Pourbaix diagrams some examples

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reaction rates, and at elevated temperatures used in practice the thermodynamic equilibria calculated at 25 °C are no longer valid. 

C. Selected Applications. Some brief examples of the types of uses that can be made of thermodynamics in electrometallurgy are given. Pourbaix diagrams, which have received rather extensive use, are excluded since they are discussed in greater detail in other papers that are a part of this symposium. 

Pourbaix diagrams have been drawn not only for elements but also for certain compounds [115], especially ones of geological and industrial importance. Metal sulphides have attracted particular attention [115-118] and Fig. 10 shows a recently published diagram for lead sulphide. Its narrow and pH-dependent stability region, bordered by the stability domains of the sulphate and of the metal, is typical for many sulphides. Not surprisingly, metal sulphides may falsely appear to catalyse or inhibit some solution reactions. For example, the rate of the redox reaction... 

The electrolyte type (acidic, neutral, or basic) should be selected, so that the semiconductor of interest does not corrode when immersed in the solution. Some general guidance in electrolyte selection can be obtained from Pourbaix diagrams. As an example, the Pourbaix diagram of WO3 is presented in Fig. 3.8. 

Pourbaix diagrams are useful, but do not tell the whole story, there are some limitations. Diagrams are made considering equilibrium among metal, water, and reaction products. The presence of other ions, for example, chloride, may result in very much different behavior and large molecules in the body may also change the situation. Prediction of passivity may in some cases be optimistic, since reaction rates are not considered. 

Pourbaix. These equilibrium diagrams relate the reversible potentials of reactions of interest in corrosion studies to the pH and the concentration of different ionic species in solution. We shall use a number of examples to illustrate the principles involved, starting with the most basic diagram relating to water and some of the ionic and molecular species at equilibrium with it. 

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