The Stabilities of Ions in Aqueous Solution

This chapter is not concerned with the thermodynamic stability of ions with respect to their formation. Rather, it is concerned with whether or not a given ion is capable of existing in aqueous solution without reacting with the solvent. Hydrolysis reactions of ions are dealt with in Chapter 3. The only reactions discussed in this section are those in which either water is oxidized to dioxygen or reduced to dihydrogen. The Nernst equation is introduced and used to outline the criteria of ionic stability. The bases of construction and interpretation of Latimer and volt-equivalent (Frost) diagrams are described. 

I hat there is a limiting value fr r any reduction prttential below w hich the reduced form of the cr uple has the capacity to reduce water to dihydrogen 

Thai ihei c are practical limits to the stabilities of oxidizing and reducing agents 

Although reduction potentials may be estimated for half-reactions, there are limits for their values that correspond to both members of a couple having stability in an aqueous system with respect to reaction with water. For example, the Na /Na couple has a standard reduction potential of —2.71 V, but metallic sodium reduces water to dihydrogen. The reduced form of the couple (Na) is not stable in water. The standard reduction potential for the Co /Co couple is +1.92 V, but a solution of Co slowly oxidizes water to dioxygen. In this ca.se the oxidized form of the couple is not stable in water. The standard 

Waller Nernsi vras awarded Ihe 1920 Nobel Prize lor Chemistry (or work in ihermochemlsiiy - 

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The subject of acids and bases is very extensive. The discussion in this book is restricted to the definitions of acids and bases in aqueous solutions and their applications to the nature of ions in aqueous solutions and their stabilities. The two main definitions are those accredited to... 

Ion size plays an important role in determining the structure and stability of ionic solids, the properties of ions in aqueous solution, and the biological ef-... 

Ion size plays an important role in determining the structure and stability of ionic solids, the properties of ions in aqueous solution, and the biologic effects of ions. As with atoms, it is impossible to define precisely the sizes of ions. Most often, ionic radii are determined from the measured distances between ion centers in ionic compounds. This method, of course, involves an assumption about how the distance should be divided up between the two ions. Thus you will note considerable disagreement among ionic sizes given in various sources. Here we are mainly interested in trends and will be less concerned with absolute ion sizes. 

The electrode constructed from silver which is in contact with Ag20 should be thermodynamically stable. The influence of higher oxides on the potential of such electrode should not be important, since the stability of Ag " in aqueous solutions is very low. In fact Ag ions practically do not exist in aqueous solutions, since for the reaction ... 

The Mn ion is so unstable that it scarcely exists in aqueous solution. In acidic aqueous solution, manganic compounds readily disproportionate to form Mn ions and hydrated manganese(IV) oxide, Mn02 2H20 in basic solution these compounds hydroly2e to hydrous manganese(III) oxide, MnO(OH). Sulfuric acid concentrations of about 400 450 g/L are required to stabilize the noncomplexed Mn ion in aqueous solutions. 

It has been recognized that sulfur donors aid the stabilization of Cu(i) in aqueous solution (Patterson Holm, 1975). In a substantial study, the Cu(ii)/Cu(i) potentials and self-exchange electron transfer rate constants have been investigated for a number of copper complexes of cyclic poly-thioether ligands (Rorabacher et al., 1983). In all cases, these macrocycles produced the expected stabilization of the Cu(i) ion in aqueous solution. For a range of macrocyclic S4-donor complexes of type... 

The equilibrium constant for this reaction depends on the stability constants of the ionophore-M+ complexes and on the distribution of ions in aqueous test solution and organic membrane phases. For a membrane of fixed composition exposed to a test solution of a given pH, the optical absorption of the membrane depends on the ratio of the protonated and deprotonated indicator which is controlled by the activity of M+ in the test solution (H,tq, is fixed by buffer). By using a to represent the fraction of total indicator (Ct) in the deprotonated form ([C]), a can be related to the absorbance values at a given wavelength as... 

During the early sixties Thompson and Loraas (77) reported the formation of mixed complexes of reasonable stability (log K 3.0—5.3) between lanthanide—HEDTA and ligands such as EDDA (N,N -ethylenediaminediacetic acid), HIMDA (N-hydroxyethyliminodiacetic acid) and IMDA (iminodiacetic acid). This observation together with the remarkably large formation constants (72) for the bis-EDDA complexes [log A2 =4.73 (La) 8.48 (Lu)] suggested a coordination number larger than six for the tripositive lanthanide ions in aqueous solution, in view of the fact that mixed chelates of the t5q>e M (HEDTA) (IMDA) axe not formed when M =Co(II), Ni(II) or Cd(II). 

Thallium(I) halides are predominantly ionic, although there is a tendency toward increasing covalent character in the series of compounds TlCl (17%), TlBr (20%), and TII (28%). This increased degree of covalency results in decreased solubility for example, TIF is soluble in water whilst the other Tl halides are only sparingly soluble. The thallium(I) halides are classical examples of incompletely dissociated 1 1 electrolytes. The stability of halide complexes of Tl is low and follows the order TIF < TlCl < TlBr < TII, where for the series of halides, Kx = -, 0.8, 2.1, 5.0 and Ki = -, 0.2, 0.7, 1.5 respectively. The fluoride ion F is preferred to perchlorate as a noncomplexing counterion. Claims have been made for T1X species with n = 3 and 4 however, the formation of complexes in aqueous solution with n > 2 seems unlikely. 

The conformational stability of biomolecules is greatly dependent on the solvent species. It is also affected by coexisting solutes such as salts (e.g., NaCl). The salt effects [47, 48, 49] on the solubility and the conformational stability of proteins in aqueous solutions are experimentally known to follow the order called the Hofmeister series. The series for anions is [S04 > CHsCOO" > Cl > Br > NOa" > CIOJ > 1 > CNS ], and that for cations is [(CH3)4N+ > NH > Rb+,K+,Na+, Cs+ > Li+ > Mg + > Ca + > Ba +j. In each of these series, the species to the left decrease the solubility of proteins and stabilize their native structures. The species to the right, on the contrary, increase the solubility and cause destabilization of the native structures. Though the Hofmeister series is not valid for acidic and basic proteins [50, 51], it is generally applicable to neutral proteins. The series, except for divalent cations, is also applicable to the other neutral substances such as benzene [52]. That is, the effects of monovalent ions on the solubility of various neutral substances follow the Hofmeister series. The microscopic mechanisms of these experimentally known properties, however, have not been elucidated yet. 

Polyphosphotungstate appears to be able to complex Cm(iv) and Cf(iv) sufficiently well to stabilize these ions in aqueous solution [81]. Since the An /An potential shift of 1.7 V in carbonate is more favourable for stabilization of An than is the shift of 1.0 V in polyphosphotungstate [82], it was expected that Cm(iv) and Cf(iv) would be readily produced in carbonate. Such was not found to be the case, however [67]. 

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