Dichromate anion

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 ... 

C20-0110. Determine the Lewis structure and draw a ball-and-stick model showing the geometry of the dichromate anion, which contains one bridging oxygen atom. 

Dichromate anions are readily absorbed under acidic conditions by wool that has been dyed with chrome dyes. The chromium(VI) on the fibre is then gradually reduced by the cystine residues in wool keratin to chromium(III) cations, which react with the dye ligands to form a stable complex. In this way the cystine disulphide bonds are destroyed, resulting in oxidative degradation of the wool fibres [71]. 

Dichromate anions are absorbed and interact with protonated amino groups in wool (Scheme 5.19), the sorption proceeding most rapidly below pH 3.5. 

A tertiary amine (Alamine 336) is used as a selective extractant of Cr(VI). The extraction mechanism involves ion pairing between the amine and the dichromate anion, HCr04. After extraction, the Cr(VI)-loaded extractant is stripped with alkali to produce a strip liquor, containing 2% Cr(VI). This solution can be recycled. 

A compound prepared and first described as nicotinium dichromate (NDC) by Palomo et al.,379 was later shown by X-ray-crystal analysis380 to be a betainic mixed anhydride of nicotinic and chromic acid (NACAA). Because of its unique structure, it deserves a close scrutiny of its oxidative properties.381 Replacement of the chloride anion in the quaternary ammonium resin, Dowex 1-X8, for the dichromate anion, leads to a polymer supported dichromate, which is able to make selective benzylic oxidations.382 Finally, poly[vinyl(pyridinium dichromate)] (PVPDC), a polymeric analogue of PDC, must be mentioned whose use in the oxidation of alcohols allows for a very easy work-up.383... 

The two major species in which chromium exists are the Cr(III) cation and the chromate and dichromate anions, Cr(VI). The Cr(VI) is considered to be considerably more dangerous in environmental samples of soil or water because it is more toxic and also considerably more soluble, and therefore more mobile in the environment. A pH 8, 0.05 M ammonia buffer is used with ultrasonication to extract the Cr(VI) from soil samples, and then the Cr(VI) is sorbed from the extract on an anion-exchange column (Dowex 1-X8). The sorbed analyte is eluted with 10 mL of pH 8, 0.5 M ammonia buffer. The Cr(III) is determined by difference from the total Cr measure-... 

In aqueous solutions chromium forms three types of ions the chromium(II) and chromium(III) cations and the chromate (and dichromate) anion, in which chromium has an oxidation state of + 6. 

The first study of Cr in natural abundance and in the liquid phase was reported by Egozy and Loewenstein (206) in 1969 from which the rate constant for the chromate-dichromate equilibrium was derived. Although the quadrupole moment of Cr is small (0-03 barn) no chromium signal could be detected for the dichromate anion in which the valence electron symmetry is only slightly perturbed relative to the tetrahedral CrO. ... 

Dichromate anion has the formula Cr207, in which chromium ion occurs in the -1-6 oxidation state. It also occurs in the same valence state (-1-6) oxidation state. It also occurs in the same valence state (4-6) in chromates, Cr04. Sodium and potassium dichromates of formula Na2Cr207 and K2Cr207 and CAS numbers [10588-01- 9] and [7778-50-9], respectively, are of commercial importance. These compounds are used for bleaching resins, oils, and waxes in chrome tanning of hides in the manufacture of chrome pigments in pyrotechniques and safety matches and as corrosion inhibitors. 

Two common oxidizing agents are potassium permanganate (KMn04) and potassium dichromate (K2Cr20v). As Figure 4.22 shows, the colors of the permanganate and dichromate anions are distinctly different from those of the reduced species ... 

In any event, the oxidations by means of solutions of potassivim dichromate in PEG 400 can be compared to other similar systems which have been reported to bring into organic solutions the dichromate anion. 

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