Oxide Ion Titrations in Molten NaCl with the Zirconia Electrode

Abstract

Stabilized zirconia electrolytes can be used to titrate O 2-into and out of molten NaC1. Relations between the oxide activity in the melt and the charge passed through the ceramic have been derived and verified. The technique is applicable to the purification of melts and the determination of oxide solu-bility products. Zirconia stabilized with several percent CaO, MgO, or Y20~ is finding increasing use in a variety of applications at high temperatures. All of these depend on the fact that charge transport through this material occurs by migration of oxide (02-) ions (1). Electrochemical applications may be divided into equilibrium methods in which the stabilized ZrO2 (SZ) is used as part of an indicator electrode, and nonequilibrium methods in which a sensible amount of charge is passed through the material. Indicator electrodes made of SZ have been used to: (i) measure 02 partial pressure in gases (2), (ii) measure solubility of O2 in metals (2), (iii) measure the dissociation pressures of solid oxides (2), (iv) measure the oxide activity in molten salts (3-5) and glasses (6). The transport of O 2-through SZ under an applied potential has been used to: (i) operate an oxygen fuel cell (2), (ii) change the O2 content of gases (7, 8), (iii) measure the diffusivity of 02 in metals (9), (iv) titrate oxygen into metals (9). Most of the applications depend on two properties of SZ: (i) at an interface of the type O2(M)!SZ, the potential-determining process is 1/2 O2(g) + 2e ~ O 2-(in SZ) [1] where M is an inert metal such as platinum. Studies of such interfaces have shown that the process is readily reversible, at least at temperatures above 500 ~ (2); (ii) applications involving the measurement of 02-activities in molten salts depend on the equilibrium O 2-(in SZ) ~ O 2-(melt) [2] The reversibility of these processes under poten-tiometric conditions (very small currents) can be regarded as established. In this case the "zirconia electrode," which can be described as O2(g) (Pt)lsz]o2-(inmelt) [3] functions as an O2-specific electrode for which the half-cell reaction is given by the sum of reactions [I] and [2] 1/2 O2 (g) (Pt) + 2e ~ 02-(in melt) [4] Thus, the zirconia electrode functions operationally as does the glass electrode for H +, although the mechanisms responsible for the potential-determining reactions are not entirely analogous. Reaction [4] has so far been applied only poten-tiometrically (3-5, 10, 11). Although the oxygen gas electrode (O2(Pt)IO 2-(in melt)) has also been used in oxide-containing melts, the advantages of the zirconia electrode are (i) specificity to the O 2-ion (no response to 022-and O2-), and (ii) the oxygen * Electrochemical Society Active Member.