Chromic acid solutions

Water-Soluble Trivalent Chromium Compounds. Most water-soluble Cr(III) compounds are produced from the reduction of sodium dichromate or chromic acid solutions. This route is less expensive than dissolving pure chromium metal, it uses high quaHty raw materials that are readily available, and there is more processing fiexibiHty. Finished products from this manufacturing method are marketed as crystals, powders, and Hquid concentrates. 

Oxide films on aluminum are produced by anodi2ing in a chromic acid solution. These films are heavier than those produced by chemical conversion and thinner and more impervious than those produced by the more common sulfuric acid anodi2ing. They impart exceptional corrosion resistance and paint adherence to aluminum and were widely used on military aircraft assembHes during World War II. The films may be dyed. A typical anodi2ing bath contains 50 to 100 g/L CrO and is operated at 35—40°C. The newer processes use about 20 volts dc and adjust the time to obtain the desired film thickness (184). 

Chromium is conventionally deposited from chromic acid solutions containing at least one anionic catalyst, which is usually the sulfate ion. The weight ratio of chromic acid to catalyst is important and, for sulfate-cataly2ed solutions, is maintained about 100 1. Formulations and conditions for operating hard chromium plating solutions are shown in Table 5. 

The end of the dropping funnel extends about 2 in. into the flask from the opening and is drawn into a capillary. This is done to ensure the introduction of the chromic acid solution in the form of small droplets. 

While keeping the collected deuterioammonia at dry ice-isopropyl alcohol temperature, lithium wire (10 mg) is added, followed by a solution of 3/3-hydroxy-5a-cholest-7-en-6-one (161 50 mg) in anhydrous tetrahydrofuran (4 ml). The reaction mixture is stirred for 20 min, the cooling bath is then removed and the ammonia is allowed to boil under reflux for 40 min. A saturated solution of ammonium chloride in tetrahydrofuran is added dropwise until the deep blue color disappears and then the ammonia is allowed to evaporate. The residue is extracted with ether and the organic layer washed with dilute hydrochloric acid and sodium bicarbonate solution and then with water. Drying and evaporation of the solvent gives a semicrystalline residue which is dissolved in acetone and oxidized with 8 N chromic acid solution. After the usual workup the residue is dissolved in methanol containing sodium hydroxide (0.2 g) and heated under reflux for 1 hr to remove any deuterium introduced at C-5 or C-7. (For workup, see section II-B). 

ChromsMure, /. chromic acid, -anhydrid, n. chromic anhydride, chromium(VI) oxide, emisch, n. chromic acid mixture. -Ibsung. /. chromic acid solution, -salz, n. salt of chromic add, chromate. 

Chromate ions, when used as inhibitors in aqueous solutions, passivate by maintaining a coherent oxide film on the metal surface. Passivation is maintained even in a boiling concentrated chromic acid solution, in which many of the oxides in bulk form are soluble. The passivity breaks down rapidly, however, once the chromate is removed. 

Note in making up the chromic acid solution it is advisable to dissolve the silver nitrate separately and add it to the boiling chromic acid to prevent excessive crystallisation of the silver chromate. The chromic acid must be free from sulphate to avoid attack on the zinc. Immerse each specimen for 15 s in a 6% solution of hydriodic acid at room temperature to remove the remaining corrosion products. Immediately after immersion in the acid bath, wash the samples first in tap water and then in absolute methanol, and dry in air. This procedure removes a little of the zinc and a correction may be necessary. 

Solid chromic acid and methanol will ignite spontaneously care should be taken that the methanol does not come in contact with any of the chromic acid solution which may have dried around the edges of the dropping funnel. 

It is helpful to empty the brown tarry residue from the distillation flask while it is still hot. The flask can then be cleaned by using a sulfuric acid-chromic acid solution. 

This flask must be cleaned with h ot chromic acid solution and then, along with all other glassware used in this preparation, soaked in a base solution, rinsed with distilled waiter, and oven dried. Thermal rearrangement of the intermediate vinyl ether in a new (untreated) flask resulted in elimination. 

Norprogesterone. A solution of 2.2 g of 19-hydroxyprogesterone in 110 ml acetone is treated with 11 ml of an 8 N chromic acid solution in dilute sulfuric acid 18a at 0° for 40 min. Then 110 g crystalline sodium acetate in 170 g water is added and the mixture extracted 3 times with benzene. The combined extracts are washed 5 times with saturated sodium chloride solution, dried and evaporated. The crystalline residue is washed with a small amount of benzene to give 2 g of the corresponding 19- acid mp 138° (dec). 

Magnesium Chromate. [CAS 13423-fil-5. MgCiCC SH.o, small readily soluble, yellow cryslals. formed by reaction ol magnesium carbonate and chromic acid solution. Use Since it does not produce a fusible alkaline residue when thermally decomposed, it is used as a corrosion inhibitor in the water coolant of gas turbine engines. Insoluble basic magnesium chromates also arc available. Their potential applications arc in the treatment of light metal surfaces. 

It has been established that most cathode metals are to some extent soluble in chromic acid solutions, and ions will enter the solution in the highest available oxidation state [e.g. copper(II), gold(III)]. Polarization of the cathode will then cause reduction to lower oxidation states [kinetic factors will prevent the prior reduction of chromate(VI)], then new low-valent species may then initiate a chemical reduction of the chromium(Vl). Chromium deposition occurs within the potential range for the evolution of dihydrogen and, indeed, the latter is the dominant cathode process with the result that typically cathode current efficiencies of only 10-20% are achieved (see equation 9). 

Detection and Determination of the Methyl Alcohol.—(A). Qualitative Test, (a) 25 c.c. of the spirit are slowly distilled and the first 3-4 c.c. of distillate mixed in a conical flask with 25 c.c. of water, 15-20 drops of dilute sulphuric acid and 3-4 c.c. of 1% chromic acid solution, and again distilled. The first 5-6 c.c. of distillate are discarded, the next 10 c.c. being treated with 1 c.c. of 4% phenylhydiazine hydrochloride solution, 0-5 c.c. of 4% ferric chloride solution and 2-3 c.c. of concentrated... 

It is also possible to prepare a chromic acid solution by treating sodium dichromate (Na2Cr207) or potassium dichromate (K C Ot) with sulfuric acid. Consequently, sodium14 and potassium15 dichromate can be used, instead of chromium trioxide, in Jones oxidations. 

Chromium trioxide in aqueous solution equilibrates with a number of species, and chromic acid, being the most abundant one under acidic conditions (see page 1). Thus, a mixture of chromium trioxide and sulfuric acid is often referred to as a chromic acid solution. Such solution can also be obtained by the action of sulfuric acid on sodium dichromate (Na2Cr207) or potassium dichromate (K2Cr207). 

Notes. (1) The chromic acid solution may be prepared as follows. Dissolve lOOg (0.33 mol) of sodium dichromate dihydrate in 300 ml of water and slowly add 134g (73 ml, 1.34 mol) of concentrated sulphuric acid (98%, d 1.84). Cool the solution and dilute to 500 ml with water in a graduated flask. 

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