Platinum Metals Rev., 1962, 6, (2), 48
Surface Phenomena at Platinum Electrodes
Adsorption of Cations at Anodic Potentials
When platinum electrodes are used for the anodic formation of persulphuric acid, the nature of the cations present has long been known to have a marked influence on the kinetics of the anodic oxidation; at any given potential, the rate of persulphate formation is diminished by the presence of alkali metal ions, the effect increasing in the order Li+<Na+<K+. Experiments on the anodic evolution of oxygen from perchloric acid, which is also influenced by the nature and concentration of the cations present, have shown that the platinum surface changes with time, chemisorbed oxygen being produced.
In a review paper recently published (Electrochimica Acta, 1961, 5, 265 - in French) Professor A. N. Frumkin, of the Institute of Physical Chemistry, Moscow, draws together a number of phenomena concerned with the surface of platinum anodes, and puts forward the view that the dipolar nature of the platinum-oxygen bond promotes the adsorption of cations. At high anodic polarisations, definite oxide films are formed, as indicated by earlier work. Cation adsorption has been directly demonstrated by the use of caesium ions labelled with Cs134.
Both the adsorption of cations and the oxidation of the surface inhibit anodic oxidations such as persulphate formation and oxygen evolution. With the alkali metal ions, the cation effect increases in the order Li+ <Na+ <K+ <Cs+, undoubtedly the order of increasing specific adsorbability, when platinum is polarised anodically in 6N sulphuric acid and in electrolytes such as 5N H2SO4, IN Li2SO4. Even 0.01N Cs+ has an appreciable influence. A similar although smaller effect of Cs+ can be found on the evolution of oxygen from the oxide-covered surface of passive iron.
In this important paper Professor Frumkin gives sixty-six references to earlier work in his own and other laboratories on the platinum anode. In view of the importance of platinum as a so-called “inert” electrode in electrolytic processes, in cathodic protection and in fundamental investigations, it is surprising that its actual “reactivity” has been relatively little studied; the paper is valuable in drawing attention to many points urgently needing further examination, as well as being an excellent summary of what is known to date.