Pourbaix diagram for water

Figure 7 E-pH (Pourbaix) diagram for water. Water is stable between lines a and b. ...

Reference to the Pourbaix diagrams for water and copper shows the stable species to be Cu2+, H+ and H2O. Corrosion is possible under these conditions. Let us consider the case of iron metal at —0.750 V vs SCE in a solution of pH 5.0 and a ferrous ion activity of 10 6 at 25°C. The electrode potential with respect to SHE is 0.509 V and reference to Pourbaix diagrams for water and iron shows the stable species to be Fe2+ and H2, and that corrosion is possible under these conditions. 

Figure 6 contains a Pourbaix diagram for water 31 the zones in this diagram are labeled by the formulas for the predominant species at the electrode potential and pH indicated on the axes. Thus the threshold (thermodynamic) potentials for the decomposition of water via ... 

With these two equilibria (eqs. 2.83 and 2.86), the Pourbaix diagram for water can be calculated and drawn (see Table 2.6 and Figure 2.3). The lines are actually equilibrium lines. If water is subject to a potential higher than that on the 02/H20 line (either by an external applied potential or by the action of a strong oxidizer), the most stable species is O ), whereas for a potential below the H20/H2 line (furnished by either an external applied potential or by the action of a strong reducer), the most stable species is H2(g). 

By plotting these two equations, the complete pE vs pH diagram for water is obtained, which is equivalent (with a y-axis displacement) to the Pourbaix diagram for water drawn above. 

Each line of a Pourbaix diagram represents conditions of thermodynamic equilibrium for some reaction. The Pourbaix diagram for water is presented in Fig. 4.1. Above line b, oxygen is evolved in accord with the reaction H2O jOz + 2H -I- 2e. For this equilibrium, the relationship between potential and pH is, from the Nemst equation. 

Figure 4.1. Pourbaix diagram for water at 25 C, showing the oxygen line, b, above which oxygen is evolved, and the hydrogen line, a, below which hydrogen is evolved, from the surface of an immersed electrode. Between these two lines, water is stable. (M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd English edition, p. 100, copyright NACE International 1974 and CEBELCOR.)...

The constmction of the E-pH Pourbaix diagram for water and oxygen is straightforward. Consider the hydrogen evolution reaction at standard conditions (T = 25 °C, Po = 1 atm)... 

Rg. 1 (b) Pourbaix diagram for water-titanium system at 25 °C. Source Ref 4... 

The Pourbaix diagram for water is shown in Figure 5.5. Above the upper dotted line shown in the figure, molecular oxygen is stable, whereas below the lower dotted line, molecular hydrogen is stable. Water is stable between the dotted lines, with protons dominating to the left of the centre line (i.e. pH 7) and hydroxide ions to the right. 

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...

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. 

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.

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.

Fig. 16.1. Simplified Pourbaix diagram for iron in pure water—ionic activities 1(T6 (a) The diagram in terms of most stable species (b) The diagram in terms of the type of reaction that can occur.

Figure 1.7 Pourbaix diagram for the system water-hydrogen-oxygen-hydrogen ion-hydroxyl ion. Activity of water is unity fugacities of hydrogen and oxygen are unity. Temperature 25° C...

Figure 1.9 is the Pourbaix diagram for iron and some of its compounds in an aqueous system at 25°C. The equilibrium potential of the reaction Fe° = Fe2+ + 2e falls outside the stability region of water represented by dashed lines. Hence, measurement of the equilibrium electrode potential is complicated by the solvent undergoing a reduction reaction, while the iron is undergoing electrochemical oxidation. This is the basis of the mixed potential model of corrosion. 

The principle of the cathodic protection may be elucidated for the case of carbon steel. The Pourbaix diagram for iron in water consisting of the plot E (potential) vs pH is shown in Figure 1.68. The regions of passivity, immunity and corrosion are seen in the figure. 

The Pourbaix diagram for platinum [109] is shown in Fig. 9. The domain of stability of water itself lies between the lower broken diagonal line a (below which water can be reduced to hydrogen) and the upper broken diagonal line b (above which water may be oxidised to oxygen). Platinum metal is stable below the lowest full diagonal line which stands for the reaction... 

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