Solubility product Oxides

CVD processing can be accompanied by volatile hot-reaction by-products such as HCl or HF, which, along with unused precursor gases, must be removed from the exhaust gas stream. This is done by scmbbing the chemicals from the gas using water to dissolve soluble products or by burning the precursor gases to form oxides. 

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. 

Dissolution of CaCOs is a congruent reaction the entire mineral is weathered and results completely in soluble products. The above reaction is driven to the right by an increase of CO2 partial pressure and by the removal of the Ca and/or bicarbonate. Any impurities present in the calcareous rock, such as silicates, oxides, organic compounds, and others, are left as residue. As the calcium and bicarbonate leach... 

The soluble products are able to form charged high-spin states after chemical and electrochemical oxidation. The high-spin character is the result of the lack of conjugative interaction between the highly distorted, orthogonally arranged aromatic subunits (decoupled rr-systems) [68]. 

Depending on electrolyte composition, the metal will either dissolve in the anodic reaction, that is, form solution ions [reaction (1.24)], or will form insoluble or poorly soluble salts or oxides precipitating as a new solid phase next to the electrode surface [reaction (1.28)]. Reacting metal electrodes forming soluble products are also known as electrodes of the first kind, and those forming solid products are known as electrodes of the second kind. 

Anodic oxidation often involves the formation of films on the surface, i.e. of a solid phase formed of salts or complexes of the metals with solution components. They often appear in the potential region where the electrode, covered with the oxidation product, can function as an electrode of the second kind. Under these conditions the films are thermodynamically stable. On the other hand, films are sometimes formed which in view of their solubility product and the pH of the solution should not be stable. These films are stabilized by their structure or by the influence of surface forces at the interface. 

Using on-line mass spectroscopy [65] carbon dioxide and formic acid were demonstrated as soluble products of methanol oxidation. The former gives the most intense MS signal according to the fact that it is the main product. There are two main problems to detect formic acid as such. In the presence of carbon dioxide most of the m/e signals of HCOOH overlap with signals of the major product. Besides this, in the presence of methanol, formic acid reacts to form the methyl ester ... 

Most lanthanide compounds are sparingly soluble. Among those that are analytically important are the hydroxides, oxides, fluorides, oxalates, phosphates, complex cyanides, 8-hydroxyquinolates, and cup-ferrates. The solubility of the lanthanide hydroxides, their solubility products, and the pH at which they precipitate, are given in Table 2. As the atomic number increases (and ionic radius decreases), the lanthanide hydroxides become progressively less soluble and precipitate from more acidic solutions. The most common water-soluble salts are the lanthanide chlorides, nitrates, acetates, and sulfates. The solubilities of some of the chlorides and sulfates are also given in Table 2. 

The cyclic voltammetry of polysilanes adsorbed on the electrode surface has also been investigated [65]. The oxidation potentials depend upon the nature of the organic groups on silicon. The electrochemical oxidation is irreversible to give soluble products which are liberated from the surface of the anode. 

In reviewing the basic solubility products for these systems, the sulfide system removes the most inorganics, with the exception of arsenic, because of the low solubility of sulfide compounds. This increased removal capability is offset by the difficulty in handling the chemicals and the fact that sulfide sludges are susceptible to oxidation to sulfate when exposed to air, resulting in resolubilization of the metals. The carbonate system is a method that relies on the use of soda ash (sodium carbonate) and pH adjustment between 8.2 and 8.5. The carbonate system, although... 

Stability constants (ethylendiamine, glycinate, oxalate), surface complex formation constants and solubility products (sulfides) of transition ions. The surface complex formation constant is for the binding of metal ions to hydrous ferric oxide =Fe-OH + Me2+ =FeOMe++ H+ K. ... 

Does Surface Precipitation occur at Concentrations lower than those calculated from the Solubility Product As the theory of solid solutions (see Appendix 6.2) explains, the solubility of a constituent is greatly reduced when it becomes a minor constituent of a solid solution phase (curve b in Fig. 6.10).Thus, a solid species, e.g., M(OH)2 can precipitate at lower pH values in the presence of a hydrous oxide (as a solid solvent), than in its absence. 

The Rate of reductive Dissolution of Hematite by H2S as observed between pH 4 and 7 is given in Fig. 9.6 (dos Santos Afonso and Stumm, in preparation). The HS" is oxidized to SO. The experiments were carried out at different pH values (pH-stat) and using constant PH2s- 1.8 - 2.0 H+ ions are consumed per Fe(II) released into solution, as long as the solubility product of FeS is not exceeded, the product of the reaction is Fe2+. The reaction proceeds through the formation of inner-sphere =Fe-S. The dissolution rate, R, is given by... 

This model may possibly be adapted to metal-water thermal explosions if one assumes that there are reactions between the molten metal and water (and substrate) that form a soluble salt bridge across the interface between the two liquids. This salt solution would then be the material which could superheat and, when finally nucleated, would initiate the thermal explosion. As noted, the model rests on the premise that there are chemical reactions which occur very quickly between metal and water to form soluble products. There is experimental evidence of some reactions taking place, but the exact nature of these is not known. Perhaps, in the case of aluminum, the hydroxide or hydrated oxides form. With substrates covered by rust or an inorganic salt [e.g., Ca(OH)2], these too could play an important role in forming a salt solution. 

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