Project Details pH Measurement in Seawater

Project No.:
Start Date:
01 May 2013
End Date:


Natural seawaters are polyelectrolyte solutions of high ionic strength where ion concentrations of the dissolved substances strongly influence the pH values.

The pH measurement and the study of proton transfer reactions in seawater are some of the metrological critical areas of marine chemistry, in need of establishing traceability of measured pH to the SI, with reduced uncertainty, essential for comparability of the measurements results and their interpretation. This demands generally accepted procedures including calibration standards for pH measurements to examine the proton effect on processes in the oceans.


The complexity of the pH concept leads to problems both with the comparability of the measurements results and with their interpretation. This situation becomes critical, especially when the seawater pH decreases by 0.002-0.003 pH / year due to climate changes. Internationally accepted references for the measurement results have not been established so far.

As compulsory prerequisite to achieve measurement results with associated uncertainties lower than the observed variations in pH generally accepted methods including calibration standards for the measurements of the free concentration or activity of the proton need to be established.

The Project will provide reference procedures and validated measurement methodologies for the determination of seawater acidity establishing traceability to SI units. The Project aims to provide harmonized pH measurement procedures and to underpin the traceability of the pH data of seawater. The development of a primary reference measurement procedure with the potentiometric Harned cell method will lead to a characterization of artificial seawater of reference composition which will be suitable as calibration standard for pH measurements.

Target deliverables are:

  • Provision of thermodynamic data of seawater calculated from the obtained measurement results.
  • Data characterizing synthetic seawater buffer,
  • Provision of a primary reference procedure for pH measurements in seawater.
  • Provision of harmonized and validated pH measurement procedures with respect to potentiometric pH measurements in seawater.
  • Provision of uncertainty budgets according to GUM, with respect to potentiometric and spectrophotometric pH measurement methods in seawater.
  • A good practice guide for pH measurements in seawater.


May 2014 – Project announcement published in Chem. Int. May-June 2014, p. 13; DOI: 10.1515/ci.2014.36.3.13a

April 2017 update – Work has been performed, discussions have taken place and publications have been produced in order to meet the established project milestones.
The task group is planning to meet 24-25 April at BIPM in Paris on the occasion of the CCQM- EAWG meeting, and to review and finalize a draft of a Technical report to submit to PAC. A further meeting at the IUPAC GA, in S. Paulo, in July 2017 will be the forum for retuning the document and plan further developments.

Relevant publications:

• R. Feistel, et al, “Metrological challenges for measurements of key climatological observables: Oceanic salinity and pH, and atmospheric humidity. Part 1: Overview”  METROLOGIA, 53, 1, (2016) pp. R1-R11;

(Part 2: oceanic salinity;

• A.G. Dickson, et al., “Metrological challenges for measurements of key climatological observables: Oceanic salinity and pH, and atmospheric humidity. Part 3: Seawater pH” METROLOGIA, 53, 1 (2016) R26-R39;

(Part 4: atmospheric relative humidity;

• Barbara Anes, Ricardo J. N. Bettencourt da Silva, Hugo F. P. Martins, Cristina S. Oliveira, M. Filomena Camoes, “Compatibility of activity coefficients estimated experimentally and by Pitzer equations for the assessment of seawater pH” Accred Qual Assur (2016) 21;

• Filomena Camões, Bárbara Anes, Hugo Martins, Cristina Oliveira, Paola Fisicaro, Daniela Stoica, Petra Spitzer “Assessment of H+ in complex aqueous solutions approaching seawater”, Journal of Electroanalytical Chemistry 764 (2016) 88–92;

• M. Filomena Camões, Bárbara Anes, Traceability of pH to the mole, Water, 7(8) (2015) 4247-4255;


July 2018 update – Progress towards publication of Technical Report/Recommendation in PAC has been discussed on 16-17 April 2018, with colleagues present at the EAWG/CCQM meeting and at the Division V Meeting in Nara, 25 April 2018. Considering both the pressure from the oceanographic community for recommendations to be issued, and the diverse research lines that are being followed by expert scientists, an attempt is being taken to organize a discussion forum where harmonization of efforts will be debated and sought. The ICPWS 2018, 2nd September in Prague, is an envisaged opportunity.

Feb 2020 update – A Technical Report entitled “Feasibility of multifunction calibration of H+ responsive glass electrodes in seawater”, by D. Stoica, B. Anes, P. Fisicaro, and M. Filomena Camões, has been submitted to PAC aiming at preparing a future recommendation on Measurement of pH in Seawater and the production of a good practice guide for pH measurements in seawater, the final target of the project.
A group meeting at the IAPWS meeting in Prague clarified several hanging issues among the chemical oceanographic community. Discussions continued at the 2019 annual IAPWS meeting at Banff, Canada. As a consequence, and application to the International Association for the Physical Sciences of the Oceans (IAPSO) grant to produce a good practice guide for pH measurements in seawater, was promoted and led by M. Filomena Camões and Andrew Dickson. Submission was successful and a first working group meeting has taken place at Scripps, U California, S.Diego.

Last update 3 Mar 2020