Die anodische oxydation von oxalsäure an platin—I. Passivierungseffekte an elektroden aus glattem platin

Abstract

The anodic passivation of a bright platinum electrode in 1 N H 2SO 4 for the reaction H 2C 2O 4 → 2CO 2 + 2H + + 2e − occurs by means of a separation of oxygen from the water at the Pt anode with the formation of a layer of chemisorbed oxygen. The passivation of the electrode for the oxalic acid oxidation is the stronger the greater the coverage with oxygen. With a complete monomolecular layer of oxygen the oxalic acid oxidation is completely halted; there is only an evolution of O 2. For the passivating action it is indifferent whether the oxygen layer is formed by means of an anodic prepolarization in the absence of oxalic acid or during the anodic oxalic acid oxidation. The chemisorbed passivating oxygen can be removed either by cathodic reduction or by slow reduction by the oxalic acid itself (self-activation). The passivating oxygen is the more difficult to reduce the greater the degree of coverage of the electrode. Concerning the question as to whether the anodic oxalic acid oxidation occurs with direct yield of electrons to the electrode, or through an intermediate active form of oxygen, indications in favour of the second possibility have been found as follows: for example, the shortening of the transition time of the anodic ε( t) J curve (galvanostatic polarization curve) and the rapid destruction of part of the chemisorbed oxygen layer, anodically formed in the absence of oxalic acid, immediately after the addition of oxalic acid.