Thermodynamic activities at 1273 K in the system cupric oxide-magnesium oxide using a solid-state galvanic cell

Abstract

Schmahl and Minzl [1] examined the phase relationships in the system CuO-MgO by the X-ray method. The phase relations at 1273K are characterized by two terminal solid solutions, the MgO-rich rock salt phase and the CuO-rich tenorite phase which are separated by three regions, i.e. the region in which the rock salt phase and a compound of CuO and MgO, the guggenite phase, coexist; the single-phase guggenite region over a varying composition range and a two-phase region in which the guggenite and the tenorite coexist. The activity-composition relations for CuO and hence the phase-boundary limits were determined at 1273 K by directly measuring the oxygen pressures in equilibrium with the phases in the presence of Cu20. Navrotsky [2] determined the activities of CuO at 1273 K by measuring the oxygen pressures for the equilibrium-phase assemblages. The activities of MgO were calculated by a Gibbs-Duhem integration. The activity-composition relations obtained by Navrotsky [2] agree well with those determined by Schmahl and Minzl [1] for the rock salt phase and the two-phase region containing the rock salt phase and guggenite. No direct measurements of activity of MgO have been reported before. In the present work, a solid-state galvanic cell with MgF2 as the solid electrolyte was used to measure MgO activities at 1273 K in the equilibrium phases of the CuO-MgO system. The suitability and reliability of this technique to measure MgO activities in solid solutions containing MgO are well documented [3-6]. Magnesium fluoride, for making the solid electrolyte pellets and electrodes, was prepared according to the method of Taylor and Schmalzried [3]. The electrolyte was prepared by pressing the powder in a cylindrical die. The green pellet was then sintered in an atmosphere of CO2-free dry oxygen gas at 1323 K for 4 h. Samples of the desired composition were weighed from dried reagent-grade CuO and MgO and ground under acetone. The equilibrium phases were prepared by sintering the oxide-mixture pellets at 1273 K in a stream of CO2-free dry oxygen gas. The product was ground and sintered again at 1273K for 30 to 72h. The formation of true equilibrium phases was confirmed by the X-ray method using CuK~ radiation. The apparatus used in the present investigations was similar to that described earlier [7]. The performance of the polycrystalline MgF 2 as the solid electrolyte was checked by operating the cell: