Chromate reactions with iron

In the presence of iodide ions the reaction of iron(II) with chromate induces the formation of iodine. The induced formation of iodine can be represented by reaction (21) and (22). 

Chromates, vanadates, and cerium salts give colour reactions with the reagent and should therefore be absent. Iron salts give a yellow colour with hydrogen peroxide, but this is eliminated by the addition of syrupy phosphoric acid. Fluorides bleach the colour (stable [TiF6]2 ions are formed), and large amounts of nitrates, chlorides, bromides, and acetates as well as coloured ions... 

It has been involved in many industrial explosions. Explodes on contact with aluminum + barium nitrate + potassium nitrate + water. Forms explosive mixtures with aluminum powder + titanium dioxide, ethylene glycol (240°C), cotton lint (245°C), furfural (270°C), lactose, metal powders (e.g., aluminum, iron, magnesium, molybdenum, nickel, tantalum, titanium), sulfur, titanium hydride. Reaction with ethanol + heat forms the explosive ethyl perchlorate. Violent reaction or ignition under the proper conditions with aluminum + aluminum fluoride, barium chromate + mngsten or titanium, boron + magnesium + silicone rubber, ferrocenium diammine-tetrakis(thiocyanato-N) chromate(l —), potassium hexacyanocobaltate(3—), A1 +... 

LEAD CHROMATE or LEAD(II) CHROMATE (7758-97-6) PbCr04 Noncombustible solid. Violent reaction with sulfur, iron(III) ferrocyanide, azo-dyes. Forms unstable, impact-sensitive and/or pyrophoric mixtures with hydrozoic acid, sodium azide, sulfur, tantalum, trinitrobenzoic acid. Incompatible with azo dyes, ammonia, hydrazine, methyl isocyanoacetate, sodium peroxyborate, urea nitrate. A known human carcinogen. 

Electrokinetically driven iron mineralization originates when Fe(III) combines with OH" ions produced at the cathode to form insoluble ferric hydroxides [Fe(OH)3(s)], hematite ( -Fe203) andgoethite (FeOOH) (e.g., Faulkner, Hopkinson, and Cundy, 2005 Mukhopadhyay, Sundquist, and Schmitz, 2007). SEM observations of soil samples taken from the anodic zones of the experimental cells reveals a ubiquitous association between the iron minerals and subsidiary quantities of chromium. Cr(III) can substitute for Fe(III) in the FeOOH structure (Eary and Rai, 1988), and Cr(VI) reduction by Fe " leads to the development of solids at nearneutral pH showing mixed iron/chromium solid solution of the form Fe Cri x(OH)3 (Eary and Rai, 1988 Fendorf and Li, 1996). Such conditions would have been met at the interface between the acidic and alkali portions of the experimental cells (Fig. 8.5). When Fe(III) is produced solely from the stochiometric reaction with chromate, the value of x is 0.75 (Batchelor et al, 1998) ... 

Excess fluoride ions prevent the reaction through the formation of stable [TiFe] ions. However, by demasking, it is possible to detect titanium in the presence of even large amounts of fluoride (see below). Large eimounts of acetates, nitrates, chlorides, bromides, and colored ions reduce the sensitivity. Chromates, vanadates, and molybdates, should not be present because they too give color reactions with hydrogen peroxide. Iron salts yield... 

Preparation of Potassium Chromate. (Perform one experiment on each table.) Melt a mixture of 1 g of potassium carbonate, 1 gof potassium hydroxide, and 2 g of potassium nitrate in an iron crucible by heating with the flame of a burner. While stirring the melt with an iron wire, introduce 1 g of finely comminuted chromite or -0.8 g of chromium oxide into the crucible. Roast the mixture for 5-10 minutes on a blowpipe. Treat the cooled melt with water. Filter the solution and evaporate it until a crystalline film appears. What is the composition of the formed crystals Why was potassium carbonate introduced into the reaction Write the equation of the reaction. 

Pour an ammonium sulphide solution into a potassium chromate or dichromate solution and heat the mixture. How does the solution s colour change What is the composition of the precipitate Write the equation of the reaction. What properties does chromium (VI) exhibit in this reaction How does an acidified potassium chromate or dichromate solution react with hydrogen sulphide, sulphur dioxide, and an iron(II) salt Write the equations of the reactions. 

Silicon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between silicon carbide and a variety of compounds at relatively high temperatures. Sodium silicate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal silicide. Silicon carbide decomposes in fused alkalies such as potassium chromate or sodium chromate and in fused borax or cryolite, and reacts with carbon dioxide, hydrogen, air, and steam. Silicon carbide, resistant to chlorine below 700°C, reacts to form carbon and silicon tetrachloride at high temperature. SiC dissociates in molten iron and the silicon reacts with oxides present in the melt, a reaction of use in the metallurgy of iron and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new silicon nitride-bonded type exhibits improved resistance to cryolite. 

The most established technology for treating contaminants by abiotic reduction reactions relies on zero-valent iron metal (82). In addition to effecting hydrogenolysis (Equation (4)) and reductive elimination (Equation (5)), Fe° readily reduces nitro aromatics (Equation (6)), azo dyes, nitrate, chromate, chlorine residual, and some radionuclides. Recently, an investigation of soils contaminated with the herbicide alachlor provided evidence for reductive N-dealkylation (as well as dechlorination) by Fe° (83). 

We have demonstrated recently that epoxidation and hydroxyl-ation can be achieved with simple iron-porphine catalysts with iodosylbenzene as the oxidant (24). Cyclohexene can be oxidized with iodosylbenzene in the presence of catalytic amounts of Fe(III)TPP-Cl to give cyclohexene oxide and cyclohexenol in 55% and 15% yields, respectively. Likewise, cyclohexane is converted to cyclohexanol under these conditions. Significantly, the alcohols were not oxidized rapidly to ketones under these conditions, a selectivity shared with the enzymic hydroxylations. The distribution of products observed here, particularly the preponderance of epoxide and the lack of ketones, is distinctly different from that observed in an autoxidation reaction or in typical reactions of reagents such as chromates or permanganates (15). 

The decarbonylation of furfural to give furan is best carried out at rather high temperatures. The following catalysts have been described Pd or Pd on charcoal,30 calcium oxide,31 32 zinc and iron chromite,33 or zinc, chromium, and manganese oxide (from ammonium chromate and manganese nitrate).34 The optimum reaction temperature with... 

A further method consists in treating a solution of chromate udth an excess of methylene white (the leuco base of methylene blue) in presence of hydrochloric acid. The methylene blue produced is then titrated with standard titanous chloride, the temperature being kept above 40° C. in order to sharpen the end-point. The whole process should be performed in an atmosphere of carbon dioxide. In applying this method to ferrochrome or chromium steel, the iron and chromium may be estimated together, the iron then being determined separately by ordinary methods and the chromium obtained by difference. The reaction may also be used colorimetrieally. ... 

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