A diagnostic criterion was developed to distinguish between an intermediate formed in an over-all reaction path and a product in a parallel electrochemical reaction. It utilizes the method of the rotating-disk electrode with a concentric ring, first proposed by Frumkin and Nekrassov. From the dependence of currents at the disk and at the ring electrode on the rate of disk rotation, individual rates of parallel reactions can be obtained.

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A rotating disk electrode with a concentric ring has been used to determine the role of hydrogen peroxide in the reduction of oxygen at platinum electrodes in alkaline solution. Oxygen reduction proceeds along two parallel reaction paths with comparable rates. In one of the paths, hydrogen peroxide is a reaction intermediate which is partially reduced to water. In the other, oxygen reduces to water without hydrogen peroxide as an intermediate. In the path with hydrogen peroxide intermediate, the first part of the probable sequence of reaction steps is shown to be O2+e−⇄O2− O2−+H2O→HO2−+OH OH+e−⇄OH− The peroxide anion is thereafter reduced to water in a path which does not involve an electron transfer before or in the rate‐controlling step. Possible paths for oxygen reduction to water without peroxide intermediates are discussed in the light of already proposed mechanisms for the reduction at prereduced and preoxidized electrodes.

Ellipsometry is applied to study the oxidation of platinum in sulfuric acid solution. Steady-state and transient measurements show that oxide-film formation commences when the potential is anodic to 0.95 V. The film is light absorbing. The film thickness increases linearly with potential. A mechanism of film growth based on place exchange is developed.

The rotating disk electrode with a concentric ring is used to analyze the role of H2O2 in the reduction of O2 to H2O in acid solutions. No H2O2 is formed as an intermediate in the reaction path of oxygen reduction to water. Only when the residual impurities from the not sufficiently purified solutions are adsorbed at the electrode does a parallel reaction path appear in which H2O2 is formed as an intermediate that does not reduce further to H2O with any significant rate. The mechanism of O2 reduction to H2O involves a first charge transfer step as rate‐controlling under Temkin conditions of adsorption.

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