Oxidation states of chromium

Chromium(VI) oxide is acidic, and the corresponding salts are the chromates and dichromates, containing the ions CrO and Cr207 . i.e. [Cr04 -I- CrOj] ". The oxidation state of chromium is -f6 in each ion (cf sulphur in and 8207 ). 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

Chromium Exposure Levels and U.S. Government Regulations. The level of exposure to chromium compounds for employees in industry and for the general population via waste disposal and industrial emissions is the subject of much regulation, research, and controversy. Some U.S. Government regulations, such as the Comprehensive Environmental Response, Compensation, and LiabiUty Act (CERCLA), also known as the Superfund Act, make no distinction as to the oxidation state of chromium (144). However, there is valence distinction in other regulations. 

MeCDOHMe, showing that the a hydrogen is removed in the rate-determining step. Note that, as in 14-6, the substrate is oxidized by three different oxidation states of chromium. 

The oxidation by chromic acid alone leads to a mixture of cyclobutanone and 4-hydroxybutyraldehyde the existence of an isotope effect for the oxidation of I-deuteriocyclohexanol suggests that Cr(VI) produces the ketone and lower oxidation states of chromium produce the cleavage product. 

Cr3+ and Cr6+ are the most stable oxidation states of chromium, but with the only difference that while +3 oxidation state is cationic where as the +6 oxdation state is oxoanionic. However, the other oxidation states of +1, +2, +4 and +5 are also known for chromium, especially in aqueous solution at different pH. Inter-conversion of these oxidation states too is very frequent. With this view, an attempt is made here to examine the effect of ultrasound on the inter-convertibility of chromium among various oxidation states in aqueous solutions. The details of this study is reported in the literature [36]. 

The overall result of second step is the reduction of HCr04 to HCrCV, a two electron (2 e ) change in the oxidation state of chromium, from Cr(VI) to Cr(IV). 

The commonest oxidation states of chromium in its complexes are III and II (electronic configuration d3 and d4, respectively), even if Cr(I)-d5 complexes are known. 

The most stable oxidation states of chromium in the subsurface environment are Cr(III) and Cr(VI), the latter being more toxic and more mobile. The oxidation of Cr(III) in subsurface aqueous solutions is possible in a medium characterized by the presence of Mn(IV) oxides. Eary and Rai (1987), however, state that the extent of Cr(III) oxidation may be limited by the adsorption of anionic Cr(VI) in acidic solutions and the adsorption and precipitation of various forms of Cr(OH). These authors also report a rapid quantitative stoichiometric reduction of aqueous Cr(VI) by aqueous Fe(ll), in a pH range covering the acidity variability in the subsurface even in oxygenated solutions. 

Initiation hy the Phillips catalyst is not well understood. Both Cr(II) and Cr(III) have been proposed as the active oxidation state of chromium. Initiation involves the formation... 

Domenech A, Torres FJ, Ruiz de Sola E, Alarcon J (2006) Electrochemical detection of high oxidation states of chromium(lV and V) in chromium-doped cassiterite and tin-sphene ceramic pigmenting systems. Eur J Inorg Chem 638-648. 

Wood, R. M., Aboud, K. A., Palenik, R. C., and Palenik, G. J. (2000). Bond valence sums in coordination chemistry. Calculation of the oxidation state of chromium complexes containing only Cr-O bonds and a redetermination of the crystal structure of potassium tetra(peroxo)chromate(V). Inorg. Chem. 

In the laboratory, acidified solutions of dichromates, in which the oxidation state of chromium is +6, are useful oxidizing agents ... 

Since the reduction of chromate(VI) to metal is a six-electron process, it would seem logical to plate the metal from an electrolyte based on a lower oxidation state of chromium. A good deal of work has been done on the development of trivalent chromium plating,8 but it is only recently that a commercial process has been available in the United Kingdom. 

What are the most common oxidation states of chromium Of these, why is chromium in the lower oxidation state generally insignificant in water ... 

A similar investigation combining UV-vis and Raman spectroscopy showed an equivalent effect of feed composition on the oxidation state of chromium in zirconia-supported chromia catalysts (Malleswara Rao et al., 2004). 

Complex 6.37 has been shown to be an active catalyst for the manufacture of HDPE. Note that 6.37 is a 15-electron complex with chromium in the formal oxidation state of 3 +. The mechanism of polymerization involves generation of coordinative unsaturation through the dissociation of a THF molecule from 6.37. The evidence for an oxidation state of 3+ in the commercial catalyst comes from the fact that complex 6.38 is active for polymerization. However, complex 6.39, identical to 6.38 in every respect except the oxidation state of the metal ion, is inactive. Note that the oxidation state of chromium in 6.39 is 2+. 

The two main oxidation states of chromium usually display opposite charges in their compounds. Chromium(VI) is particularly toxic and all operations are subject to stringent legislation in most countries with significant process industries. Chromium(III) is oxidized by hydrogen peroxide to chromium(VI) in neutral or alkaline conditions, but reduced from chromium (IV) to (III) in acid (Figure 6.4). 

A comparison of the isoelectronic CpFe(CO)2R an CpCr(NO)2R is also noteworthy. When R = CHaPh or Ph, the latter system inserts SO2 much more rapidly than the former (61, 71). This may be a result of the lower formal oxidation state of chromium(O) than of iron(II) and is consistent with an electrophilic nature of the insertion. 

Although the actual oxidation state of chromium in the active catalyst is unclear, the reaction has again been interpreted as an ethylene dimerization leading to a metallacycle, in this case followed by ethylene addition and p-hydrogen elimination (Figure 16). 

Calculate the change in oxidation state of chromium during the following reaction ... 

Fortunately, it has been established that the reduction of the oxidized precursors can also be performed with a simpler reductant such as CO, with formation of a single oxidation product (CO2) that is not adsorbed on the sample (3,182,185,195-197). This simpler reduction procedure allows one to obtain a simplified version of the catalyst, whereby the oxidation state of chromium and the surface hydroxylation are much better controlled. According to the literature data, no significant difference in the polymerization products has been found between the CO-reduced system and the ethene-reduced one (3,182,198). Therefore, this CO-reduced catalyst, containing predominantly anchored Cr(II), has been considered as a model catalyst. ... 

The most common oxidation states of chromium in inorganic componnds are 11, III, and VI. By comparison, there are few stable compounds of tetravalent or pentavalent chrominm these oxidation states are largely represented by reactive intermediates in redox reactions. 7r-Acid ligands like CO (see Tt -A del Ligand) stabilize chromium in very low formal oxidation states (down to -IV ). Table 3 lists oxidations states and coordination environments for representative chromium compounds. 

The Oxidation States of Chromium. The principal oxidation states of chromium are represented in the following diagram ... 

Oxidation states of chromium - -2, - -3, and -f-6. Oi es of chromium chronate, FeCr204, and crocoite, PbCr04. Chromium metal and its alloys ferrochrome, alloy steels, stainless steel. The aluminothermic process (Goldschrtiidt process). Electrolytic chromium. Chromium trioxide, chromic acid, dichromic acid, potassium chromate, potassium didiromate, sodium chromate, lead chromate. Equilibrium between chromate ion and dichromate ion. Chrome-tanned leather. Chromic oxide (chrome green) chromic ion, chrome alum, chromic chloride, chromic hydroxide, chromite ion. Chromous compounds. Peroxy-chromic acid. 

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