Atmospheric CO₂ absorbed by seawater causes acidification as evidenced by a reduction in the amount of total H⁺ (pH_T). This causes decreased calcification, respiratory difficulties, and reproductive changes in marine species worldwide. The European Marine Strategy Framework Directive targets a healthy marine environment through establishment of robust measuring systems. Internationally the Global Ocean Acidification Observing Network and UNECSO’s SDG 14 both recognise the need to decrease sea acidification. ISO 18191:2015 details methods for pH_T measurements but lacks specifications for metrological traceability, uncertainty evaluation and method validation leading to different strategies for pH_T measurement uncertainty and reduced data comparability.  

The EMRP project “Ocean” developed pH_T methodology based on the Harned cell primary system, linking spectrophotometric pH_T measurement to higher order standards. However, comparability of pH_T values and the degree of equivalence obtained at different National Metrology Institutes (NMI) using this procedure was not demonstrated.

This project has provided traceable seawater pH_T measurements in the range 7.4 to 8.2 at 10 to 30 ^oC and a salinity range 5 to 40. Traceable standard reference solutions characterised with the Harned cell were also established. A comprehensive uncertainty model for spectrophotometric pH_T measurements will be developed with associated software tool(s). It will also improve methods for measuring seawater pH_T by field laboratories, validated by interlaboratory comparison. A revised ISO 18191:2015 will be produced supporting evidence on NMI’s ability to perform high precision pH_T measurements. Results will improve the reliability of pH_T data included in European and international databases, increase confidence in climate trend models and aid the design of more effective remediation measures.