Reaction path model titration

In the first step, acidic water of about pH 4.5 was first neutralized to a pH of 6.6 by titrating calcite using phreeqc (see 8.3). Neutralization of the acid water caused the precipitation of gypsum, and A1 and Fe hydroxides. This kind of reaction path modeling will be discussed in Chapter 8. Here, we only present the resulting compositions of the neutralized water and precipitated solids in Table 7.8. 

Figure 8.6. Reaction path model of solution composition as potassium feldspar is titrated into rainwater.

Fig. 2.3. Configuration of a reaction path as a titration model. One or more reactants are gradually added to the equilibrium system, as might occur as the grains in a rock gradually react with a pore fluid.

Simple reactants are those added to (or removed from) the system at constant rates over the reaction path. As noted in Chapter 2, we commonly refer to such a path as a titration model, because at each step in the process, much like in a laboratory titration, the model adds an aliquot of reactant mass to the system. Each reactant Ar is added at a rate nr, expressed in moles per unit reaction progress, . Negative values of nr, of course, describe the removal rather than the addition of the reactant. Since is unitless and varies from zero at the start of the path to one at the end, we can just as well think of nr as the number of moles of the reactant to be added over the reaction path. 

Most applications in the regulatory environment have used speciation-solubility models. A few used surface complexation models. Applications of surface complexa-tion models mostly used the model and data from Dzombak and Morel (1990). Reaction path calculations are mostly limited to the titration and mixing calculations of two fluids. 

Many of the original convergence problems of PHREEQE were redox-related. Improvements to the convergence criteria of PHREEQE (noted in the January, 1985 version) have been incorporated in PHRQPITZ. Precautions and comments on the use of ion exchange, and titration/mixing reactions in PHREEQE, and the lack of uniqueness of modeled reaction paths (2, 39) also apply to PHRQPITZ. 

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