Behaviour of oxalic acid in the anodic oxidation of aluminium. The role of anodically initiated reduction processes in coloration and photoluminescence

Abstract

Anodization of aluminium in aqueous oxalic acid produces a yellow to dark red anodic film and anolyte, both of which luminesce under 365 nm excitation. The luminous substances separated from the anodic film and anolyte by means of paper chromatography showed an e.s.r. signal and have been identified as Al[Al(C₂O₄)₃]nH₂O (n= 3 or 4). Yellowish coloration, luminescence and the existence of radicals, however, are not inherent to Al[Al(C₂O₄)₃]nH₂O. These phenomena have been explained by the formation of C₂O^{2[graphic omitted]}₄ produced by intramolecular charge transfer between the low valency aluminium ion and oxalato ligand within the complex. Reduction of oxalic acid at the anode was observed and a reduced product, glyoxylic acid, was isolated from the anolyte. This reductive process has also been explained by an intramolecular charge transfer reaction which gives rise to H atoms.