Detection of a complex intermediate in the oxidation of ascorbic acid by the copper (II) ion.

A complex intermediate of the copper (II) ion with the ascorbate anion was detected by using a rapid scanning spectrophotometer with a stopped-flow apparatus. The intermediate was confirmed to be a copper (II) complex ion by electron paramagnetic resonance measurements. The dependence of the absorbance of the complex on the concentrations of reactants and on the pH of the reaction solutions indicates that the complex is Cu(II)Ha+ where Ha is the ascorbate anion ligand. The absorption maximum, the molar absorption coefficient, and the formation constant of the complex are 410 nm, 33 +/- 1 M-1 cm-1 at 420nm, and 42 +/- 6 M-1, respectively. The values are comparable to those in the other metal complexes of the ascorbate anion.