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Metrology for standardised seawater pHT measurements in support of international and European climate strategies

Short Name: SApHTIES, Project Number: 20NRM06
Image showing a school of fish in a tropical reef in the west pacific ocean
School of fish in west Pacific Ocean

Developing measurements and standards to protect the oceans.

Atmospheric CO2 absorbed by seawater causes acidification as evidenced by a reduction in the amount of total H+ (pHT). 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 pHT measurements but lacks specifications for metrological traceability, uncertainty evaluation and method validation leading to different strategies for pHT measurement uncertainty and reduced data comparability.  
The EMRP project “Ocean” developed pHT methodology based on the Harned cell primary system, linking spectrophotometric pHT measurement to higher order standards. However, comparability of pHT values and the degree of equivalence obtained at different National Metrology Institutes (NMI) using this procedure was not demonstrated.


This project will provide traceable seawater pHT 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 will be also established. A comprehensive uncertainty model for spectrophotometric pHT measurements will be developed with associated software tool(s). It will also improve methods for measuring seawater pHT 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 pHT measurements. Results will improve the reliability of pHT data included in European and international databases, increase confidence in climate trend models and aid the design of more effective remediation measures.



Project website
Other Participants
Helmholtz Zentrum für Ozeanforschung Kiel (Germany)
Institut Français de Recherche pour l'Exploitation de la Mer (France)


in progress