Metal hydroxides solubility products, Table

The data given in Tables 1.9 and 1.10 have been based on the assumption that metal cations are the sole species formed, but at higher pH values oxides, hydrated oxides or hydroxides may be formed, and the relevant half reactions will be of the form shown in equations 2(a) and 2(b) (Table 1.7). In these circumstances the a + will be governed by the solubility product of the solid compound and the pH of the solution. At higher pH values the solid compound may become unstable with respect to metal anions (equations 3(a) and 3(b), Table 1.7), and metals like aluminium, zinc, tin and lead, which form amphoteric oxides, corrode in alkaline solutions. It is evident, therefore, that the equilibrium between a metal and an aqueous solution is far more complex than that illustrated in Tables 1.9 and 1.10. Nevertheless, as will be discussed subsequently, a similar thermodynamic approach is possible. 

Values for the solubility products of metallic hydroxides are, however, not very precise, so that it is not always possible to make exact theoretical calculations. The approximate pH values at which various hydroxides begin to precipitate from dilute solution are collected in Table 11.2. 

If the ion product [Cd ][S ] exceeds the solubility product, K p, of CdS (10 Table 1.1), then, neglecting kinetic problems of nucleation, CdS will form as a solid phase (see Chap. 1). If the reaction is carried out in alkaline solution (by far the most common case), then a complex is needed to keep the metal ion in solution and to prevent the hydroxide from precipitating out (but see later). Since the decomposition of the chalcogenide precursor can be controlled over a very wide range (by temperature, pH, concentration), the rate of CdS formation can likewise be well controlled. Of course, the CdS should form a film on the substrate and (at least ideally) not precipitate in the solution. This... 

Precipitation of Hafnium Hydroxide. In order to interpret the adsorption data it was necessary to determine the conditions which lead to the precipitation of hafnium hydroxide. It is not usually advisable to depend on the solubility product because the information on this quantity is often unreliable for hydroxides of polyvalent metal ions. In addition, "radiocolloids may apparently form much below saturation conditions in radioactive isotope solutions. In the specific case of hafnium hydroxide only two measurements of the solubility seem to have been reported. According to Larson and Gammill (16) K8 = [Hf(OH)22+] [OH ]2 — 4 X 10"26 assuming the existence of only one hydrolyzed species Hf(OH)22+. The second reported value is Kso = [Hf4+] [OH-]4 = 3.7 X 10 55 (15). If one uses the solubility data by Larson and Gammill (Ref. 16, Tables I and III) and takes into consideration all monomeric hafnium species (23) a KBO value of 4 X 10 58 is calculated. 

The data of Table 3.6.4 lead to the conclusion that the use of BaO instead of alkali-metal hydroxides for the titration results in an overestimation of the value of the solubility product. This seems to be caused by Ba2+ cations possessing enhanced acidic properties which arise in the melt, thus leading to the fixation of some quantity of oxide ions from the melt and, consequently, to the increase in the solubility of the studied metal oxide. A similar conclusion may be drawn if we attribute the composition BaCl2-KCl-NaCl to the melt studied. As seen in Part 3, such a melt possesses enhanced acidity that causes an increase in metal-oxide solubilities. We can estimate the oxobasicity index of the melt KCl-NaCl + 0.05 mol kg-1 BaCl2 as +0.23, although this value is comparable with the accuracy of the solubility calculations. 

TABLE 4.8.7 Solubility Product of the Hydroxides of Alkaline Earth Metals [18,101-103]... 

The anion Y is usually hydroxide (OH ), sulfide (S ) or carbonate (C03 ) and M is usually an alkali metal or alkaline earth. Broadly speaking, the effectiveness of a particular precipitant for a particular toxic metal M can be estimated by looking-up the solubility product of the precipitated metal salt M Y . The lower the solubility product the more effective the precipitant. Table 14.5 lists the solubility products of some common metal salts. 

The physical and chemical properties of elemental thorium and a few representative water soluble and insoluble thorium compounds are presented in Table 3-2. Water soluble thorium compounds include the chloride, fluoride, nitrate, and sulfate salts (Weast 1983). These compounds dissolve fairly readily in water. Soluble thorium compounds, as a class, have greater bioavailability than the insoluble thorium compounds. Water insoluble thorium compounds include the dioxide, carbonate, hydroxide, oxalate, and phosphate salts. Thorium carbonate is soluble in concentrated sodium carbonate (Weast 1983). Thorium metal and several of its compounds are commercially available. No general specifications for commercially prepared thorium metal or compounds have been established. Manufacturers prepare thorium products according to contractual specifications (Hedrick 1985). 

A base is a substance which neutralises an acid, producing a salt and water as the only products. If the base is soluble the term alkali can be used, but there are several bases which are insoluble. It is also a substance which accepts a hydrogen ion (see p. 119). In general, most metal oxides and hydroxides (as well as ammonia solution) are bases. Some examples of soluble and insoluble bases are shown in Table 8.4. Salts can be formed by this method only if the base is soluble. 

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