Chromium salts mechanism

To 3600 g. of concentrated sulfuric acid, in a 5-I. flask placed in an empty water bath, are added 360 g. of technical trinitrotoluene, while the mixture is stirred mechanically. Sodium dichromate (Na2Cr207 2 H20) is now added in small quantities (precaution see Notes), with constant stirring, until the temperature of j he mixture reaches 40° the empty water bath i now filled with cold water and the addition of sodium dichromate continued at such a rate that the temperature remains at 45-55°. In all 540 g. of sodium dichromate are added, the addition taking one to two hours. When all has been added, the mixture, which has now become very thick, is stirred for two hours at 45-55°, and poured into a crock containing 4 kg. of crushed ice. The insoluble trinitrobenzoic acid is filtered off, and carefully washed with cold water until free from chromium salts. On drying it weighs 320-340 g. 

Traditionally, the transformation of the raw skin into leather is effected by means of a series of chemical and mechanical operations. Chemical processes take place in chemical reactors (tumblers) in which the skins react with different chemicals (acids, alkalis, chromium salts, tannins, solvents, sulfides, dyes, auxiliaries, etc.) in aqueous solution. 

Chromium/zeolite catalysts have been prepared both by ion-exchange and by impregnation with chromium salts and the activity of the various samples was measured, as a function of time, at various temperatures and pressures. The rate of monomer consumption presented a typical shape, with a sharp increase at the beginning of the run, followed by a decrease that tended to an almost stationary rate. These results are discussed in terms of the possible mechanism for the reaction. 

Chromium III derivatives have been used for catalyzing epoxy-carboxy reactions (see Table f). Unfortunately, most publications are patents which describe the process without any information on the mechanisms and the kinetics. Let Cr(OOC R ) be an activated chromium tricarboxylate salt , i.e., a chromium salt having readily available coordination sites. Steele et al. propose the following mechanism ... 

Equip a I litre three-necked flask with a mechanical stirrer and a thermometer, and immerse the flask in a bath of ice and salt. Place 306 g. (283 ml.) of acetic anhydride, 300 g. (285 ml.) of glacial acetic acid and 25 g. of p-nitrotoluene in the flask, and add slowly, with stirring, 42 5 ml. of concentrated sulphuric acid. When the temperature has fallen to 5°, introduce 50 g. of A.R. chromic anhydride in small portions at such a rate that the temperature does not rise above 10° continue the stirring for 10 minutes after all the chromium trioxide has been added. Pour the contents of the flask into a 3 litre beaker two-thirds filled with crushed ice and almost fill the beaker with cold water. Filter the solid at the pump and wash it with cold water until the washings are colourless. Suspend the product in 250 ml. of cold 2 per cent, sodium carbonate solution and stir mechanically for 10-15 minutes filter (1), wash with cold water, and finally with 10 ml. of alcohol. Dry in a vacuum desiccator the yield of crude p-nitrobenzal diacetate is 26 g. (2),... 

Chromium-containing wood preservatives and their chemical compositions are Hsted ia Table 13 (199). Chromium compounds have a triple function ia wood preservation (200). Most importantiy, after impregnation of the wood the Cr(VI) compounds used ia the formulations react with the wood extractives and the other preservative salts to produce relatively insoluble complexes from which preservative leaches only very slowly. This mechanism has been studied in the laboratory (201—206) and the field (207). Finally, although most of the chromium is reduced to chromium (ITT), there is probably some slight contribution of the chromium (VT) to the preservative value (208). 

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

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