Eh-pH diagram

Eh-pH diagram for the speciation of plutonium in equilibrium with Pu02 in water (10). 

D. G. Brookins, Eh-pH Diagrams for Geochemistry, Springer-Verlag, New York, 1988. 

Manning, G.D. Melling, J. "Potential (Eh)-pH Diagrams at Elevated Temperatures - A Survey", Warren Spring Laboratory LR 128(ME), 1971. 

Blum JS, Bindi AB, Buzzelli J, Stolz J, Oremland RS (1998) Bacillus arsenicoselenatis, sp. nov., and Bacillus selenitireducens, sp. nov two haloalkaliphiles from Mono Lake, California that respire oxyanions of selenium and arsenic. Arch Microbiol 171 19-30 Brookins DG (1988) Eh-pH Diagrams for Geochemistry. Springer-Verlag, New York... 

Figure 2. Eh-pH diagram of dissolved Mo speciation in the system M0-H2O-S. ZMo = 10 M ES = 10 M. Modified after Manheim and Landergren (1974), using molybdate protonation constants from Baes and Mesmer (1986). H2M0O4 is related to Mo(OH)g by addition of two water molecules (see text). MoO +, included in earlier Eh-pH diagrams, is omitted because this and other Mo(V) species are typically unstable except as dimers (e.g., Mo20/ ) at higher EMo than common in natiwe. Speciation at Eh below fiie SO/ - H2S ftansition is not well characterized and is commonly out of equilibrium. The boundary between MoS/ and MoO/ is based on Erickson and Helz (2000) intermediate oxythiomolybdates are metastable and hence not indicted.

Brookins DG. 1997. Eh-pH diagrams for geochemistry. Berlin Springer-Verlag. 

Figure 8.20 Eh-pH diagram for the Ce-H20 system (modified from Pourbaix, 1966). Figures on limiting curves are base 10 logarithms of solute activity unitary activity (i.e., one-molal solution) is identified by zero.

The main usefulness of Eh-pH diagrams consists in the immediacy of qualitative information about the effects of redox and acid-base properties of the system on elemental solubility. Concerning, for instance, cerium, figure 8.20 immediately shows that, within the stability field of water, delimited upward by oxidation boundary curve o and downward by reduction boundary curve r, the element (in the absence of other anionic ligands besides OH groups) is present in solution mainly as trivalent cerium Ce and as soluble tetravalent hydroxide Ce(OH)2. It is also evident that, with increasing pH, cerium precipitates as trivalent hydroxide Ce(OH)3. 

The interpretation of Eh-pH diagrams implies assumption of complete equilibrium among the various solutes and condensed forms. Although this assumption is plausible in a compositionally simple system such as that represented in figure 8.20, it cannot safely be extended to more complex natural systems, where the various redox couples are often in apparent disequilibrium. It is therefore necessary to be cautious when dealing with the concept of the system Eh and the various redox parameters. 

Table 8.16 Standard molal Gibbs free energies of formation from the elements for aqueous ions and complexes and condensed phases, partly adopted in constructing the Eh-pH diagrams in figure 8.21. Data in kcal/mole. Values in parentheses Shock and Helgeson s (1988) tabulation. Sources of data (1) Wagman et al. (1982) (2) Garrels and Christ (1965) (3) Pourbaix (1966) (4) Berner (1971)...

Figure 8.21C shows the Eh-pH diagram for phosphorus at a solute total molality of 10 ". Within the stability field of water, phosphorus occurs as orthophos-phoric acid H3PO4 and its ionization products. The predominance limits are dictated by the acidity of the solution and do not depend on redox conditions. 

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

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. 

Figure 8.22A shows the Eh-pH diagram of iron in the Fe-O-H system at T = 25 °C and P = 1 bar. The diagram is relatively simple the limits of predominance are drawn for a solute total molality of 10 . Within the stability field of water, iron is present in the valence states 1+ and 3-I-. In figure 8.22A, it is assumed that the condensed forms are simply hematite Fe203 and magnetite Fe304. Actually, in the 3-1- valence state, metastable ferric hydroxide Fe(OH)3 and metastable goe-thite FeOOH may also form, and, in the 1+ valence state, ferrous hydroxide Fe(OH)2 may form. It is also assumed that the trivalent solute ion is simply Fe ", whereas, in fact, various aqueous ferric complexes may nucleate [i.e., Fe(OH), Fe(OH)2+, etc.]. 

Figure 8,22 Eh-pH diagrams for iron-bearing aqueous solutions. Chemical complexity increases from A to F.

Reaction 8.220 takes place in the silicate bands present in iron ores iron silicates occur in such bands in coexistence with opaline amorphous silica (Klein and Bricker, 1972). A complex Eh-pH diagram for the Fe-C-S-O-H system is shown in figure 8.22E. The stability field of pyrrhotite FeS disappears and is replaced by... 

The predominance limits shown in figure 8.22 are analytically summarized in table 8.17. Compare figures 8.22 and 8.21 to better visualize the redox state of the anionic ligands at the various Eh-pH conditions of interest (particularly the sulfide-sulfate transition and carbonate limits). We remand to Garrels and Christ (1965) for a more detailed account on the development of complex Eh-pH diagrams. 

Figure 8.23A shows a simplified Eh-pH diagram for the Mn-O-H system. Within the stability field of water, manganese occurs in three valence states (2+, 3 +, and 4+). Figure 8.23A shows the condensed phases relative to the three valence states as the hydroxide pyrochroite Mn(OH)2 (2+), multiple oxide haus-mannite Mu304 (2+, 3 + ), sesquioxide Mu203 (3 + ), and oxide pyrolusite Mn02 (4+). 

Fig.1. Eh-pH diagram for the system Fe-U-S-C-H2O at 25 °C showing the mobility of uranium under oxidizing conditions, the relative stability of iron minerals, and the distribution of aqueous sulfur species. Heavy line represents the boundary between soluble uranium (above), and insoluble conditions (below), assuming 1 ppm uranium in solution.

Fig. 2. Eh-pH diagram for the system K-U-V-C-S-H2O at 25 °C showing the stability of secondary carnotite.

Fig. 1 Eh-pH diagram for the sulfur-water system at 298 K with concentrations of 0.2 mM for S(—II) species and 1 pM for polysulfide species [45].

Figure 8 The Eh/pH diagram for the U/02/C02/H20 system at 25 °C with a partial C02 pressure of 10 2bar.27 Solid-solution boundaries are drawn at 10-6 M dissolved uranium species U02am = amorphous U02...

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