Chromate anion

Chromate Anion Polystyrene matrix Tertiary or quaternary ammonium functional groups Sodium carbonate or alkaline NaCl... 

In order that a chromate film may be deposited, the passivity which develops in a solution of chromate anions alone must be broken down in solution in a controlled way. This is achieved by adding other anions, e.g. sulphate, nitrate, chloride, fluoride, as activators which attack the metal, or by electrolysis. When attack occurs, some metal is dissolved, the resulting hydrogen reduces some of the chromate ion, and a slightly soluble golden-brown or black chromium chromate (CtjOs CrOs xHjO) is formed. 

Steel objects are often protected from corrosion by electroplating with chromium. The most straightforward process would be to electrolyze a solution of Cr cations. This fails because aqueous Cr ions are not reduced at a useful rate. Instead, solutions containing chromate anions are used ... 

A special case of anodizing in chromic acid is characterized by the absence of incorporated anions.162,173 It has been shown in a number of studies4 that chromate anions are accumulated at the outer oxide surface and do not penetrate into the oxide body. 

Example calculation <-> "Cr substitution in the chromate anion. An example calculation on the chromate anion [Cr04] makes use of tabulated G and F matrix elements. 

The chromate anion is a highly soluble, toxic tetrahedral complex (point group Tj) that occurs in oxidized, neutral-basic solutions. It is also one of a small number of aqueous complexes that have been thoroughly characterized by spectroscopic measurements on numerous isotopic compositions (Muller and Kbniger 1974), so it will be possible to check the vibrational model against real data. Here the MUBFF is applied under the assumption that aqueous chromate can be approximately modeled as a gas-phase molecule. 

This technique of MEUF has also been successfully employed for the recovery of thuringiensin [258], removal of cresols [262], extraction of chromate anion [257], removal of dissolved organic pollutants [256], removal of -alcohols [263],preconcentration and removal of iron [260], and preconcentration of aniline derivatives [261].Kandori and Schechter [264] have given a detailed account of selecting surfactants for MEUF. The design characteristics of micellar enhanced utrafilters and cross-flow ultrafiltration of micellar surfactant solutions have been described by Markets et al. [265]. 

By definition,4 all oxyanions, [XO ]m, contain a central element X (generally a non-metal, such as sulfur in a sulfate anion, [S04]2", but X may also be a metal,5 such as chromium in the chromate anion, [Cr04]2-) and a number of terminal oxygen atoms (Table 1). Each terminal oxygen atom involves up to three lone pairs of electrons and consequently each oxyanion [XO ]m may be involved in coordination to n different metal atoms if each terminal oxygen atom coordinates to only one metal atom. If more than one lone pair on each terminal oxygen atom is used in coordination... 

Opening of the epoxide by attack of a chromate anion leads to an intermediate that suffers a carbon-carbon bond breakage, resulting in a stabilized carbocation that evolves to give... 

The initially formed chromate ester is fragmented, producing an allylic cation that can be attacked at two positions by a chromate anion. The resulting allylic chromates evolve by producing two isomeric ketones. 

Formation of a chromate ester is followed by the opening of cyclopropane, driven by attack of chloride and elimination of chromate anion. 

The toxicity of chromium is dependent on the oxidation state of the chromium atom, with chromium(VI) being significantly more toxic than chromium(III). One of the factors believed to contribute to this increased toxicity is the greater ability of chromium(VI) to enter cells, compared to chromium(III). Chromium(VI) exists as the tetrahedral chromate anion at physiological pH, and resembles the forms of other natural anions, such as sulfate and phosphate, which are permeable across nonselective membrane channels. Chromium(III), however, forms octahedral complexes and cannot easily enter through these channels. Therefore, the lower toxicity to chromium(III) may be due in part to lack of penetration through cell membranes. It follows that extracellular reduction of chromium(VI) to chromium(III) may result in a decreased penetration of chromium into cells, and therefore, a decreased toxicity. 

The various simple chromate anions including chromate, Cr042-, dichromate, Cr2072, and bichromate, HCr04, have all been reported to be potent corrosion inhibitors when they are present as soluble species in solution. Chromate is an effective inhibitor for Al, Fe, Mg, Cd, Sn, and many other metals and alloys. The specific form of the anion present in solution depends on its concentration and solution pH (21). Because evidence of corrosion inhibition has been detected over very wide ranges of soluble Cr(VI) ion concentration and pH, each of these species appears to possess inhibiting properties. 

Another commonly used extraction reagent for metal ions is diethylhexyl phosphoric acid (DEHPA) [55,56]. See Figure 12.3c. In this case a pH gradient must exist over the membrane, so the acceptor is kept more acidic than the donor (typically pH 1 and pH 3, respectively). Speciation of different chromium species (chromate and chromium ion) has been performed by the combination of two membrane extraction systems, one working with DEHPA for extraction of Cr + and the other with Aliquat-336 for extraction of the chromate anions [51]. Using the same principles, also organic compounds, such as amino acids [57] and polyamines [58], can be extracted. 

Chromium may cause adverse health effects following inhalation, ingestion, or dermal exposure. The toxicity of chromium is mainly caused by hexavalent compounds as a result of a higher cellular uptake of chromium(VI) compounds than chromium(III). This is explained by the fact that the chromate anion (Cr04) can enter the cells via facilitated diffusion through nonspecific anion channels (similarly to phosphate and sulfate anions). Absorption of chro-mium(III) compounds is via passive diffusion and phagocytosis. 

Three comii rcially available longaliphatic amines, Primene JM-T (primary), Amberlite LA-2 (secondary), and trilaurylamine (TLA, tertiary), were tested as membrane carriers for nitrate, pertechnetate and chromate anions. Long-chain aliphatic amines, dissolved in an (H ic diluents, are known to extract acids according to the reaction... 

The availability of the selenate, vanadate, arsenate, and chromate anions decreases with the soil s Fe and Al hydroxyoxide content and increases with pH, since anion retention decreases as pH Increases. The pH response for anions, however, is generally less dramatic than for cations, which adsorb/precipitate as hydroxyoxides in soils. 

Of these three types of poly (organo) chromate anions, only the hexaorgano species tolerates a variety of ligand types bound to chromium. Hexamethylchromate has been reported,as well as an unusual spirocyclic compound, Li3[Cr(l,4-C4Hg)3] (Et20)2.s. Notable polyaryls... 

Table 1. Assignment of FT-Raman bands for chromate anions (cm- ). ...

A Raman spectrophotometer was used to measure in situ the polymerisation rate of acrylamide photoinitiated by hydrogen chromate anion in aqueous solution upon irradiation by laser beam (ionised argon)... 

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