The pyrophosphate-assisted reduction of chromium(vi) by manganese(ii) and its reverse reaction

Abstract

We report the discovery of a novel reaction that behaves in an unexpected way. Although Cr(VI) and Mn(II) do not react in the absence of stabilizing ligands, they react in aqueous perchloric acid provided that pyrophosphate [a stabilizing ligand for Mn(III)] is present in the medium. The reaction [whose stoichiometry is Cr(VI) + 3 Mn(II) → Cr(III) + 3 Mn(III)] takes place in a reversible manner, but the kinetic plots are not as those expected for conventional reversible reactions. On the contrary, a period of decay of the Cr(VI) concentration [and formation of Mn(III)] is followed by a period in which that reactant is actually regenerated (and the product consumed). During the course of the reaction, pyrophosphate is slowly hydrolyzed to phosphate. This hydrolysis constitutes the driving force for the backward reaction. A mechanism involving three consecutive 1-electron redox steps is proposed for the reduction of Cr(VI) to Cr(III), in each step a Mn(II) being oxidized to Mn(III), whereas colloidal manganese dioxide might be a key intermediate for the reoxidation of Cr(III) to Cr(VI).