Chromium reductions, table

A 5% solution of chromium trioxide-pyridine complex in dry methylene chloride is prepared. The alcohol (0.01 mole) is dissolved in dry methylene chloride and is added in one portion to the magnetically stirred oxidizing solution (310 ml, a 6 1 mole ratio) at room temperature. The oxidation is complete in 5-15 minutes as indicated by the precipitation of the brownish black chromium reduction products. The mixture is filtered and the solvent is removed (rotary evaporator) leaving the crude product, which may be purified by distillation or recrystallization. Examples are given in Table 1.1. 

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

Moreno-Castilla and coworkers [139,140] did clarify the relationship between carbon surface chemistry and chromium removal. Table 3 summarizes some of the key results. Upon oxidation of carbon M in nitric acid (sample MO), the surface has become much more hydrophilic and more acidic, and the uptakes increased despite a decrease in total surface area. The enhancement in Cr(III) uptake was attributed to electrostatic attraction between the cations and the negatively charged surface. The enhancement in Cr(VI) uptake (at both levels of salt concentration) was attributed to its partial reduction on the surface of carbon MO (perhaps due to the presence of phenolic or hydroquinone groups), which is favored by the lower pH. The increase in uptake on carbon MO with increasing NaCl concentration is consistent with this explanation, from a straightforward analysis of the Debye-Hvickel and Nemst equations the decrease in uptake on carbon M was attributed to the competition of specifically adsorbed Cl and CrOj- ions on the positively charged surface. 

The increase in thickness of chromium and molybdenum coatings up to 2 pm results in some grain size growth and entail to nanohardness reduction (Table 1). However, its values remain much above tabulated values of nanohardness of these metals in a massive state and nanohardness of chromium coatings still remains above molybdenum. 

Chromium oxide is mixed with aluminum powder, placed in a refractory-lined vessel, and ignited with barium peroxide and magnesium powder. The reaction is exothermic and self-sustaining. Chromium metal of 97—99% purity is obtained, the chief impurities being aluminum, iron, and silicon (Table 4). Commercial chromium metal may also be produced from the oxide by reduction with silicon in an electric-arc furnace. 

Preparation and chemistry of chromium compounds can be found ia several standard reference books and advanced texts (7,11,12,14). Standard reduction potentials for select chromium species are given ia Table 2 whereas Table 3 is a summary of hydrolysis, complex formation, or other equilibrium constants for oxidation states II, III, and VI. 

Table 2. Standard Reduction Potentials for Chromium Species ...

Ghromium(III) Compounds. Chromium (ITT) is the most stable and most important oxidation state of the element. The E° values (Table 2) show that both the oxidation of Cr(II) to Cr(III) and the reduction of Cr(VI) to Cr(III) are favored in acidic aqueous solutions. The preparation of trivalent chromium compounds from either state presents few difficulties and does not require special conditions. In basic solutions, the oxidation of Cr(II) to Cr(III) is still favored. However, the oxidation of Cr(III) to Cr(VI) by oxidants such as peroxides and hypohaUtes occurs with ease. The preparation of Cr(III) from Cr(VI) ia basic solutions requires the use of powerful reducing agents such as hydra2ine, hydrosulfite, and borohydrides, but Fe(II), thiosulfate, and sugars can be employed in acid solution. Cr(III) compounds having identical counterions but very different chemical and physical properties can be produced by controlling the conditions of synthesis. 

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

In addition to the heavy metals stated in Table 22.10, ferro- and ferricyanide are also part of the pollutants in the wastewater generated in a chrome pigment plant. These wastes are generally combined and treated through reduction, precipitation, equalization, and neutralization to be followed by clarification and filtration processes. Most of the heavy metals are precipitated using lime or caustic soda at specific pH. Chromium is reduced by S02 to a trivalent form, wherein it is precipitated as chromium hydroxide at specific pH. Sodium bisulfide is also employed to precipitate some of the metals at a low pH. The treated water is recycled for plant use while the sludge is sent to landfills (Figure 22.7). 

Selenium (masses 74, 76, 77, 78, 80, and 82 Table 1) and chromium (masses 50, 52, 53 54 Table 1) are treated together in this chapter because of their geochemical similarities and similar isotope systematics. Both of these elements are important contaminants in surface and ground water. They are redox-active and their mobility and environmental impact depend strongly on valence state and redox transformations. Isotope ratio shifts occur primarily during oxyanion reduction reactions, and the isotope ratios should serve as indicators of those reactions. In addition to environmental applications, we expect that there will be geological applications for Se and Cr isotope measurements. The redox properties of Se and Cr make them promising candidates as recorders of marine chemistry and paleoredox conditions. 

Table 16.1 Characteristics of the investigated soils and parameters affecting redox of chromium TOC total organic carbon, SOM soluble organic matter. Reprinted with permission from Kozuh N, Stupar 1, Gorenc B (2000) Reduction and oxidation processes of chromium in soils. Environ Sci Technol 34 112-119. Copyright 2000 American Chemical Society...

The products of reduction of Fe(NCS)2+ by Cr2+ in aqueous solution containing thiocyanate ions (equation 36), include the isomers of [Cr(NCS)(SCN)(H20)4]+, which contains both bland S-bonded thiocyanate ligands.641 These isomers undergo spontaneous decomposition by parallel aquation (loss of the S-thiocyanato ligand) and isomerization (Cr—SCN— Cr—NCS) reactions. Details of the solution visible spectra of these isomers as well as those of similar chromium(III) complexes for comparison are listed in Table 68. From the compiled data it is apparent that the S-bonded thiocyanate ligand lies very close to Br- in the spectrochemical series while the N-bonded form lies between Nj and NO. 

A detailed kinetic study performed on Cr2C3 and Cr7C3 showed that the experimental data was consistent with equation (16.1) Table 16.5 presents the rate constants for the oxidation and reduction steps on the surface of the chromium carbides as well as the activation energies of these steps. The... 

Little is understood of the mechanism involved in the improvement of a wood surface after treating with chromium-containing chemicals and the subsequent durability of applied finishes (5). The data in Table II show that the reduction of the rate of weathering (as measured by loss of springwood) is directly related to chromium concentration. 

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