from several factors: (1) the preponderance of the calcium complex in the speciation of EDTA, (2) the competition of calcium and copper for reaction with any free EDTA formed by the dissocation of CaEDTA, and (3) the slow kinetics of direct attack of copper on CaEDTA compared to reaction with free or protonated EDTA species. Additional study results are discussed.