EMF STUDY OF THE PYRRHOTITE-MAGNETITE EQUILIBRIUM USING THE AG/AG2S GAS BUFFER

Abstract

The equilibrium 3/1 - xFe1 - xS + 6/1 - xAg + 2O2 = Fe3O4 + 3/1 - xAg2S (i) was studied over a temperature interval of 800-1050 K by measuring the oxygen partial pressure in a solid-state galvanic cell with the use of an O2--conductive solid electrolyte (Y2O3 stabilized with ZrO2). The measured temperature dependence of p(O2) is described by the equation log p (O2) = 67.815 - 29026.11/T - 8.116ln T (σ = ±0.03). The experimental data obtained were utilized to calculate the free energy of reaction (i) and the free energy of pyrrhotite (po) formation (kJ/mol). The fitting of the data by least squares yielded the equations ΔrG°(i) = - 1168.53 + 3.09666T - 0.37492Tln T, ΔfG°(po) = - 7.256 - 1.1837T + 0.15939Tln T. The free energies were calculated for the actual chemistry of pyrrhotite in equilibrium with the Ag/Ag2S buffer as (1 - x) = 1.181 - 241.2/T (K), which was experimentally determined for the temperatures of 1008 and 1073 K.