Thermodynamic functions by e.m.f. measurements on solid galvanic cells in non-stationary conditions: system Al + Al2O3

Abstract

E.m.f. measurements were carried out on the cell,[graphic ommitted] after a cathodic reduction of the aluminum surface-oxide layer, in order to bring the aluminum activity at the interface equal to that of the pure phase. After the polarization current was cut off, the decay e.m.f. against time curves were recorded and were found to be step-shaped. The first plateau corresponds to the formation of a more oxidized two-phase system at the anode interface. The system is rapidly destroyed by iron diffusing from the bulk which restores the original system and the e.m.f. rapidly decrease; then the slope becomes very slight and can be accounted by the decrease of the aluminum activity at the cathode interface. The difference of these two plateaus corresponds to the predicted variation of the Gibbs function in the transition from Fe₃O₄+Fe₂O₃ to Fe-wüstite. By estrapolating the last portion of the curve at the time at which the current is switched off, the standard molar free energy of formation of γ-Al₂O₃ is calculated.