Equilibrium potentials, lead oxides

A survey of the thermodynamic situation is provided by so-called Pourbaix diagrams [10], which show equilibrium potentials versus the pH value. Figure 1 shows such a diagram for lead and its oxides in a very simplified form that considers only the standard concentrations of the dissolved components. The complete diagram contains a great number of parallel lines that express the various concentrations. 

PbO and Pb304 are oxides of lead that are important primary products. Under certain circumstances, PbO is also observed in lead-acid batteries (cf. Sec. 4.4.5.1). Fig. 1 shows that these oxides are only stable in a neutral or alkaline environment. Their equilibrium potentials are represented by curves C-F and their standard values are compiled in Table 3. 

Oxidation potentials lead to a value of 7.9 x 10 for the equilibrium constant. Kinetic data for the reaction (from 0 to 55.6 °C) in acid perchlorate solutions (over the range 0.047-1.0 M) have been obtained spectrophotometrically by following the disappearance of V(V) (which absorbs strongly between 305 and 350 m/i) as a function of time. The second-order nature of the rate law... 

Thus films can be divided into two groups according to their morphology. Discontinuous films are porous, have a low resistance and are formed at potentials close to the equilibrium potential of the corresponding electrode of the second kind. They often have substantial thickness (up to 1 mm). Films of this kind include halide films on copper, silver, lead and mercury, sulphate films on lead, iron and nickel oxide films on cadmium, zinc and magnesium, etc. Because of their low resistance and the reversible electrode reactions of their formation and dissolution, these films are often very important for electrode systems in storage batteries. 

Except for the electrochemical reaction (2.6), all other reactions depend on the pH of the solution. A number of electrochemical reactions proceed in this system, which form different electrode systems, depending on lead ion valency, solution composition and pH, and electrode potential. These reactions cover a potential range of 2.0 V. Table 2.4 summarises the electrochemical reactions involving Pb, lead oxides, PbS04 and basic lead sulfates, and the equilibrium potentials of the respective electrode systems. The reactions and the equilibrium potentials for the hydrogen and oxygen electrodes are also given in the table. Several chemical reactions in which basic lead sulfates take part are also included in Table 2.4. 

Above the equilibrium potential of the positive electrode, lead is oxidized to... 

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