Ocean Acidification

pH is a major variable in complex aquatic ecosystems, influencing biological activity, metal speciation and more. The gold standard method to routinely measure pH is potentiometric measurements with a glass electrode connected to a reference Ag/AgCl element in contact with the sample through a liquid junction. However, it has been largely replaced by optical pH assays in the field of seawater pH measurements because much better precisions could be achieved. Glass electrodes also suffer from bias such as liquid junction potential changes and temperature influence on the inner solution pH that generate inaccuracies. Moreover, the Nernstian relationship between observed potential and pH results in limited sensitivity. To overcome these limitations, an alternative readout called constant potential coulometry is implemented in the measurement system for increased sensitivity. A symmetrical pH cell is proposed in which two identical glass electrodes, separated by an open 3 M KCl channel, are measured against each other. One of them is kept in a NIST buffer while the other is used to measure the pH of the sample or the calibrant solution. Glass electrodes are evaluated at different temperatures in NIST buffers versus a classical reference electrode and versus each other in a symmetrical flow cell. The latter features open junctions that should improve the repeatability of the liquid junction potential. The determined measurement repeatability is as low as 0.3 mpH with a precision of 0.001 pH, which is drastically improved over routinely available pH probes.

This study allowed us to establish a procedure for determining the pH of seawater by the primary method using cell without junction (Harned cell) in artificial seawater without sulfate ASW-SO₄ which is taken as a reference buffer solution featuring the natural seawater at salinity 35 and an ionic strength equal to 0.6731 mol kg^-1. The determination of the coefficient average activity (γ ± HCl) of HCl was taken from two approaches. The first one is an experimental approach in which we introduced the potential standard value (E°) calculated in dilute media. In the second approach, we applied and adapted the Pitzer model in function of the temperature (15, 25 and 35 °C) taking into account all the parameters of the solution in ionic interactions and we demonstrated its feasibility for the solution ASW-SO₄. The choice was made for the first option which was preferred because of its simpler implementation.

Highlights

• A novel ISM-free Pb/PbCO₃ electrode is developed via in-situ electrochemical plating.
• PbCO₃ nanosheet layer integrates ion recognition and signal transduction functions.
• The electrode shows near-Nernstian response, fast response time, and high selectivity.
• Successful in-situ detection of CO₃²⁻ reveals diurnal variation in natural waters.

Abstract

Accurate determination of carbonate ions (CO₃²⁻) plays a pivotal role in water quality monitoring and the conservation of marine ecosystems, particularly in tracking ocean acidification processes and maintaining aquatic balance. Traditional potentiometric CO₃²⁻ sensors based on all-solid-state ion selective electrodes (ASS-ISEs), face fundamental limitations such as multi-interface structural complexity, persistent water-layer formation, and high fabrication costs. In this study, we propose a novel ISM-free Pb/PbCO₃ electrode for CO₃²⁻ determination by electrochemically developing an in-situ PbCO₃ nanosheet crystalline membrane. This solid-contact membrane integrates ion recognition and signal transduction synergistically, serving as both the ion-selective membrane and transducer, thereby eliminating interfacial instabilities inherent to traditional bilayer ASS-ISEs. This electrode exhibits a near-ideal Nernstian response slope of −32.7 ± 1.2 mV·decade⁻¹, with a linear range from 10 ^{to 5.15} to 10^–3.13 mol·L⁻¹ and a rapid response time of 35 s. It also demonstrates good anti-interference capabilities, with low selectivity for common anions like NO₃⁻ and SO₄²⁻ enabling effective monitoring of CO₃²⁻ dynamics in natural water samples. The design avoids conventional polymeric membranes, suppressing water-layer formation and enhancing mechanical stability. This study provides a cost-effective, scalable solution for in situ CO₃²⁻ detection and presents a new approach to solid-contact ion sensors design through functional integration, improving real-time monitoring capabilities.

