The Construction of E-pH Diagrams

In order to construct an E-pH diagram one needs to follow eight basic 

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N. de Zoubov published Atlas d equUibres electrochimiques, a collection of E-pH diagrams for 90 chemical elements. This volume was translated into English in 1966 by J. A. Franklin and published as Atlas of Electrochemical Equilibria in Aqueous Solutions. Subsequently other investigators published computer programs for constructing the diagrams L. Santoma ... 

The eight steps described in the previous section will now be applied to the construction of a Ga E-pH diagram. 

Select an element of interest showing a simple cation or anion and construct the E-pH diagram for the element at a soluble species equilibrium concentration of 10 M. 

Element selection. The Ag E-pH diagram is selected for study because silver exhibits a single soluble cation species the Ag" " ion. The Ag E-pH diagram is then constructed for a dissolved species concentration of 10 4.o using the method discussed in Chapter 2. This diagram appears as Figure 4.1. 

These references have extensive listings of inorganic complexes and their aqueous stabilities. Special note If the complexing agent is one which protonates, the concentrations of the non-protonated agent (as a function of pH) must be calculated. These values are to be used in the construction of the E-pH diagram. 

An E-pH diagram is now constructed using a Pd concentration of lO Then the Pd" " label on the diagram is replaced with the label PdCl4 since this species is in equilibrium with the M Pd" ", but is now the... 

This equation permits the construction of a table which will identify the predominant species at a given Cl concentration, and will facilitate the construction of an E-pH diagram through use of the simple Pd" " concentration in equilibrium with the predominant species. The table will contain columns for [Cl ], each of the terms in the denominator of the above equation, and the resulting [Pd" " ]. The initial [Pd ] is 10 M, so a range ofincreasing [Cl ] values will be used starting with [Cl ] less than the [Pd" " ] at lO" M. See Table 4.2. 

Figure 8.13. Equilibrium concentrations of biochemically important redox components as a function of pe at a pH of 7.0 (a) nitrogen (b) nitrogen, with elemental nitrogen N2 ignored (c) iron and manganese (d) sulfur (e) carbon. These equilibrium diagrams have been constructed from equilibrium constants listed in Tables 8.6a and 8.6b for the following concentrations Cr (total carbonate carbon) = 10 M [HjSCaq)) + [HS ] -I- [SOri = 10 M [NOj-] + [NOj"] + [NH ] = 10 M = 0.78 atm and thus [NjCaq)] = 0.5 x 10 M. For the construction of (b) the species NH4 , NOj, and NO are treated as metastable with regard to Nj.

The construction of a pe-pH or E/ pH diagram is the next step in complexity in the graphical representation of redox equilibria. In such diagrams we establish areas of predominance in a pe-pH coordinate system for various species involved in redox, acid-base, precipitation, and complexation equilibria. 

The E-pH diagram of aluminum and zinc are quite similar and surely amongst the simplest E-pH diagrams of all metals. The Pourbaix diagram of aluminum will be used here to demonstrate how such diagrams are constructed from basic principles. In the following discussion, only four species containing the aluminum element will... 

The next phase for constructing the aluminum E-pH diagram is to consider all possible reactions between the four chemical species containing aluminum retained for this exercise, that is, Al, Al Oj-HjO, AP, and AlO -. These reactions are summarized in Table 4.12. A computer program that would compare all possible interactions and rank the chemical species involved in terms of their thermod5maiiuc... 

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