Electrochemical studies in oxide formation on some metals
Part I: Anodic and cathodic behaviour of palladium, rhodium and iridium electrodes in dilute sulphuric acid and dilute sodium hydroxide:(1) (a) A study has been made of the anodic and cathodic polarization of palladium (p.49- p.24), rhodium (p.33-p.34), and iridium (p.41-p.42) electrodes in dilute sulphuric acid, with current densities between 1 and 50 microamps. (b) It is found that an approximately monatomic layer of adsorbed oxygen is formed at these electrodes during anodic polarization, at potentials less positive than that required for the liberation of oxygen. (p.46- p.48).(2) (a) A study has been made of the electrochemical behaviour of palladium (p.29- p.32), rhodium (p.35- p40)ß iridium (p.42) and platinum (p.43), in dilute sodium hydroxide, with current densities between 100 and 1000 microamps. (b) There is substantial evidence of oxide formation at palladium and rhodium electrodes when anodically polarized with these larger currents. (p.51- p.57).(3) (a) The amount formed is small; not much more than one molecular layer. (b) It is formed very slowly, and probably results from interaction between adsorbed oxygen atoms and metal atoms. (c) A striking increase in the oxidisability of the electrodes, is noted after long anodic polarizations. This is brought about by a change in the forces existing between the metal atoms at the electrode surface. The effect is especially marked in the case of rhodium where oxidation occurs with small currents, with which it previously could not be detected, after such long anodic polarizations.Part II: (A) Electrochemical behaviour of nickel and iron in N/5 sodium hydroxide. (B) Electrochemical behaviour of lead in M/lO sulphuric acid.(A) (1) The anodic and cathodic behaviour of ( nickel in N/5 sodium hydroxide has been studied (p.63). (2) A fresh nickel electrode is inert in this solution owing to the presence of an air -adsorbed oxide film NiO, which is only removed at very negative potentials. (3) At high positive potentials this is oxidised to Ni203. (4) This oxide is reduced to Ni0 during cathodic polarization at Eh =-0.60 volt. (5) A hysteresis effect similar to that studied in the earlier work is observed here. (6) The anodic and cathodic behaviour of iron in N/5 sodium hydroxide has been studied (p.71). (7) Iron is quite inert in this solution but adsorbs a considerable amount of hydrogen during cathodic polarization. This is subsequently removed when the electrode is made anodic.(B) (1) The electrochemical behaviour of lead in M /10 sulphuric acid has been studied (p.82). (2) Initially a lead electrode is very inert rapidly attaining high positive potentials with the subsequent formation of lead peroxide. (3) After lead peroxide has been formed two depolarization processes occur during a cathodic polarization (p.83). (a) The first occurring at +1.45 volt is due to reduction of lead peroxide to lead sulphate. (b) The second occurring at -0.30 volt is due to reduction of lead sulphate to metallic lead (p84). (c) These two electrochemical reductions are not quantitative (p.85), owing to the appreciable thickness of the solid films and to the poor electrical conductivity of the lead sulphate. (4) The alternate oxidation and reduction leaves a surface film of lead on the electrode, which is very active passing into solution with great ease. (5) This activity becomes very marked after some time, when oxygen dissolved in the electrolyte is able to bring the lead into solution as lead sulphate by an ordinary corrosion process (p.90) (6) The process, lead<=>lead sulphate should be quantitatively reversible, and was studied in greater detail. It was found that: (a) The amount of lead which went into solution anodically as plumbous ion was equal to the amount which had previously been deposited during the cathodic polarization (p.86). (b) Reverse did not hold true, that is, the quantity of electricity required to reach some arbitrary potential during the cathodic polarization was not equal to the amount of electricity passed during the anodic solution of lead (p.88) . (c). The above phenomena have been explained by a theory based on the appreciable thickness of the active layer of lead at the surface of the electrode.