The thermodynamics of the carbonate system in seawater

Frank J. Millero

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The apparent constants (K'i) for the ionization of carbonic acid in seawater at various salinities (S,%.) have been fit to equations of the form ln K'i = ln Ki + AiS 1 2 + BiSwhere Ki is the thermodynamic ionization constant in water, Ai, and Bi are adjustable parameters. The temperature dependence (TK) of Ki, Ai and Bi were of the form, a0 + a1/T + a3 ln T. Equations of similar forms have been used to analyze the ionization constants for water and boric acid and the solubility product of calcite in seawater. The effect of pressure on the apparent constants ( Kpi Koi) have been fit to equations of the form ln ( Kpi Koi) = - (ΔVP + 0.5 ΔK P2)/RT where the volume (ΔV) and compressibility (ΔK) changes are polynomial functions of temperature. The equations generated for various açids in seawater have been used to examine the carbonate system in seawater. Equations relating the NBS and Tris pH scales have been derived as well as equations of pH as a function of temperature and pressure. The equations from Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Mehrbach et al. (1973, Limnol. Oceanogr. 18, 897-907) have been used to examine the components of the carbonate system. At a fixed total alkalinity and total carbon dioxide, differences of ±0.01 m-equiv kg-1 in HCO-3 and CO2-3 were found; however, the [CO2] and Pco2 are nearly the same. The contribution of borate ion, B(OH)-4 determined from the equations of Hansson (1972, Ph.D. Thesis, University of Göteborg, Sweden) and Lyman (1957, Ph.D. Thesis, University of California, Los Angeles) differ by ±0.01 m-equiv kg-1 for waters with the same salinity and temperature.

Original languageEnglish (US)
Pages (from-to)1651-1661
Number of pages11
JournalGeochimica et Cosmochimica Acta
Issue number10
StatePublished - Oct 1979


ASJC Scopus subject areas

  • Geochemistry and Petrology

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