Metrological Challenges for pH Measurements in Natural Waters

Frank Bastkowski1, Steffen Seitz1, Andrew G Dickson2, Daniela Stoica3 and Jens Daniel Müller4, (1)Physikalisch-Technische Bundesanstalt, Department of Physical Chemistry, Braunschweig, Germany, (2)Scripps Institution of Oceanography, UCSD, La Jolla, United States, (3)Laboratoire national de métrologie et d’essais, LNE, Department of Biomedical and Inorganic Chemistry, Paris, France, (4)Leibniz Institute for Baltic Sea Research, Department of Marine Chemistry, Rostock, Germany
Abstract:
Measurement of pH in the ocean plays an important role for the quantification of acidification of the ocean due to anthropogenic CO2 uptake. Although frequently performed for a long time, pH measurements in the ocean are still challenging. Mostly, pHT (pH on the total scale) measurements of seawater are performed spectrophotometrically.

Recently, traceability to a higher order pHT standard has been established in the salinity range 5-35 allowing well-constrained spectrophotometric pHT measurements. Linking the results to the International System of Units (SI) is however still an unresolved challenge. Moreover, uncertainties are – if stated at all – often only roughly estimated in practice leading to questionable results. Even if performed with properly calibrated setups, measurements can lead to erroneous results due to conceptual limitations. Two examples are: (1) The buffer system of the calibration standards affecting the composition of the artificial seawater (ASW) matrix particularly at low salinities is one reason for erroneous results, because it implies that the composition of the sample solution will always differ from that of the calibration solution. Hence a well-adapted calibration strategy is particularly important. (2) Natural variations of the sample composition on pHT determination is another influence. This is particularly important in brackish waters like the Baltic Sea due to large variations in their ionic composition. First steps towards a quantification of this problem by the help of Pitzer models are under way.

It is particularly challenging to determine the pH of seawater according to its IUPAC definition that is based on the hydrogen activity a(H+): pH = - log a(H+). While determination of pH is well established for low ionic strength buffer solutions (≤ 0.1 mol/kg), determination of pH of highly saline seawater is still a matter of investigation due to challenges related to the high ionic strength of seawater. In our talk, we will discuss these metrological issues of current pH measurements of seawater.

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