Impedance measurements of a chalcogenide membrane iron(III)-selective electrode in contact with aqueous electrolytes

Abstract The electrochemical characterization of a chalcogenide-based iron(III)-selective electrode [Fe(III) ISE] [i.e., Fe 2.5 (Se 60 Ge 28 Sb 12 ) 97.5 ] was achieved using impedance spectroscopy. The influence of electrolyte composition (i.e., NO 3 − , Cl − , and pH) on the membrane oxidation reaction has been examined, and a mechanism for its action is proposed. Equivalent circuit analysis was undertaken to determine the interfacial charge transfer resistance and corresponding double layer capacitance as a function of electrolyte composition and immersion time. Variations were detected in the charge transfer time constant, and this was attributed to changes in the dielectric/conduction properties of the surface layer. It was found that the Fe 2.5 (Se 60 Ge 28 Sb 12 ) 97.5 oxidation kinetics depend on the pH, and the interfacial reaction is dictated by sluggish charge transfer. By contrast, chloride was shown to accelerate the rate of membrane oxidation presumably via the formation of soluble metal-chloride complexes. Electrochemical impedance spectroscopy (EIS) aging studies of the Fe 2.5 (Se 60 Ge 28 Sb 12 ) 97.5 membrane in chloride electrolyte under alkaline conditions showed that the charge transfer resistance decreases with exposure time. However, extended aging revealed a change in the rate of oxidation, which was attributed to a combined diffusion/passivation effect. It is proposed that the development of a modified surface layer (MSL) and passive surface layer (PSL) are partly responsible for the electrochemical stability of the chalcogenide membrane. This paper attempts to clarify and address some of the misconceptions/issues reported previously in the literature on the chalcogenide iron(III)-selective electrode.