CHEMICAL-POTENTIAL OF OXYGEN FOR IRON-RUTILE-ILMENITE AND IRON-ILMENITE-ULVOSPINEL EQUILIBRIA

The chemical potential of oxygen corresponding to the iron-rutile-ilmenite (IRI) and iron-ilmenite-ulvospine (IIU) equilibria has been measured employing solid-state galvanic cells, Pt, Fe + TiO2, + FeTiO3//(Y2O3) ZrO2//Fe + "FeO", Pt and Pt, Fe + FeTiO3 + Fe2TiO4//(Y2O3) ZrO2//Fe + "FeO", Pt in the temperature range of 875 to 1275 K and 900 to 1373 K, respectively. The cells are written such that the right-hand electrodes are positive. The electromotive force (emf) of both the cells was found to be reversible and to vary linearly with temperature over the entire range of measurement. The chemical potential of oxygen for IRI equilibrium is represented by DELTA-mu(O2)(IRI) = -550,724 - 29,445T + 20.374T In T(+/- 210) J mol-1 (875 less-than-or-equal-to T less-than-or-equal-to 1184 K) = - 620,260 + 369.593T - 27.716T In T(+/- 210) J mol-1 (1184 less-than-or-equal-to T less-than-or-equal-to 1275 K) and that for IIU equilibrium by DELTA-mu(O2)(IIU) = -501,800 - 49.035T + 20.374T In T(+/- 210) J mol-1 (900 less-than-or-equal-to T less-than-or-equal-to 1184 K) = - 571,336 + 350.003T - 27.716T In T(+/- 2 10) J mol-1 (1184 less-than-or-equal-to T less-than-or-equal-to 1373 K) The standard Gibbs energy changes for IRI and IIU equilibria have been deduced from the measured oxygen potentials. Since ilmenite contains small amounts of Ti3+ ions, a correction for the activity of FeTiO3 has been incorporated by assuming ideal mixing on each cation sublattice in the FeTiO3-Ti2O3 system. Similarly, the ulvospinel contains some Fe3+ ions and a correction for the activity of Fe2TiO4 has been included by modeling the Fe2TiO4-Fe3O4 system. The third-law analysis of the results obtained for IRI equilibrium gives DELTA-H298-degrees = -575 (+/- 1.0) kJ mol-1 and for IIU equilibrium yields DELTA-H298-degrees = -523.7 (+/- 0.7) kj mol-1. The present results suggest that Fe 2+ and Ti4+ cations mix almost ideally on the octahedral site of spinel lattice in Fe2TiO4, giving rise to a configurational contribution of 2R In 2 (11.5256 J mol-1 K-1) to the entropy of Fe2TiO4.