Acid-base concepts ionic potential

The strength of the complexation is a function of both the donor atom and the metal ion. The solvent medium is also an important factor because solvent molecules that are potential electron donors can compete for the Lewis acid. Qualitative predictions about the strength of donor-acceptor complexation can be made on the basis of the hard-soft-acid-base concept (see Section 1.2.3). The better matched the donor and acceptor, the stronger is the complexation. Scheme 4.3 gives an ordering of hardness and softness for some neutral and ionic Lewis acids and bases. 

Figure 18. In the same way as the concentration of protonic charge carriers characterizes die acidity (basicity) of water and in the same way as the electronic charge carriers characterize the redox activity, the concentration of elementary ionic charge carriers, that is of point defects, measure the acidity (basicity) of ionic solids, while associates constitute internal acids and bases. The definition of acidity/basicity from the (electrochemical potential of the exchangeable ion, and, hence, of the defects leads to a generalized and thermodynamically firm acid-base concept that also allows to link acid-base scales of different solids.77 (In order to match the decadic scale the levels are normalized by In 10.) (Reprinted from J. Maier, Acid-Base Centers and Acid-Base Scales in Ionic Solids. Chem. Eur. J. 7, 4762-4770. Copyright 2001 with permission from WILEY-VCH Verlag GmbH.)...

Electrocatalysis in metallic corrosion may be classified into two groups Adsorption-induced catalyses and solid precipitate catalyses on the metal surface. In general, the bare surface of metals is soft acid in the Lewis acid-base concept and tends to adsorb ions and molecules of soft base forming the covalent binding between the metal surface and the adsorbates. The Lewis acidity of the metal surface however may turn gradually to be hard as the electrode potential is made positive, and the bare metal surface will then adsorb species of hard base such as water molecules and hydroxide ions in aqueous solution. Ions and molecules thus adsorbed on the metal surface catalyze or inhibit the corrosion processes. Solid precipitates, on the other hand, are produced by the combination of hydrated cations of hard acid and anions of hard base forming the ionic bonding between the cations and the anions on the metal surface. 

The second method considers the surface acidity to result from the electron acceptor characta- of the oxide surface (29). This is related to the Lewis acid-base concept where, for an ionic oxide, the acid entity is the cation with the base being the oxygen anions. For an oxide M ,Oy, the surface acidity has been shown to be related to the ionization potential (IP) of the metal M according to... 

The ionic-potential (Ip) concept discussed in Chap. 3 best describes element behavior at intermediate pH s. Most of the metal oxides and hydroxide minerals formed by cations with Ip values between about 3 and 8.2 become significantly soluble in acid waters, where the high H+ concentrations can break metal-0 or metal-OH bonds to form water and release metal cations to solution. Some of these otherwise insoluble metal oxides and hydroxides (for example, those of Al and Fe " ) are also solubilized by high pH s. In other words, the mobilities of these metal cations depend on the acid-base properties of the water, as well as on their ionic potentials. The rate of chemical weathering is also greatly accelerated in strongly acid waters. 

Earlier work (7 10>ll) described the chemical behavior of the slag constituents in terms of acids and bases. Vorres ]) used the concept of ionic potential (ratio of ionic charge to crystallographic radius) to differentiate slag constituents on the basis of their ability to attract a common anion (oxide ions in these systems). The strongest acids attract anions most strongly or are most able to effectively compete for anions to complete a regular close packed coordination. The bases are not able to compete for anions, and serve primarily as oxide ion donors. 

In order to overcome the difficulty due to the discrepancy of the values of activities and concentrations, which may be important, the concept of formal potentials E° has been devised. (The symbol E° of formal potentials is, unfortunately, the same as that used for standard biological potentials.) The formal potential is that which is experimentally observed with solutions containing both Ox and Red forms of the couple at the unit concentration and that also may contain other species whose concentrations are specified. They take into account the variations in activity coefficients with the ionic strength, the acid-base equilibria involving the Ox and/or Red form(s), and their possible complexations with the other solution species. They are experimentally determined using electrochemical cells of the classes described above, after measurements of their zero-current cell potentials. The formal potentials can be used only when the experimental conditions of the redox reaction under study are the same as those under which they have been determined. 

Owing to the size of the subject, it has proved necessary in the following chapters, to be selective in the choice of material presented. Earlier chapters are concerned with ionics and its applications. Here are considered ion interactions in solution, acid-base equilibria, transport phenomena, and the concept of reversible electrode potential. This last named leads to the development of reversible cells and their exploitation. Here one is dealing with electrochemical thermodynamics - with the rapid attainment of equilibrium between species at an electrode surface and charged species in solution. 

The term electromembrane process is used to describe an entire family of processes that can be quite different in their basic concept and their application. However, they are all based on the same principle, which is the coupling of mass transport with an electrical current through an ion permselective membrane. Electromembrane processes can conveniently be divided into three types (1) Electromembrane separation processes that are used to remove ionic components such as salts or acids and bases from electrolyte solutions due to an externally applied electrical potential gradient. (2) Electromembrane synthesis processes that are used to produce certain compounds such as NaOH, and Cl2 from NaCL due to an externally applied electrical potential and an electrochemical electrode reaction. (3) Eletectromembrane energy conversion processes that are to convert chemical into electrical energy, as in the H2/02 fuel cell. 

The concept of assisted ion transfer is, of course, applicable to proton-transfer reactions assisted by the presence of an acid or base, hydrophilic or lipophilic. As pioneered by Kontturi and Murtomaki [165], voltammetry at ITIES has proved to be an excellent method to measure the log values of protonated or deprotonated molecules. Indeed, for therapeutic molecules, the logF values, which are related the Gibbs energy of transfer as shown by Equations 1.11 and 1.12, provide an important physical parameter to assay the toxicity of a molecule. If a molecule is lipophilic, that is, logF >2, it is potentially toxic. In fact, with the concept of ionic distribution diagrams (vide infra) it is even possible to measure the logF values of the neutral associated bases. The application of voltammetry at ITIES to the study of therapeutic molecules has been one of the success stories of electrochemistry at liquid-liquid interfaces. The field has been reviewed over the years [166,167] and very recently by Gulaboski et al. [168]. 

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