Kinetic Modeling Assisted Analysis of Vitamin C‑Mediated Copper Redox Transformations in Aqueous Solutions

The kinetics of oxidation of micromolar concentrations of ascorbic acid (AA) catalyzed by Cu(II) in solutions representative of biological and environmental aqueous systems has been investigated in both the presence and absence of oxygen. The results reveal that the reaction between AA and Cu(II) is a relatively complex set of redox processes whereby Cu(II) initially oxidizes AA yielding the intermediate ascorbate radical (A•–) and Cu(I). The rate constant for this reaction was determined to have a lower limit of 2.2 × 104 M–1 s–1. Oxygen was found to play a critical role in mediating the Cu(II)/Cu(I) redox cycle and the oxidation reactions of AA and its oxidized forms. Among these processes, the oxidation of the ascorbate radical by molecular oxygen was identified to play a key role in the consumption of ascorbic acid, despite being a slow reaction. The rate constant for this reaction (A•−+O2→DHA+O2•−) was determined for the first time with a calculated value of 54 ± 8 M–1 s–1. The kinetic model developed satisfactorily describes the Cu/AA/O2 system over a range of conditions including different concentrations of NaCl (0.2 and 0.7 M) and pH (7.4 and 8.1). Appropriate adjustments to the rate constant for the reaction between Cu(I) and O2 were found to account for the influence of the chloride ions and pH on the kinetics of the process. Additionally, the presence of Cu(III) as the primary oxidant resulting from the interaction between Cu(I) and H2O2 in the Cu(II)/AA system was confirmed, along with the coexistence of HO•, possibly due to an equilibrium established between Cu(III) and HO•

Kinetics of Cu(II) Reduction by Natural Organic Matter

The kinetics of Cu­(II) reduction by Suwannee River fulvic acid (SRFA) at concentrations from 0.25 to 8 mg L^–1 have been investigated in 2 mM NaHCO₃ and 0.7 M NaCl at pH 8.0. In the absence of oxygen, SRFA reduced Cu­(II) to Cu­(I) in a biphasic manner, with initial rapid formation of Cu­(I) followed by a much slower increase in Cu­(I) concentration over time. When present, oxygen only had a noticeable effect on Cu­(I) concentrations in the second phase of the reduction process and at high [SRFA]. In both the absence and presence of oxygen, the rate of Cu­(I) generation increased with increasing [SRFA]. At 8 mg L^–1 [SRFA], nearly 75% of the 0.4 μM Cu­(II) initially present was reduced to Cu­(I) after 20 min, although the yield of Cu­(I) relative to [SRFA] decreased at [SRFA] > 1 mg L^–1. Two plausible kinetic modeling approaches were found to satisfactorily describe the experimental data over a range of [SRFA]. Despite some uncertainty as to which approach is correct, common features of both approaches were complexation of Cu­(II) by SRFA and reduction of Cu­(II) by two different electron donor groups within SRFA: a relatively labile electron donor (with a concentration of 1.1 × 10^–4 equiv of e^– (g of SRFA)⁻¹) that reduced Cu­(II) relatively rapidly and a less labile donor (with a concentration of 3.1 × 10^–4 equiv of e^– (g of SRFA)⁻¹) that reduced Cu­(II) more slowly

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