Electroreduction of peroxycitric acid coexisting with hydrogen peroxide in aqueous solution

Abstract

The electrochemical reduction of peroxycitric acid (PCA) coexisting with citric acid and hydrogen peroxide (H 2O 2) in the equilibrium mixture was extensively studied at a gold electrode in acetate buffer solutions containing 0.1 M Na 2SO 4 (pH 2.0–6.0) using cyclic and hydrodynamic voltammetric, and hydrodynamic chronocoulometric measurements. The reduction of PCA was characterized to be an irreversible, diffusion-controlled process, and the cyclic voltammetric reduction peak potential ( E p c ) was found to be more positive by ca. 1.0 V than that of the coexisting H 2O 2, e.g., the E p c values obtained at 0.1 V s −1 for PCA and H 2O 2 were 0.35 and −0.35 V, respectively, vs. Ag|AgCl|KCl (sat.) at pH 3.3. The E p c of PCA was found to depend on pH, i.e., at pH > 4.5, the plot of E p c vs. pH gave the slope (−64 mV decade −1) which is close to the theoretical value (−59 mV decade −1) for an electrode process involving the equal number of electron and proton in the rate-determining step, while at pH < 4.5, the E p c was almost independent of pH. The relevant electrochemical parameters, Tafel slope, number of electrons, formal potential ( E 0′), cathodic transfer coefficient and standard heterogeneous rate constant ( k 0′) for the reduction of PCA and the diffusion coefficient of PCA were determined to be ca. 100 mV decade −1, 2, 1.53 V (at pH 2.6), 0.29, 1.2 × 10 −12 cm s −1 and 0.29 × 10 −5 cm 2 s −1, respectively, and except for E 0′, the obtained values were almost independent of the solution pH. The overall mechanism of the reduction of PCA was discussed.