Chalcogenide glass chemical sensors: Relationship between ionic response, surface ion exchange and bulk membrane transport

Electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy study of the response mechanism of the chalcogenide glass membrane iron(III) ion-selective electrode in saline media

The response mechanism of the iron(III) chalcogenide glass membrane ion-selective electrode (ISE) in saline media has been studied using electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). EIS equivalent circuits and XPS surface compositions for the FeIII ISE are consistent with the presence of two surface films probably comprising a outer surface layer (OSL) and an Fe-deficient modified surface layer (MSL), along with a low-frequency charge-transfer impedance that is attributable to the reduction of Fe3+. In accordance with literature data for the conductivity of low-bearing iron(III) chalcogenide glasses, a high-impedance MSL is internally consistent with XPS data for an Fe-deficient MSL. It is evident that the impedance of the MSL diminishes on exposure to solutions containing Fe3+, and this finding is consistent with the ion exchange of Fe3+ within the MSL. Likewise, the charge-transfer impedance also decreases at elevated levels of Fe3+, demonstrating that Fe3+ is a participant in the reversible charge-transfer reaction occurring at the electrolyte/electrode interface. The kinetics of charge transfer are facilitated by Fe chelating agents (e.g., citrate, salicylate, EDTA, etc.) due presumably to the complexation of the products of the charge transfer process (possibly Fe2+). It is shown unequivocally that the response of the FeIII ISE in saline buffers is independent of pH, demonstrating that the ISE is responding directly to Fe3+, not H+. A mechanism involving a combination of charge transfer and ion exchange of FeIII, at the electrode diffusion layer, has been proposed to explain the 30 mV/decade slope of the FeIII ISE.