Sliding pH path

Fig. 14.8. Concentrations (mmolal) of sites on a hydrous ferric oxide surface exposed at 25 °C to a 0.1 molal NaCl solution, calculated using a sliding pH path.

FIG. 12.10 Concentrations (znmolal) of surface species on hydrous ferric oxide exposed at 25°C to a solution containing Ca, SO4, Hg, Cr, As, and Zn, calculated using a sliding pH path. 

Fixed-activity and sliding-activity paths (Sections 14.2-14.3) are analogous to their counterparts in fugacity, except that they apply to aqueous species instead of gases. Fixed-activity paths are useful for simulating, for example, a laboratory experiment controlled by a pH-stat, a device that holds pH constant. Sliding-... 

Fig. 14.6. Effect of C02 fugacity on the solubility of calcite (top) and on pH (bottom), calculated at 25 °C using a sliding fugacity path.

We set a 0.1 molal NaCl solution and define a sliding activity path in which pH varies from 4 to 12... 

In a final example of the use of a sliding activity path, we calculate a speciation diagram, plotted versus pH, for hexavalent uranium in the presence of dissolved phosphate at 25 °C. We take a 10 mmolal NaCl solution containing 1 mmolal each ofUO +, the basis species for U(VI), and HPO4... 

FIG. 12.11 Fractions of various components sorbed onto a hydrous ferric oxide surface at 25°C, as functions of pH, for the sliding activity path shown in Fig. 12.10. 

The resulting fluid has a pH of about 8.3. To vary the CO2 fugacity, we set a path in which fco2 slides from the initial atmospheric value... 

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