Reduction by chromium

The rate behaviour for the reduction of salicylato and several substituted salicylato derivatives of [(NH3)5Co] was consistent with the intervention of chelated precursor complexes formed from the two cobalt(iii) centres with loss of H, after which internal electron-transfer competes with non-productive dissociation of the precursor. The kinetics of reduction by chromium(ii) of a series of 2-hydroxy-benzenato derivatives of [(NH3)5Co] were compared with those of the 2-amino-benzoate derivatives. A study of the kinetics of dissociation of oxalatobis(phenan-throline)cobalt(iii) into cis-d iaquobis(phenanthroline)cobalt(iii) in aqueous HCl-KCl media found the rate was first order with respect to The photo-redox... 

Kinetics are reported for aquation and for reduction by chromium(n). The stoicheio-metry is... 

Photosensitive Reactions. The reduction of chromium (VI) by organic compounds is highly photosensitive, and this property is used ia photosensitive dichromate-coUoid systems. 

Cephachlor (35) became accessible when methods for the preparation of C-3 methylenecephalosporins became convenient. The allylic C-3-acetoxyl residue characteristic of the natural cephalosporins is activated toward displacement by a number of oxygen- and sulfur-containing nucleophiles. Molecules such as can therefore be prepared readily. Subsequent reduction with chromium(II) salts leads to the desired C-3... 

A voltaic cell consists of two half-cells. One of the half-cells contains a platinum electrode surrounded by chromium(III) and dichromate ions. The other half-cell contains a platinum electrode surrounded by bromate ions and liquid bromine. Assume that the cell reaction, which produces a positive voltage, involves both chromium(III) and bromate ions. The cell is at 25°C. Information for the bromate reduction half reaction is as follows ... 

It is possible to titrate two substances by the same titrant provided that the standard potentials of the substances being titrated, and their oxidation or reduction products, differ by about 0.2 V. Stepwise titration curves are obtained in the titration of mixtures or of substances having several oxidation states. Thus the titration of a solution containing Cr(VI), Fe(III) and V(V) by an acid titanium(III) chloride solution is an example of such a mixture in the first step Cr(VI) is reduced to Cr(III) and V(V) to V(IV) in the second step Fe(III) is reduced to Fe(II) in the third step V(IV) is reduced to V(III) chromium is evaluated by difference of the volumes of titrant used in the first and third steps. Another example is the titration of a mixture of Fe(II) and V(IV) sulphates with Ce(IV) sulphate in dilute sulphuric acid in the first step Fe(II) is oxidised to Fe(III) and in the second jump V(IV) is oxidised to V(V) the latter change is accelerated by heating the solution after oxidation of the Fe(II) ion is complete. The titration of a substance having several oxidation states is exemplified by the stepwise reduction by acid chromium(II) chloride of Cu(II) ion to the Cu(I) state and then to the metal. 

Reductant equivalent weights of, 847 Reduction 409 by chromium(II) salts, 409 by hydrogen sulphide, 416 by Jones reductor (zinc amalgam), 410 by liquid amalgams, 412 by silver reductor, 414 by sulphurous acid, 416 by tin(II) chloride, 415 by titanium(II[), 410 by vanadium(II), 410 see also Iron(III), reduction of Reduction potentials 66 Reference electrodes potentials, (T) 554 Relative atomic masses (T) 819 Relative error 134 mean deviation, 134... 

The kinetics of NO reduction by hydrogen and CO was studied by Ayen and Peters. Hydrogen reduction of NO over oxides of copper, zinc, and chromium was studied at 375-425°C. The products formed include... 

We are of the opinion that the reaction can be explained without assuming the reduction of chromium(rV) to chromium(Il) by arsenic(III), although this step is... 

Methanol is sometimes used to prepare dichromium trioxide by the reduction of chromium oxide (Vi). Since these attempts lead to spontaneous ignition and detonation, it is preferable to use the thermal decomposition of ammonium dichromate, provided it is carried out with caution (see p.200). 

The production of chromium metal by the aluminothermic reduction of chromium sesquioxide can be represented by the equation ... 

Appendix D presents the U.S. EPA data on a full-scale treatment of aluminum forming waste-water by chromium reduction, chemical precipitation, and sedimentation clarification. Chromium reduction, as described in detail by Wang, Hung and Shammas,9 is an important step prior to... 

Full Scale Treatment of Aluminum Forming Wastewater by Chromium Reduction, Chemical Precipitation, and Sedimentation Clarification... 

As an example, we consider the reduction by dihydrogen sulfide (H2S) of hexava-lent chromium (CrVI) to its trivalent form (Crm). Kim et al. (2001) report when pH is in the range 6.5-10, this process can occur via a reaction,... 

Kim C., Q. Zhou, B. Deng, E. C. Thornton and H. Xu, 2001, Chromium(VI) reduction by hydrogen sulfide in aqueous media stoichiometry and kinetics. Environmental Science and Technology 35, 2219-2225. 

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). 

In some cases the methods may be combined. Examples would include the biotechnological precipitation of chromium from Cr(VI)-containing wastes from electroplating factories by sulfate reduction to precipitate chromium sulfide. Sulfate reduction can use fatty acids as organic substrates with no accumulation of sulfide. In the absence of fatty acids but with straw as organic substrate, the direct reduction of chromium has been observed without sulfate reduction [43]. 

Divalent chromium salts show very strong reducing properties. They are prepared by reduction of chromium(III) compounds with zinc [187] or a zinc-copper couple and form dark blue solutions extremely sensitive to air. Most frequently used salts are chromous chloride [7SS], chromous sulfate [189], and less often chromous acetate. Reductions of organic compounds are carried out in homogeneous solutions in aqueous methanol [190], acetone [191], acetic acid [192], dimethylformamide [193] or tetrahydrofuran [194] (Procedure 37, p. 214). 

H. A. Headlam and P. A. Lay, EPR spectroscopic studies of the reduction of chromium(VI) by methanol in the presence of peptides. Formation of long-lived chromium(V) peptide complexes, Inorg. Chem., 40 (2001) 78-86. 

M. Branca and G. Micera, Reduction of chromium(VI) by D-galacturonic acid and formation of stable chromium(V) intermediates, Inorg. Chim. Acta, 153 (1988) 61-65. 

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