Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure

• Main Authors: C. Mehrbach, C. H. Culberson, J. E. Hawley, R. M. Pytkowicx
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1973
• Subjects: Ari; Paren’; Carbonic acid
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.506.9035; http://www.aslo.org/lo/toc/vol_18/issue_6/0897.pdf

The apparent dissociation constants of carbonic acid in seawater were determined as functions of temperature (2-35°C) and salinity ( 19-43%) at atmospheric pressure by measurement of K’1 and the product K’, K’,. At 35sa salinity and 25°C the measured values were pE1 = 6.600 and pK’2 = 9.115; at 35 % and 2°C the measured values were pK’1 = 6.177 and pKPz = 9.431. We have determined the apparent dis-sociation constants of carbonic acid in sea-water at atmospheric pressure as functions of temperature and salinity, because of discrepancies in previous published results. These constants are defined (Lyman 1956) by equations I and 2 (Table 1). The paren-theses represent concentrations in moles per kilogram of seawater, arI is the hydrogen ion activity defined in the NBS buffer scale (Pytkowicz et al. 1966; Pytkowicz 1969), and C02 * is the sum of the concentrations of molecular carbon dioxide and carbonic acid. The use of thermodynamic dissociation constants for the calculation of CO2 species’ concentrations requires estimates of single ion activity coefficients, which are poorly known functions of temperature, salinity, pressure, and ion association. The direct determination of apparent constants in sea-water obviates this problem. Apparent constants depend on temperature, pressure, ion association, and on the free concen-trations of the major ions in seawater (Weyl 1961; Kester and Pytkowicz 1967). They are useful because the concentrations of the major ions are insensitive to changes in pH or to processes such as photosynthesis and the solution and precipitation of car-bonates. Essentially, apparent constants can be used for processes that do not have