Assessing the dynamic temperature and salinity responses of the Honeywell Durafet's internal and post-factory added external reference electrodes
Date
2024
Authors
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Publisher
University of Delaware
Abstract
The work described herein comprises field- and laboratory-based assessments of dynamic errors in electrode response of the Honeywell Durafet and its internal (Ag/AgCl reference electrode containing a saturated KCl reference gel - pHINT) and post-factory added external (solid-state chloride ion-selective electrode, Cl-ISE - pHEXT) reference electrodes. Data collected by the Durafet-based pH sensors used here were then utilized to improve our understanding of electrode response in nearshore waters that experience wide ranges of and rates of change in pH, temperature, and salinity. ☐ In Chapter 2, empirical analyses of pH and environmental data from the Murderkill Estuary-Delaware Bay System collected by a deployed sensor revealed that tidally-driven dynamic errors in the temperature and salinity responses of the internal and external reference electrodes, respectively, were introduced into our pH timeseries. Dynamic errors in reference electrode response drove large anomalies between pHINT and pHEXT (denoted ΔpHINT−EXT) that reached >±0.8 pH when the lowest water temperatures and maximum tidal salinity variability occurred in the winter. A clear linear relationship was demonstrated between the ∆pHINT−EXT and the rate of salinity change between sensor measurements (dSalt/dt) thereby making dSalt/dt the strongest limiting factor of reference electrode response in our application. A dynamic sensor response correction for the Cl-ISE was also developed and applied in the voltage domain. After application, this correction substantially reduced ΔpHINT−EXT anomaly ranges and it removed the first-order salinity dependence of the ΔpHINT−EXT anomalies. However, additional work is needed to refine the Cl-ISE dynamic sensor response correction and develop a corresponding correction to address dynamic errors in the temperature response of the internal reference electrode to improve pH measurement accuracy in nearshore waters. ☐ In Chapter 3, to further scrutinize the suitability of the Cl-ISE for pH measurement as the reference electrode, a half-cell reaction approach for pH calculation using Cl-ISE as the chloride ion (Cl−)-sensitive reference electrode and the ion-sensitive field effect transistor (ISFET) of the Honeywell Durafet as the hydrogen ion (H+)-sensitive measuring electrode was developed. This new approach split out and isolated the independent responses of the Cl-ISE to Cl− (and salinity) and the ISFET to H+ (and pH), and calculated pH directly on the total scale (pHtotalEXT) in molinity (mol (kg-soln)-1) concentration units. The new half-cell and existing complete cell (where the responses of the Cl-ISE and ISFET are combined) reaction approaches were then applied to calculate pHtotalEXT using measurements made using two SeapHOx sensors between salinity and pH of 1 and 31 and 6.9 and 8.1, respectively, over a sixday period in a test tank. When splitting out and calibrating raw pH sensor timeseries as needed according to salinity, pHtotalEXT had root-mean squared errors ranging between ±0.0026 and ±0.0168 pH calculated using both reaction approaches relative to pHtotal of co-located discrete bottle samples (pHtotaldisc). These results are notably in contrast to those of the few in situ field deployments over similar environmental conditions that demonstrated pHtotalEXT calculated using the Cl-ISE as the reference electrode had larger uncertainty in nearshore waters. Therefore, additional work beyond the correction of variable temperature and salinity conditions in pH calculation using the Cl-ISE is needed to constrain the impacts of other external stimuli on in situ Cl-ISE response. Furthermore, increased scrutiny of the ISFET as the H+-sensitive measuring electrode for pH measurement in natural waters is also needed. ☐ In Chapter 4, dynamic electrode response was assessed over a six-day period in a test tank over wide ranges of and rates of change in salinity between 1 and 31 and - 11.21 and +10.66 (0.5 h)-1, respectively. To do this, measurements made using two SeapHOx sensors (designed and assembled by Todd R. Martz of Scripps Institution of Oceanography (La Jolla, CA, USA)) and two SeaFET V2 sensors (designed and assembled by Sea-Bird Scientific (Bellevue, WA, USA)) were used. After employing a new calibration approach designed to reconstruct the electrodes’ equilibration periods following the salinity change each day, clear differences between the dynamic responses of electrodes integrated into each sensor model emerged. Dynamic errors in electrode response were substantially greater and persisted longer for the SeaFET V2 sensors than the SeapHOx sensors; for which these errors were greatly reduced or not present at all. Salinity-driven dynamic errors in electrode response for the SeaFET V2 sensors were greatest directly following the salinity change each day (at Hour 0) and produced large uncorrected Hour 0 salinity-driven anomalies between pHtotaldisc and sensor-measured pH (pHtotalelec) (referred to as ∆pHtotal,uncorrdisc−elec anomalies) of > ±0.1 pH that also persisted for multiple hours after the salinity change. ☐ The differences in electrode response between different sensors models are attributable to the different voltage measurement sequences that are carried out over measurement periods of different lengths. Here, the SeapHOx sensors use an average of 20 voltage measurements collected over a 16 sec period and the SeaFET V2 sensors use the final of four voltages collected over a 0.5 sec period for pH calculation. For the SeaFET V2 sensors, this may truncate the electrode response period after salinity changes each day and produce the dynamic errors in electrode response we observed. Therefore, further work is needed to optimize the voltage measurement sequence for the SeaFET V2 sensors for nearshore waters. However, large salinity-driven ∆pHtotal,uncorrdisc−elec anomalies for SeaFET V2 sensors were ultimately corrected using post-calibration secondary pH corrections that utilized the smooth exponential relationships between ∆pHtotal,uncorrdisc−elec anomalies and time. After correction, corrected ∆pHtotaldisc−elec (∆pHtotal,corrdisc−elec) anomalies for the SeaFET V2 sensors substantially improved and largely met oceanographic community-standard pH data quality thresholds while associated improvements were minimal for the SeapHOx sensors since their pH corrections were few. Here, the power and necessity of secondary pH correction ultimately underpins the need for the development of similar pH correction methods for field data collected in nearshore waters.
Description
Keywords
Ion selective electrodes, Ocean acidification, Potentiometry, Sensor