Acidity of Hydrated Metal Ions

The aqueous solutions of certain metal ions are acidic because the hydrated metal ion transfers an H ion to water. Consider a general metal nitrate, M(N03), as it dissolves in water. The ions separate and become bonded to a specific number of surrounding H2O molecules. This equation shows the hydration of the cation (M ) with H2O molecules hydration of the anion (N03 ) is indicated by aq)  

If the metal ion, M , is small and highly charged, it has a high charge density and withdraws sufficient electron density from the 0—H bonds of these bonded water molecules for a proton to be released. That is, the hydrated cation, M(H20)/, acts as a typical Brpnsted-Lowry acid. In the process, the bound H2O molecule that releases the proton becomes a bound OH ion  

Each type of hydrated metal ion that releases a proton has a characteristic value. Some common examples appear in Appendix C. 

Aluminum ion, for example, has the small size and high positive charge needed to produce an acidic solution. When an aluminum salt, such as A1(N03)3, dissolves in water, the following steps occur  

Note the formulas of the hydrated metal ions in the last step. When is released, the number of bound H2O molecules decreases by 1 (from 6 to 5) and the number of bound OH ions increases by 1 (from 0 to 1), which reduces the ion s pos- 

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It should be noted that one cannot necessarily compare the acidity of the bivalent ion of one metal with that of the trivalent ion of another metal in this way, however. There appears to be no good general rule concerning the acidities of hydrated metal ions at the present time, although some attempts have been made at correlations. 

The strength of an acid depends on its ability to donate a proton, which depends in turn on the strength of the bond to the acidic proton. In this section, we apply trends in atomic and bond properties to determine the trends in acid strength of nonmetal hydrides and oxoacids and discuss the acidity of hydrated metal ions. 

Acid Strength of Nonmetal Hydrides 601 Acid Strength of Oxoadds 602 Acidity of Hydrated Metal Ions 602... 

Both inorganic (e.g., metals) and organic substances may be subject to a hydrolysis reaction in waters. Examples of several hydrolyzable functional groups are given in Table 13.2. Water is a weak acid and the acidity of the water molecules in the hydration shell of a metal ion usually is greater than that of the water. The acidity of aqueous metal ions is expected to increase with a decrease in the radius and an increase in the charge of the central ion. In the case of Fe(III), for example. 

With the exception of the Ru1 complex, a variety of metal ions activate the hydration reaction to a very consistent extent (106-108-fold). The relative constancy of the rate acceleration and the anomaly of the Ru11 complex are consistent with the view that it is the Lewis acidity of the metal ion which is essential for activation. In the Ru11 complex, considerable metal h-ligand -bonding is expected, resulting in back donation of electron density from the metal centre into the C=N bond. This view is supported by measurements of the C=N stretching frequency of free nitriles and of their complexes with Ru11 and the other metal ions.317... 

Table 1 Basic features of hydrated metal ions involved in metal-nucleic acid interactions...

The catalysts preparable with swelling layer lattice silicates are classified into four types, as illustrated in Fig. 1. The intercalate of hydrated metal ion (a. Fig. 1) is easily obtained by a simple ion-exchange reaction in an aqueous medium. The intercalate acts as a Br0nsted acid catalyst because... 

We discussed the Lewis acid-base properties of hydrated metal ions, which are a type of complex ion, in Section 18.8, and we examined complex-ion equilibria in Section 19.4. In this section, we consider the bonding, structure, and properties of complex ions. 

The pH found in Example 5-2 is quite low, showing that the mercuric ion has the property of a weak acid of approximately the same strength as acetic acid. The acid strength of some metal ions is quite considerable. For example, the pH of equimolar H3PO4 and solutions are similar. The strong acid properties of hydrated metal ions become evident in processes such as coagulation/flocculation, where hydrated aluminum sulfate (alum) or ferric chloride is added to a natural water that is buffered with bicarbonate. The metal ion "titrates" (or reacts with) a stoichiometric amount of alkalinity, just as the addition of an equal amount of strong mineral acid would. 

Hydrated metal ions, particularly those with a charge of +3 or more, are Bronsted acids because they tend to lose H+ in aqueous solution. The acidity of a metal ion increases with charge and decreases with increasing radius. Hydrated iron(III) ion is a relatively strong acid, ionizing as follows ... 

The tendency of hydrated metal ions to behave as acids may have a profound effect upon the aquatic environment. A good example is acid mine water (see Chapter 12, Section 12.8), which derives part of its acidic character from the tendency of hydrated iron(III) to lose H ... 

Consequently they cannot be prepared by the addition of sulphide ions to a solution of the metal salt, the hydrated metal ions being so strongly acidic that the following reaction occurs, for example... 

Antimony trioxide is insoluble in organic solvents and only very slightly soluble in water. The compound does form a number of hydrates of indefinite composition which are related to the hypothetical antimonic(III) acid (antimonous acid). In acidic solution antimony trioxide dissolves to form a complex series of polyantimonic(III) acids freshly precipitated antimony trioxide dissolves in strongly basic solutions with the formation of the antimonate ion [29872-00-2] Sb(OH) , as well as more complex species. Addition of suitable metal ions to these solutions permits formation of salts. Other derivatives are made by heating antimony trioxide with appropriate metal oxides or carbonates. 

On this basis = 0.0170 sec , = 0.645 sec , and K = 0.739 mole.P at 25 °C. The corresponding activation parameters were determined also by Es-penson. By a method involving extrapolation of the first-order rate plots at various wavelengths to zero time, the absorption spectrum of the intermediate was revealed (Fig. 1). Furthermore, the value of K obtained from the kinetics was compatible with that derived from measurements on the acid dependence of the spectrum of the intermediate. Rate data for a number of binuclear intermediates are collected in Table 2. Espenson shows there to be a correlation between the rate of decomposition of the dimer and the substitution lability of the more labile metal ion component. The latter is assessed in terms of the rate of substitution of SCN in the hydration sphere of the more labile hydrated metal ion. 

Certain other metal ions also exhibit catalysis in aqueous solution. Two important criteria are rate of ligand exchange and the acidity of the metal hydrate. Metal hydrates that are too acidic lead to hydrolysis of the silyl enol ether, whereas slow exchange limits the ability of catalysis to compete with other processes. Indium(III) chloride is a borderline catalysts by these criteria, but nevertheless is effective. The optimum solvent is 95 5 isopropanol-water. Under these conditions, the reaction is syn selective, suggesting a cyclic TS.63... 

In general, complexation of an aquometal ion occurs when the ligand is a stronger base than H20, and analogously may be considered an acid-base reaction. The stability (or formation) constant, KMl, is used to describe the interaction of the metal ion (Mz+, shown here with the hydration sheath surrounding the metal ion omitted for reasons of clarity) with a complexant (L" ) ... 

Calcium sulfate crystals were precipitated in a Continuous Mixed Suspension Mixed Product Removal (CMSMPR) crystallizer by mixing of calcium phosphate and sulfuric acid feed streams. The formed calcium sulfate hydrate (anhydrite, hemihydrate and dihydrate) mainly depends on the temperature and the solution composition. The uptake of cadmium and phosphate ions in these hydrates has been studied as a function of residence time and solution composition. In anhydrite, also the incorporation of other metal ions has been investigated. The uptake was found to be a function of both thermodynamics and kinetics. 

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