Determination of carbonate ion concentration and inner sphere carbonate ion pairs in seawater by ultraviolet spectrophotometric titration

We describe a novel method for determination of carbonate ion concentration in seawater by acidimetric titration with UV detection. Because CO32− absorbs light at wavelengths of less than ~ 250 nm, it is feasible to titrate most carbonate-containing natural waters with acid and observe an increase in %Transmittance. The observed signal is proportional to the concentration of carbonate ion in the original sample. Present technology permits a theoretical precision in the determination of [CO32−] in natural seawater background of ~ 0.5% (at 10 cm pathlength, 200 μM CO32−, ± 0.0001 AU). The procedure has been tested at 1 and 10 cm pathlengths using single and multipoint titration methods, respectively. Results using natural seawater test solutions indicate a resolution in [CO32−] of 3.6% in a standard 1 cm cuvette using a very simple manual method, and 0.7% using a custom-built 10 cm closed titration cell. Estimates of the relative distribution of CO32− between inner and outer sphere complexes with Mg2+ and Na+ were also determined and the equilibrium constants agree with published values. This method provides a new tool for studies of several fundamental aspects CO2 chemistry, including the second dissociation constant of carbonic acid, CO32− ion pairing, and can be used to directly measure the distribution of carbonate ion in seawater and many other types of natural waters.