Effects of pH, Chloride, and Bicarbonate on Cu(I) Oxidation Kinetics at Circumneutral pH

The oxidation kinetics of nanomolar concentrations of Cu(I) in NaCl solutions have been investigated over the pH range 6.5-8.0. The overall apparent oxidation rate constant was strongly affected by chloride, moderately by bicarbonate, and to a lesser extent by pH. In the absence of bicarbonate, an equilibrium-based speciation model indicated that Cu+ and CuClOH- were the most kinetically reactive species, while the contribution of other Cu(I) species to the overall oxidation rate was minor. A kinetic model based on recognized key redox reactions for these two species further indicated that oxidation of Cu(I) by oxygen and superoxide were important reactions at all pH values and chloride concentrations considered, but back reduction of Cu(II) by superoxide only became important at relatively low chloride concentrations. Bicarbonate concentrations from 2 to 5 mM substantially accelerated Cu(I) oxidation. Kinetic analysis over a range of bicarbonate concentrations revealed that this was due to formation of CuCO3-, which reacts relatively rapidly with oxygen, and not due to inhibition of the back reduction of Cu(II) by formation of Cu(II)-carbonate complexes. We conclude that the simultaneous oxygenation of Cu+, CuClOH-, and CuCO3- is the rate-limiting step in the overall oxidation of Cu(I) under these conditions.