The various oxides

A generalized overview of the occurrence of the different Fe oxides in various soils (see Schwertmann, 1985) is given in Table 16.2. 

There are two ways by which goethite can be formed in soils. If iron is released from solid Fe compounds such as Fe silicates, carbonates and sulphides or, alternatively, from existing Fe oxides by microbial reduction, the Fe will be oxidized in an 

Anaerobic, clayey, non-calcareous soils of cooler and temperate regions. 

Groundwater and stagnant water soils (gleys and pseudogleys) and podzols of temperate and cool regions. Paddy soils. 

Aerobic soils of subtropical, mediterranean and humid to subhumid tropical regions (lateritic and plinthitic soils, red mediterranean soils, oxisols, ultisols). Usually absent in soils of temperate and cool regions. 

---

Metal Cleaning. About 204 thousand metric tons of HCl (100% basis) was consumed in 1993 for steel pickling, wherein the hydrochloric acid readily dissolves all of the various oxides present in the scale formed during the hot rolling process. Using suitable inhibitors such as alkyl pyridines, HCl reacts very slowly with the base metal rendering the surface so clean that it must be passivated with a mild alkaline rinse. 

Ternary compounds are also named by citing the more electropositive constituent first. The various oxidation states of the more electropositive element are designated by a system of prefixes and terminations added to a stem characteristic of the element, except in the case of coordination compounds (qv). Examples are as follows (see Chlorine oxygen acids and salts) ... 

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

Lower Oxides. A number of hard, refractory suboxides have been prepared either as by-products of elemental boron production (1) or by the reaction of boron and boric acid at high temperatures and pressures (39). It appears that the various oxides represented as B O, B O, B22O2, and B23O2 may all be the same material ia varying degrees of purity. A representative crystalline substance was determined to be rhombohedral boron suboxide, B12O2, usually mixed with traces of boron or B2O3 (39). A study has been made of the mechanical properties of this material, which exhibits a hardness... 

A number of metals, such as copper, cobalt and h on, form a number of oxide layers during oxidation in air. Providing that interfacial thermodynamic equilibrium exists at the boundaries between the various oxide layers, the relative thicknesses of the oxides will depend on die relative diffusion coefficients of the mobile species as well as the oxygen potential gradients across each oxide layer. The flux of ions and electrons is given by Einstein s mobility equation for each diffusing species in each layer... 

When an element is present on the surface of a sample in several different oxidation states, the peak characteristic of that element will usually consist of a number of components spaced close together. In such cases, it is desirable to separate the peak into its components so that the various oxidation states can be identified. Curve-fitting techniques can be used to synthesize a spectrum and to determine the number of components under a peak, their positions, and their relative intensities. Each component can be characterized by a number of parameters, including position, shape (Gaussian, Lorentzian, or a combination), height, and width. The various components can be summed up and the synthesized spectrum compared to the experimental spectrum to determine the quality of the fit. Obviously, the synthesized spectrum should closely reproduce the experimental spectrum. Mathematically, the quality of the fit will improve as the number of components in a peak is increased. Therefore, it is important to include in a curve fit only those components whose existence can be supported by additional information. 

The fact that oxides can exist as metastable phases is illustrated by the Ni-HjO diagram (Fig. 1.18) in which the curves for the various oxides of nickel have been extrapolated into the acid region of Ni stability, and this diagram emphasises the fact that nickel can be passivated outside the region of thermodynamic stability of the oxides". 

The reaction of MnC03 is of particular interest because of the possibility of the formation and interconversion of the various oxides of manganese [733,1276], Westerdahl and Leader [762] identify the initial process as... 

In this review the nature of the various oxidation species in solution are reviewed and generalizations drawn about their complexing behavior. 

Speciation of plutonium leached from the glass cubes is shown in Figure 1. The first bar represents the total amount of insoluble plutonium and is the summation of suspended plutonium (the difference between the values for filtered and unfiltered waters) and sorbed plutonium—viz., the amount removed from the cubes by a 0.1 M perchloric acid wash, normalized to the volumes of leachant solutions so that it is comparable to the other values in the graphs. For simplicity, the insoluble fractions are combined in one bar, whereas the various oxidation states in the soluble fraction are represented by separate bars. It should be noted that the ordinate scale varies among the graphs. 

