An evaluation of the performance of an ISFET pH sensor in a dynamic estuarine system
Date
2016
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Publisher
University of Delaware
Abstract
The feasibility of the Honeywell Durafet to the measurement of pH in a dynamic, productive, high-fouling, highly-turbid estuarine environment was investigated. Three different configurations of the SeapHOx sensor equipped with a Honeywell Durafet and its integrated internal (Ag/AgCl reference electrode containing a 4.5M KCl gel liquid junction) and external (solid-state chloride ion selective electrode, Cl-ISE) reference electrodes were deployed between April 2015 to August 2015 and September 2015 to August 2016 in the Murderkill Estuary-Delaware Bay System (Delaware, USA). Comprehensive sensor maintenance was performed every 1–2 weeks to ensure proper continuous sensor operation. Discrete bottle samples for dissolved inorganic carbon (DIC), total alkalinity (TA), and pHNBS were collected every 30 minutes to calibrate raw sensor output on more than 20 tidal cycle sampling trips. During these sampling trips, the full range of biogeochemical exchange between the fresher Murderkill Estuary outflow and the more saline Delaware Bay water endmembers was captured. The sensor pH collected during the summer of 2016 using the furthest refined SeapHOx configuration exhibited good agreement with the independent reference pH used. Accordingly, the sensor pH had a root-mean squared error (RMSE) ranging between 0.011 and 0.036 pH units across the full salinity range of the deployment environment relative to both pHT calculated from measured DIC and TA and pHNBS measured with a glass electrode corrected to pHT at in-situ conditions. ☐ In this environment, the Honeywell Durafet proved capable of making high-resolution, high-frequency pH measurements ranging between 6.8 and 8.4. Natural pH fluctuations ranging from <0.5 pH units to >1 pH unit were routinely captured. A number of deficiencies in existing deployment guidelines and calibration protocol for Durafet-based biogeochemical sensors specific to estuarine and coastal ocean systems were identified and highlighted. Further, aspects of electrode response requiring further investigation were highlighted. A set of recommendations for the future utilization of these sensors in estuarine and coastal systems also resulted from the present work. The present work effectively demonstrated the viability of the Honeywell Durafet to the accurate and dependable measurement of pH as a part of future estuarine and coastal ocean CO2 chemistry studies. The present work was a vital step in linking the parallel emerging trends in seawater pH metrology associated with the spectrophotometric measurement of pH using purified colorimetric indicator dyes and the electrochemical measurement of pH using the Honeywell Durafet over the full temperature and salinity range of natural waters. When these trends finally converge, the excellent accuracy of the Honeywell Durafet characterized under open-ocean conditions (<0.01 pH units) should be consistently achievable in estuarine and coastal ocean systems as well.