Carbonate equilibria calculating the pH of seawater

We have now described the system of equations necessary for determining the pH of seawater and the distribution of carbonate species. By including the definition and numerical value of the alkalinity to the system of equations used to determine the curves in Fig. 4.2, we have constrained the location on the plot to a single pH. The equations necessaiy to determine this location are summarized in Appendix 4.1 for the progressively more complicated definitions using the three forms of the alkalinity, Ac, Ac b. and Aj. 

In order to solve the equations and determine pH and the concentrations of the species that make up the alkalinity, the apparent equilibrium constants, F, must be accurately known. These constants have been evaluated and re-evaluated in seawater over the past 50 y. The pH scales and methods of measuring pH during these experiments have been different, and this has complicated comparisons of the data until recently, when many have been converted to a common scale. Equations for the best fit to carbonate system equilibrium constants as a function of temperature and salinity are presented by Luecker et al. (2000), DoE (1994) and Millero (1995) (see Appendix 4.2). 

The pH and carbonate species distribution for waters from different locations in the ocean (Table 4.4) are calculated by using data for At and DIG and the equilibrium constants. The equilibrium equations were solved with the computer program of Lewis and Wallace (1998) using the carbonate equilibrium constants fCj and K 2 of 

Column (I)° is the calculation utilizing all species in the total alkalinity, Aj. Ai andAp (bottom row) are the alkalinities due to silicate and phosphate species. Column (11) is the calculation assmning the total alkalinity does not include Si and P species, Ax=Ac b- Concentrations and Die are in units of pmol kg and alkalinity values, Aj, are in peq kg .  

Parameter Surface Water Water Water Water  

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