Emf's across CdI2 ‐ Cd2 I 2 Electrolytes Using Electrodes of Bare W, Molten Cd, and Molten Au‐Cd: Bare Electrode Effects, Use of n ≠ 2 , Premonitory Phenomena and Liquid Structure in Au‐Cd, and the Cd ‐ CdX2 Phase Diagrams

Abstract

Emf measurements have been made on cells with mixed‐valence electrolytes (a) to establish structural and thermodynamic properties for molten Au‐Cd alloys and (b) to study the source and behavior of some unexpected voltages associated with bare W electrodes dipping into these electrolytes. Changes with temperature and composition of the partial molal entropies, enthalpies, and free energies of the Cd component in the Au‐Cd melts point out the existence of premonitory changes a few degrees above the liquidus as well as of liquid structural changes far from the liquidus; these changes correlate with earlier studies by vapor pressure and by electrical conductivity in the Au‐Cd system at higher temperatures. is used to evaluate the effective valence for these electrolytes when saturated with Cd; this appears in calculation of . Apparently the mixed‐valence electrolyte, plus probable oxide impurities, reacts with the bare electrode (in this case W) surfaces by direct chemical action to form surface films which protect the metal below against direct chemical attack. These films are in equilibrium with the electrolytes. When, however, electrochemical attack is permitted, as when an emf measurement is made, then the underlying metal becomes subject to attack and another reactant enters the reaction. This effect leads to anomalous voltages in electrodes which are normally considered to be inert in molten salts, e.g., W, Mo, and graphite. If these sources of voltage are recognized, then proper determinations of activities and phase boundaries can be made; some earlier measurements of this type which neglected these voltages, however, must be reevaluated. Application of these proper techniques to the system is made.