An All-Metal Cell Technique for the Measurement of the EMF of Molten Metal-Metal Halide Solutions Corrosive to Ceramics

Abstract

A new technique is described which permits the measurement of emf using an all-metal cell. The applicability of the method to both aqueous systems and molten metal-metal halide systems is demonstrated. This technique will now permit measurements of the emf of systems such as the molten alkaline earth-alkaline earth halides and the rare earth-rare earth halides which are extremely corrosive to ceramics.The measurement of equilibrium cell potentials has long been an important means of determining thermodynamic properties of systems. In addition, equilibrium emf measurements can yield information as to the species present and the number of electrons involved in the potential-determining process. The application of this fundamental measurement to a unique group of solutions, namely, the alkaline earth metals, and the rare earth metals dissolved in their respective molten halides (1, 2) has been reported for the lanthanum-lanthanum chloride (3, 4) and the cerium-cerium chloride (5) solutions. However, the results are subject to question because of the corrosiveness of these solutions to ceramic material (6, 7). If dependable emf measurements are to be made on these solutions, a technique must be employed which eliminates the use of ceramic containers and insulators.This paper describes such a technique which permits the measurement of emf in an all-metal cell. Discussion of TechniqueAqueous cells such as Hg ~ Hg2SO4/H~SOJAg2SO4, Ag ~ are contained in glass apparatus. Each electrode compartment is in communication with the sulfuric acid bridging solution by means of a capillary or a sintered glass frit. Electrode leads are platinum wire in contact with the mercury of the Hg-Hg~SO4 halfcell and the silver wire of the Ag, Ag2SO~ electrode. The open-circuit potential of this cell is measured in the normal manner. The current is comprised of electron transport from the Hg-Hg2SO4 electrode, 2Hg --> Hg2 +2 -4-2e-, via the platinum lead through the potential measuring instrument to the Ag-Ag2SO4 electrode, 2Ag + -~ 2e---> 2Ag ~ with the sulfuric acid acting as the transfer medium for the sulfate ion. The over-all cell reaction being 2Hg o -4-Ag2SO4 --> Hg2SO4 q-2Ag ~ If the same half-cell mixtures Hg o, Hg2SO4(sond), saturated aqueous solution and Ag ~ Ag2SO~(sond), saturated aqueous solution are placed in separate tantalum tubes, these containers have now the same function as the platinum and silver wires of the glass cells. The tubes are partially immersed in a sulfuric acid solution and communication of the solution in each tube with the bridging liquid is provided by means of a sintered porous tantalum plug welded in the side of each tube. If there were no such communication, a current would not be expected to flow, since the potential difference between the two electrodes is much less than the potential necessary for decomposition of the sulfuric acid (H20) and that of the anodic dissolution of tantalum metal. Concomitant with this requirement is the absence in the bridging liquid of any redox process which can occur wit...

Electromotive force measurements on solutions of rare earth metals in their molten halides. I. The cerium-cerium chloride, praseodymium-praseodymium chloride, and neodymium-neodymium chloride solutions

“…It has been demonstrated previously that the potential of the reference electrode system including its junction with the metal-metal halide solution does not change within the time necessary to complete the experiment or with variation of the concentration of the studied solution. 19 Therefore, the magnitude of the liquid junction potential has no effect on the reliability of the present measurements as a true reflection of the properties of these solutions. The emf results may now be directly related to other observations on rare earth metal-rare earth halide solutions.…”

Galvanic Cell Studies Involving Calcium Carbide Solutions

“…(5) have found an irreversible (non-Faradaic) transfer of alkali metal from the anode to the bimetallic cathode to take place. Equation [7] may be shown to be valid for t ~> 15, 4.5, and 1.0 hr at 800 ~ 900 ~ and 1000~ respectively (18). They concluded that the self-discharge was largely due to the presence of dissolved alkali metal in equilibrium with alkali metal ions and electrons in the electrolyte resulting in partial electronic conduction in the electrolyte.…”

“…The design of the calcium electrode for cell [III] was similar to that used by Bronstein (7) for various metal/metal halide electrodes. It was fabricated from a steel thimble 6.4 mm ID, 9.5 mm OD, by 7 cm long, in the side of which were incorporated two layers of steel felt (Feltmetal Grade FM 1501 rolled to about 65% density ex Huyck Metals Company).…”

“…Equation [7] may be shown to be valid for t ~> 15, 4.5, and 1.0 hr at 800 ~ 900 ~ and 1000~ respectively (18).…”