Oxidation-reduction solubility

In electroless deposition, the substrate, prepared in the same manner as in electroplating (qv), is immersed in a solution containing the desired film components (see Electroless plating). The solutions generally used contain soluble nickel salts, hypophosphite, and organic compounds, and plating occurs by a spontaneous reduction of the metal ions by the hypophosphite at the substrate surface, which is presumed to catalyze the oxidation—reduction reaction. 

The values of these ratios change appreciably by passing from the heterogeneous (suspension) to the homogeneous (DMF) system. In the case of copolymerization in suspension in the presence of the K2S208—AgN03 oxidation-reduction system at 30—40 °C, the ratios were found to be ry = 0,77 0,2 and r2 = 1,09 0,04, whereas in the case of the copolymerization in solution they are = 0,52 and r2 = 1,7. The difference in these values seems to be the result of the different solubility of the monomers in water and of the different rate of diffusion of the monomers to the surface of the precipitated copolymer20. From this it follows that 4 is the more reactive monomer in this binary system. 

Although the exact chemical mechanism for the direct oxide reduction reaction has not yet been fully characterized, it has been well established that the reaction goes to completion when excess calcium is present, sufficient CaCl2 is available to dissolve the CaO produced, and adequate stirring is used. As calcium metal is soluble to about 1 wt% in CaC12 at 835°C, excess Ca insures that the reaction is driven to completion by mass-action effects. 

Physicochemical properties and stability of minerals decide many vital processes relevant to their treatment. The solubility of minerals in various media and oxidation-reduction reactions involving minerals and various reagents are all very significant in the technology of mineral raw material processing. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

Oxidation-reduction electrodes. An inert metal (usually Pt, Au, or Hg) is immersed in a solution of two soluble oxidation forms of a substance. Equilibrium is established through electrons, whose concentration in solution is only hypothetical and whose electrochemical potential in solution is expressed in terms of the appropriate combination of the electrochemical potentials of the reduced and oxidized forms, which then correspond to a given energy level of the electrons in solution (cf. page 151). This type of electrode differs from electrodes of the first kind only in that both oxidation states can be present in variable concentrations, while, in electrodes of the first kind, one of the oxidation states is the electrode material (cf. Eqs 3.1.19 and 3.1.21). 

An evaluation of the fate of trace metals in surface and sub-surface waters requires more detailed consideration of complexation, adsorption, coagulation, oxidation-reduction, and biological interactions. These processes can affect metals, solubility, toxicity, availability, physical transport, and corrosion potential. As a result of a need to describe the complex interactions involved in these situations, various models have been developed to address a number of specific situations. These are called equilibrium or speciation models because the user is provided (model output) with the distribution of various species. 

The necessary roughening of the electrodes is usually produced by oxidation-reduction cycles. For this purpose the electrode surface is first oxidized, so that metal cations or poorly soluble salts like AgCl... 

The species of components present will also be affected by oxidation-reduction, and pH. For example, iron is primarily in the Fe3+ (oxidized) or the Fe2+ (reduced) state depending on the oxidation-reduction potential of the soil. Speciation, which depends, in part, on the oxygen status of soil, is of environmental concern because some species are more soluble, such as Fe2+, and are thus more biologically available than others. The occurrence of a specific species is related to the chemistry occurring in a soil, which is related to its features. Thus, large features must be taken into consideration when studying soil chemistry and when developing analytical and instrumental methods. 

The color of soil gives an indication of its oxidation-reduction conditions and the amount of OM present. Well-aerated soils will be under oxidizing conditions iron will be in the Fe3+ state, less soluble and thus less available for chemical reaction. Under water-saturated conditions, soil will be under reducing conditions as indicated by increased yellow colorings, gleying, and mottling. Iron will be in the Fe2+ state, which is more soluble and thus more available for chemical reaction. Under these conditions, reduced species such as methane (CH4), hydrogen, (H2), and sulfides will be found. 

Most commonly, iron is discussed as being in either the ferrous (Fe2+) or ferric (Fe3+) state. Changes between these two depend on the soil s pH and Eh (where Eh is a measure of the oxidation-reduction potential of soil) as discussed in Chapter 9. Add conditions and low Eh values tend to lead to the production of ferrous ion, while high pH and high Eh values result in the predominance of ferric ion. It should be noted that the ferrous ion is more soluble than the ferric ion and, thus, it will be more available to plants. 

The metallic properties increase down any column and towards the left in any row on the periodic table. One important metallic property is that metal oxides are base anhydrides. A base anhydride will produce a base in water. These are not oxidation-reduction reactions. Many metal oxides are too insoluble for them to produce any significant amount of base. However, most metal oxides, even those that are not soluble in water, will behave as bases to acids. A few metal oxides, and their hydroxides, are amphoteric. Amphoteric means they may behave either as a base or as an acid. Amphoterism is important for aluminum, beryllium, and zinc. Complications occur whenever the oxidation number of the metal exceeds +4 as in the oxides that tend to be acidic. 

Density, AGf, solubility, dissociation const, and standard potentials of oxidation-reduction reactions for aqueous species... 

Fig. 16.4 Kinetics of oxidation of soluble Cr(III) in various soils, at initial Cr(III) concentrations of 100 jig g Reprinted with permission from Kozuh N, Stupar J, Gorenc B (2000) Reduction and oxidation processes of chromium in soils. Environ Sci Technol 34 112-119. Copyright 2000 American Chemical Society...

The solubility and the hydrolysis constants enable the concentration of iron that will be in equilibrium with an iron oxide to be calculated. This value may be underestimated if solubility is enhanced by other processes such as complexation and reduction. Solubility is also influenced by ionic strength, temperature, particle size and by crystal defects in the oxide. In alkaline media, the solubility of Fe oxides increases with rising temperature, whereas in acidic media, the reverse occurs. Blesa et al., (1994) calculated log Kso values for Fe oxides over the temperature range 25-300 °C from the free energies of formation for hematite, log iCso fell from 0.44 at 25 °C to -10.62 at300°C. 

Physical and Chemical Properties. Some of the physical and chemical properties (i.e., K°w K°<= and Henry s law constant) that are often used in the estimation of environmental fate of organic compounds are not useful or relevant for most inorganic compounds including thorium and its compounds. Relevant data concerning the physical and chemical properties, such as solubility, stability, and oxidation-reduction potential of thorium salts and complexes have been located in the existing literature. 

Figure 3.14 E-pH diagram for water system with commonly used oxidants and reductants. Soluble species and most solids are hydrated. No agents producing complexes or insoluble compounds are present other than HOH and OH .

---