Mesure de pO2? au moyen d'une electrode a membrane de zircone stabilisee et determination potentiometrique de constantes d'equilibre d'echange de O2? dans le melange equimolaire NaCl-KCl fondu

A galvanic cell with a Ag/AgCl reference electrode and a pO^2? indicating electrode—made of a solid-state membrane in zirconia stabilized by calcia and filled by a mixture of Ni + NiO—has been constituted for working in the molten mixture NaCl-KCl (1:1), from 700 to near 1000°C. The response of the zirconia electrode in the fused salt has been tested by known variations of pO^2?, due to coulometric titrations of free oxide ions, then of hydroxide ions, by means of metallic ions (Ni²⁺, Cu⁺) forming scarcely soluble metallic oxides. The results of measurements were in a good accordance with the variation laws theoretically established, giving the proof that measurements of pO^2?, between about 2 and 12, are possible in the electrolytic and temperature range considered, by means of this system. An alteration of the electrode can arise in media with too low a pO^2? value.Equilibrium constants have been determined: solubility products (K_s) of the oxides NiO and Cu₂O, constant values of the equilibria 2OH^? ? H₂O (g) + O^2? (K₁) and H₂O (g) + 2Cl^? ? O^2? + 2HCl (g) (K₂); as functions of the absolute temperature T/°K and in the molar fraction scale (partial pressure of the gazeous substances in atm):

The solubility product values (at 727°C) for a few metallic oxides have been calculated from the free enthalpy values of formation and experimental values of the activity coefficients of the metallic ions and O^2? dissolved in NaCl-KCl.Limitations to the ability of the membrane electrode for measuring pO^2? have been established by an analysis based on the chemical and electrochemical behaviour of zirconia. A potential-pO^2? diagram shows the area inside which one has to work.