Oxidation of chromium

In the case of chromium in 1 N H2SO4 transpassivity occurs at about 1 1 V (below the potential for oxygen evolution, since the equilibrium potential in acid solutions at pH 0 is 1 23 V and oxygen evolution requires an appreciable overpotential) and is associated with oxidation of chromium to dichromate anions ... 

Fig. 12.24 Rate of oxidation of chromium-diffused mild steel (0- l5 l oC) when heated in air at...

Fig. 12.25 Influence of carbon content on the rate of oxidation of chromium-diffused steel...

Determination of chromium as lead chromate (precipitation from homogeneous solution) Discussion. Use is made of the homogeneous generation of chromate ion produced by the slow oxidation of chromium(III) by bromate at 90-95 °C in the presence of excess of lead nitrate solution and an acetate buffer. The crystals of lead chromate produced are relatively large and easily filtered the volume of the precipitate is about half that produced by the standard method of precipitation. 

The kinetics of the oxidation of chromium(II) by vanadium(ni) in acid perchlorate media have been studied spectrophotometrically between 0.2° and 35.0 °C over a range of 0.027-0.500 M HC104 . The oxygen-sensitivity of both reactants meant that the air had to be excluded in all kinetic runs. Also, since V(III) slowly reduces perchlorate ion, fresh solutions of V(III) were required for each experiment. In terms of stoichiometry the reaction conforms accurately to... 

Vanadium forms numerous oxides, the most important of which are vanadium monoxide, vanadium sesquioxide, vanadium dioxide and vanadium pentoxide. In the earlier examples (e.g., oxides of chromium and of niobium) the enthalpy values for the aluminothermic reduction of each of the oxides was given for the purpose of illustration. Normally, the consideration can be restricted to only those oxides which are readily obtained and which can be handled freely without any special or cumbersome precautions. In the case of vanadium for example, it is sufficient to consider the reduction of the sesquioxide (V203) and the pentoxide (V2Os). The pertinent reactions are ... 

Kolasa et al. [562] have reported changes in the degree of oxidation of chromium (from Cr6+ to Cr3+) in the course of probe mineralisation of PE for AAS analysis. HPLC-ICP-MS has been used as a selenium-specific detector [563]. Other selenium speciation work by ICP-MS has been reported [564]. Numerous other examples of speciation analysis have been described for the most appropriate techniques (Chapter 7). 

The black powder ignites if ground or heated in air [1], The oxide obtained by oxidation of chromium amalgam is pyrophoric [2],... 

Balthis and Bailar6 obtained tris (ethylenediamine) chromium-(III) complexes by the oxidation of chromium(II) solutions, using a procedure somewhat similar to that used for the synthesis of cobalt (III) com plexes. Mori7 described the preparation of hexaamminechromium(III) salts from the oxidation of chromium (II) salts in the presence of ammonia. The results obtained in both syntheses have been erratic.8,9 Berman noted that the foregoing syntheses are rendered dependable by the use of a catalyst of activated platinum on asbestos. Schaeffer,100 in a subsequent study, independently used colloidal platinum as a catalyst but reported some difficulty in separating it from the product.106 The procedures recommended and described here are based on the use of platinized asbestos as the catalyst. 

Sodium chromate is produced as part of the process for obtaining Cr203, and it is probably the most important chromium compound. Although there are other oxides of chromium, Cr203 is very important because of its catalytic properties. One way to obtain this oxide is by the decomposition of ammonium dichromate,... 

The acidity and the covalent nature of transition metal oxides generally increases with increasing oxidation state of the metals. This is shown by the oxides of chromium. 

Common catalyst compositions include oxides of chromium or molybdenum, or cobalt and nickel metals, supported on silica, alumina, titania, zirconia, or activated carbon. 

The first source of chromium was found in the mineral crocoite. Today it is obtained from the mineral chromite (FeCr O ), which is found in Cuba, Zimbabwe, South Africa, Turkey, Russia, and the Philippines. Chromite is an ordinary blackish substance that was ignored for many years. There are different grades and forms of chromium ores and compounds, based on the classification of use of the element. Most oxides of chromium are found mixed with other metals, such as iron, magnesium, or aluminum. 

Chromiain(ii) Complexes.—The oxidation of chromium(ii) in alkaline solution has been studied polarographically and the reaction shown to be irreversible with = — 1.65 V vs. S.C.E. In the presence of nitrilotriacetic acid, salicylate, ethylenediamine, and edta the values were determined as —1.075, —1.33, — 1.38, and —1.48 V, respectively. The production of [Cr(edta)NO] from [Cr (edta)H20] and NO, NOJ, or NO2 suggests that this complex is able to react via an inner-sphere mechanism in its redox reactions. ... 

Since the properties of erythronium closely resembled those which Fourcroy had ascribed to the recently discovered metal chromium, del Rio lost confidence in the importance of his discovery and concluded that his supposed new element was, after all, nothing but chromium (11). In a note to his translation of Kars ten s Mineralogical Tables he wrote (7, 9,12) .. . but, knowing that chromium also gives by evaporation red or yellow salts, I believe that the brown lead is a yellow oxide of chromium, combined with excess lead also in the form of the yellow oxide. ... 

This electron transfer is even more general with highly-charged positive ions thus, if 0rO3, containing a hexavalent Cr ion, reacts with H2S, sulphur and a lower oxide of chromium are formed according to the equation... 

Metals can be used as sacrificial anodes in the electrochemical synthesis of anhydrous salt and complexes. This method frequently produces chromium(IH) complexes, but electrochemi cal oxidation of chromium in a mixture of concentrated aqueous HBF4 and acetonitrile yield [CKMeCN KBF,),.82... 

Dowex 50W-X2). The compounds were tentatively identified as Cr02Cr4+ and Cr02Cr02Cr5+ species their decompositions were not simple processes. Attempts to synthesize these or similar species by the oxidation of chromium(II) with oxygen were unsuccessful. The compound [(H20)5Cr02Cr(H20)5]4+ was also reported earlier. 

Green may be produced either by protoxide of iron, protoxide of copper—Cn 0—or oxide of chromium— Cra Oj. Tho tint produced by the Erst of these substances has little brilliancy. The oxide of copper yields... 

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