The solvolytic disproportionation of mixed-valence compounds: III. Pb3O4

The chemical transformation corresponding to the equation Pb,O,(s) + 4H+(aq) = PbO>(s) + 2PbYaq) + 2H,O(I) has been studied chemically but especially by high-resolution electron microscope (HREM) observations on specimens quenched at regular intervals during reaction. It was determined that attack by the acid solution is most rapid along {101} directions with a substantial component along [001]. “PbO” as well as PbO2 go into solution as Pb2+ and water. This requires electrons to be supplied to the reacting surface. As a consequence, a receding (101) edge, free of PbO2 precipitate, is usually maintained thin by the reaction. The precipitation of PbO2 occurs as an independent process separated spatially from the dissolution reaction according to the equation . Pb’+(aq) + 2HzO(l) = PbO:(s) + 4H-(aq) + 2c This reaction is reversible and the rapid exchange between Pb2+(aq) and PbO2 promotes simultaneous and continuous recrystallization. This mechanism, elucidated by HREM techniques, is also consistent with the gross chemical kinetic and X-ray studies