Measurements of Dissociation Constants of Carbonic Acid in Synthetic Seawater by Means of a Cell Without Liquid Junction

Abstract

Using the “total hydrogen ion concentration scale”, the first, K 1, and second, K 2, stoichiometric dissociation constants of carbonic acid have been determined in synthetic seawater for temperature and salinity ranges of 0–30 °C and 1.5–40, respectively. The values of the K 1 have been determined potentiometrically by means of a cell without liquid junction, composed using a pH-glass electrode and silver–silver chloride electrode. This cell was equilibrated with CO2 at 1 atm total pressure. The same cell was used for the determination of K 2. Dissolved inorganic carbon was measured by the coulometric method, total alkalinity was known from the preparation of the synthetic seawater, and pH measurements provided the data required for the calculations of K 2. Estimated precision is about ±0.003 pK and uncertainty less than 0.01 pK for K 1 and twice that for K 2. Our K 1 data agree with “best” published data within ±0.01 for log10 K 1 in the 30–40 salinity range obtained for natural seawater, but become progressively higher as the salinity decreases. Our results for the second stoichiometric dissociation constants agree within ±0.015 for log10 K 2 in the 30–40 salinity range with available “best” published data for natural seawater, but become progressively lower as the salinity decreases. Since constants obtained in this paper are reliable for the high salinity range (25–40) and strive to the thermodynamic constants at 0 salinity, they are recommended for study of carbonate system in estuaries.