A half-cell reaction approach for pH calculation using a solid-state chloride ion-selective electrode with a hydrogen ion-selective ion-sensitive field effect transistor

Here, we explicitly define a half-cell reaction approach for pH calculation using the electrode couple comprised of the solid-state chloride ion-selective electrode (Cl-ISE) as the reference electrode and the hydrogen ion-selective ion-sensitive field effect transistor (ISFET) of the Honeywell Durafet as the hydrogen ion $\left(H^{+}\right)$-sensitive measuring or working electrode. This new approach splits and isolates the independent responses of the Cl-ISE to the chloride ion $\left({\mathrm{Cl}}^{-}\right)$ (and salinity) and the ISFET to $H^{+}$ (and pH), and calculates pH directly on the total scale $\left({\mathrm{pH}}_{\text{total}}^{\mathrm{EXT}}\right)$ in molinity (mol (kg-soln)⁻¹) concentration units. We further apply and compare ${\mathrm{pH}}_{\text{total}}^{\mathrm{EXT}}$ calculated using the half-cell and the existing complete cell reaction (defined by Martz et al. (2010)) approaches using measurements from two SeapHOx sensors deployed in a test tank. Salinity (on the Practical Salinity Scale) and pH oscillated between 1 and 31 and 6.9 and 8.1, respectively, over a six-day period.

In contrast to established Sensor Best Practices, we employ a new calibration method where the calibration of raw pH sensor timeseries are split out as needed according to salinity. When doing this, ${\mathrm{pH}}_{\text{total}}^{\mathrm{EXT}}$ had root-mean squared errors ranging between ±0.0026 and ±0.0168 pH calculated using both reaction approaches relative to ${\mathrm{pH}}_{\text{total}}$ of the discrete bottle samples $\left({\mathrm{pH}}_{\text{total}}^{\text{disc}}\right)$. Our results further demonstrate the rapid response of the Cl-ISE reference to variable salinity with changes up to ±12 (30 min)⁻¹. Final calculated ${\mathrm{pH}}_{\text{total}}^{\mathrm{EXT}}$ were ≤±0.012 pH when compared to ${\mathrm{pH}}_{\text{total}}^{\text{disc}}$ following salinity dilution or concentration. These results are notably in contrast to those of the few in situ field deployments over similar environmental conditions that demonstrated ${\mathrm{pH}}_{\text{total}}^{\mathrm{EXT}}$ 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 $\mathrm{pH}$ calculation using the Cl-ISE is needed to examine the effects of other external stimuli on in situ electrode response. Furthermore, whereas past work has focused on in situ reference electrode response, greater scrutiny of the ISFET as the $H^{+}$-sensitive measuring electrode for pH measurement in natural waters is also needed.