Autocatalytic Reaction Pathway on Manganese Dioxide Colloidal Particles in the Permanganate Oxidation of Glycine

The progress of the reaction of permanganate ion with the amino acid glycine in near-neutral aqueous solutions was monitored with a UV-vis spectrophotometer at two different wavelengths in order to observe the decay of the oxidant (MnO4-, at 526 nm) and the formation of one of the reaction products (colloidal MnO2, at 418 nm). The use of a phosphate buffer resulted in the stabilization of MnO2 as a soluble colloid during the kinetic runs. The experimental data are consistent with the hypothesis that of the two conjugated buffer ions H2PO4-/HPO42- the acidic one (H2PO4-) is the predominant species responsible for that stabilization, suggesting that the formation of hydrogen bonds between the MnO2 oxygen atoms and the hydrogen atoms from phosphate ions might be involved in the process. An increase of the phosphate concentration resulted in a notable decrease of both the molar absorption coefficient and the size of the MnO2 colloidal particles. The reaction showed an autocatalytic behavior, but the kinetic plots deviated from the differential rate law usually employed for the study of autocatalytic reactions. The experimental data suggest that these deviations might be caused by the competitive adsorption of both phosphate ions and glycine on the surface of the MnO2 colloidal particles, with the adsorption of the two species being too slow for the quasi-equilibrium approximation to hold, and resulting in an inhibition of the autocatalytic reaction pathway by phosphate ions.