Electrochemical Study of Metallic Oxides in Fused Lithium Chloride-Potassium Chloride Eutectic

Abstract

The oxygen electrode does not behave reversibly in LiC1-KC1 at temperatures of 400~176 and at oxide ion concentrations of 0.1-0.2M. The following electrode systems behave reversibly as electrodes of the second kind: Cu/ Cu~O,O=; Pt/PtO,O=; Pd/PdO,O=; Bi/BiOC1,O=. The Ni/NiO,O= and Bi/Bi~O~,O: electrodes did not show reversible behavior. Because of the relatively high solubilities of the metal oxides, the applicability of the metal oxide electrodes to the measurement of oxide ion activity is limited to solutions containing oxide ion in concentrations comparable with, or greater than, that contributed by dissolution of the oxides. This limit is 10-sM or higher, depending on the system. The relatively high solubility of heavy metal oxides in this solvent is attributed to the great complexing tendency of chloride ion. It is probable that these oxides would be much less soluble in nitrate and sulfate melts and thus their utility as acidity electrodes could be extended to lower oxide concentrations. A molten alkali metal chloride solvent may be said to be "buffered" with respect to the addition of Lewis acids such as A1C1, because of the presence of a large excess of the weakly basic chloride ion. On the other hand, such a solvent can be rendered basic by the addition of substances such as oxide ion which are more strongly basic than chloride ion. The purpose of the present investigation was to determine the feasibility of using the oxygen-oxide ion electrode or metal-metal oxide-oxide ion electrodes for the electrochemical measurement of oxide ion activity in molten lithium chloride-potassium chloride eutectic at 450 ~ A secondary objective was to determine the solubilities of metal oxides from emf measurements. There is considerable evidence for the reversi-bility of the oxygen-oxide ion electrode at temperatures of the order of 1000~ in various melts containing oxyanions. The early work of Baur and Ehrenberg (1) on borates, silicates, etc., and of Treadwell (2) on metal oxides, using quartz or porcelain as an electrolyte and silver as the indicator electrode, had indicated that the oxygen-oxide ion was exhibiting reversible behavior. Cs~ki and Dietzel (3) reported on emf measurements in cells involving platinum indicator electrodes in borate melts, but their interpretation of the results has been criticized by Flood, Forland, and Motzfeld (4). Antipin (5) used a graphite electrode, which had been anodized to make it behave as an oxygen electrode, as a reference electrode in cryolite at 1050~ The investigations of Lux (6) largely in sulfate and phosphate systems at 950~ were the first which clearly showed the feasibility of oxide ion activity measurements using a platinum electrode with oxygen gas. Similar measurements were made by Flood and co-workers (4, 7, 8) who demonstrated that the dependence of the potential on oxide ion activity obeyed the Nernst equation. Work at lower temperatures has been much more limited in scope. Rose, Davis, and Ellingham (9) made measurements involving oxygen electrodes in molten NaOH containing small concentrations of tin or lead at 4000-700 ~ and found that the cells behaved reversibly. Hill and co-workers (10) measured emfs of cells of the type M/MO/CaO in Li~SO4-K~SOJPt,O~ [A]