Photo-oxidation of Water by Ceric Ions

Abstract

IN view of the importance which is attached to the photochemical oxidation of water in the understanding of the process of photosynthesis¹, we would like to submit a scheme, which, as we show, agrees well with the data reported in the literature and our own experience in this field of research. Weiss and Porret² were the first to investigate the photo-oxidation by ceric ions (a qualitative observation to this effect had been made by Baur³ in 1908). A more detailed study was carried out by Heidt and Smith⁴. The latter assumed the formation of an active ceric dimer being deactivated by cerous ions to account for their otherwise inexplicable observations as to dependence of the quantum yield on the concentration of ceric and cerous ions in solution. Spectrophotometric measurements carried out by us (to be published later) show conclusively that there is no dimer formation, and that in ceric perchlorate solutions in normal perchloric acid approximately 92 per cent of the ceric ion is present as the ion-pair complex Ce⁴⁺OH⁻. On the other hand, Heidt and Smith's kinetic results can be accounted for qualitatively and quantitatively on the basis of the following reaction scheme, which makes it also possible to explain in quantitative terms why in the case of ferric ions the quantum yield should be much smaller, and therefore under normal laboratory conditions scarcely measurable; as in our work on photo-initiated polymerization^5,6 and oxidation⁷, we consider the primary step as an electron-transfer excitation with the ion-pair complex as the active species. This is the scheme we suggest: From stationary-state kinetics we obtain: