Oxidation of Cr

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. 

Chromium (TV) oxide [12018-01 -8], is obtained from the hydrothermal decomposition of mixed oxides of Cr(III) and Cr(VI). A mixed oxide... 

Griskin et reported that there is no apparent effect of steam pressure on the rate of oxidation of Cr-Ni steels at temperatures between 600°C and 650°C at 10.1-20.2 MPa. Similar observations for Cr-Mo and Cr-Mo-V steels between 500°C and 600°C have been made by Wiles" . She compared low-alloy steel samples exposed to 101 kPa steam with power plant components that had operated for up to 150000b in steam at 17.25 MPa and found no significant difference in the oxidation rates (Fig. 7.11). 

Anderson and Bonner made the first detailed kinetic study on the exchange using the isotopic method ( Cr) and a separation method based on the conversion of Cr(II) into Cr(IIl) oxalate and an ion-exchange treatment. To prevent oxidation of Cr(II) during exchange a hydrogen atmosphere was maintained over the reaction mixture. The rate law found to be obeyed for the concentration ratio range Cr(III)/Cr(II) of between 3.3 x 10 and 2.0 in perchlorate media was... 

The oxidation of Cr(Il) by Fe(ril) in perchloric acid is markedly catalysed by chloride ion. Taube and Myers found that Cr(H20)5Cl is formed along with Cr(Fl20)5 as products of the oxidation, the relative proportions of the two species depending on concentrations of H and Cl . The suggestion was made that Cr(H20)5Cl is produced by reaction of Cr(H20)g with a chloro complex of Fe(III), viz. 

Although the oxidation of Cr(lII) by Ce(lV) is a rapid reaction in perchlorate solutions, the oxidation of Cr(III) by Co(III) in the same media occurs at a rate similar to or slower than that of the thermal decomposition of Co(lll) in perchloric acid (3 M). However, the Co(lII)- -Cr(III) reaction is subject to catalysis by Ag(I) ion and Kirwin et al have made a kinetic study of this system, viz. 

The Cr(III) compounds with the formula Cr(R2cftc)3 are easily prepared for various R groups. They are stable and well-studied (2). Electronic spectra (20, 21) and EPR data (22, 23) are in accordance with an octahedral coordination of six S atoms around the metal. Voltametric data for the reduction and oxidation of Cr(R2[Pg.91]

The following mechanism for the oxidation of Cr(III) to Cr(VI) on a Pb02 surface has been postulated. Pb02 operates as oxygen carrier anode... 

The redox cycling of these elements have pronounced effects on the adsorption of trace elements onto oxide surfaces and trace element fluxes under different redox conditions. The hydrous Mn(III,IV) oxides are important mediators in the oxidation of oxidizable trace elements e.g., the oxidation of Cr(lll), As(III) and Se(III) is too slow with 02 however, these elements subsequent to their relatively rapid adsorption on the Mn(III,IV) oxide are rapidly oxidized by MnfUI.IV). 

The main kinetic consideration is the time-scale of the redox reaction if the relevant electron-transfer reaction is slow, then we run the risk that measurements are taken before a true equilibrium has been attained after the addition of each aliquot. In practice, however, most analytes are oxidized or reduced within a very short time-scale - probably within microseconds if mixing is efficient, and provided that the oxidant (e.g. H2O2, Mn04, Ce or Cr20j ) or reductant (e.g. chromous ion, Cr, dithionite, 8204, or thiosulfate, S203 ) is sufficiently powerful. Note that while oxidation of Cr to form is fast, the... 

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. 

Cr203 has some tendency to reaction with H2O. High surface area facilitates the oxidation of Cr(III) to Cr(VII). Proper destruction of the used catalyst is rather expensive. 

This mechanism can be illustrated by the reaction of ferrous ions with hydrogen peroxide (42), the reduction of organic peroxides by cuprous ions (63), as well as by the reduction of perchlorate ions by Ti(III) (35), V(II) (58), Eu(II) (71), The oxidation of chromous ions by bromate and nitrate ions may also be classified in this category. In the latter cases, an oxygen transfer from the ligand to the metal ion has been demonstrated (8), As analogous cases one may cite the oxidation of Cr(H20)6+2 by azide ions (15) (where it has been demonstrated that the Cr—N bond is partially retained after oxidation), and the oxidation of Cr(H20)6+2 by 0-iodo-benzoic acid (6, 8), where an iodine transfer was shown to take place. 

Another interesting case is the oxidation of a system resulting in the simultaneous oxidation of the central atom and the ligand. Reference is made here to the oxidation of certain aquated transition metal ions, at their low states of oxidation by the action of HsO4". The oxidation of Mn+ by H30+, which was shown to involve a hydride transfer from the ligand (P), follows this type of behavior. The oxidation of Cr(II) by H2O proceeds by an analogous mechanism (14). 

It is of interest to compare the influence of cerium(III) on the cerium(IV)-ar-Cr(OH2)2(0204)2" reaction with the similar retarding action by cerium(III) which Tong and King (43) found in their careful kinetic investigation of the cerium(IV) oxidation of Cr(OH2)6+3 in acidic-sulfate media. The observed rate law for the latter reaction may be written in the form... 

Not until similar information is obtained on the effect of cerium(III), hydrogen ion, and sulfate ion on the oxidations of Cr(C2C>4)3 3 and Cr(OH2)4C204+, will it be appropriate to discuss the relative reactivities of the three oxalato complexes toward cerium(IV). 

The chemistry in this area (6-8) has been approached from two different directions reduction of Cr(VI) or oxidation of Cr(II) it is only recently that an overall, self-consistent picture has emerged (7,9). The key experiment is the observation that the reaction between Cr(VI) and alcohols in acid solution under an 02 atmosphere yields [(H20)5 Cijii02]2+, a Cr(III) superoxo complex,1 according to the sequence (1). 

Sequential steps are proposed (75- 77) with two monomeric units combining to form first a single hydroxo-bridged dimer, which in turn closes to give the double hydroxo-bridged dimer. This latter species [(H20)4Cr(0H)2Cr(0H2)4]4+ is one of the major products from the 02 oxidation of [Cr(OH2)6]2+ (11) (Section I,A), and the crystal structure of the p-toluenesulfonate salt has been determined (78). 

The oxidation of [Cr(SH)(H20)5]2+ by I2 or Fe3+ under aerobic conditions in acid solutions gives the disulfido-bridged complexes [(H20)5CrS2Cr(H20)5]4+ and [(H20)sCr(S2H)Fe-(H20)5]4+ respectively (Scheme 100).967,968 The latter complex can also be obtained by substitution of chromium(III) in the former complex by iron(II) under acid conditions. The product distribution in the iron(UI) oxidation of [Cr(SH)(H20)5]2+ is pH dependent and at 298 K, pH = 1 the heteronuclear dimer [(H20)5Cr(S2H)Fe(H20)5]4+ constitutes over 80% of the product mixture. The rate of this reaction shows a [H+] 1 dependence, an observation consistent with [CrS(H20)5]+ being the kinetically active species. 

The oxidation of Cr(MPDME) in solution by 02 or H202 can be reversed by sodium dithionite. Since /ie = 2.84 BM in the solid and 5.19 BM in CHC13, there are axial interactions in the solid state which are removed on dissolution. 

One oxochromium(V) complex, CrO(TETMC), containing the trinegative anion of a corrole (279), has been characterized as the solid.1266 It is prepared (Table 102) simply by exposure to air of a solution presumably containing a Cr11 complex. Aerial oxidation of Cr (TPP) produces the oxochromium(IV) complex CrO(TPP) so the corrole ligand apparently facilitates autoxida-tion. The redox behaviour of CrO(TETMC) has been examined by cyclic voltametry.1267... 

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