Metrological challenges for measurements of key climatological observables. Part 3: seawater pH

Abstract

Water dissolves many substances with which it comes into contact, leading to a variety of aqueous solutions ranging from simple and dilute to complex and highly concentrated. Of the multiple chemical species present in these solutions, the hydrogen ion, H + , stands out in importance due to its relevance to a variety of chemical reactions and equilibria that take place in aquatic systems. This importance, and the fact that its presence can be assessed by reliable and inexpensive procedures, are the reasons why pH is perhaps the most measured chemical parameter. In this paper, while examining climatologically relevant ocean pH, we note fundamental problems in the definition of this key observable, and its lack of secure foundation on the International System of Units, the SI. The metrological history of seawater pH is reviewed, difficulties arising from its current definition and measurement practices are analysed, and options for future improvements are discussed in conjunction with the recent TEOS-10 seawater standard. It is concluded that the International Bureau of Weights and Measures (BIPM), in cooperation with the International Association for the Properties of Water and Steam (IAPWS), along with other international organisations and institutions, can make significant contributions by developing and recommending state-of-the-art solutions for these long standing metrological problems.

Thermodynamic properties of seawater, ice and humid air: TEOS-10, before and beyond

“…Several textbooks on ice were published in the past, such as those by Dorsey (1968), Hobbs (1974) and Petrenko and Whitworth (1999), offering separate empirical equations for selected properties. The first Gibbs functions for ice Ih were developed by Hagen (1995, 1998) and Tillner-Roth (1998), valid in the vicinity of the melting curve.…”

“…As an urgent problem beyond TEOS-10, seawater pH belongs to the climatological key parameters depicted as metrological challenges by JCS, the successor of WG127 Dickson et al, 2016). For suspectedly disastrous effects on marine ecosystems, the acidification of the oceans quantified in terms of seawater pH values, has become a severe public, ecological and political concern (Feely et al, 2004;Le Quéré et al, 2015) far beyond mere curiosity of a few scientists.…”

Metrology for pH Measurements in Brackish Waters—Part 1: Extending Electrochemical pHT Measurements of TRIS Buffers to Salinities 5–20

“…However, it must be emphasized that effects (1)-(4) do not affect the calibration of pH instruments, if they include the extrapolation of calibrated parameters, e.g., the dissociation constant of mCP , to pure ASW conditions. The extrapolated quantity refers to an exact definition of the total hydrogen ion concentration scale without constraints (Nemzer and Dickson, 2005;Dickson et al, 2015). Therefore, we recommend calibration of pH instruments in the salinity range 5-20 at the three TRIS/TRIS·H + molalities reported in this study and extrapolation of the results to zero TRIS/TRIS·H + molality.…”

“…(1) Changes in the value of K HSO (2) Changes in the activity coefficients of H + and Cl − This limitation was previously discussed (e.g., Nemzer and Dickson, 2005;Dickson et al, 2015) and applies to all currently available experimental pH T measurements of TRIS buffered ASW solutions. The activity changes can only be corrected with Figure 4), pH T = pH T, measured -pH T, model , as a function of salinity (x-axis), temperatures (in K, indicated at the top of the panels), and TRIS/TRIS·H + molalities (color).…”

“…The definition of such concentration scales requires the definition of a standard composition of seawater, because hydrogen ions are in equilibrium with other acid base components in seawater. pH T , where the index T denotes the total pH scale, has become a widely accepted convention within the scientific community (Dickson et al, 2015). According to its definition, pH T = −log 10 H + 1 + .…”