Chromic acid, purification

Traditionally, sodium dichromate dihydrate is mixed with 66° Bh (specific gravity = 1.84) sulfuric acid in a heavy-walled cast-iron or steel reactor. The mixture is heated externally, and the reactor is provided with a sweep agitator. Water is driven off and the hydrous bisulfate melts at about 160°C. As the temperature is slowly increased, the molten bisulfate provides an excellent heat-transfer medium for melting the chromic acid at 197°C without appreciable decomposition. As soon as the chromic acid melts, the agitator is stopped and the mixture separates into a heavy layer of molten chromic acid and a light layer of molten bisulfate. The chromic acid is tapped and flaked on water cooled roUs to produce the customary commercial form. The bisulfate contains dissolved CrO and soluble and insoluble chromic sulfates. Environmental considerations dictate purification and return of the bisulfate to the treating operation. 

Gouthro, R.P. and Vaz, L., Recovery and purification of nickel salts and chromic acid using the RECOFLO system, Eco-Tec, Technical Paper 145, presented at the Metal Finisher s Association of India, Mumbai, India, September 1999. Available at http //www.eco-tec.com/main/electroplate.htm. 

Dried with MgSC>4, CaSC>4, NajSC or Linde type 13X molecular sieves, then distd. Cyclohexanol and other oxidisable impurities can be removed by treatment with chromic acid or dil KMnC>4. More thorough purification is possible by conversion to the bisulphite addition compound, or the semicarbazone, followed by decompn with Na2CC>3 and steam distn. [For example, equal weights of the bisulphite adduct (crystd from water) and NajCC are dissolved in hot water and, after steam distn, the distillate is saturated with NaCl and extracted with benzene which is then dried and the solvent evaporated prior to further distn]. 

Chromium metal is commercially produced in the United States by the reduction of chromite ore with carbon, aluminum, or silicon, and subsequent purification. Sodium chromate and dichromate are produced by roasting chromite ore with soda ash. Most other chromium compounds are produced from sodium chromate and dichromate (Hartford 1979 Westbrook 1979). For example, basic chromic sulfate (Cr(0H)S04), commonly used in tanning, is commercially produced by the reduction of sodium dichromate with organic compounds (e.g., molasses) in the presence of sulfuric acid or by the reduction of dichromate with sulfur dioxide. Lead chromate, commonly used as a pigment, is produced by the reaction of sodium chromate with lead nitrate or by reaction of lead monoxide with chromic acid solution (IARC 1990). 

Another approach to achieve purification of rinses and recovery in one step, electrodialysis has been suggested for chromic acid recovery and removal of metallic impurities [108]. As the authors point out there are two main process limitations first, the poor stability of most anion-exchange membranes against the oxidative chromic acid solution and secondly the increase in membrane resistance due to the formation of polychromates in the membrane. 

Acrylic acid (of Compagnie Nobel-Bozel or Uglior) was used without further purification. 4-Vinylpyridine (Fluka) was distilled twice under vacuum (b.p., = 58°C. at 12 mm. Hg) and stored in a refrigerator under nitrogen. Small pieces of PTFE, 10 X 20 to 10 X 40 mm.2 were cut from commercial films 0.1- and 0.05-mm. thick. They were immersed for 24 hours in either chromic acid or methanol at room temperature, washed, and dried under vacuum. 

Acetic acid (bp 118°) is a versatile solvent which is of particular value in carrying out oxidation reactions such as chromic acid oxidation and epoxidation. The commercial material contains a small amount of acetaldehyde and other oxidizable compounds. This is best removed by heating the solvent with chromium trioxide (1 to 2 per cent by weight) and then distilling, preferably under reduced pressure. For the purification to be successful, some unreacted chromic acid should be left in the distillation flask at the end of the distillation. 

Liquid phase oxidation. In spite of the difficulties attending the purification of anthracene by the solvent extraction method, the first vat colors made in this country were from anthraquinone obtained by the oxidation of anthracene.10 Anthracene crystallized from pyridine is distilled with superheated steam or sublimed to render it in a finely divided form suitable for oxidation. Oxidation with chromic acid is simple and almost quantitatively to anthraquinone. However, disposition of the chromic sulfate resulting from the process presents serious difficulties."... 

