Formation of Peroxodiphosphate by the Copper(II)-Catalyzed Oxidation Reaction of the Phosphinate Ion by the Molecular Oxygen. Kinetics and Mechanisms in Aqueous Solutions

Article abstract of DOI:10.1246/bcsj.69.613

It was found that peroxodiphosphate ions (denoted by PDP and meaning all forms of P₂O₈^4-, HP₂O₈^3-, HP₂O₈2^3-, etc.) were formed by the oxidation of the phosphinate ion (PH₂O₂^-) by molecular oxygen (O₂) using trace amounts of the copper(II) ion as a catalyst in aqueous solutions. The formation rate was dependent on the acidity being maximum at around pH 3.5, and was inhibited by the presence of radical scavengers or a complex-forming substance of ethylene-diaminetetraacetic acid (EDTA). The formation of PDP was not observed when ions such as Ir^IV and Ce^IV were used in place of the copper(II) ion. Thus, this reaction is specific only for the copper(II) ion, which can be easily reduced to Cu⁺ by the phosphinate ion, where the reduced species are capable of reacting with molecular oxygen to efficiently produce the superoxide of O^-.₂ or HO^.₂. After some PDP had been formed in the presence of air or oxygen, if the reacting solution was changed to be anaerobic by bubbling nitrogen through the solution, the PDP began to decompose according to the first-order rate law with respect to the concentrations of PDP. The kinetics and mechanisms for not only the formation, but also the decomposition of PDP, are presented.