Potassium dichromate Chromic acid

In addition, the toluene must be practically free from olefines or members of the di-olefine series. This demands that the toluene be washed several times with concentrated sulphmic acid to remove these compounds. The laboratory test for olefines consists in agitating some of the toluene with a certain percentage of concentrated sulphuric acid. If olefines are present, the acid layer will acquire a yellow to red color. This color must not be deeper than whatever shade the certain plant has adopted as its standard. The comparison standard colors consist of definite concentrations of solutions of potassium dichromate, chromic acid, etc. 

The hexammino-chromic salts are formed by reducing potassium dichromate in acid solution with alcohol. Zinc is added so that reduction... 

Chromic acid, dipotassium salt. See Potassium dichromate Potassium chromate Chromic acid, disodium salt. See Sodium dichromate Sodium chromate Chromic acid green. See. Chromium oxide (ic) Chromic acid, lead (2+) salt (1 1). See Lead chromate... 

Lead nitrate can be used for the titration of tartrates > or glycerol phosphates in addition to sulphates already mentioned. A standard solution of lead nitrate is also used for the final titration in an indirect determination of morphine. > The method consists of treatment of the sample containing morphine with an excess of a standard solution of potassium dichromate in acid solution. Morphine is oxidized and the chromate reduced to the chromic state. The excess of dichromate is determined by titration with a standard solution of a lead salt at pH 4-2. The method was used for the analysis of Tinctura Opii. 

By Oxidation of Alcohols.—Barium manganate is readily available and stable, and has been recommended for the oxidation of primary and secondary alcohols to aldehydes and ketones respectively. Chromic acid adsorbed on to silica geP and the recyclable poly[vinyl(pyridinium chromate)] effect the same changes, while acid-stable primary and secondary alcohols are oxidized rapidly using potassium dichromate, sulphuric acid, methylene chloride, and a phase-transfer catalyst." Benzeneseleninic anhydride is an alternative reagent for the oxidation of alcohols under essentially neutral conditions. ... 

CAUTION. Ethers that have been stored for long periods, particularly in partly-filled bottles, frequently contain small quantities of highly explosive peroxides. The presence of peroxides may be detected either by the per-chromic acid test of qualitative inorganic analysis (addition of an acidified solution of potassium dichromate) or by the liberation of iodine from acidified potassium iodide solution (compare Section 11,47,7). The peroxides are nonvolatile and may accumulate in the flask during the distillation of the ether the residue is explosive and may detonate, when distilled, with sufficient violence to shatter the apparatus and cause serious personal injury. If peroxides are found, they must first be removed by treatment with acidified ferrous sulphate solution (Section 11,47,7) or with sodium sulphite solution or with stannous chloride solution (Section VI, 12). The common extraction solvents diethyl ether and di-tso-propyl ether are particularly prone to the formation of peroxides. 

The primary Cr—O bonded species is cbromium (VT) oxide, CrO, which is better known as chromic acid [1115-74-5], the commercial and common name. This compound also has the aliases chromic trioxide and chromic acid anhydride and shows some similarity to SO. The crystals consist of infinite chains of vertex-shared CrO tetrahedra and are obtained as an orange-red precipitate from the addition of sulfuric acid to the potassium or sodium dichromate(VI). Completely dry CrO is very dark red to red purple, but the compound is deflquescent and even traces of water give the normal mby red color. Cbromium (VT) oxide is a very powerful oxidi2er and contact with oxidi2able organic compounds may cause fires or explosions. 

Manufacture The primary iadustrial compounds of chromium made directly from chromite ore are sodium chromate, sodium dichromate, and chromic acid. Secondary chromium compounds produced ia quantity include potassium dichromate, potassium chromate, and ammonium dichromate. 

Potassium and ammonium dichromates are generally made from sodium dichromate by a crystallization process involving equivalent amounts of potassium chloride or ammonium sulfate. In each case the solubiHty relationships are favorable so that the desired dichromate can be separated on cooling, whereas the sodium chloride or sulfate crystallizes out on boiling. For certain uses, ammonium dichromate, which is low in alkaH salts, is required. This special salt may be prepared by the addition of ammonia to an aqueous solution of chromic acid. Ammonium dichromate must be dried with care, because decomposition starts at 185°C and becomes violent and self-sustaining at slightly higher temperatures. 

The primary routes of entry for animal exposure to chromium compounds are inhalation, ingestion, and, for hexavalent compounds, skin penetration. This last route is more important in industrial exposures. Most hexavalent chromium compounds are readily absorbed, are more soluble than trivalent chromium in the pH range 5 to 7, and react with cell membranes. Although hexavalent compounds are more toxic than those of Cr(III), an overexposure to compounds of either oxidation state may lead to inflammation and irritation of the eyes, skin, and the mucous membranes associated with the respiratory and gastrointestinal tracts. Skin ulcers and perforations of nasal septa have been observed in some industrial workers after prolonged exposure to certain hexavalent chromium compounds (108—110), ie, to chromic acid mist or sodium and potassium dichromate. 

The NIOSH recommended exposure limit for carcinogenic hexavalent chromium is 1 lg/m Cr(VI) as a 10-h TWA, and for noncarcinogenic Cr(VI) the 10-h TWA is 25 lg/m Cr(VI), including a 15-min maximum exposure of 50 lg/m Cr(VI). According to NIOSH, the noncarcinogenic Cr(VI) compounds are chromic acid and the chromates and dichromates of sodium, potassium, lithium, mbidium, cesium, and ammonia. NIOSH considers any hexavalent chromium compound that does not appear on the preceding Hst carcinogenic (145). 