In some forms of generators for the gas, which are used commercially, the finely powdered carbide runs slowly into a reservoir of water. A better regulation of the gas supply is effected by using this method, and overheating, which may lead to an explosion, is avoided. Acetylene prepared from commercial calcium carbide contains ammonia, hydrogen sulphide, and phosphine. These substances must be removed when the gas is to be used for indoor illumination. The purification is effected by washing the gas with water and a mixture of slaked lime and bleaching powder, or with a solution of chromic acid in hydrochloric acid. In the laboratory a solution of mercuric chloride in dilute hydrochloric acid can be conveniently used for the purpose. 

Pyridine is a tertiary amine its aqueous solution shows an alkaline reaction and precipitates the hydroxides of metals, some of which are soluble in an excess of the amine. Salts of pyridine like those of other amines form characteristic double salts with metallic halides. The ferrocyanide of pyridine and the addition-product of pyridine and mercuric chloride are difficultly soluble in water these compounds are used in the purification of the base. Pyridine is a very stable compound it can be heated with nitric acid or chromic acid without undergoing change but at 330° it is converted by a mixture of nitric acid and fuming sulphuric acid into nitropyridine, a colorless compound that melts at 41° and boils at 216°. At a high temperature pyridine is converted into a sulphonic acid by sulphuric acid. Chlorine and bromine form addition-products, e.g., C5H5N.CI2, at the ordinary temperature when these are heated to above 200°, substitution-products are formed. The hydroxyl derivative of pyridine is made by fusing the sulphonic acid with sodium hydroxide it resembles phenol in chemical properties. The three possible carboxyl derivatives of pyridine are known. The a-acid is called picolinic acid, the jS-acid nicotinic acid (664), and the 7-acid isonicotinic acid. 

Distillation Methods Distillation methods have been widely used in iodine isotope production. Since iodine may be converted to a volatile form (I2), either wet distillation or dry distillation has been employed. A general distillation procedure for carrier-free purification has been reported earlier by Kahn and Freedman (1954). In a wet distillation method (IAEA, 1966), irradiated Te metal is dissolved in a chromic acid-H2S04 mixture. After complete dissolution, the iodate (IO3) formed is reduced to elemental iodine (I2) with oxalic acid and then distilled off from the solution. The distillate is trapped in alkaline sulfite solution. This solution is then purified by an oxidation-reduction cycle and finally redistilled into dilute alkaline solution. In another wet distillation method, irradiated Te02 is dissolved in NaOH and the sodium tellurite is oxidized with H2O2 in the presence of a catalyst, sodium molybdate. The mixture is then acidified with H2SO4 and the iodine is distilled off and trapped in ice-cold water. 

The identity of the ketone (179) could not be confirmed by direct comparison owing to the non-availability of an authentic specimen of the ketone (179). In order to confirm the identity of the ketone (179), its carbonyl group was eliminated by Clemmensen reduction [75]. The resulting deoxygenated material without purification was subjected to oxidation with chromic acid in acetic acid and demethoxylation with silicon... 

Oxidations-The use of a two phase ether-aqueous chromic acid system for oxidation of secondary alcohols to ketones has been reported to afford, in a rapid fashion, ketones which are remarkably free of isomerized and side products. Amine oxides can be prepared in 90% yield from the corresponding amines, using m-chloroperbenzoic acid in chloroform, followed by purification of the reaction mixture on an alumina column. ... 

Frenzel, L, Stamatialis, D.F. and Wessling, M. 2006. Water recycling fiom mixed chromic acid waste effluents by membrane technology. Sep. Purif. Technol. 49(1) 76-83. 

The use of ED can be used in the treatment of process solutions from plating baths containing chromium [6]. Dilute rinse water solutions can be concentrated by the removal of the CrOv " ions across an anion exchange membrane into the chromic acid concentrate. ED is also used as a method of purification of plating solutions by the removal of contaminant ions which are picked up during the plating operation. 

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