Chromium complexes oxide fluorides

Chromic acid strontium salt Chromic acid strontium salt (1 1). See Strontium chromate Chromic acid, zinc salt. See Zinc chromate Chromic anhydride. See Chromium trioxide Chromic chloride. See Chromium chloride (ic) Chromic chloride stearate. See Stearatochromium chloride complex Chromic fluoride. See Chromium fluoride (ic) Chromic hydrate Chromic hydroxide Chromic (III) hydroxide. See Chromium hydroxide (ic) Chromic nitrate. See Chromium nitrate Chromic oxide. See Chromium oxide (ic) Chromic oxide hydrated. See Chromium hydroxide green... 

Redox potentials of chromium complexes Refractory metal electrodeposition Residual oxide in fluoride melts... 

A very rapid oxidative disintegration of chromium-bearing minerals, rocks and alloys is obtained by fusing or sintering the finely pulverized material with potassium bifluoride (platinum spoon). Potassium chromate results and may be detected by means of the diphenylcarbazide reaction. The fluoride disintegration is particularly recommended for the detection of chromium in steels or special alloys, which are likely to contain molybdenum. The latter, in the form of molybdate ions, reacts with diphenylcarbazide to yield a red-violet color and thus impairs the test for chromium. However, the fluoride method yields no M0O4" ions, but instead complex [M0O3F2] ions, which do not react with diphenylcarbazide. 

Sheldon and co-workers have circumvented this problem to some extent by three approaches 46 the use of sulfuric acid to reduce the pH, by addition of ammonium fluoride and by addition of ammonia. Ammonia stabilizes monomeric chromium(III) species via the formation of amine complexes, and the fluoride effects dissolution of the silica at near-neutral pH. The three catalysts that were synthesized were evaluated in the oxidative cleavage of styrene with 35% m/m hydrogen peroxide in 1,2-dichloroethane at 70 °C (Table 4.4). The... 

The complex m-[Cr(en)2F2]I is a useful starting material for the preparation of compounds of the type m-(Cr(en)2FX]I (X = CP, Br, NCS, HjO, and NHs). " The initial preparation of m-[Cr(en)2F2]I involved the reaction of anhydrous 1,2-ethanediamine with CrFj-xHjO that was suspended in a dry diethyl ether solution of hydrogen fluoride. Workup of the crude product from hydroiodic acid gave the desired compound. The present method, which is similar to that reported previously, utilizes the direct reaction of chromium(lll) fluoride-water (1/3.5) with excess dry 1,2-ethanediamine to yield m-[Cr(en)2F2](Cr(en)F4)-xH20, from which the desired cation can be easily isolated. This method avoids the difficulty of preparing easily oxidized Cr(II) compounds and working in an atmosphere of nitrogen. 

The tendency of halides to form metal halide complex is very important in understanding the stabilization of corrosion pit by prevention of the repassivation of a defect site within the passive layer. Among the halides, fluoride forms strong complexes with metals. The resistance of chromium to localized corrosion is because of slow dissolution kinetics of Cr(III) salts. Higher-valence oxides are the best passivators (films) because of their slow rates of dissolution. 

For the yellow chromating of aluminum, solutions containing chromium(VI) compounds as well as simple or complex fluorides and activators are used to accelerate layer formation. The pH value is 1.5-2.5 at total bath concentrations of 5-20 g/L. The conversion layers consist of oxides or hydrated oxides of trivalent and hexava-lent chromium and aluminum. The color of the layer may range from colorless through yellowish iridescent to yellowish brown, corresponding to an increase in the surface weight from 0.1 to 3 g/m. ... 

These studies differ in that, whereas Wells and co-workers have spectrally characterised an intermediate and suggested from the data that the substitution rate for Mn" is 5 x 10 1 mol s" (c/. the recent study by Diebler on the formation of fluoride species), since it should lie between that for the chromium(ii) species and the oxidation rate for the Mn" -hydroquinone complex, Davies and Kustin postulate in the acid range 0-6—3-60 mol 1 an inner-sphere mechanism with hydrogen atom transfer as the predominant mode of reaction for MnOH +, with rates not too different from those derived from studies on other systems (Table 1). The reaction is first-order in each reactant and the observed second-order rate constant is invariant with [Mn" ]o, [H2Q], and [Mn"]. A mechanism consistent with intermediate formation may be written... 

Chrome baths always contain a source of hexavalent chromium ion (e.g., chromate, dichromate, or chromic acid) and an acid to produce a low pH which usually is in the range of 0-3. A source of fluoride ions is also usually present. These fluoride ions will attack the original (natural) aluminum oxide film, exposing the base metal substrate to the bath solution. Fluoride also prevents the aluminum ions (which are released by the dissolution of the oxide layer) from precipitating by forming complex ions. The fluoride concenfration is critical. If the concentration is too low, a conversion layer will not form because of the failure of the fluoride to attack the natural oxide layer, while too high a concentfa-tion results in poor adherence of the coating due to reaction of the fluoride with the aluminum metal substrate. 

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