Reinvestigation of the Surface Chemistry of the Iron Chalcogenide Glass Ion-Selective Electrode in Seawater: Studies of Copper Interference, Seawater Aging, Influence of Flow, and Iron Sorption Properties

This study reinvestigated the surface chemistry of the iron chalcogenide glass ion-selective electrode (ISE) in seawater using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and synchrotron radiation X-ray photoelectron spectroscopy (SR XPS). Aging of ISE membranes in seawater under static conditions revealed, for the first time, a seawater matrix effect originating from membrane fouling by a 0.6 nm-thick layer of a scale comprising calcium, magnesium, silicate, and chloride, necessitating the use of a FeEDTA-spiked seawater calibrant, while Fe contamination from membrane dissolution, adsorption, and accumulation was problematic with a static ISE. Conversely, ISE membranes subjected to turbulent flow in continuous flow analysis (CFA) displayed significant declines in Fe contents compared to a polished membrane (“Fe suctioning”) and amelioration of electrode scaling under these conditions. This CFA behavior is consistent with ablation of the membrane surface due to turbulent flow effects, as well as cleansing of the ISE diffusion layer (DL) of membrane-released Fe 3+ through flow-induced DL thinning. Iron L edge NEXAFS semiquantitative analysis revealed a strong proportionality against pFe in buffered media, which is consistent with a flatband potential theoretical model for semiconducting ISE membranes. Last, the Cu 2+ interference poisoning reaction was observed at high Cu 2+ concentrations, and despite the Cu 2+ ISE selectivity coefficient implying a strong Cu 2+ interference at low Cu 2+ activities ≈10 –11 M in seawater, the interference effect was unobservable due to kinetic limitations of the poisoning reaction at ≈10 –11 M Cu 2+ . Overall, this is the first direct surface spectroscopic evidence providing mechanistic validation of our new CFA seawater method calibrated using FeEDTA-spiked seawater standards.