Metrological needs for traceable oceanic pH measurements
by Dr. Daniela Stoica, Dr. Frank Bastkowski, Dr. Igor Maksimov, Dr. Steffen Seitz, Dr. Jason Waters, Prof. Simon Clegg, Prof. Andrew Dickson, Prof. David Turner

Abstract

The uptake of CO2 by the oceans, and consequent reduction in pH, has ecological, economic, and social consequences that need to be better understood. The effects of CO2 uptake need to be monitored and quantified globally, by cooperation between scientists within a framework of international bodies which support engagement in ocean research across multiple disciplines and over the long time scales needed. The accurate measurement of seawater acidity, and its use in chemical models, is fundamental to evaluating chemical speciation in the oceans and for monitoring and understanding the complex dynamics of the ocean CO2 system and its effects. The operationally defined 'total' pH scale, pHT, is used for measurements in seawaters above 20 salinity (and for freshwaters, a different scale is used). Practical requirements of oceanographers include quantification of the difference between conventional total hydrogen ion concentration and pHT, and the extension of this scale to lower salinities and eventually relating it to the measurement of freshwater pH. The development of chemical speciation models of ion concentrations and activities in both Tris buffers (used to calibrate pHT) and seawater media, with quantified uncertainties, can assist these aims as well as providing an essential tool for societal applications. In order to link the individual measurement results to a common higher order reference or measurement standard, there is a need to assign pHT values to standard saline buffer solutions over a wide range of salinities and temperatures. This requires: (i) the definition of the measurement model of pHT; (ii) the definition of the route of traceability of pHT measured by end-users to agreed primary pHT standards or even to the international system of units (SI); and (iii) the establishment of a metrologically sound uncertainty budget of pHT measured by end-users. Also important is the development of a speciation model of Tris buffers which is likely to assist in making pHT traceable to the SI, by quantifying assumptions inherent in the definition and calibration of the scale. Measurements with Harned cells carried out by the NMIs, for which they have a unique capability, can contribute to these goals. The above aims have begun to be addressed in a collaboration between National Metrology Institutes (NMIs) representative of EAWG within CCQM/BIPM, the Joint Committee on the Properties of Seawater (JCS), and the Scientific Committee on Oceanic Research (SCOR particularly Working Group 145) that combines metrological, oceanographic, and speciation modelling expertise.