The thermodynamics of the carbonate system in seawater

Abstract

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 K i + A iS 1 2 + B iS where K i is the thermodynamic ionization constant in water, A i , and B i are adjustable parameters. The temperature dependence (TK) of K i , A i and B i were of the form, a 0 + a 1/ T + a 3 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 ( K p i K o i ) have been fit to equations of the form ln ( K p i K o i ) = − (ΔVP + 0.5 ΔK P 2)/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 CO 2− 3 were found; however, the [CO 2] and P co2 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.