1 The following is a description
of the chemical equilibrium model and the resulting titration curve shown in the
graph below (figure 1 for pure seawater). The equilibrium model may include the
effect of adding an additional alkali to the seawater (e.g. NaOH).
2 The
titration curve in figure 1 is prepared by using a chemical equilibrium model for
seawater. The model includes inorganic carbon, boric acid, sulphate, fluoride and
dissolved SO2 equilibria; the equilibrium constants are functions of
salinity (ionic strength) and temperature. The apparent pKa values for the
equilibrium reactions are found in general oceanography literature, e.g. An
Introduction to the Chemistry of the Sea, Michael E.Q. Pilson,
Cambridge University Press (2013), and in the publication ʺThe solubility
of SO2 and the dissociation of H2SO3 in NaCl
solutionsʺ, F. Millero, P. Hershey, G. Johnson and J. Zhang, Journal of
Atmospheric Chemistry, 8 (1989). pH is given on the NBS scale.
3
Basis for the computed curve:
-
.1 Released CO2 retained in solution, i.e. no forced stripping of
CO2;
-
.2 10% of dissolved S(IV) oxidized to S(VI) inside EGCS;
-
.3 Seawater alkalinity 2.2 mmol/L;
-
.4 Seawater salinity 35 psu;
-
.5 Seawater pH 8.2; and
-
.6 Seawater temperature 32°C.
4 Fit equation. The fit equation for pure seawater is provided based on an
empirical equation fit to the EM curve. The equation is:
where the variable SO2 is defined
as SO2 absorbed in mmol/kg seawater.
The "fit equation" is used
for the determination of the dilution factor.
Figure 1 – pure seawater titration
curve