Eh-pH Diagrams for Main Anionic Ligands

Sulfur (figure 8.21D) is present in aqueous solutions in three oxidation states (2—, 0, and 6+). The field of native S, at a solute total molality of 10, is very limited and is comparable to that of carbon (for both extension and Eh-pH range). Sulfide complexes occupy the lower part of the diagram. The sulfide-sulfate transition involves a significant amount of energy and defines the limit of predominance above which sulfates occur. 

The extension of the field of native elements (S, C) depends on the adopted 

The above-discussed anionic ligands are the most important ones from the viewpoint of redox properties. For the sake of completeness, we can also consider the redox behavior of halides and boron. Halides stable in water are the simple 

For all the other halides, Eh-pH conditions have no influence. Boron occurs in water mainly as boric acid H3BO3 and its progressive ionization products at increasing pH. Redox conditions do not affect the speciation state of boron. 

Information on the speciation states of solutes and their equilibria with condensed phases furnished by Eh-pH diagrams is often simply qualitative and should be used only in the initial stages of investigations. The chemical complexity of natural aqueous solutions and the persistent metastability and redox disequilibrium induced by organic activity are often obstacles to rigorous interpretation of aqueous equilibria. 

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Figure 8.21 Eh-pH diagrams for main anionic ligands. From Brookins (1988). Reprinted with permission of Springer-Verlag, New York.

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