Models, proposed by Pavlov and Ruetschi to describe the multiphase electrode structure of anodically oxidized lead/sulfuric acid solutions, have been quantitatively tested, modified, and elaborated in accordance with the experimental evidence. The current, during oxidation at constant potential in the region of lead dioxide formation, is shown to be controlled sequentially by at least three diffusion processes, followed by at least two electrocrystallization stages and further intervals of diffusion control. The natures and concentrations of the diffusing species are quantitatively characterized on the basis of kinetic and thermodynamc data. The corresponding values of pH in the electrode pores are calculated. A reversal of the pH gradient in the pores apparently occurs in close association with the onset of
β‐PbO2
production. It is following this stage of the anodic oxidation that the charging kinetics become qualitatively dependent on temperature. At normal temperatures, growth of
β‐PbO2
at the expense of lead sulfate occurs. At very low temperatures this process is blocked, resulting in poor subsequent charge acceptance.
A multiphase model of anodic films on lead in sulfuric acid, previously developed to describe the oxidation of lead at room temperature, has been applied to potentiostatic transient data at very low temperatures (0° to −50°C). Freezing of the dilute pore electrolyte is proposed, such that ionic diffusion and equilibria must occur in a thin layer of adsorbed water between the ice and pore walls The experimentally determined activation energies and entropies of diffusion indicate that desorption of water or dehydration of ions plays an important role in the activation process. Except for the effects of the more limited electrolyte volume within the pores upon freezing, the electro‐oxidation processes follow a similar sequence above and below the freezing point. However, eventual reactant depletion, due to kinetically hindered equilibria and reduced electrolyte volume in the pores, leads to cessation of charge acceptance at very low temperatures, before appreciable
β‐PbO2
formation can occur.
Reference electrodes, suitable for use at high pressures in chloride or hydrogen containing solutions, were applied to the study of various buffer equilibria at pressures from 0 to 2.5 kbar. Partial molal volumes of reaction obtained are compared and interpreted in terms of their effects on the solvent structure. Results obtained with anodes of magnesium alloys (AZ 61 and AP 65), aluminum alloys (GB 420 and GB 80S) and pure aluminum, in phosphate and borate buffered and unbuffered chloride solutions, are presented and discussed. Anode potentials were found to be simultaneously controlled by more than one oxidation reaction. In the case of the magnesium anodes, the relative contributions of the three oxidation reactions were clearly determined by both the alloy composition and the ambient pressure. In the case of aluminum this was also true, but the actual reactions taking place were more difficult to identify.
Vergleichselektroden, die bei hohen Drücken in Chlorid oderWasserstoff enthaltenden Lösungen verwendet werden können, werden in verschiedenen Chloridmedien getestet bei Drücken zwischen 0 und 2,5 kbar.
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