Symmetrical pH Electrochemical Cell Coupled to Constant Potential Coulometry for Improved Sensitivity and Precision: Part 2. Submersible Probe for In Situ Measurements.

Seawater pH is a critical parameter influencing many processes in the ocean. Today it is mainly measured by indicator-based spectrophotometry to allow for high precision. This, however, is at the expense of traceability and systematic errors originating from changes in temperature, salinity, and other matrix effects. Moreover, in routine practice, this approach is not performed in situ and requires sampling and manual manipulations, which are prone to introduce additional errors, including gas exchange with the atmosphere. Unfortunately, in the last few decades, the electrochemical sensing community has failed to make efforts to improve the performance of the main method, which is potentiometric detection with pH glass electrodes. To address this, we aim here to improve the sensitivity and precision of submersible pH probes on the basis of pH glass electrodes by minimizing systematic errors from temperature changes and by implementing a recently described coulometric method. The electrodes are mounted in a symmetrical cell reported in part 1 of this work to reduce sensor drift and minimize inaccuracies due to liquid junction potential variations and pH changes of the inner solution from temperature fluctuations. The development and construction of the probe are explained. The circuit is evaluated, and the sensors are calibrated over a range of temperatures, approaching ideal behavior. The submersible probe was deployed in situ in April 2025 in the vertically stratified Krka River Estuary in Croatia. The precision of the probe was evaluated in situ by stability experiments in the seawater layer. The determined precision is 0.001 pH unit, which is significantly better than that reported earlier for routine pH probes. A recalibration procedure with synthetic seawater is also evaluated for minimizing drift. A depth profile with a changing salinity was performed and compared with multiparameter probes.