Complexation, solubility and redox equilibria

A complex is formed by reactions between two or more species that are capable of independent existence. Often this is between a metal ion, M and a coordinating molecule L. 

Ionic reactions producing a compound that is insoluble in the chosen solvent used may be used for analysis. 

Where one species is reduced while the other is simultaneously oxidized, the reactions are termed redox reactions and are useful in many analytical methods. 

The formation of stable compounds and complexes is important in analytical chemistry, since many species may be formed in a real sample. The amounts and nature of the species present are analyzed to study spedation. For example, in a natural water sample, the metal ions may form complexes with water molecules, carbonate species, plant acids or pollutants. Complexes may be used for titrations, both directly and for masking imwanted reactions. 

The formation of a complex compound between an acceptor species, most usually a metal ion, M, and a coordinating species, or donor ligand, L, involves the formation of coordinate bonds, for example, hexamino cobalt (III) 

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In this chapter we have applied the methods of chapter 4 to ionic equilibria other than those between acids and bases. Of course, complexation, extraction, solubility, precipitation, and redox equilibria may also involve acid-base equilibria, which is why we treated acid-base equilibria first. The examples given here illustrate that the combination of exact theory with the computational power of a spreadsheet allows us to solve many problems that occur in quantitative chemical analysis, and to analyze experimental data accordingly. Even quite complicated titrations, such as the multi-component precipitation titrations, the von Liebig titration, and redox titrations involving many species and complicated stoichiometries, can be handled with ease. 

The review of Martynova (18) covers solubilities of a variety of salts and oxides up to 10 kbar and 700 C and also available steam-water distribution coefficients. That of Lietzke (19) reviews measurements of standard electrode potentials and ionic activity coefficients using Harned cells up to 175-200 C. The review of Mesmer, Sweeton, Hitch and Baes (20) covers a range of protolytic dissociation reactions up to 300°C at SVP. Apart from the work on Fe304 solubility by Sweeton and Baes (23), the only references to hydrolysis and complexing reactions by transition metals above 100 C were to aluminium hydrolysis (20) and nickel hydrolysis (24) both to 150 C. Nikolaeva (24) was one of several at the conference who discussed the problems arising when hydrolysis and complexing occur simultaneously. There appear to be no experimental studies of solution phase redox equilibria above 100°C. 

Any consideration of sovent effects on rates or equilibria must start from solvent activity coefficients, VI for reactants, transition states and products (Wiberg, 1964 Laidler, 1950 Parker, 1966). Once solvent activity coefficients are available, or can be predicted, it is highly probable, as indicated at the end of this article, that an enormous amoimt of information on the kinetics of reactions in solution and on equilibrium properties such as solubility, acid-base strength, ion-association, complexing, redox potentials and kinetics of reactions in different solvents (Parker, 1962, 1965a, 1966) can be reduced to a relatively small number of constants which can then be used in appropriate linear free energy relationships. 

We can write equilibrium constants for many types of chemical processes. Some of these equilibria are listed in Table 6.1. The equilibria may represent dissociation (acid/base, solubility), formation of products (complexes), reactions (redox), a distribution between two phases (water and nonaqueous solvent—solvent extraction adsorption from water onto a surface, as in chromatography, etc.). We will describe some of these equilibria below and in later chapters. 

Although the standard potentials are the fundamental values for all thermodynamic calculations, in practice, one has more frequently to deal with the so-called formal potentials. The formal potentials are conditional constants, very similar to the conditional stability constants of complexes and conditional solubility products of sparingly soluble salts (see [2c]). The term conditional indicates that these constants relate to specific conditions, which deviate from the usual standard conditions. Formal potentials deviate from standard potentials for two reasons, i.e., because of nonunity activity coefficients and because of chemical side reactions . The latter should better be termed side equilibria however, this term is not in common use. Let us consider the redox system iron(II/ni) in water ... 

Sillen, L. G. and A. E. Martell, Stability Constants of MetaUIon Complexes, Special Publications No. 17, 1964, and No. 25 (Supplement), 1971, The Chemical Society, London. This is the main source of compiled experimental results for acid-base, solubility, redox, and ion-ligand equilibria. 

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