Journal ArticleComparison of open cell and single‐step total alkalinity titration methods and implications for organic alkalinityWoosley, Ryan J [Center for Sustainability Science and Strategy Department of Earth Atmospheric and Planetary Sciences Massachusetts Institute of Technology Cambridge Massachusetts USA] (ORCID:0000000220087751); Bruno, Jessica A [Center for Sustainability Science and Strategy Department of Earth Atmospheric and Planetary Sciences Massachusetts Institute of Technology Cambridge Massachusetts USA] (ORCID:0009000038938844); Neithardt, Daina [Center for Sustainability Science and Strategy Department of Earth Atmospheric and Planetary Sciences Massachusetts Institute of Technology Cambridge Massachusetts USA] (ORCID:0009000281979021); Wang, Zhaohui Aleck [Marine Chemistry and Geochemistry Woods Hole Oceanographic Institution Woods Hole Massachusetts USA] (ORCID:000000020540662X); Fujiki, Nagisa [Department of Marine and Earth Sciences, Marine Works Japan LTD. Yokosuka Japan]; Shigemitsu, Masahito [Research Institute for Global Change Japan Agency for Marine‐Earth Science and Technology Yokosuka Japan] (ORCID:000000018160265X); Murata, Akihiko [Research Institute for Global Change Japan Agency for Marine‐Earth Science and Technology Yokosuka Japan] (ORCID:0000000259312784)<title>Abstract</title> <p>An open question in marine carbon chemistry is if organic alkalinity (or some other unidentified species) is present in non‐negligible quantities in the open ocean. If organic alkalinity is indeed present, different methods for total alkalinity (TA) analysis with different titration endpoints could titrate different amounts depending on the dissociation constants (pK_a) of the acids present, resulting in meaningful differences or offsets between methods. Two commonly used methods, open‐cell titration with non‐linear least squares fitting and single‐step titration with spectrophotometric endpoint detection, might titrate different amounts of organic alkalinity, if present, depending on their pK_a. We test this hypothesis using paired samples collected on two cruises, one in the northwest Pacific and one in the western Arctic, and analyze the TA using both methods. We found the differences to be statistically indistinguishable (∆TA_{[Open‐Cell−Single‐Step]} = 0.5 ± 3.9 <italic>μ</italic>mol kg⁻¹_swmean and standard deviation<italic>N</italic> = 206). Adjustment of the single‐step TA to certified reference material could be obscuring a difference in the methods. The good agreement between methods indicated that the analytical method is not the cause of offsets in Pacific TA identified by the Global Ocean Data Analysis Project version 2. From these results, the presence of organic alkalinity in open ocean waters remains inconclusive but suggests that if present, the concentration is either very low or both methods titrate similar amounts.</p>Association for the Sciences of Oceanography and Limnology, Wiley Periodicals, LLC2025-12-0110672674Limnology and Oceanography: Methods2312960 to 9721541-5856https://doi.org/10.1002/lom3.700022148468; 2342986National Science Foundation