Programs Designed for Specific Systems

As mentioned in the introduction to this section, there is a second class of programs that solve for chemical equilibrium in pre-defined systems. These contain built-in equilibrium constants, reactions, and sets of species. Many such programs are currently available and a good many of these have been summarized by Nordstrom and Ball (1984). Most of these programs describe chemical equilibria in mineral-water systems. 

We will illustrate this class of programs with one of the earliest, and still most used, examples-the program WATEQ. This was described originally by i ruesdell 

The general algorithm used by this program was designed by Carrels and Thompson (1962) for one of the first calculations of the equilibrium speciation of seawater. We will summarize the original description by Truesdell and Jones (1974) of the numerical method. 

The concentrations or activities of anionic species are calculated first from measured total concentrations and pH and estimated activity coefficients. Using silica as an example, the mass balance on dissolved silica species is 

The mass action or equilibrium constant expressions for ionization of silicic acid, 

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