Thermodynamic study of Fe2O3-Fe2(SO4)3 equilibrium using an oxyanionic electrolyte (Na2SO4-I)

The emf of the cell, Pt, Ar + O₂ + SO₂ + SO₃/Na₂SO₄-I/Fe₂O₂ + Fe₂(SO₄)₃, Pt, has been measured in the temperature range 800 to 1000 K, using a gas mixture of known input composition as the reference electrode. The equilibrium composition of the reference gas at the measuring temperatures was computed using the thermodynamic data on the gaseous species reported in the literature. A mixture of ferric oxide and sulfate was kept in a closed system to ensure establishment of equilibrium partial pressure at the electrode. The cell was designed to avoid physical contact between Fe₂(SO₄)₃ and Na₂SO₄ electrolyte. Uncertainties arising from the formation of sulfate solid solution were thus eliminated. The Gibbs’ energy of formation of ferric sulfate calculated from the emf is discussed in comparison with data reported in the literature. There is no evidence for the formation of oxysulfates in the Fe-S-0 system. Based on the results obtained in the present study for Fe₂(SO₄)₃ and literature data for other phases, chemical potential diagrams have been constructed for the Fe-S-O system at 900 and 1100 K.