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pH of sea water

Results of different measurement procedures made comparable for the first time

PTBnews 3.2018
02.10.2018
Especially interesting for

oceanography

climate research

In cooperation with the Leibniz Institute for Baltic Sea Research Warnemünde, pH values of average-salinity sea water (5 to 20 g/kg sea water) have been measured at PTB for the first time in such a way that they can be traced to PTB’s primary pH standard. This ensures comparability of pH values measured in the sea, which is indispensable for applications such as the reliable quantification of sea water acidification.

PTB’s measurement setup for the electrochemical measurement of the pHT value. The system contains Harned cells filled with artificial sea water samples whose voltages are measured under temperature-controlled conditions.

CO2 emissions are a problem not only for the global climate but also for the oceans. Carbon dioxide dissolves in seawater, releasing hydrogen ions that lead to acidification. This has an impact on most biochemical and biological processes that take place in sea water; coral growth, for instance, is extremely sensitive to acidification. Long-term measurements in areas such as the North Pacific near Hawaii have shown that the pH value is presently decreasing by approx. 0.003 per year; the uncertainty of the most accurate pH measurements currently available is approx. 0.002.

Moreover, the fact that different definitions of pH exist makes it difficult to compare the measurement results. In metrology (and in its associated industrial applications), the pH value is measured electrochemically. The measurements are based on the fact that an electrode submerged in an aqueous solution generates a measurable voltage compared to a reference electrode; this voltage is determined by the free hydrogen ions present in the solution. This electrochemical procedure allows not only the pH value, but also the so-called pHT value to be measured. The index T stands for “total” and indicates that, in addition to hydrogen ions, hydrogen sulfate ions also contribute to the transfer of hydrogen ions, which is particularly relevant in sea water. In oceanography, an optical method has established itself for the measurement of the pHT value. In this method, a dye with a pH-dependent absorption spectrum is added to the sea water.

Within the scope of a cooperation project between PTB and the Leibniz Institute for Baltic Sea Research Warnemünde (IOW), pHT values obtained by means of the optical method have been linked to the metrologically defined – i.e. electrochemically measured – pHT value for the first time. This took place for the salinity that is typical of the Baltic sea, namely 5 g to 20 g per kilogram of sea water, and at temperatures ranging from 5 °C to 45< °C. After having developed a procedure together with IOW, PTB produced artificial sea water standards against which the pHT reference values were measured with PTB’s primary electrochemical measuring device. Then, IOW applied the optical measurement method in the same temperature range. Combining both values closes the gap in the traceability chain for optical pHT value measurements in the Baltic sea and ensures reliable comparability of the measured values.

In cooperation with PTB’s “Mathematical Modelling and Data Analysis” department, the next step will be to calculate a metrologically sound mathematical relation between the pHT values measured, the salinity and the temperature to make the results available to researchers in the field of oceanography.

Contact

Steffen Seitz
Department 3.1
Metrology in Chemistry
Phone: +49 531 592-3019
Opens window for sending emailsteffen.seitz(at)ptb.de

Scientific publication

J. D. Müller, F. Bastkowski, B. Sander, S. Seitz, D. R. Turner, A. G. Dickson, G. Rehder: Metrology for pH measurements in brackish waters – part 1: Extending electrochemical pHT measurements of TRIS buffers to salinities 5–20. Front. Mar. Sci. 5,176 (2018)