Equilibrium electrode potentials Pourbaix diagram

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. 

If we multiply eq. (8-31) by —3 and add it to eq. (8-32), we can see that if d G2 — 3 d <0, then a F6304 surface layer will be stable. However, if d G2 — 3 d Gj > 0, then iron will be in equilibrium with Fe " ions in aqueous solution. By choosing a suitable standard state, we can relate the free energies d G to the equilibrium electrode potentials by the equation d G = — zFEn i. Since the d G are functions of the chemical potentials jU,- of the metal ions and of the hydrogen ions, the equilibrium lines in the Pourbaix-diagram will be dependent upon p. ... 

A simplified Pourbaix diagram for the zinc electrode at pH between 0 and 14 is shown as an example in Fig. 3.1. The vertical axis is that of the values of electrode potential on the SHE scale for activities of the Zn + and HZnO ions of 1 mol/kg. The segments of solid lines correspond to the equilibrium potentials of the following electrode reactions ... 

The thermodynamic information is normally summarized in a Pourbaix diagram7. These diagrams are constructed from the relevant standard electrode potential values and equilibrium constants and show, for a given metal and as a function of pH, which is the most stable species at a particular potential and pH value. The ionic activity in solution affects the position of the boundaries between immunity, corrosion, and passivation zones. Normally ionic activity values of 10 6 are employed for boundary definition above this value corrosion is assumed to occur. Pourbaix diagrams for many metals are to be found in Ref. 7. 

Interpretation of the Pourbaix diagram in Fig. 2.11 requires discussion of the experimental conditions under which, at least in principle, it would be determined. The coordinates are pH and electrode potential, and it is implied that each of these may be established experimentally. Their values will locate a point on the diagram, and from this point the equilibrium state of the system is determined. It is assumed that the pH may be established by appropriate additions of an acid or base. 

Formation of ions during equilibrium electrochemical reaction depends on the pH of the solution and electrode potential. The relationship between electrode potential and pH of the solution can be represented by a phase diagram that is known as the Pourbaix diagram [5]. If a metal is made anodic in an aqueous solution, several reactions can occur depending on the change in free energy. For example, if zinc is made anode in water, the following possible reactions may take place ... 

Thus the stability of the passive fihn depends on two parameters, the electrode potential and the pH value. Pourbaix developed special diagrams of stabUity regions of oxides on metal surfaces as function of electrode potential and pH value. The diagrams were calculated from thermodynamic equilibrium values for selected reactions between the metal and aqueous electrolyte. A Pourbaix diagram for iron is shown as example in Figure 10.11 (Kaesche ). 

The main objective of this chapter is to introduce students to one of the most important subjects of the book, equilibrium electrochemistry, which is mainly based on equilibrium thermodynamics. Equilibrium electrochemistry is usually the first and required step in analyzing any electrochemical system. How to estimate the equilibrium potential of a half-reaction and the electric potential difference of an electrochemical cell are described in this chapter. One of the most fundamental equations of electrochemical science and engineering, the Nemst equation, is introduced and anployed for composing the potential-pH (Pourbaix) diagrams. Temperature dependence of the electrode potential and the cell potential difference is also described. 

In the lead-acid battery, sulfuric acid has to be considered as an additional component of the charge-discharge reactions. Its equilibrium constant influences the solubility of Pb and so the potential of the positive and negative electrodes. Furthermore, basic sulfates exist as intermediate products in the pH range where Figure 6.1 shows only PbO (cf. corresponding Pourbaix diagrams in Ref. [5], p. 37, or in Ref [11] the latter is cited in Ref. [8]). Table 6.2 shows the various compounds. 

Pourbaix plotted electrochemical equilibrium diagrams of metals in water as a function of the potential E with respect to the hydrogen electrode, and as a function of pH (Figure B.1.10). Several domains can be identified in these diagrams corrosion, passivation and immunity (see Section B.1.6). 

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