Terephthalic acid has been obtained from a great many /)-disubstituted derivatives of benzene or cyclohexane by oxidation with permanganate, chromic acid, or nitric acid. The following routes appear to have preparative value from />-toluic acid, />-methylacetophenone,2 or dihydro-/)-tolualdehyde by oxidation with permanganate from f>-cymene by oxidation with sodium dichromate and sulfuric acid from />-dibromobenzene or from /i-chloro- or -bromobenzoic acid by heating at 250° with potassium and cuprous cyanides and from />-dibromo-benzene, butyllithium, and carbon dioxide. ... 

Chromium trioxide (chromic anhydride) [1333-82-0] M 100.0, m 197°, dec at 250° to Cr203, d 2.70 (pK 0.74, pK 6.49, for H2Cr04, chromic acid). Red crystals from water (0.5mL/g) between 100° and -5°, or from water/conc HNO3 (1 5). It separates when potassium or sodium dichromate are dissolved in cone H2SO4. Dried in a vacuum desiccator over NaOH pellets hygroscopic, powerful oxidant, can ignite with organic compounds. It is a skin and pulmonary IRRITANT. 

Chromic acid (hexavalent chromium) Potassium dichromate... 

Oxidation of secondary alcohols to ketones (Section 15.10) Many oxidizing agents are available for converting secondary alcohols to ketones. PDC or PCC may be used, as well as other Cr(VI)-based agents such as chromic acid or potassium dichromate and sulfuric acid. 

Trimethylacetic acid may be made by the hydrolysis of tert-butyl cyanide with weak hydrochloric acid at ioo0.1 It is also obtained by oxidation of trimethylpyroracemic acid with silver oxide or potassium dichromate and sulfuric acid,2 by oxidation of tertf-butylethylene with permanganate solution,3 or by oxidation of dimethyl 2,2-propanol with chromic acid.4 Schroeter reports the formation of trimethylacetic acid by rearrangement of the oxime of trimethylacetophenone to give the anilide of trimethylacetic acid, which can be hydrolyzed to give the acid.5... 

A method which is frequently used consists in filling the apparatus with chromic acid cleaning mixture (CARE), a nearly saturated solution of powdered sodium dichromate or potassium dichromate in concentrated sulphuric acid, and allowing it to stand for several hours, preferably overnight the acid is then poured off, the apparatus thoroughly rinsed with distilled water, and allowed to drain until dry. [It may be mentioned that potassium dichromate is not very soluble in concentrated sulphuric acid (about 5 g per litre), whereas... 

Safety. The addition of concentrated sulphuric acid on sodium (or potassium) dichromate is to produce chromium trioxide, which is a powerful oxidizing agent capable of oxidizing carbon to carbon dioxide. The solution is loosely called chromic acid, but although true chromic acid H CrO has not been isolated, the aqueous solution contains dichromic acid, The... 

Thiocyanato-pentammino-chromic Salts, [Cr(NH3)5(SCN)]It2, are prepared by replacing chlorine in chloro-pentammino-chloridc with (SCN) by treating a concentrated aqueous solution containing acetic acid with excess of potassium thiocyanate, warming the solution, and thus obtaining the thiocyanate, [Cr(NH3)5(SCN)](SCN)2. From this the other salts are prepared by double decomposition. The thiocyanate, which has not been obtained pure, is reddish brown in colour and does not crystallise easily it is soluble in cold water, and the aqueous solution gives with hydrochloric acid, potassium bromide, potassium nitrate, potassium iodide, and potassium dichromate, the corresponding salts. 

Dithiocyanato-diethylenediamino-chromic Chloride, [Cr en2 (SCN)2]C1.HC1, is prepared from the thiocyanate on treatment with concentrated hydrochloric acid, filtering off undissolved substance, and cooling the filtrate, when a crystalline powder separates. This is recrystallised from water and obtained in ruby-red glistening needles. From a solution of this salt may be precipitated the nitrate, sulphate, bromide, or dichromate by means of the corresponding acid or potassium dichromate. 

Phenanthrenequinone has been prepared by treatment of phe-nanthrene with chromic acid in acetic acid 5 potassium dichromate in sulfuric acid 3-6 hydrogen peroxide in acetic acid 6 7 and selenium dioxide above 250°.8 It can also be prepared from benzil with aluminum chloride at 120° 9 and from biphenyl-2,2 -dialdehyde with potassium cyanide.10... 

Preparation of Chromic Anhydride. Place 1 g of potassium dichromate into a porcelain bowl, dissolve it in 10 ml of water, and add ml of concentrated sulphuric acid while stirring with a glass rod. What substance precipitates Cool the solution and filter off the precipitate on a funnel with a glass filtering bottom. Write the equation of the reaction. 

If chromic chloride is not available as a starting material for this procedure, the compound may be prepared in solution by reducing BOg of potassium dichromate with a mixture of 250ml of concentrated hydrochloric add and either 100ml of 95% alcohol, or HOg of oxalic acid 2-hydrate. The reduced solutions are then used directly in the reduction procedure only the zinc need be added. 

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