10.1007/s11175-008-2015-4

Abstract

Using the method of phase modeling, the pH values of solutions corresponding to the uncharged surface of passive iron and ferric oxide γ-Fe2O3 (pH0) are compared. According to the theory of connected places, the charge of metal oxide surface is determined by the adsorption or desorption of hydrogen ions leading to a change in the potential drop at the oxide/solution interface. Preliminarily passivated iron electrode was washed with twice-distilled water and placed into 0.5 M NaNO3 solution with various pH values; the variation in the potential (ΔE) with time was studied. The pH0 value for passive electrode under the open-circuit conditions was determined by the dependence of ΔE on the pH value (pH0 6.2 ± 0.1). The pH0 value was close to that for γ-Fe2O3 (pH0 6.2), which was determined by the method of potentiometrical titration of oxide suspension in the nitrate solution. The introduction of surface-active ions Ba2+ and Cl− changes the charge of passive iron surface: Ba2+ ions increase the electrode potential, while Cl− ions decrease it. Comparing the pH0 values for passive electrode and metal oxides, one can identify the composition of passive electrode surface.