Mechanism of the Anodic Oxidation of Lead in Sulphuric Acid Solutions

Abstract

AbstractThe processes of the anodic oxidation of lead in 1 n H2SO4 were studied by means of successive polarizations at constant potential and subsequent observation of the open‐circuit potential transients. The behaviour of the electrode was investigated at steps of 100 mV in the range lying between the equilibrium potentials of the Pb/PbSO4 and the PbSO4/PbO2 electrodes. Following each oxidation run the electrode was observed on the electron microscope. It was established that by oxidation up to — 450 mV (vs Hg/HgSO4 in 1 n H2SO4) crystals of PbSO4 are obtained. At potentials between — 450 and + 800 mV the pH of the solution in the intercrystalline spaces is raised and reaches values, where 5PbO · 2H2O and 4PbO · PbSO4 are formed. The composition of the layer changes along the depth of the layer according to the solubility product of the compounds obtained. At open circuit, the H2SO4 diffusion in the intercrystalline spaces causes a process of sulphatization of lead oxide and of the basic lead sulphates, which is reflected by the form of the open‐circuit potential transients. At potentials higher than + 800 mV, PbO2 is obtained in the intercrystalline spaces. At higher potentials the oxidation reactions of PbSO4, Pb and basic lead sulphates proceed with a high over‐voltage. At oxidation potentials above + 1000 mV the oxidation rate shows maximums.