Chromium iodide

Mukherjee studied the gas phase equilibria and the kinetics of the possible chemical reactions in the pack-chromising of iron by the iodide process. One conclusion was that iodine-etching of the iron preceded chromis-ing also, not unexpectedly, the initial rate of chromising was controlled by transport of chromium iodide. Neiri and Vandenbulcke calculated, for the Al-Ni-Cr-Fe system, the partial pressures of chlorides and mixed chlorides in equilibrium with various alloys and phases, and so developed for pack aluminising a model of gaseous transport, solid-state transport, and equilibria at interfaces. 

A classical example of this process is given by the van Arkel method for the preparation (purification) of several metals. If impure Cr, for instance, is contained together with a small quantity of iodine in a vacuum tube maintained at a temperature at which chromium iodide volatilizes, and a hot zone is created by means of, say, a W filament heated by an electric current, the following reaction will be observed ... 

In the case of dibenzene chromium and its cation, the most complete investigation of the electronic spectra is due to Yamada and co-workers (76) who report both solution and crystal spectra in detail for dibenzene chromium iodide and ditoluene chromium iodide. Four intense bands were observed in the region 3 to 6 eV, and a detailed discussion of these in terms of molecular-orbital assignments has been given by Berry (78), although no direct comparison with the energy-level scheme of Shustorovich and Dyatkina (75) is reported. The intense band at 5.51 eV (log max = 4.14) for... 

The sandwich structure of bis (benzene) chromium (XXV) prepared by this new route was soon confirmed by Weiss and Fischer (242). Moreover, Fischer and Sens (98) prepared bis (phenyl) chromium iodide (XXII) by the Friedel-Crafts-type method, starting with biphenyl in place of benzene, and were able to show that the product obtained in this manner was identical to bis (biphenyl) chromium iodide (XXII) isolated via Heines Grignard route. A longstanding and vexing problem in coordination chemistry was thus finally solved. 

Dibenzenechromium iodide is very soluble in water. Benzene-biphenyl-chromium iodide is fairly soluble in water, but bis-biphenylchromium iodide is insoluble. Why ... 

Tris- r-allyl chromium polymerizes butadiene giving 1,2-polybutadiene, whilst bis- r-allyl chromium iodide causes cyclotrimerization to cyclo-dodeca-1,5,9-trienes. 

The dichromate ion oxidises iron(II) to iron(III), sulphite to sulphate ion, iodide ion to iodine and arsenic(III) to arsenic(V) (arsenate). Reduction of dichromate by sulphite can be used to prepare chrome alum, since, if sulphur dioxide is passed into potassium dichromate acidified with sulphuric acid, potassium and chromium(III) ions formed are in the correct ratio to form the alum, which appears on crystallisation ... 

Chemical ingenuity in using the properties of the elements and their compounds has allowed analyses to be carried out by processes analogous to the generation of hydrides. Osmium tetroxide is very volatile and can be formed easily by oxidation of osmium compounds. Some metals form volatile acetylacetonates (acac), such as iron, zinc, cobalt, chromium, and manganese (Figure 15.4). Iodides can be oxidized easily to iodine (another volatile element in itself), and carbonates or bicarbonates can be examined as COj after reaction with acid. 

CH3I (methyl iodide) principal axes, 103 If rotation, 113 CH2NH (methanimine) interstellar, 120 Cr203 (chromium trioxide) in alexandrite laser, 347ff in ruby laser, 346ff HC3N (cyanoacetylene) interstellar, 120 HCOOH (formic acid) interstellar, 120 NH2CN (cyanamide) interstellar, 120... 

Table 3. Mechanical Properties of Room Temperature Swaged Iodide Chromium ...

The vacuum melting process can upgrade chromium at a modest cost the other purification processes are very expensive. Thus iodide chromium is about 100 times as expensive as the electrolytic chromium and, therefore, is used only for laboratory purposes or special biomedical appHcations. 

The anhydrous hahdes, chromium (II) fluoride [10049-10-2], chromium (II) bromide [10049-25-9], CrBr2, chromium (II) chloride [10049-05-5], CrCl2, and chromium (II) iodide [13478-28-9], 03x1, are prepared by reaction of the hydrohaUde and pure Cr metal at high temperatures, or anhydrous chromium (II) acetate [15020-15-2], Cr2(CH2COO)4, atlower temperatures, or by hydrogen reduction of the Cr(III) hahde at about 500—800°C (2,12). 

The anhydrous halides, chromium (ITT) fluoride [7788-97-8], CrF, chromium (ITT) chloride [10025-73-7], CrCl, chromium (ITT) bromide [10031-25-1], CrBr, and chromium (ITT) iodide [13569-75-0], Crl, can be made by the reaction of Cr metal and the corresponding halogen at elevated temperatures (12,36). Other methods of synthesis for the haUdes are also possible (36—38). All of the haUdes have a layer stmcture and contain Cr(III) in an octahedral geometry. They are only slightly soluble in water but dissolve slowly when Cr(II) or a reducing agent such as Zn or Mg is added. 

Wet-Chemical Determinations. Both water-soluble and prepared insoluble samples must be treated to ensure that all the chromium is present as Cr(VI). For water-soluble Cr(III) compounds, the oxidation is easily accompHshed using dilute sodium hydroxide, dilute hydrogen peroxide, and heat. Any excess peroxide can be destroyed by adding a catalyst and boiling the alkaline solution for a short time (101). Appropriate ahquot portions of the samples are acidified and chromium is found by titration either using a standard ferrous solution or a standard thiosulfate solution after addition of potassium iodide to generate an iodine equivalent. The ferrous endpoint is found either potentiometricaHy or by visual indicators, such as ferroin, a complex of iron(II) and o-phenanthroline, and the thiosulfate endpoint is ascertained using starch as an indicator. 

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