A Kinetic Study of the Chromium(VI)−Hydrogen Peroxide Reaction. Role of the Diperoxochromate(VI) Intermediates

The kinetics of the Cr(VI)−H2O2 reaction has been studied in aqueous solutions (pH = 4.6−7.3) in the presence of either phosphate or acetate buffers. In those media, Cr(VI) behaves both as a catalyst (being recovered unchanged) for the dismutation of H2O2 and as an oxidant (being reduced to Cr(III)). High concentrations of H2O2, buffer, or H+ favor the reduction Cr(VI) → Cr(III). A combination of spectrophotometric and titrimetric measurements has allowed us to obtain the initial rates of disappearance of both Cr(VI) (vc,o) and H2O2 (vp,o) as well as an apparent value of the overall initial rate of formation (v'i,o) of two relatively stable, violet intermediates, identified in this work as two diperoxochromate(VI) complexes differing in the absence (C1) or presence (C2) of a phosphate (or acetate) ligand. Both vc,o and v'i,o are of first order in both Cr(VI) and H2O2, whereas vp,o follows the law vp,o = ((kp[H2O2]o + k'p[H2O2]o2)[Cr(VI)]o)/(1 + k''p [H2O2]o2), kp, and being pseudo rate constants dependent on both the total buffer concentration and pH. The three initial rates increase with rising total buffer concentration, the effect caused by phosphate ions being greater than that caused by acetate ions; vc,o and v'i,o show acid catalysis, whereas the vp,o vs pH plots are bell shaped. Anomalous Arrhenius plots have been found in the three cases. A mechanism is proposed where the intermediates C1 and C2 both have an active role in the conversion H2O2 → O2 and where C2 is involved in the generation of most of the Cr(III) formed under the experimental conditions of our work. Of some biological relevance might be the finding that adenine seems to stabilize some peroxochromium intermediate complex, the adenine molecule being destroyed during the course of the Cr(VI)−H2O2 reaction.