CCQM EAWG Task Group on Metrological traceability for seawater pH and pHT values (CCQM-EAWG-TG-SEAWATER)

Date Established: April 2023

Status: Active

Terms of Reference

The overall goals of the Task Group are to deliver:

• Definition of the metrological traceability for pH and pHT measurement results
• Implementation of the pH and pHT measurements in the CIPM-MRA framework
• Preparation of a liaison between international stakeholder organisations relevant to ocean pH and pHT measurements with CCQM, such as GOA-ON or the GOOS.

Specific goals are:

1. To describe the state of the art of pH and pHT measurements of saline buffers with the Harned cell system. Following aspects will be tackled and further included in a position paper:
   1. Establishment of a complete measurement model.
   2. Evaluation of speciation model(s) for their use in obtaining thermodynamic consistent pH and pHT values instead of operational values.
   3. Quantification of the current assumptions and estimation of an uncertainty associated with proposed conventions
   4. Establishment of an uncertainty budget for Harned cell measurement results, showing the best accuracy level that can possibly be achieved for buffer solutions at NMIs level.
2. The results of existing projects will be used to establish and document the traceability of in-situ and in-lab pHT measurements of seawater, including both potentiometric (such as a glass or ISFET electrodes) and optical (UV-Vis spectrophotometer) devices. A group of experts will be formed to write a key paper on the traceability of pH and pHT of saline standard buffer solutions to internationally agreed reference methods as defined in items 1.
3. To agree upon criteria necessary for the establishment of future CMCs (specific matrix, buffer composition, ionic strength, ranges for pH/pHT) and pave the way for NMIs to deliver robust measurement and calibration services aimed to support quantification and evaluation of changes in ocean acidity e.g. through the provision of internationally consistent standard buffer solutions. The outcome will be implemented in the CMC guidelines of EAWG.
4. To promote the uptake of the metrological concepts by oceanographic community. To this end, the task group will establish contacts with stakeholders responsible for the measurement, storage and quality of pHT measurement results in global observation systems, such as the Global Ocean Acidification Observation Network (GOA-ON), the Global Climate Observation System (GCOS) or the Global Ocean Observation System (GOOS), aiming at persistent links between the communities.

Activities of this task group align with all strategic objectives defined by EAWG. Members of this TG will include representatives from participating NMIs in EAWG as well as external experts having recognized international expertise at both theoretical and practical level.