Chromium as chromate

Svancara, 1., Eoret, P, and Vytfas, K. (2004) A study on the determination of chromium as chromate at a carbon paste electrode modified with surfactants. Talanta, 64, 844-852. 

A host of inorganic colored compounds still has been left unaccounted for. For example, some mercury, bismuth and lead compounds exhibit brilliant colors, but they are not transition element compounds. Chromium as chromate is bright yellow and manganese as permanganate is deep purple, yet in neither of these cases do the metals have available /-electrons to be promoted to excited states. Since an absolute requirement of color production in chemical species is the presence of the proper and available energy levels, color in such compounds can be accounted for, at least in part, by recourse to the charge-transfer phenomenon. 

Where appropriate, the direct precipitation of hexavalent chromium with barium, and recovery of the Cr(VI) value can be employed (166). Another recycling (qv) option is ion exchange (qv), a technique that works for chromates and Finally, recovery of the chromium as the metal or alloy is... 

The most common toxic metals in industrial use are cadmium, chromium, lead, silver, and mercury less commonly used are arsenic, selenium (both metalloids), and barium. Cadmium, a metal commonly used in alloys and myriads of other industrial uses, is fairly mobile in the environment and is responsible for many maladies including renal failure and a degenerative bone disease called "ITA ITA" disease. Chromium, most often found in plating wastes, is also environmentally mobile and is most toxic in the Cr valence state. Lead has been historically used as a component of an antiknock compound in gasoline and, along with chromium (as lead chromate), in paint and pigments. 

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. 

Discussion. Small amounts of chromium (up to 0.5 per cent) may be determined colorimetrically in alkaline solution as chromate uranium and cerium interfere, but vanadium has little influence. The transmittance of the solution is measured at 365-370 nm or with the aid of a filter having maximum transmission in the... 

Hexavalent chromium-bearing wastewaters are produced in the metal finishing industry in chromium electroplating, in chromate conversion coatings, in etching with chromic acid, and in metal finishing operations carried out on chromium as a basis material. 

Still, these coatings are far from ideal and are not yet sufficiently stable for technical application. Their reoxidation is strongly hindered, necessitating a relatively high overpotential of several hundreds of millivolts and their stability is insufficient, with chromium bleeding out as chromate. Yet the results obtained by the method that allows fixing with an easily applicable method a soluble redox mediator to an oxidic anode is encouraging. The an-... 

Water-Soluble Chromium(VI). Chromium(VI) is determined photometrically in an aqueous extract obtained by hot or cold extraction using the color produced with diphenylcarbazide, or directly as chromate. For standards, see Table 1 ( Matter soluble in water ). Apparatus equipment for shaking conical flasks, photometer with glass cells, pH meter. 

Whether the chromium atoms are isolated as chromate, or exist in pairs as dichromate, has proven a difficult problem despite its importance to our understanding of the catalyst. Some researchers believe polymerization can occur only when the chromium atoms are paired, and elaborate polymerization mechanisms are proposed involving two chromium atoms (2-11). This was advanced as the reason why unsupported Cr(VI) esters have failed to exhibit activity, because they are always chromate-like structures (2). 

Spectroscopy has not proven to be very conclusive in solving this problem. Similarities between the visible spectrum of the calcined catalyst and that of bulk dichromates have been noted (5,12-14). In the end, however, there is always doubt about the interpretation of spectra because no adequate reference data exist for these surface bound species (76). Krauss and coworkers have carefully studied the luminescence of Cr/silica and concluded that at least a portion of the chromium is present as chromate (75). 

When the silica has been calcined a 400°C or lower, most or even all of the chromyl chloride attaches as chromate, losing two chlorides per Cr. All of the chromium remains hexavlent. These chromate catalysts exhibit nearly identical activity to Cr03 on silica activated at the same temperature. The kinetics... 

The reverse reaction has also been studied (18). Treating the calcined Cr03/silica with dry HC1 strips off the chromium as chromyl chloride vapor leaving one OH for each former point of attachment. The silica base is otherwise unaffected. Hydroxyl measurements before and after the stripping give a direct indication of the bonding of Cr(VI), because chromate leaves two hydroxyls per chromium removed and dichromate leaves only one. 

It seems likely that the chromate species can exist on the silica surface and acts as parent for an active site. Thus, pairing of chromium atoms is not a requirement for polymerization. Chromium trioxide (Cr03) probably binds to the silica as chromate initially, at least at the ordinary 1 % loading. But some rearrangement to dischromate at high temperatures may occur. If so, it could account for the change in color from yellow to orange, and even to red in some modified catalysts. 

There is now much interest in the biology of chromium,1086"1088 which has been suggested to be involved not only in the action of insulin, but also in the activation of certain enzymes, and, possibly, the stabilization of nucleic acids. Chromium is also known to be carcinogenic and mutagenic at high concentrations, particularly as chromate. Chromate is reduced in rat liver microsomes to Crm and Crv. Crv is labile and may well be the carcinogenic form.1089... 

Polyethylene (chromium catalyst). The chromium on silica catalyst is quickly reduced from Cr(VI) to Cr(II). The active site consists of a single chromium ion present as silyl chromate before reduction with ethylene. Ethylene adds to the chromium as indicated. 

The typical Phillips catalyst comprises chemically anchored chromium species on a silica support. The formation of a surface silyl chromate, and eventually silyl dichromate [scheme (29)], is significant during the catalyst preparation, because at the calcination temperature chromium trioxide would decompose to lower-valent oxides. Chromium trioxide probably binds to the silica as the chromate initially, at least for the ordinary 1% loading. However, some rearrangement to the dichromate at high temperature may occur. It is incorrect to regard only one particular valence state of chromium as the only one capable of catalysing ethylene polymerisation. On the commercial CrOs/silica catalyst the predominant active species after reduction by ethylene or carbon monoxide [scheme (59)] is probably Cr(II), but other species, particularly Cr(III), may also polymerise ethylene under certain conditions ... 

Chromium-containing mesoporous silica molecular sieves (Cr-HMS) with tetrahedrally coordinated isolated chromium oxide (chromate) moieties can operate as efficient photocatalysts for the decomposition of NO and the partial oxidation of propane with molecular oxygen under visible light irradiation (Yamashita et al., 2001). 

Conversion coatings such as chromates or phosphates may be used. Molybdates may be used in place of chromates because of the environmental toxicity of hexavalent chromium. Paint can also be applied over a conversion coating.46... 

Chromated zinc chloride—a mixture of sodium dichromate and zinc chloride having the composition hexavalent chromium as Cr03, 20% and zinc as ZnO, 80%. 

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