Measurement of Gibbs energy of formation of Ca₂PbO₄ 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 Ca₂PbO₄ was found to be present. The compound coexists with CaO and PbO. The intermetallic compounds Ca2Pb, Ca5Pb3 and CaPb and liquid alloys are in equilibrium with CaO. The standard Gibbs energies of formation of Ca₂PbO₄ (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 Ca₂PbO₄, 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: 2CaO+PbO+½O₂→Ca₂PbO₄ Δ_rG°/J mol^-1=(-128 340+93.21T /K)±200 3PbO+½O₂→Pb₃O₄ ΔG°/J mol^-1=(-70 060+77.5T /K)±150.