| Abstract
| - The response mechanism of the iron(III) chalcogenideglass membrane ion-selective electrode (ISE) in salinemedia has been studied using electrochemical impedancespectroscopy (EIS) and X-ray photoelectron spectroscopy(XPS). EIS equivalent circuits and XPS surface compositions for the FeIII ISE are consistent with the presence oftwo surface films probably comprising a outer surfacelayer (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+. Inaccordance with literature data for the conductivity of low-bearing iron(III) chalcogenide glasses, a high-impedanceMSL is internally consistent with XPS data for an Fe-deficient MSL. It is evident that the impedance of the MSLdiminishes on exposure to solutions containing Fe3+, andthis finding is consistent with the ion exchange of Fe3+within the MSL. Likewise, the charge-transfer impedancealso decreases at elevated levels of Fe3+, demonstratingthat Fe3+ is a participant in the reversible charge-transferreaction occurring at the electrolyte/electrode interface.The kinetics of charge transfer are facilitated by Fechelating agents (e.g., citrate, salicylate, EDTA, etc.) duepresumably to the complexation of the products of thecharge transfer process (possibly Fe2+). It is shownunequivocally that the response of the FeIII ISE in salinebuffers is independent of pH, demonstrating that the ISEis responding directly to Fe3+, not H+. A mechanisminvolving a combination of charge transfer and ion exchange of FeIII, at the electrode diffusion layer, has beenproposed to explain the 30 mV/decade slope of the FeIIIISE.
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