Thermodynamics of oxide-sulfate melts: the system PbO-PbSO₄

Activities in the PbO-PbSO₄ melts at 1253 K have been measured by emf and gas-equilibration techniques. The activity of PbO was directly obtained from the emf of the solid oxide cell, Pt, Ni-NiO/CaO-ZrO₂/Auo.92PbO.08, PbOx-PbSO4(1-x), Ir, Pt for 1.0 >XPbO > 0.6. The melt and the alloy were contained in closed zirconia crucibles. Since the partial pressure of SO₂ gas in equilibrium with the melt and alloy was appreciable (>0.08 atm) atXPbO < 0.6, activities at lower PbO concentrations were derived from measurements of the weight gain of pure PbO under controlled gas streans of Ar + SO₂ + O₂. The partial and integral free energies of mixing at 1253 K were calculated and found to fit a subregular model: ΔGEPbO =X²PbSO₄ {-42,450 + 20,000X2PbSO₄} J mol-1 ΔGEPbO =X²pbSO {-12,450 - 20,000XPbS} J mol^-1 ΔG^EpbSOXPbSO₄ {-32,450XPbS - 22,450XPbSO₄ } J mol^-1. The standard free energy of formation of liquid PbSO₄ from pure liquid PbO and gaseous SO₃ at 1 atm at 1253 K was evaluated as -88.02 (±0.72) kJ mol^-1