MEASUREMENT OF GIBBS ENERGY OF FORMATION OF CA2PBO4 USING A SOLID-STATE CELL WITH 3 ELECTRODES

Phase relations in the system Ca-Pb-O at 1100 K have been determined b y equilibrating 18 compositions in the ternary and identifying the pha ses present in quenched samples by X-ray diffraction and energy disper sive X-ray analysis (EDX). Only one ternary compound Ca2PbO4 was found to be present. The compound coexists with CaO and PbO. The intermetal lic compounds Ca2Pb, Ca5Pb3 and CaPb and liquid alloys are in equilibr ium with CaO. The standard Gibbs energies of formation of Ca2PbO4 (880 - 1100 K) and Pb3O4 (770 - 910 K) were determined using solid-state c ells based on yttria-stabilized zirconia as the solid electrolyte. Pur e oxygen gas at 0.1 MPa was used as the reference electrode. For measu rements on Ca2PbO4, a novel cell design with three electrodes in serie s, separated by solid electrolyte membranes, was used to avoid polariz ation of the electrode containing three solid phases. Two three-phase electrodes were used. The first absorbs the electrochemical flux of ox ygen from the reference electrode to the measuring electrode. The othe r three-phase electrode, which is unaffected by the oxygen flux throug h the solid electrolyte, is used for electromotive force (EMF) measure ment. The results from EMF studies were cross-checked using thermograv imetry (TG) under controlled oxygen partial pressures. The stability o f Pb3O4 was investigated using a conventional solid-state cell with Ru O2 electrodes. The results can be summarized by the following equation s: 2CaO + PbO +1/2O(2) --> Ca2PbO4 Delta(r)G degrees/J mol(-1) = (- 12 8340 + 93.21 T/K) +/- 200 3PbO + 1/2O(2) --> Pb3O4 Delta(r)G degrees/J mol(-1) = (- 70060 + 77.5 T/K) +/- 150