Chronopotentiometric study of oxygen reduction at a platinum wire cathode

Abstract

This study provides conclusive proof that in both acid and basic aqueous media oxygen is reduced at a platinum cathode all the way to water rather than to hydrogen peroxide. It was discovered that rapid, diffusion-controlled reduction of oxygen occurs only when there is some platinum oxide on the platinum cathode. It is not necessary that the electrode be fully covered by a platinum oxide (PtO + PtO 2) film; an amount of platinum oxide corresponding to about So μcoulombs/cm 2 (i.e., about one-tenth the amount on a fully oxidized electrode) is enough to maintain diffusion control. When the quantity of platinum oxide is much smaller than this the overall reaction becomes kinetically-controlled. but the net reaction is still a 4-electron reduction of oxygen. The platinum-platinum ion (or platinum oxide) couple must be intimately involved in whatever the detailed kinetic mechanism may be. Apparently the primary step (or steps) in the reduction of oxygen is direct “chemical” reaction between the oxygen and the platinum, and the platinum ion(s) or oxide(s) thus formed are rapidly reduced back to metallic platinum in subsequent electron-transfer steps to yield the observed current. Very probably the rate-controlling step is associated with the primary “chemical” reduction of the oxygen by the metallic platinum. The characteristics of the chronopotcntiograms of oxygen with a platinum wire cathode in both acid and basic media are very well suited to the practical determination of oxygen. In such analitycal applications the platinum electrode should be freshly oxidised by brief anodic polarization before each trial to insure that the transition time will be diffusion-controlled, and thus will be governed by the concentration of oxygen.