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 commercialised 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). Characterization 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 °C, 25 °C and 30 °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. •First global uncertainty budget for pHT reference material (RM) is presented.•The uncertainty reached meets requirements for the study of ocean acidification.•All steps are performed following international standards for RM production.•Stability and homogeneity are rigorously studied with Harned cell measurements.•The water mass fraction is identified as a key issue for the assessment of pHT.