G. N. Botukhova

Contemporary understanding of the peroxodisulfate reduction at a mercury electrode

Specific aspects of the kinetics of anion electroreduction at high overvoltages are addressed by various traditional procedures to treat experimental data. The expansion of the Frumkin correction concept is proposed in terms of the reaction volume which increases with increasing negative electrode charge for anionic reactants. A molecular level approach based on quantum chemical calculations of the work terms and the electrode–reactant electronic coupling is employed for calculations of the reaction volume for the case of peroxodisulfate S2O82− and the ion pair Na+·S2O82−, the species which are to be considered as possible reactants in Na2S2O8 solutions with sodium salts as supporting electrolytes. Estimates of the partial rate constants for the anion and ion pair are reported as well. New experimental data are presented for solutions of equal ionic strength and equal degree of association but with different total reactant concentration. These data confirm the possibility of local ion pair formation at low negative charge. In parallel the previously ignored problem of correction for mass transport limitations is discussed for a system with two simultaneously discharging species of various charges. It follows from this reconsideration that previous data cannot be interpreted for sure as the absence of an ion pair contribution to the total current. The problem of separation of contributions from corresponding parallel steps to the total current is discussed, and the fast formation of an ion pair preceding the electron transfer was found to be rather probable.

Electroreduction of peroxodisulfate anion at platinum rotating disc electrode in the cyclic voltammetry mode

Electroreduction of peroxodisulfate anion at smooth polycrystalline and platinized (at different deposition potentials) platinum in perchloric acid and sulfuric acid solutions is studied by rotating disc electrode and cyclic voltammetry techniques. Dependences of the process rate on the electrode rotating velocity, the potential scan rate, the anodic limit of the scanning, the peroxodisulfate anion concentration in the solution and the platinizing conditions are found. The suggestion on the complications in the peroxodisulfate anion reduction caused by adsorbate formation is corroborated, at least, for certain potential region. The reaction structure sensitivity is evidenced, which makes it possible to use the reaction for characterization of the platinized Pt surface structure. The comparing of obtained results with literature data concerning smooth platinum and the single-crystal platinum basis faces allows concluding that the peroxodisulfate anion reduction maximal rate in sulfuric-acid solutions occurs at the potentials close to those observed for the (110) face. When the platinized Pt surface roughness factor exceeds ~30, the peroxodisulfate anion reduction reaction proceeds under the inner-diffusion limitation control. The platinized-Pt rotating disc electrode can serve as model tool in the studying of properties of disperse material microdeposits.

Mechanism of Electroreduction of Peroxodiphosphate Anions

The kinetic regularities of peroxodiphosphate anion reduction on dropping mercury and 40% thallium amalgam electrodes are studied. The reaction rate is found to increase with an increase in the supporting electrolyte and peroxodiphosphate anion concentrations, with the adsorption of inorganic cations on the electrode, and when the electrode exhibits a more negative zero-charge potential. The data obtained are analyzed within the Frumkin theory of slow discharge.