Oceanic Alkalinity and titration Alkalinity: a novel straightforward approach in keeping with common general chemistry

We present exact concepts of Oceanic Alkalinity and Titration Alkalinity that are in keeping with common chemistry. Oceanic Alkalinity is the difference between the sum of charges of strong cations and the sum of charges of strong anions in seawater. This difference is compensated by the sum of charges of the weak anions minus the sum of charges of the weak cations. The Titration Alkalinity of a natural seawater sample is defined as the concentration of all bases able to accept a hydrogen ion at the equivalence point of the carbonic acid system. Ideally, acid titration of all weak ions would yield a value of total consumption of added hydrogen ions that then would equate perfectly with the net sum of the charges of the strong ions in Oceanic Alkalinity. However, some adjustments are necessary. At the equivalence endpoint of the titration, hydrogen ions have also been absorbed by small portions of the strong SO 4 2- and F - anions. Additionally, at the endpoint several phosphate anions are not completely neutralized to H 3 PO 4 . Due to this non-ideal situation, there is an exact difference between Oceanic Alkalinity and Titration Alkalinity: Alk Oceanic = Alk Titration + 1.021 [DIP] - 0.0003966 [SO 4 2− ] - 0.010457 [F - ] or in terms of salinity: Alk Oceanic = Alk Titration + 1.021 [DIP] - 0.3409 S where DIP is total dissolved inorganic phosphorus. This exact difference in the order of 12 to 15 μmol.kg -1 is highly significant versus the best feasible ∼ 1 μmol.kg -1 precision of the titration. Finally, because the concentration of Dissolved Inorganic Carbon is determined independently by coulometry, there is no need for non-linear curve fitting of the complete acid titration curve. It suffices to only fit the second part of the titration curve to determine the second equivalence point, yielding a more straightforward, and in principle more precise, determination of Titration Alkalinity.