Chromium ions, oxidation

For metal compound categories (e.g.,chromiumcompounds), report releases of only the parent metal. Forexample, a user of various inorganic chromium salts would report the total chromium released in each waste type regardless of the chemical form (e.g., as the original salts, chromium ion, oxide) and exclude any contribution to mass made by other species in the molecule. 

Since the kinetics in these redox sterns seem to be rather complex, owing to the formation of either unstable Cr (V) or Cr (IV) transient species through the reduction of chromium ion oxidant from hexavalent state to the trivalent ones, determination of the stoichiometiy becomes very important. 

Chromium(VI) oxide is acidic, and the corresponding salts are the chromates and dichromates, containing the ions CrO and Cr207 . i.e. [Cr04 -I- CrOj] ". The oxidation state of chromium is -f6 in each ion (cf sulphur in and 8207 ). 

Chromium Removal System. Chlorate manufacturers must remove chromium from the chlorate solution as a result of environmental regulations. During crystallization of sodium chlorate, essentially all of the sodium dichromate is recycled back to the electrolyzer. Alternatively, hexavalent chromium, Cr, can be reduced and coprecipitated in an agitated reactor using a choice of reducing agents, eg, sodium sulfide, sulfite, thiosulfate, hydrosulfite, hydrazine, etc. The product is chromium(III) oxide [1333-82-0] (98—106). Ion exchange and solvent extraction techniques have also... 

When sufficient hydroxide is added to an aqueous solution of the trivalent chromium ion, the precipitation of a hydrous chromium (ITT) oxide,... 

Cr2 03 - 112 0, of indefinite composition occurs. This compound is commonly misnamed as chromic or chromium (ITT) hydroxide [1308-14-1], Cr(OH)2. A tme hydroxide, chromium (ITT) hydroxide trihydrate [41646-40-6], Cr(OH)2 3H20, does exist and is prepared by the slow addition of alkaU hydroxide to a cold aqueous solution of hexaaquachromium(III) ion (40). The fresh precipitate is amphoteric and dissolves in acid or in excess of hydroxide to form the metastable Cr(OH). This ion decomposes upon heating to give the hydrous chromium (ITT) oxide. However, if the precipitate is allowed to age, it resists dissolution in excess hydroxide. 

The addition of a small amount of cluomium at concentrations less than 1 %, increase the oxidation rate proportionately to the cluomium content. This is to be expected since the replacement of tluee nickel ions in NiO by two chromium ions in Ci 203 will introduce one cation vacancy/CiaOs molecule. 

In compounds, the important oxidation numbers of Cr are +2, +3, and +6. In all of these states the chromium ions are colored and, in fact, the element got its name from this property (ichroma is the Greek word for color). The +2 state is not frequently encountered but it can be made quite easily as the beautiful blue chromous ion in solution by dripping a solution containing CrM over metallic zinc. Air has to be excluded since O rapidly converts Cr1-5 back into Cr. ... 

So far the problem of active center formation in chromium oxide catalysts amounted mainly to a discussion of the oxidation number of chromium that is necessary for catalytic activity. As an active species chromium ions having practically every possible oxidation number—... 

Two ions of the transition metal take part in this reaction. However, in the case of supported one-component catalysts the formation of the active bond seems to occur on the interaction of the monomer with isolated ions of the transition metal. That may be illustrated by the data showing that the activity of chromium oxide catalysts decreases linearly with decreasing chromium content (or even increases per chromium ion) to the rather low (0.01%) chromium concentrations on the catalyst surface (62, 69). In... 

Complex oxide film is formed in a basic solution, whereas the films described earlier are formed in an acidic solution. Complex oxide conversion coating reactions do not contain either hexavalent or trivalent chromium ions. However, the sealing rinse contains much greater quantities of hexavalent and trivalent chromium ions than do the sealing rinses associated with phosphate conversion coatings and chromate conversion coatings. 

However the second question, whether the Cr+3 species either underwent some chemical change so that they became inert in the solution or Cr+3 ions were not available to DPC for complexation from the existing dichromate ions remain to be explained. Since either oxidation (c) or reduction (b) would occur in the solution in the given set of experimental condition, another experiment was performed to ascertain the cause of decomposition of Cr-DPC complex resulting into the decolourisation. A current of N2 gas was purged into the decolourised solution for about 10 min to remove all dissolved 02 gas from the solution and create an oxidation free atmosphere in and above the solution in the flask. The solution was sealed and left for an hour. The colourless solution changed to feebly pinkish colour and intensified over night (about 10 h). This confirmed the restoration of chromium ions to +3... 

The first laser produced was the ruby laser, invented in 1960. Rubies are crystals of aluminum oxide (corundum, AI2O3), containing about 0.5% chromium ions Cr3+, as substitution impurities, CrA, and laser action, as well as color, is entirely due to these... 

D—The dichromate ion oxidizes the sulfide ion to elemental sulfur, as the sulfide ion reduces the dichromate ion to the chromium(III) ion. Chromium goes from +6 to +3,while sulfur goes from -2 to 0. The hydrogen remains at +1, so it is neither oxidized nor reduced. 

In this state, chromium compounds are usually coloured yellow or red (but due to charge transfer (p. 60) and not to the presence of d electrons on the chromium ion). The only halide known is the unstable chromium(VI) fluoride CrF6, a yellow solid. However, oxide halides are known, for example Cr02Cl2 ( chromyl chloride ), formed as a red vapour when concentrated sulphuric acid is added to a chromate(VI) (or dichromate) mixed with a chloride ... 

The EPD functions at the oxygen atoms are also available for interaction with one more chromium trioxide molecule. Since the oxygen in the chromate ion has weaker EPD properties than the uncoordinated oxide ion, its interaction with the second chromium (VI)-oxide molecule is weaker than with the first one, and the chromium-oxygen bond distances in the newly attached CrOa unit show a less marked increase. At the same time the formation of a strong coordinate link from one oxygen of the chromate ion induces EA function in the chromium so that it... 

Chromium forms chromium(VI) oxide in which the metal is in -1-6 oxidation state. In acid medium it yields yeUow chromate ion, Cr04, and the red-orange dichromate ion,... 

The aqueous solution may be analyzed for chromium by AA or ICP techniques. Chromium(III) may be measured by ion chromatography. Additionally, the compound may be decomposed thermally to chromium(lll) oxide, Cr203, which can be identified by x-ray techniques. Water content of the hydroxide may be measured by gravimetry. 

Chromiumflll) oxide is amphoteric. Although insoluble in water, it dissolves in acid to produce hydrated chromium ion, [Cr(H20)6] +. It dissolves in concentrated alkah to yield chromite ion. When heated with finely divided aluminum or carbon it is reduced to chromium metal ... 

In the above reaction the equilibrium, however, hes far to the left. Therefore the chromium(VI) oxide solution also contains trace amounts of chromate ion,... 

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