Up to one-third of the anthropogenic atmospheric carbon dioxide dissolves into the oceans, disrupting the chemical equilibria in seawater [1]. This process is often referred to as ocean acidification (-0.002 pH y^-1) and is particularly pronounced in estuarine areas due to elevated organic matter concentration [1]. As pH controls many biogeochemical processes in aquatic environments, accurate monitoring is crucial for a better comprehension of environmental systems [1]. Routine pH measurements are typically performed using zero-current potentiometry with glass electrodes. Their phase-boundary potential, measured against a reference electrode, relates to the sample pH following the Nernst equation. This intrinsic property results in a limited sensitivity that is insufficient for precise environmental pH monitoring. Additionally, changes in ionic strength can introduce significant accuracy errors due to residual liquid-junction potential at the reference electrode.

Our group tackled the sensitivity limitation by developing an alternative readout named constant potential coulometry [2]. During this protocol, a high impedance input voltage follower and capacitor are implemented in the electronic readout and the cell potential is kept constant. Thus, any potential change at the glass electrode induces an opposite one on the capacitor and a transient current arises. The latter is then integrated to obtain the charge, resulting in an increased sensitivity compared to potentiometry. This method is successfully applied for pH sensing with benchtop instruments in narrow pH ranges comparable to environmental samples with a precision up to 64 μpH. Notably, the system retains the ability to perform conventional potentiometric measurements, allowing direct comparison between the two readouts. The inaccuracies caused by liquid-junction potential variations can be minimized using open-junction designs as previously reported [3].

This work presents the integration of the coulometric readout and open-junction principle into a submersible probe developed for in situ measurements in natural waters, along with the results from the first field deployment in April 2025 in the Krka estuary, Croatia. This stratified environment was selected for its stable pH in the bottom seawater layer for signal stability and precision evaluation, while its pronounced vertical salinity gradient enables evaluation of measurement accuracy under large ionic strength variations through comparison with a commercial multi-parameter probe. For all experiments, the performance of potentiometric and coulometric readouts are compared. 

[1] S. F. Gonski, W-J. Cai, W. J Ullman, A. Joesoef, C. R. Main, D. T. Pettay, T. R. Martz, Estuar. Coast. Shelf Sci., 2017, 200, 152-168.

[2] R. Nussbaum, S. Jeanneret, E. Bakker, Anal. Chem., 2024, 96 (16), 6436-6443.

[3] A. K. Covington, P. D. Whalley, W. Davison, Anal. Chim. Acta, 1985, 169, 221–229.