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Development of a spectrophotometric pH-measurement system for monitoring in the Baltic Sea

01.04.2014 - 31.03.2017
Project manager:
Prof. Dr. Gregor Rehder
Focus 2: Basin-scale ecosystem dynamics
Focus 4: Coastal seas and society

“Ocean acidification” due to increasing atmospheric CO2 levels has become an important issue in chemical and biological oceanography. This refers in particular to the Baltic Sea which in most regions is characterized by a low buffer capacity.

Thus the need for precise, accurate, and traceable pH measurements on a uniform and internationally accepted scale has gained importance during the last decades.

Additionally, investigation and determination of the entire marine CO2 system are useful tools to study biogeochemical processes in the Baltic Sea. However, using pH for the calculations of the CO2 system requires a high accuracy and must refer to the “total” scale which is the basis for the currently best available dissociation constants.

In the Baltic pH-monitoring was carried out during recent decades. Attempts were undertaken to detect trends in pH. But the results were ambiguous and revealed severe inconsistencies.

To meet the demands for both effective monitoring and biogeochemical research, we propose to develop and to construct a flow-through pH measurement device suitable for continuous measurements on platforms such as VOS lines as well as for the measurement of discrete samples. The determination of the pH will be based on spectrophotometry using m-cresol purple as indicator dye. The method has been successfully applied to ocean waters. However, additional investigations are required to adapt spectrophotometric pH measurements to the specific hydrochemical characteristics of the Baltic Sea.

The outcome of the project will be the development of a prototype system for accurate pH-measurements, investigations of chemical parameters and their theoretical and mathematical description and additionally the further hard- and software development.


  • Müller, J. D., B. Schneider, S. Aßmann and G. Rehder (2018). Spectrophotometric pH measurements in the presence of dissolved organic matter and hydrogen sulfide. Limnol. Oceanogr. Meth. 16: 68-82, doi: 10.1002/lom3.10227
  • Müller, J. D., F. Bastkowski, B. Sander, S. Seitz, D. R. Turner, A. G. Dickson and G. Rehder (2018). 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, doi: 10.3389/fmars.2018.00176
  • Müller, J. D. and G. Rehder (2018). Metrology of pH measurements in brackish waters - Part 2: Experimental characterization of purified meta-Cresol purple for spectrophotometric pHT measurements. Front. Mar. Sci. 5: 177, doi: 10.3389/fmars.2018.00177
  • Staudinger, C., M. Strobl, J. P. Fischer, R. Thar, T. Mayr, D. Aigner, B. J. Müller, B. Müller, P. Lehner, G. Mistlberger, E. Fritzsche, J. Ehgartner, P. W. Zach, J. S. Clarke, F. Geißler, A. Mutzberg, J. D. Müller, E. P. Achterberg, S. M. Borisov and I. Klimant (2018). A versatile optode system for oxygen, carbon dioxide, and pH measurements in seawater with integrated battery and logger. Limnol. Oceanogr. Meth. 16: 459-473, doi: 10.1002/lom3.10260
  • Wahl, M., S. S. Covachã, V. Saderne, C. Hiebenthal, J. D. Müller, C. Pansch and Y. Sawall (2018). Macroalgae may mitigate ocean acidification effects on mussel calcification by increasing pH and its fluctuations. Limnol. Oceanogr. 63: 3-21, doi: 10.1002/lno.10608
  • Fritzsche, E., P. Gruber, S. Schutting, J. P. Fischer, M. Strobl, J. D. Müller, S. M. Borisov and I. Klimant (2017). Highly sensitive poisoning-resistant optical carbon dioxide sensors for environmental monitoring. Anal. Methods 9: 55-65, doi: 10.1039/C6AY02949C
  • Müller, J. D., B. Schneider and G. Rehder (2016). Long-term alkalinity trends in the Baltic Sea and their implications for CO2-induced acidification. Limnol. Oceanogr. 61: 1984-2002, doi: 10.1002/lno.10349