Diagram predominance

In the preceding sections the phase diagrams have been represented in terms of composition. Alternatively, the chemical potential of one or more of the components may be used as variables. This gives rise to a range of similar diagrams that 

The stability fields for the condensed phases correspond to F = 2, which means that both temperature and the partial pressure of 02 can be varied independently. 

For a binary oxide like Fe203 the reaction in question is 

Assuming that the metal and oxygen are in their standard states, the equilibrium constant corresponding to reaction (4.50) is given as 

If the oxygen partial pressure is lower than Pq2 = 1IK the reactant (in our case Fe) is stable. If it is higher, the product (in our case FC2O3) is formed. 

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As an additional example, reference is drawn to oxide systems in which one of the peer examples is shown in Figure 4.4 (A) which depicts the predominance diagram for oxides of iron in contact with C0/C02 mixture. It can be well used to obtain the answer of (i) consequence of contacting Fe203 with a gas mixture such that the composition and temperature correspond to the point of X (ii) the minimum temperature required at which reduction of Fe304 with C0/C02 mixtures causes wustite (FeO) to form and (iii) the reaction product expected to form first as a result of oxidation of iron with C02 at 1200 K. The Figure 4.4 (A) is very often studied with a line superimposed to indicate the equilibrium in reaction... 

Figure 4.4 (A) Predominance diagram of iron and its oxides in contact with carbon monoxide/dioxide...

Figure 4.25 log pSQ2 versus log p0 predominance diagram for the system Fe-S-O at 527 °C. Thermodynamic data are taken from reference [21]. 

FIGURE 13.32. A predominance diagram of silver and palladium species as a function of pH. 

Fig. 2. Predominance diagram of the uranium system showing the influence of pe and pH in the system in the absence of carbonate. In brackets are the aqueous species in equilibrium with U02(s). (Database taken from Grenthe et al. (1992) and Silva el al. (1995).)...

The Eh-pH predominance diagram for Asv and As111 species is shown in Fig. 5.9. The upper and lower boundaries represent the stability field for water. 

Figure 5.10 The Eh-pH predominance diagram for Sevl, Selv, Se° and SeHI species.

The FACTSage thermochemical database was used to identify the thermodynamically stable phases that could exist in a system comprised of a pure metal, oxygen, HC1 and Cl2 at 500°C (Suppiah, 2008). The predominant Fe, Ni, Cu and Cr phases in an 02/HCl/Cl2 environment were determined. The equilibrium reaction boundary was plotted as a function of the partial pressures of 02 and HC1, for a constant Cl2 partial pressure. The resulting predominance diagrams were plotted over an 02 and HC1 partial pressure range of 10-20 to 1 atm for Cl2 partial pressures between 10-6 and 1 atm. The predominant Ni and Cr species are solids, suggesting that a corrosion resistant protective layer could be formed on the metal. 

These three equations are merely algebraic combinations of Eq. 3.18. Inspection suggests that the activities of H20, Mg2+, Fe3, and H4Si04 are candidates for the dependent aqueous-phase variable in a predominance diagram. To preserve comparability with Fig. 3.5, (H,Si04) will be chosen, with the other three activities fixed as before. These choices reduce the general log K equations... 

Fig. 3.7. Predominance diagram for the same set of solid phases as in Fig. 3.5. At pH 5, the sequence of stability with increasing silicic acid activity is gibbsite -> kaolinite - smectite.

Equilibria involving reductive dissolution reactions add to the complexity of mineral solubility phenomena in just the way that pE-pH diagrams are more complicated than ordinary predominance diagrams, like that in Fig. 3.7. The electron activity or pE value becomes one of the master variables whose influence on dissolution reactions must be evaluated in tandem with other intensive master variables, like pH or p(H4Si04). Moreover, the status of microbial catalysis under the suboxic conditions that facilitate changes in the oxidation states of transition metals has to be considered in formulating a thermodynamic description of reductive dissolution. This consideration is connected closely to the existence of labile organic matter and, in some cases, to the availability of photons.26... 

For an introduction to the two types of diagram, see, for example, Chaps. 5 and 6 in G. Sposito, The Chemistry of Soils, Oxford University Press, New York, 1989. Activity-ratio and predominance diagrams are discussed with many examples in Chaps. 5 and 7 of W. Stumm and J. J. Morgan, Aquatic Chemistry, Wiley, New York, 1981... 

Prepare activity-ratio and predominance diagrams for the Al(III) minerals whose dissolution reactions are described in Eqs. 3.18-3.20. Set pH = 6 for the activity-ratio diagram, but otherwise use fixed activity data as given in connection with Figs. 3.5 and 3.7. Repeat your calculations for (H20) = 0.5 instead of unit water activity. What is the effect on mineral stability (Answer. 

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