Total alkalinity (TA) measurements in seawater are crucial for characterizing and monitoring the oceanic carbonate system. While international best practices and guidelines exist, the field still lacks widely available traceable reference materials and a well-established uncertainty budget of the measurement method. In this study, we applied key metrological principles—development of reference materials, inter-laboratory comparison and uncertainty quantification—to TA measurements. We developed two reference materials, including an artificial material with a rigorously characterized reference value and an associated uncertainty budget, being potentially traceable to the International System of units (SI). These materials were tested in an inter-laboratory comparison involving five laboratories and demonstrated the applicability and interest of the reference materials developed for quality control. Additionally, we established an uncertainty budget for the TA measurement method using two metrological approaches. The resulting expanded uncertainty was 5 µmol kg⁻¹ (k = 2) in TA, approaching the 4 µmol kg⁻¹ target set by the Global Ocean Acidification Observing Network for climate monitoring. These findings mark a significant step toward improving the quality and comparability of TA measurements, thereby strengthening long-term ocean carbonate system monitoring.

The partial pressure of CO₂ (p(CO₂)) dissolved in seawater is often indirectly determined through a mathematical calculation involving two variables of the carbonate system, which requires the use of expensive equipment, materials, and reagents, and is usually demanding in terms of human resources. In this study, we present a simple, low-cost method for directly measuring dissolved CO₂ (p(CO₂)) in coastal waters using a small gas equilibrator coupled with a non-dispersive infrared (NDIR) detector in a closed-loop system. The cost of this methodology can be further reduced by utilizing a low-cost NDIR detector and inexpensive materials. Its analytical performance was validated against the conventional indirect approach based on pH and dissolved inorganic carbon (DIC), showing high accuracy (91%), excellent linearity (r² = 0.998), and a quantification limit (∼21 μatm) approximately 20 times lower than the average surface seawater p(CO₂). The method proved robust to typical variations in salinity (15–45 PSU) and temperature (20–25 °C), though a new calibration is recommended for salinities below 15 PSU. Application to samples from a tropical coastal lagoon and adjacent oceanic area showed statistically comparable results with both methodologies. This approach could enable direct measurement in the field, thereby reducing labor, equipment, and material costs. This low-cost approach enhances in situ p(CO₂) monitoring, offering significant potential for long-term ocean acidification studies in resource-limited settings.

During the continuous monitoring of pH in environmental water, the temperature of the environmental water changes at each moment. Coral reefs are likely to be strongly affected by ocean acidification, so accurate and continuous pH monitoring is important. On the other hands, the water temperature in coral reef areas varies greatly not only due to solar radiation but also due to the mixing of open ocean water and land water. This temperature variation affects the accurate pH measurement. Preparing calibration values for the pH electrodes at all water temperatures is impossible, but the monitoring accuracy could be improved by predicting the constituent values at the on-site water temperature from the electrode calibration results at one or two temperatures. Therefore, we calibrate the pH at one or two temperatures and use the result for estimating the calibration value at another temperature. For this purpose, we estimated the pH change of the internal liquid of the electrode (assumed as a phosphate buffer) under saturated and unsaturated conditions of the coexisting KCl in the buffer.

As a key index to measure the buffering capacity of marine carbonate systems, the total alkalinity (TA) of seawater plays an important role in monitoring and predicting ocean acidification. Given the limitation of the existing TA measuring techniques to laboratory environments and the difficulty in continuous real-time monitoring of marine fields, the existing TA analyzer was optimized based on single-step titration spectrophotometry. Then, a hardware system covering sample collection, measurement and analysis, data processing and storage, power supply, and control system was established by integrating the overall architectural design and development of the raft microcirculation system, dual-path photoelectric detection system, and mini-instrument. Besides, the corresponding instrument control, data processing and analysis software and user-friendly interface were developed, because of which the detection error and precision of the analyzer were < 1 % and ± 3.6 μmol/kg, respectively. The study results provide a high-precision and high-efficiency seawater TA measuring tool for scientific marine research and environmental monitoring.