Measurement of Gibbs energy of formation of Ca2 PbO4 using a solid-state cell with three electrodes

Abstract

Phase relations in the system Ca–Pb–O at 1100 K have been determined by equilibrating 18 compositions in the ternary and identifying the phases present in quenched samples by X-ray diffraction and energy dispersive X-ray analysis (EDX). Only one ternary compound Ca₂ PbO₄ was found to be present. The compound coexists with CaO and PbO. The intermetallic compounds Ca₂ Pb, Ca₅ Pb₃ and CaPb and liquid alloys are in equilibrium with CaO. The standard Gibbs energies of formation of Ca₂ PbO₄ (880–1100 K) and Pb₃O₄ (770–910 K) were determined using solid-state cells based on yttria-stabilized zirconia as the solid electrolyte. Pure oxygen gas at 0.1 MPa was used as the reference electrode. For measurements on Ca₂ PbO₄ , a novel cell design with three electrodes in series, separated by solid electrolyte membranes, was used to avoid polarization of the electrode containing three solid phases. Two three-phase electrodes were used. The first absorbs the electrochemical flux of oxygen from the reference electrode to the measuring electrode. The other three-phase electrode, which is unaffected by the oxygen flux through the solid electrolyte, is used for electromotive force (EMF) measurement. The results from EMF studies were cross-checked using thermogravimetry (TG) under controlled oxygen partial pressures. The stability of Pb₃O₄ was investigated using a conventional solid-state cell with RuO₂ electrodes. The results can be summarized by the following equations: