Hein J.W. de Baar, Mario Hoppema , Elizabeth M. Jones

We present novel exact concepts of Oceanic Alkalinity and Titration Alkalinity that are in keeping with common chemistry.
Oceanic Alkalinity is the small difference of 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.
Titration Alkalinity is operationally defined as “The Titration Alkalinity of a natural water sample is defined as the concentration of all bases able to accept a hydrogen ion at the equivalence point of the carbonic acid system species.”
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. Unfortunately, some adjustments are necessary. At the equivalence endpoint of the titration, hydrogen ions are also absorbed by small portions of the strong SO42- and F- anions. Additionally, at the endpoint several phosphate anions are not completely neutralized to the neutral H3PO4 state. Due to this non-ideal situation, there is an exact difference between Oceanic Alkalinity and Titration Alkalinity:
AlkOceanic = AlkTitration + 1.021 [DIP] - 0.0003966 [SO42−] - 0.010457 [F-]
or in terms of salinity:
AlkOceanic = AlkTitration + 1.021 [DIP] - 0.3409 S
where DIP is total phosphate. This allows the derivation of Oceanic Alkalinity, that is the pivotal variable to unravel biogeochemical processes in the oceans.
In addition it is shown that upon decomposition of organic matter, the ensuing increase of Dissolved Inorganic Phosphorus does not affect the value of Alkalinity, this resolving the long time confusion in the literature. Finally, because nowadays the concentration of Dissolved Inorganic Carbon is determined independently by coulometry, there is no need anymore for non-linear curve fitting of the complete acid titration curve. Nowadays it suffices to only fit the second part of the titration curve to determine the second equivalence point, this yielding a more straightforward determination of Titration Alkalinity.