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Spectrophotometric Measurement of Carbonate Ion in Seawater over a Decade: Dealing with Inconsistencies



Elisa Guallart, Noelia Fajar, Maribel García-Ibáñez, Mónica Castaño-Carrera, Rocío Santiago-Domenech, Abed El Rahman Hassoun, Fiz Pérez, Regina Easley, Marta Álvarez


The spectrophotometric methodology for carbonate ion determination in seawater was first published in 2008 and has been continuously evolving in terms of reagents and formulation. Although the method proves to be fast, relatively simple and affordable, giving it the potential to easily be implemented in different platforms and facilities for discrete and autonomous observations, its use is not widespread in the ocean acidification community. This study uses a merged overdetermined CO2 system dataset (carbonate ion, pH and alkalinity) obtained between 2009 to 2020 to assess the differences between the five published approaches by means of an internal CO2 consistency analysis and discussion of uncertainty sources. Overall results show that neither approach meets the climate goal for ocean acidification studies (±1% standard uncertainty) but usually fulfils the weather goal (±10% standard uncertainty). The inconsistencies observed between approaches compromise the consistency of datasets between regions and in time, pointing to the need to develop a validated standard operational procedure as done for the other CO2 variables.
Environmental Science and Technology


ocean acidification, carbonate, spectrophotometry, lead carbonate


Guallart, E. , Fajar, N. , García-Ibáñez, M. , Castaño-Carrera, M. , Santiago-Domenech, R. , El Rahman Hassoun, A. , Pérez, F. , Easley, R. and Álvarez, M. (2022), Spectrophotometric Measurement of Carbonate Ion in Seawater over a Decade: Dealing with Inconsistencies, Environmental Science and Technology, [online],, (Accessed June 20, 2024)


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Created June 7, 2022, Updated February 21, 2023