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

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 releva...

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Bibliographic Details
Published in:Metrologia 2016-02, Vol.53 (1), p.R26-R39
Main Authors: Dickson, A G, Camões, M F, Spitzer, P, Fisicaro, P, Stoica, D, Pawlowicz, R, Feistel, R
Format: Article
Language:English
Subjects:
Aqueous solutions
Bates-Guggenheim convention
Chemical reactions
Climate
Climatology
Dissolution
Hydrogen
hydrogen-ion activity
International System of Units
Ions
Measurement
Metrology
operational definition
potentiometric measurement
Sea water
Seawater
seawater pH
spectrophotometric measurement
State of the art
traceability
Water
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Summary: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.
ISSN:0026-1394
1681-7575
DOI:10.1088/0026-1394/53/1/R26