CHEMICAL AND ELECTROCHEMICAL-BEHAVIOR IN MOLTEN ALKALI HYDROXIDES .4. CHROMIUM SPECIES IN AN ACIDIC NAOH + KOH EUTECTIC MIXTURE

The chemical and electrochemical behaviour of Cr(VI), Cr(V) and Cr(III) in an acidic molten NaOH + KOH (49 mol%) mixture has been investigated. Cr(VI) and Cr(V) are dissolved in the melt respectively as CrO2-4 and CrO3-4 ions; the corresponding redox system behaves reversibly at a platinum working micro-electrode. At low pH2O values, Cr(V) disproportionates into Cr(VI) and Cr(III). Cr(III) precipitates as NaCrO2. Although the electrochemical study shows that the Cr(V)/Cr(III) redox system is not reversible, the peak potential for the reaction CrO3-4 + 2 e- + Na+ + 2 H2O → NaCrO2 + 4 OH- depends linearly on the pH2O value of the melt. This was explained assuming that water molecules are involved in the rate-determining step either directly or as a complexing agent of the reactant ion. In order to account for the electrochemical results and also to explain why only small amounts of insoluble NaCrO2 are formed on the cathode during the electrolysis of Cr(V)-containing melts, it was assumed that Cr(III) is produced during the electrochemical reaction as CrO3-3. The decomposition of this species yielding NaCrO2 occurs at some distance from the electrode. © 1990.