Production of a reference material for seawater pHT measurements by a National Metrology Institute

Measurements of seawater pH on the total scale, noted pHT, contribute to the assessment of ocean acidification and its impact on the marine environment if a sufficient data quality is achieved. In order to achieve this, reference materials known as TRIS buffers (equimolal buffer of 2-Amino-2-(hydroxymethyl)-1,3-propanediol (TRIS) and TRIS hydrochloride (TRIS.HCl) in artificial seawater of salinity 35) have been developed and com-mercialised in the last decades. This paper describes, for the first time, the production of this reference material according to the ISO guide 35 and ISO standard 17034 guidelines that includes a thorough uncertainty budget evaluation carried out following the Guide to the expression of Uncertainty in Measurement (GUM). Charac-terization with Harned Cells at three temperatures as well as homogeneity and long-term stability assessments were performed and taken into account in the final uncertainty budget. The global standard uncertainty budget of the produced reference material for seawater pHT measurements is of 0.0025, 0.0024 and 0.0022 at, respectively, 15 degrees C, 25 degrees C and 30 degrees C for a shelf life of six months. The level of uncertainty obtained meets the requirements of the climate quality objective requested by the Global Ocean Acidification Observing Network (GOA-ON) for the monitoring of ocean acidification.

Automated summary

This paper describes the production of TRIS buffers as reference materials for seawater pHT measurements according to ISO guidelines. The uncertainty budget evaluation, characterization, and stability assessments were carried out to meet the requirements of the Global Ocean Acidification Observing Network (GOA-ON) for climate quality monitoring of ocean acidification.