Equilibrium in Electrochemical Systems

Electrochemical potential — (SI unit J mol-1) The notion introduced by -> Butler [i] and Guggenheim [ii] for consideration of equilibria in electrochemical systems with participation of charged species on the basis of the relationship for the electrochemical -> Gibbs energy G ... 

Holze R, Lechner MD (to be published in 2014) Electrochemistry, Subvolume B Electrical conductivities and equilibria of electrochemical systems. Springer, Berlin, Heidelberg... 

Perchloric acid is an extremely strong acid in aqueous solution (see Table 7.3). Although [ 104] (Fig. 17.12b) does form complexes with metal cations, the tendency to do so is less than for other common anions. Consequently, NaC104 solution is a standard medium for the investigation of ionic equilibria in aqueous systems, e.g. it is used as a supporting electrolyte in electrochemical experiments (see Box 8.2). Alkali metal perchlorates can be obtained by disproportionation of chlorates (eq. 17.76) under carefully controlled conditions traces of impurities can catalyse decomposition to chloride and O2. Perchlorate salts are potentially explosive and must be handled with particular care. For example, solid NH4CIO4 decomposes at 298 K according to eq. 17.79, and mixtures of ammonium perchlorate and aluminium are standard missile propellants. 

Electrochemical corrosion of metals Since the aggressiveness of salt melts is governed by redox equilibria, and is often controlled by composition of the external atmosphere, effects analogous to electrochemical or oxygen-concentration corrosion in aqueous systems can occur in salt melts. Tomashov and Tugarinov determined cathodic polarisation curves in fused chlorides and concluded that the cathodic reactions of impurities could be represented as ... 

Side reactions (and side reaction coefficients) — Reactions that accompany a main reaction or electrochemical system. Example When the redox system Fe(II)/Fe(III) in aqueous solutions is studied, the following redox equilibrium is the main reaction Fe3+ + e" Fe2+. Since the standard potential of that system relates to the two aqua complexes [Fe(Fl20)6]3+ and [Fe(H20)6]2+, a number of possible side reactions (in fact equilibria ) have to be taken into account the Fe(III) hexaqua complex is prone to acid-base reactions according to [Fe(H20)6]3+ + H20 [Fe(H20)50H]2+ + H30+)etc.)... 

The thermodynamic equilibrium constants of all equilibria in the chromatographic system can be derived from basic thermodynamic considerations. When the charge status of the analyte and HR are the same, or if the analyte is neutral, ion-pairing equilibria do not apply. For the condition of equilibrium, it holds that the electrochemical potentials (ju) of species in the stationary and mobile phases are equal ... 

Conformational equilibria involving rotations around single bonds are usually too fast to be detected by electrochemical techniques. However, when steric interactions are increased, usually brought about by the restrictions introduced in cyclic systems, the conformers may be detected by differences in the electrochemical response. A classic example of this behaviour involves apparently slow electron transfer to cyclo-octatetraene (COT) to form first the radical anion and then the dianion (Allendoerfer and Rieger, 1965) as in (32). The... 

Several experimental methods based on diflferent principles have been proposed for the study of polyion-small ion-binding equilibria they include an electrochemical method, mainly through application of ion-specific electrodes [40-44], an equilibrium dialysis method [45], a dye method [46-51], and an ion-exchange distribution method [52-56]. In any case, the free ion concentration in the system under investigation is determined. They permits the evaluation of 0 with Eq. (4). The binding quotient, Ka. used to express the overall (apparent) equilibrium is then given by Eq. (5). 

The thermodynamic basis of ionic equilibria in oil + water systems may be summarized as follows. The electrochemical potential of a univalent positive ion M+ in a phase whose potential is may be expressed... 

Hunt LP (1987) Thermodynamic equilibria in the Si-H-Cl and Si-H-Br systems. In Cullen GW (ed) Proceedings of the 10th international conference on chemical vapour deposition, Honolulu, HI. Electrochemical Society, Pennington, NJ, ppll2-121... 

Finally, the systems discussed in this chapter have provided an excellent way of demonstrating how important the electrical aspects of equilibria involving charged species are. Modern research in this area involves the study of both the thermodynamic and kinetic aspects of electrochemical reactions. This is especially true for fuel cells, a very important area of contemporary research. In these systems the electrode reactions are complex and the kinetics of the individual... 

In 1966, Pourbaix published his Atlas of Electrochemical Equilibria in Aqueous Solutions, which contains electrode-potential/pH diagrams for many elements and a critical analysis of the data on which the diagrams are based (Ref 9). Figures 2.13 and 2.14 are from this publication and represent the iron/water system, assuming the solid phases to be iron and iron oxides in the first case and iron and iron hydroxides in the second case. It should be noted that the two diagrams differ only in relatively small detail, which results from the relatively small difference between the GFEs of a hydroxide and the oxide related to it. This can be demonstrated by writing ... 

Other authors have drawn attention to the fact that textbooks and teachers often present complex equilibria in a simplified way, to make these equilibria easier to handle for (beginning) students of chemistry. Hawkes (1998) has discussed this problem as a pedagogical dilemma that one either has to teach simplified ideas, which do not have much meaning in reality, or one has to adopt a rigorous treatment, which is normally beyond the scope of an introductory course. Hawkes concluded that, in any case, it is better to confront students with the complexity of phenomena, than to teach them simplified tmths. Obviously, problems associated with simplifications and idealisations can occur in many fields of application of the Equilibrium Law, such as solubility (Clark Bonicamp, 1998), pH and buffers, and electrochemical systems. 

Real electrodes sometimes represent extremely complicated systems, which can include several interfaces, each locating a certain potential drop. These interfaces, together with the phases in contact, take part in the equilibria established in the electrochemical system. This is why the electrode potential is one of the most important notions of electrochemical thermodynamics (to say nothing about its role in electrochemical kinetics), which describes the equilibrium phenomena in the systems containing charged components. The term component as... 

It can be seen that such examinations in simple systems may lead to a greater understanding of the intricacy of the solvent effect. However, the more complicated the system, the less suitable are conductometric methods for its understanding and characterization. In the investigation of the solvent dependence of several-step complex equilibria or processes associated with solvent substitution in systems with complicated solvate spheres, conductometry may at best be a source of supplementary qualitative information. Examples of this may be seen in the section dealing with the use of electrochemical methods for characterization of the donor strengths of solvents. 

Hence, the purpose of this book is to provide a unified basis for a wide range of problems relevant to the electrochemistry of many-electron processes in ionic melts and in other media as well. Equilibria in many-electron systems, non-stationary many-electron processes, electrochemical processes in mixed conductors, aspects of the electrodeposition of polyvalence elements and anode processes are considered. No arbitrary assumptions like one-step many-electron transfers or discrete discharge of complex species are involved— the consideration is based on a few very general ideas. 

Figure 4.3. Pourbaix diagram for the aluminum-water system at 25°C [2]. (M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd English edition, p. 171, copyright NACE International 1974 and CEBELCOR.)...

---