Electro-Oxidation of Carbonate in Aqueous Solution on a Platinum Rotating Ring Disk Electrode

The kinetics of electro-oxidation of carbonate aqueous solution on a platinum electrode were examined by a rotating ring (platinum) – disk (platinum) electrode in aqueous sodium carbonate/bicarbonate solutions (25 °C, pH = 10.75) with the peroxide stabilizing additives sodium silicate, magnesium sulphate and sodium DTPA such as are used in brightening mechanical wood pulp. A theoretical model based on the experimental data is proposed to describe this electro-oxidation process. The model shows consistency between the experimental results and the expectation from the proposed reaction sequence. The first-order rate constants at 25 °C, pH = 10.75 in 1.13 M (Na2CO3 + NaHCO3) for carbonate electro-oxidation to percarbonate (a.k.a. peroxidicarbonate C2O62-), percarbonate hydrolysis to hydrogen peroxide and hydrogen peroxide electro-oxidation to oxygen are estimated from the experimental data, respectively, as (1.43±0.1) × 10−11cm s−1 (extrapolated to the equilibrium electrode potential which is estimated as ca. 0.21  (vs SCE)), (1.1 ± 0.2) × 10−2 s−1 and (5.6±0.5) × 10−8cm s−1 (extrapolated to the equilibrium electrode potential of −0.019 V (vs SCE)).