Example Applications to Concentrated Brines

One of the first applications of the Pitzer equations to highly complex natural systems were made by Weare, Harvie, and their associates on marine evaporites (Harvie and 

1980 Eugster et al., 1980 Harvie et al., 1982, 1984 Brantley et al., 1984 Weare, 1987). These calculations, coupled with the free energy minimization method for computing chemical equilibria (described in Chapter 19) successfully predicted the mineral assemblages formed during the evaporation of seawater to almost complete desiccation. It is difficult to conceive of a more complicated test of the Pitzer model. Remember that these calculations represent a prediction from experimental data on systems containing no more than two different salts at a time. 

To illustrate the use of the equations, we will take the simpler problem of the solubility of anhydrite in a concentrated NaCl solution. In natural solutions, such as evaporitic brines, you can analyze the total Ca content and the total sulfate, along with everything else, but to compare the ion activity product (ftca + so ) solubility product in order to determine if the solution is over-saturated or under-saturated, you need the activity coefficients of Ca and SO4 . This is where Pitzer equations come in. Let s say we want to know if anhydrite is over- or undersaturated in a solution 3m in NaCl and 0.01m in CaS04. For the activity coefficient of Ca + in this solution, equation (17.41) becomes 

The equation giving ln7jQ2- (17.42) looks quite similar. Filling in the values of the parameters from Table 17.5 and solving, we find 7ca2+ = 0.3499,750 - = 0.03190, 

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