Equilibrium constraints on chemical activities

Because reactions among ionic species in solution are rapid, thermo-d5mamic calculations are used to constrain the activities of dissolved chemical species at equilibrium. Garrels and Thompson (1962) were the first to calculate the speciation of the major ions in seawater by determining the extent to which each species is involved in ion pairing with each counter-ion. This information is necessary to establish the percentages of free major ions available in chemical equilibrium calculations. This section presents an example of how such multiple equilibrium systems can be constrained. 

The example system consists of ions released by the complete dissolution of 1.00 mol each of MgS04(s) and CaF2(s) in 1.001 of solution at 25 °C and latm of pressure, resulting in concentrations of total Mg, SO4 and Ca of 1 molar, m, and total F of 2 m. The goal is to use 

The other needed constraints can be obtained from equilibrium equations for the formation of each of the four ion pairs  

For example, the equilibrium constant for the formation of [MgSO ] can be determined by entering standard free energies of formation for Mg, MgSO , and SO4, into Eq. (3.12) to obtain the standard free energy of the ion pair formation reaction, and then by using that AG° value in Eq. (3.18) to determine a = 10 . 

of the major seawater ions using the ion pairing model  

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