Chemical and electrochemical behaviour in molten hydroxides. VII. Chromium species in sodium hydroxide

The chemical and electrochemical behaviour of ions containing chromium in molten NaOH has been investigated as a function of the acidobasicity of the melt. Chromium (VI) dissolved as CrO42- is stable in acidic solution where it is electrochemically reduced successively to CrO43- and to solid NaCrO2. CrO42- is unstable in basic melts and is reduced to CrO43- by O2- after about 3 h yielding stable green solutions. CrO43- is reduced electrochemically to soluble CrO44-. CrO(4)(3- )is also stable in neutral melts and may be oxidized or reduced, respectively, to CrO42- at the anode and to CrO(4)(4- )at the cathode. The results obtained in this work allow us to propose the stability diagram of species containing chromium in molten NaOH at 623 K. (C) 1998 Elsevier Science Ltd. All rights reserved.