The electroreduction of chromic acid was studied in solutions with added molybdate ions by recording potentiodynamic curves and by quantitative determination of the reaction products. It was found that the reduction of Cr3O10(2-) to Cr3+ ions is accelerated at potentials below those of chromium deposition when Na2MoO4, and particularly H2MoO4, is added. The rate of hydrogen evolution remains unchanged at the potentials of incomplete chromate ion reduction, while the current of molybdate ion reduction can practically not be measured. In solutions without sulfate ions, the rate of hydrogen evolution increases when molybdate ions are added to the chromic acid. The results obtained were explained in terms of conversion of the cationic form of Mo(VI) to an anionic form which occurs owing to the pH rise of the solution layer next to the cathode during reduction of the Cr3O10(2-) ions and owing to changes in the state of the electrode surface at the start of electrolysis resulting from the formation of reduction products of the molybdate ions.
