Calibration of a chalcogenide glass membrane ion-selective electrode for the determination of free Fe3+ in seawater I. Measurements in UV photooxidised seawater

Calibration of a chalcogenide glass membrane, Fe(III)ISE [Fe-2.5(Ge28Sb12Se 60)(97.5)], in buffered saline media has been undertaken in order to assess the suitability of this ISE for seawater analyses. The electrode slopes in saline citrate and salicylate buffers were 26.3 and 28.2 mV/decade, respec tively, for Fe3+ concentrations ranging from 10(-10) M to less than 10(-25) M Fe3+. The calibration lines in the citrate and salicylate buffers were e ssentially collinear with the response in unbuffered chloride-free standard s containing > 10(-5) M Fe3+, demonstrating that the response of the FeISE is unaffected by chloride ions, A mechanism involving a combination of char ge transfer and ion-exchange of Fe(III), at the electrode diffusion layer, can be used to explain the approximate to 30 mV/decade slope of the FeISE. The response of the FeISE in UV photooxidised seawater containing 8 nM tota l Fe was measured as the pH was changed from 8.27 to 3.51. The slope of the response was 24.2 mV/decade [Fe3+] calculated as a function of pH using Fe (III) hydrolysis constants for seawater: Moreover, the response was essenti ally collinear with that in citrate buffers and in unbuffered solutions con taining > 10(-5) M Fe3+ and the slope for the combined data was 26.2 mV/dec ade. This study was restricted to organic-free seawater because the certain ty in Fe(III)-ligand stability constants is insufficient to warrant the sel ection of an ideal calibration buffer system, and there is evidence that po werful chelating ligands (e.g., EDTA along with humic and fulvic acids) may alter the response of the Fe(III)ISE, The Fe dissolution rate of the FeISE in UV photooxidised seawater was found to be 1.6 X 10(-2) nmol Fe/min, as measured by cathodic stripping voltammetry (CSV). This would contaminate a 100-ml sample by 0.8-1.6 nM Fe over a typical measurement period of 5-10 mi n obtained using a stability criterion of 0.5 mV/min. Various methods are p roposed for reducing the level of contamination in open ocean samples that contain sub-nanomolar concentrations of iron. The FeISE has the potential t o detect free Fe3+ at concentrations typically found in natural seawater. ( C) 2000 Elsevier Science B.V, All rights reserved.