Seawater pH Measurement: A Critical Metrological Assessment of Operational Scales and the Unified pH Scale

Ocean acidification, driven by rising anthropogenic CO2, causes a measurable decrease in seawater pH with consequences for marine ecosystems. Accurate, traceable pH measurement is essential for ocean monitoring sensors. This critical review examines five pH scales: the IUPAC activity scale (pH), the free proton scale (pHF), the total proton scale (pHT), the seawater scale (pHSWS), and the unified scale (pHabs). For each scale, the analytical methods, metrological assumptions, traceability chain to the SI, and measurement uncertainties are analyzed. The review updates the foundational 2016 work with subsequent developments and focuses particularly on the application of pHabs to saline solutions. In its current differential-potentiometric realization, pHabs yields an expanded uncertainty U(pH) ≈ 0.2 in seawater. GOA-ON expresses its goals as standard uncertainties (k = 1): 0.003 (climate) and 0.02 (weather), equivalent to U ≤ 0.006 and U ≤ 0.04 for k = 2. The reported U(pH) ≈ 0.2 is therefore approximately 33 times the climate-quality target and approximately 5 times the weather-quality target, compared at the same coverage. The dominant contribution is the residual liquid-junction potential; the standard uncertainty of 6.3 mV (k = 1) reported in dilute solutions corresponds, at 25 °C, to u(pH) ≈ 0.1 and U(pH) ≈ 0.2 for k = 2, consistent with the reported U(pH) ≈ 0.2. However, this dilute-solution estimate is not a validated uncertainty for seawater and should not be extrapolated to seawater without experimental validation. Liquid-junction effects are therefore a priority for potentiometric sensors and for the current pHabs realization. Spectrophotometric pHT sensors avoid a liquid junction but have distinct uncertainty contributions related to indicator characterization, optical measurement, temperature, salinity, and sample perturbation.