Comparison of open cell and single-step total alkalinity titration methods and implications for organic alkalinity

Abstract: 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 μmol kg⁻¹_sw mean and standard deviation N = 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.