The pyrophosphate-assisted reduction of chromium(VI) by manganese(II) and its reverse reaction

We report the discovery of a novel reaction that behaves in an unexpected w ay. Although Cr(VI) and Mn(II) do not react in the absence of stabilizing l igands, they react in aqueous perchloric acid provided that pyrophosphate [ a stabilizing ligand for Mn(III)] is present in the medium. The reaction [w hose 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 co nventional 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). Dur ing the course of the reaction, pyrophosphate is slowly hydrolyzed to phosp hate. This hydrolysis constitutes the driving force for the backward reacti on. A mechanism involving three consecutive 1-electron redox steps is propo sed for the reduction of Cr(VI) to Cr(III), in each step a Mn(II) being oxi dized to Mn(III), whereas colloidal manganese dioxide might be a key interm ediate for the reoxidation of Cr(III) to Cr(VI).