Seawater Calibration Systems for Oceanographic Implementation of Iron(III) Ion-Selective Electrodes

Current analytical techniques for assessing the ocean iron (Fe) system are rarely able to provide direct information about Fe-speciation at ambient seawater physicochemical conditions. The chalcogenide glass Fe 3+ ion-selective electrode (Fe-ISE) has emerged as a promising tool for in situ oceanographic Fe analyses given its ability for rapid and passive measurements of the Fe-complexation tendencies of an aqueous system. However, routine oceanographic implementation of the Fe-ISE has been limited by calibration issues arising from the use of artificial calibration systems. Here, we address these challenges with controlled laboratory experiments utilizing the Fe-ISE in an improved continuous-flow analyzer (CFA) with in-line pH and temperature sensors. A variety of calibration standards are tested and an optimal system is presented, based on a filtered seawater solution comprising 10 μM added Fe and 100 μM added EDTA. This system reproducibly generates slopes near 26 mV/dec, which is in excellent agreement with previous calibrations of the Fe-ISE, yet does not impart the potential offset to seawater measurements that has been observed in calibrations utilizing artificial iron buffer matrices. Calibration plots are revisualized as acidimetric titrations to illustrate the strong agreement between modeled Fe 3+ activity and Fe-ISE response in seawater media, provided that chemical speciation models adequately account for Fe solubility effects. These titrations also suggest that the Fe-ISE may be a powerful tool for evaluating the pH-dependent Fe-binding properties of the natural seawater ligand pool. This work establishes procedures for efficient and reliable sensor calibration to enable Fe-ISE measurements of natural seawater.