Chemical and electrochemical behaviour of chromium in molten chlorides

The chemical and electrochemical behaviour of chromium ions in the equimola r CaCl2 + NaCl mixture at 550 degreesC has been studied by electrochemical techniques (i.e. cyclic and square wave voltammetry, chronopotentiometry, c hronoamperometry and potentiometry) using various substrates (Mo, W and gla ssy carbon) as working electrodes. Some of the results have been compared t o data obtained in the eutectic LiCl + KCl mixture at 450 degreesC. The sta ble 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 subst rates, a new electrochemical wave appears, which has been related to an enh ancement 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 ch arge transfer of the electrochemical reduction and oxidation of Cr(II) ions as well as of the Cr(III) reduction process were also obtained. Chronoampe rometric studies did not show evidence of nucleation phenomena in the case of the equimolar CaCl2 + NaCl mixture when using any of the substrates. How ever, in the eutectic LiCl + KCl it has been shown that nucleation of metal lic chromium, especially on glassy carbon substrates, controls the electror eduction process. The electrochemical behaviour of some chromium oxo-anions , i.e. Cr2O72- and CrO42-, has also been studied. The results show that bot h complexes lead to the same stable Cr(VI) species in the melt, whose reduc tion at glassy carbon electrodes implies a single one-electron process givi ng Cr(V) ions which are present in the melt as soluble species. Potentiomet ric 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 stabi lised with yttrium oxide (YSZME). The values of pK(s) for the different Cr- O compounds stable in the melt, i.e. CrO, Cr2O2+ and Cr2O3, were obtained, (C) 2000 Elsevier Science B.V. All rights reserved.