This is the origin of the various values for self-exchange rate constants. We may now attempt to rationalize some of these in terms of the /-electron configurations of the various oxidation states. Consider the self-exchange rate constants for some iron complexes. 

The various oxidation states of sulfur have been determined by polarography. The electrochemical oxidation of sulfide ions in aqueous solution may lead to the production of elementary sulfur, polysulfides, sulfate, dithionate, and thiosulfate, depending on the experimental conditions. Disulfides, sulfoxides, and sulfones are typical polarographically active organic compounds. It is also found that thiols (mer-captans), thioureas, and thiobarbiturates facilitate oxidation of Hg resulting thus in anodic waves. 

The authors ascribed the large loss of solution OH primarily to its consumption in the various oxidation processes ... 

Table 3.1 Peak potential, charge passed and corresponding oxide coverage for the various oxide regions in the anodic sweep of the Pt cyclic voltammogram...

With the exception of antimony (V), which requires the presence of iodide for its reduction, all species can be reduced in an acid medium at a pH of 1 -2. However, the reduction of some species, including antimony (III), arsenic (III), and all tin species, will also proceed at higher pH, where arsenic (V) and antimony (V) are not converted to their hydrides. This effect permits the selective determination of the various oxidation states of these elements [714, 716]. In the case of tin, reduction can be achieved at the pH of the Tris-HCl... 

Neumann and Khenkin28 review most of the various oxidation methods of dienes and polyenes and their mechanisms. They obviously emphasize the difference between non-conjugated and conjugated dienes and polyenes in selected oxidation reactions. 

The results indicate that Ce02 addition improves the WGS rate. Surface oxygen mobility was also quantified over the various oxides, with and without Rh promoter,... 

The various oxidation states of plutonium exhibit characteristic absorption spectra in the ultraviolet, visible and infrared regions. Each oxidation state is sufficiently distinct that its reaction can be monitored during hydrolysis and complex formation. Various research groups have studied the relationship between oxidation and absorption spectra (6-9). The absorption spectra may respond to complex formation or hydrolysis Nebel (10) has shown that the absorption peak of Pu(IV) shifts from 470 nm to 496 nm when Pu(IV) complexed with two molecules of citrate. 

Based on the standard Gibbs free energy of the various oxides, three triple points can be calculated WO2.72. WO2.9, WO2 at about 600°C, W02,9, WO3, WO2 at about 270°C, and WO2, WO2.72, W at 1480°C. Using this data, a phase diagram can be constructed. The stability of the various oxides is shown in Fig. 8.2 with respect to the partial pressures of H2O and H2, and temperature. Because aU of these compositions are equilibrium compositions, any of them can be produced simply by annealing W or WO3 at the given partial pressure ratio and temperature. 

The open circuit potential data for the B210/NVP system mirrors the behavior of the rust ratings over the temperature range examined. A plausible explanation of the change of the open circuit potential is as follows. As temperature is increased, the composition of the various oxides and hydroxides which make up the zinc phosphate conversion layer and the base iron oxide layer undergo changes. 

The advantages of the various oxidation processes include nonselective and rapid destruction of pollutants, absence of residues, and improved biodegradabUity of the effluents. Some of the disadvantages are extremely short half-hfe of the oxidants and high expense of their generation. 

That the Mn402 is capable of adopting at least three isolable oxidation levels thus lends support, albeit circumstantial, to the possibility that it is to be found as the redox component in the WOC, if only at the lower S states. In support of this, the various oxidation levels of the synthetic materials have been found to be electrochemically interconvertible, and a [Mn402] level has been detected electrochemically (31). 

Difficulties arise, however, in mixtures and the magnetization curves will then depend on the proportions of the various oxides, including the antiferromagnetic ones (hematite, goethite) and on the crystal type (SD, MD). Positive identification is then only possible with the help of other methods, such as XRD (see for example Resende et al., 1986 Allan et al., 1988). 

---