Electrochemical Reduction of Potassium Chromate in the Presence of Zinc(ll) and Cobalt(ll) in Molten Lithium

The electrochemical reduction of K2CrO4 in molten LiC1-KC1 eutectic is shifted to more positive potentials by the addition of ZnC12 or COC12. In the case of added ZnC12 three electrons are consumed per mole of K2CrO4 and an insoluble reduction product having the empirical formula LiZn2CrO4 is ob-tained. In the presence of CoCI~, the reaction product depends on the elec-trolytic conditions, especially the cathodic potential. The l imiting processes appear to be a two-electron process, favored by low cathodic potential and high cobalt(I I) concentration to yield Co2CrO4, and a three electron process, favored by high cathodic potential and low cobalt(I I) concentration to yield LiCo2CrO4. A mechanism consistent with these observations is pro-Dosed. Laitinen and Bankei-t (1) used chronopotentiometry to study the electrochemical reduction of K2CrO4 in molten LiC1-KC1 and observed a single, irreversible, diffusion-controlled, three-electron reduction step. In the presence of Mg (II), ~he chromate reduction poten-tial is shifted from--1.1V vs. a 1M Pt ( I I) /P t reference to--0.2V and a highly insoluble, chemically resistant electrode deposit is obtained at the cathode. Propp and Laitinen (2) characterized the magnesium deposit and described it as a single unstoichiometric com-pound having the empirical formula LixMgyCrO4, where x-5 2y---- 5. The values of x and y were de-pendent on electrolysis conditions; typically 0.3 < x < 0.5. Popov and Lait inen (3) have recently shown that a similar deposit is obtained by electrolysis of Ni (II) 9 Cr (VI) mixtures. The nickel deposit is slightly more stoichiometric than the corresponding magnesium one (0.6 < x < 0.85) but has nearly the same lattice structure and thermal stability as LixMgyCrO~. The results of an extensive characterization of the electroreduction f K2CrO4 in the presence of Zn(I I) and Co(II) are the subject of this paper. In addition, possible mechanisms for the electroreduction of chromate in the presence of divalent metal ions are discussed