Pe-pH diagrams

We present the pe/pH diagram for the sulfur system in Fig. 5-3, which represents the sulfur system for a total sulfur concentration of 0.010 M. Note that as sulfur concentration decreases, the curve for pas will displace upward and the curve for pe4 will displace downward. The stability region for S(0) will be decreased and may even vanish. 

Most redox reactions consume or produce protons and the stoichiometry is often such that pe is very sensitive to pH, as the examples in the previous section show. A simple method for determining which species will predominate under particular conditions of pe and pH in an unknown redox system is to construct pe-pH diagrams . This is done as follows. Consider the following redox half reaction involving H+ ... 

Solubility data (pA sp) for two dozen hexacyanoferrate(II) and hexacyanoferrate(III) salts, and Pourbaix (pe/pH) diagrams for iron-cyanide-water, iron-sulfide-cyanide-(hydr)oxide, iron-arsenate-cyanide-(hydr)oxide, and iron-copper-cyanide-sulfide-(hydr)oxide, are given in a review ostensibly dedicated to hydrometallurgical extraction of gold and silver. " The electrochemistry of Prussian Blue and related complexes, in the form of thin films on electrodes, has been reviewed. ... 

D. W. King, A General Approach for Calculating Spedation and Poising Capacity of Redox Systems with Multiple Oxidation States Application to Redox Titrations and the Generation of pe-pH Diagrams, J. Chem. Ed. 2002, 79, 1135. 

Reference to step 4, above as applied to Eq. 3.25a indicates that the boundary line separating Fe3+(aq) from Fe2, (aq) in the pE-pH diagram will be a horizontal one at pE = 13, the maximum value observed in soil systems (see Section 2.2). Thus Fe3+ (aq) is not a likely redox species and can be excluded from the diagram. Equation 3.25e implies that equilibrium between ferric hydroxide and ferrihydrite is possible only at (HzO) t 0.1, regardless of pE or pH. If (HjO) > 0.1, ferrihydrile can also be excluded from the pE-pH diagram. This leaves Hqs. 3.25b, 3.25d, and 3.251 to define (lie stability fields... 

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... 

Brookins, D. G., Eh-pH Diagrams for Geochemistry, Springer-Verlag, Berlin, 1988. A comprehensive pictorial review of pE-pH diagrams for almost all of the Periodic Table, including compilations of the thermodynamic data used to construct the diagrams. 

In chapter 1.1.5.2.3, using the example of the system Fe-02-H20, it was demonstrated that the distributions of species can be determined analytically under different pH- and redox conditions and illustrated in a pE-pH-diagram. In the following examples, the numerical solution will be modeled with the help of PHREEQC. 

The program takes into account that aquatic species are limited in every pE-pH-diagram by the stability field of water. Therefore, the program deletes automatically all pE-pH combinations lying above the line of transformation 02-H20 or below the line of transformation H20-H2. Assistance to the program can be found within the menu HELP. 

Create a pE-pH-diagram of the predominant iron species in a solution, which contains 10 mmol/L Fe and 10 mmol/L Cl. Vary pH and pE values from 0 to 14 and from -10 to +20, respectively, in steps of 1 as well as of 0.5. 

How does the pE-pH diagram created in chapter 3.1.3.1 change when 10 mmol/L S(6) or 10 mmol/L C (4) in solution are taken into account ... 

Fig. 53 pe-pH diagram for the system iron (initial solution 10 mmol Fe + lOmmol Cl) Variation of pE, pH in steps of 1 (above) and 0.5 (below, higher raster resolution, numbers as indicated in the figure above)... 

Relation to pH, 242-257 Electrical conductivity (EC), 80-82 Electrochemistry, 117, 229 Electron activity, 234 See pe-pH diagrams Electronegativity, 9-12 Elemental composition, 103-119 Enthalpy, 224... 

Oxidation kinetics, 292-293 Reduction kinetics, 288 Removal from solution, 443-445 pe-pH diagram, 256,441 Manganese carbonate, 59,433 Manganese oxides, 131 Methane, 257-258, 324 Mica, 102-108 Layer charge, 113 Structure, 115 Molecular Weight, 13,14 Mole fraction, 202 Equivalent fraction, 202 Montmorillonite., 102,104, 109, 123 C-axis spacings, 171 Layer charge, 120 Structure, 171 Composition, 104 Physical properties, 123-124 Chemical properties, 123-124 Muscovite, 104, 123 Structure, 108 Composition, 104... 

Radius ratio of Al3+ and Si4, 118 Rates of reactions, 274 Redox potential in soils, 258 Redox reactions, 229-231 pe-pH diagrams, 245-251... 

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