Chemical and electrochemical behaviour of chromium in molten chlorides

The chemical and electrochemical behaviour of chromium ions in the equimolar CaCl2+NaCl mixture at 550°C has been studied by electrochemical techniques (i.e. cyclic and square wave voltammetry, chronopotentiometry, chronoamperometry and potentiometry) using various substrates (Mo, W and glassy carbon) as working electrodes. Some of the results have been compared to data obtained in the eutectic LiCl+KCl mixture at 450°C. The stable oxidation states of chromium species were found to be the same in both melts. However, in the case of the CaCl2+NaCl melt and when using W substrates, a new electrochemical wave appears, which has been related to an enhancement of the oxidation of trace amounts of O2− ions on the W material by the presence of Cr ions. Kinetic parameters characterising the mass and charge transfer of the electrochemical reduction and oxidation of Cr(II) ions as well as of the Cr(III) reduction process were also obtained. Chronoamperometric studies did not show evidence of nucleation phenomena in the case of the equimolar CaCl2+NaCl mixture when using any of the substrates. However, in the eutectic LiCl+KCl it has been shown that nucleation of metallic chromium, especially on glassy carbon substrates, controls the electroreduction process. The electrochemical behaviour of some chromium oxo-anions, i.e. Cr2O72− and CrO42−, has also been studied. The results show that both complexes lead to the same stable Cr(VI) species in the melt, whose reduction at glassy carbon electrodes implies a single one-electron process giving Cr(V) ions which are present in the melt as soluble species. Potentiometric titration of both Cr(III) and Cr(II) solutions were performed by using an oxide selective ion electrode, which consist of a membrane of ZrO2 stabilised with yttrium oxide (YSZME). The values of pKs for the different CrO compounds stable in the melt, i.e. CrO, Cr2O2+ and Cr2O3, were obtained.