Measurements of total carbon dioxide and alkalinity by potentiometric titration in the GEOSECS program

Approximately 6000 determinations of the alkalinity and total carbon dioxide content of seawater have now been made in the Atlantic, Pacific and Indian Oceans as part of the GEOSECS program by a computer-controlled potentiometric titration technique. The equations used to locate the equivalence points of the carbonic acid system on this titration curve were developed in 1971 but have not previously been published. These functions may be represented by:$\begin{array}{l}{F}_1=\frac{\left(V_2?V\right)}{V_0}N{H}^{+}]/K_{1C}+\frac{\left(V_0+V\right)}{V_0}\left(H^{+}]+HS{O}_4^{?}]+HF]?B{\left(OH\right)}_4^{?}]\right)\\ \times \left(1+H^{+}]/K_{1C}\right)\\ F_2=\frac{\left(V_0+V\right)}{V_0}\left(H^{+}]+HS{O}_4^{?}]+HF]?HC{O}_3^{?}]\right)\end{array}$Upon inspection, these functions are analogous to the modified Gran functions of Hansson and Jagner 25] with the omission of the contributions of OH^?] and CO₃^2?], and with the contribution of B(OH)₄^? being assessed at a chlorinity of 19‰ for all samples. Reprocessing the original titration e.m.f.-volume data with appropriate corrections and modified Gran functions reveals an error of about +12 μmol/kg in the GEOSECS total carbon dioxide data. In addition, the protonation of dissolved phosphate species during the titration results in a contribution to measured total carbon dioxide equal to the total phosphate concentration. Differences in the application of the GEOSECS functions between the Atlantic and the Pacific-Indian Oceans expeditions are also to be found so that the error deriving from this source for the Atlantic expedition was only +5 μmol/kg. The application of the correct functions increases precision enabling smaller differences, such as those attributable to fossil fuel carbon dioxide, potentially to be observed, and increases accuracy so that the error in titrator total carbon dioxide previously diagnosed by Takahashi 14] can be logically accounted for.