Major ion speciation

The concentrations of the major inorganic ions in seawater are well known in estuarine and coastal areas as well as in interstitial waters anomalies in their constant ratios may occur. The major cations are Na+, Mg2+, Ca2+, K+ and Sr2+, the major anions Cl-, SCty2-, HCO3", B(OH) ", F" and Br". Ion pairs involving these elements and H+, OH", CO32-, POif3- and SiOz -. Under anoxic conditions the S2- - ion and bi- and polysulphides become important. A summary of the major ion speciation in seawater is given by Kester et al. (1975). 

The species distributions shown in Table 12.2 depict results from one of the earliest (Garrels and Thompson, 1962) and one of the most recent (Millero and Schreiber, 1982) seawater ion pairing models. These results and others (Kester, 1975a) are consistent with the following general characteristics of major ion speciation in seawater ... 

In another example, five test cases were computed by PHREEQE and EQ3/6 and the same thermodynamic database was run for each program (INTERA, 1983) to test for any code differences. The five examples were speciation of seawater with major ions, speciation of seawater with complete analysis, dissolution of microchne in dilute HCl, reduction of hematite and calcite by titration with methane, and dedolomitization with gypsum dissolution and increasing temperature. The results were nearly identical for each test case. Test cases need to become standard practice when using geochemical codes so that the results will have better credibility. A comparison of code computations with experimental data on activity coefficients and mineral solubilities over a range of conditions also will improve credibility (Nordstrom, 1994). 

What is perhaps more important than the comparison of the various speciation models are the main conclusions that result from all major ion speciation models ... 

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