(a) Oxidation of BP over time with different kinds of metal ions. (b) Oxidation rates of BP at different Cu²⁺ concentrations. (c) Pseudo-first-order rate plots for oxidation of BP. (d) Relationship between pseudo-first-order rate constants and Cu²⁺ concentration. (b–d) Experimental conditions were as follows: the absorbance of BP was 0.5, [Cu²⁺] = 1–20 μmol/L, pH = 7.0, 25°C. Error bars represent standard deviations (n = 3).