Water Pourbaix diagram

Figure C2.8.1. Simplified /pH diagram (Pourbaix diagram) for the iron-water system at 25°C. The diagram is...

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

Fig. 1. Pourbaix diagram for the iron—water system at 25°C, considering as solid substances only Fe, Fe(OH)2, and Fe(OH)2, where (-), line a, represents...

Potential-pH Equilibrium Diagram (Pourbaix Diagram) diagram of the equilibrium potentials of electrochemical reactions as a function of the pH of the solution. The diagram shows the phases that are thermodynamically stable when a metal reacts with water or an aqueous solution of specified ions. 

It is considered useful to include here the potential-pH diagram for some redox systems related to oxygen (Fig. 2.1) [4]. Lines 11 and 33 correspond to the (a) and (b) dashed lines bounding the stability region of water, as depicted in all the subsequent Pourbaix diagrams. 

In the Pourbaix diagram, solid sulfur appears to be stable in a very narrow triangular domain, which lies completely within the stability domain of water. Sulfur is therefore stable in the presence of water and in acid solutions free from oxidizing agents. It is unstable, however, in alkaline solutions, in which it tends to disproportionate to give HS , (and polysulfides), SO , and other oxidation products. In... 

The thermodynamic principles of the Cd-Te-water system are depicted in the Pourbaix diagram of Fig. 3.5 [82]. The corresponding electrochemical reactions of CdTe reduction and oxidation are shown in Table 3.1. 

The redox behavior in the silver-selenium-water system at ambient and higher (85 °C) temperatures was studied by Petrov and Belen kii [163]. According to their voltammetry results, and the presented Pourbaix diagram (Fig. 3.9), the stability... 

Two lines on the Pourbaix diagrams (Figure 5.2) deserve reference. These are the sloping dashed lines, and they represent the stability limits for water they are essentially the domain of thermodynamic stability of water superimposed on potential-pH diagrams. At the upper line, oxidation of water goes according to ... 

It is important to recognize some of the limitations of the Pourbaix diagrams. One factor which has an important bearing on the thermodynamics of metal ions in aqueous solutions is the presence of complex ions. For example, in ammoniacal solutions, nickel, cobalt, and copper are present as complex ions which are characterized by their different stabilities from hydrated ions. Thus, the potential-pH diagrams for simple metal-water systems are not directly applicable in these cases. The Pourbaix diagrams relate to 25 °C but, as is known, it is often necessary to implement operation at elevated temperatures to improve reaction rates, and at elevated temperatures used in practice the thermodynamic equilibria calculated at 25 °C are no longer valid. 

Table 2.1 states the redox relations at standard conditions. Extended information on the distribution of the redox pairs — still under equilibrium conditions but under varying redox potential and pH — is given in a Pourbaix diagram. Figure 2.4 is an example of such a diagram for the binary sulfur and oxygen system in water at 1 atm and 25°C with the sum of the concentrations of... 

FIG. 9. Pourbaix diagrams describing (A) Cd, (B) Te, (C) CdTe, and (D) the underpotential deposition of Cd and Te on CdTe in water. The diagrams were calculated using an activity of 10 M for all soluble species. The hatched areas in (D) represent the differences in potentials, UPD, associated with deposition on CdTe as opposed to deposition on the pure elements. (From Ref. 155.)... 

The Pourbaix diagram for the O2-H2O couple is presented in Figure 7.7, along with the (,J. -pH conditions characteristic of various natural and polluted waters. The equations for the boundary lines are calculated as follows. The redox half reaction that defines the upper boundary is given by Eq. 7.3T Its equilibrium constant is... 

Pourbaix diagram for O2-H2O coupie showing conditions characteristic of various naturai and poiiuted waters. The upper hatched zone represents the stabiiity band for the O2-H2O coupie. 

The ranges of Eh and pH over which a particular chemical species is thermodynamically expected to be dominant in a given aqueous system can be displayed graphically as stability fields in a Pourbaix diagram,10-14 These are constructed with the aid of the Nernst equation, together with the solubility products of any solid phases involved, for certain specified activities of the reactants. For example, the stability field of liquid water under standard conditions (partial pressures of H2 and 02 of 1 bar, at 25 °C) is delineated in Fig. 15.2 by... 

Figure 15.3 Fh-pH (Pourbaix) diagram for the iron-water system at 25 °C for a maximum concentration of 0.001 mol L-1 dissolved iron. The shaded area shows the Fh-pH range of natural waters.

In addition to the above information, Fig. 15.3 includes the standard-state stability field of liquid water itself (broken lines) from Fig. 15.2, as well as the I h-pH ranges found in natural waters (shaded area). The usefulness of the Pourbaix diagram now becomes apparent ... 

Fe2C>3 film to soluble FeC>42, requires a high impressed EMF and is only feasible in alkaline conditions (cf. the Pourbaix diagram of the iron-water system, Fig. 15.3). 

Figure 15.19 A Pourbaix diagram showing the ranges of pH and Eh values in natural waters.

Pourbaix (16) has prepared theoretical stability diagrams of potential vs. pH for many common metals and nonmetalloids. A review of these results indicates that semiconductor compounds of Au, Ir, Pt, Rd, Ru, Zr, Si, Pd, Fe, Sn, W, Ta, Nb, or Ti should serve as relatively acid-stable photoanodes for the electrolysis of water. Indeed, all of the stable photo-assisted anode materials reported in the literature, as of March, 1980 (see Table III) contain at least one element from this stability list, with the exception of CdO. Rung and co-workers (18) observed that the CdO photoanode was stable at a bulk pH of 13.3. The Pourbaix diagram for Cd (16) shows that an oxide film passivates Cd over the concentration range 10.0 < pH < 13.5. Hence the desorption of the product H+ ion for the particular case of CdO must be exceptionally facile without producing an effective surface pH lower than 10.0. This anamolous behavior for CdO is not well understood. 

Figure 7.13 Simplified Pourbaix diagram for iron/water.

Figure 7.19 Simplified Pourbaix diagram for copper/water.

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