Difference between revisions of "Chromic acid"

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Chromic acid is the name given to the compound resulted from the addition of concentrated sulfuric acid to a dichromate salt, such as potassium dichromate. The resulting product however is impure, and also tends to contain other compounds such as chromium trioxide or dichromic acid. The generally recognized formula for chromic acid is H₂CrO₄.

Properties

Chemical

Chromic acid is a powerful oxidizer. Solutions of chromic acid are used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones.

Addition of a reducing agent, such as ascorbic acid, will reduce it to Cr(III).

Physical

Chromic acid is a dark red to red-brownish viscous liquid, that decomposes on boiling.

Availability

Chromic acid is generally not sold, and has to be prepared in situ.

Preparation

Chromic acid can be prepared by adding concentrated sulfuric acid to a cooled aqueous solution of a dichromate salt:

H₂SO₄ + K₂Cr₂O₇ → H₂CrO₄ + K₂SO₄

Very pure chromic acid can be made by adding chromium trioxide to pure water.

CrO₃ + H₂O ⇋ H₂CrO₄

Derivations

Chromosulfuric acid

Made by adding an excess of sulfuric acid to a supersaturated solution of potassium dichromate. One standard recipe involves adding 93 ml of concentrated (95-98%) sulfuric acid to a cooled 7 ml solution of supersaturated K₂Cr₂O₇. If the solution is not cooled, chromium trioxide will precipitate. The resulting liquid has a red-brownish color, it is viscous and has a weak unpleasant rotten eggs smell (though it's best not to inhale chromic solution vapors).

It is also known as sulfochromic mixture.

Chromonitric acid

This special mixture is made by adding concentrated nitric acid to a dichromate salt. It is/was widely used in the countries of the Soviet bloc, instead of the more common piranha solution. May release nitrogen oxides fumes during preparation.

Chromonitric acid is known to ignite organic materials and pose an explosion hazard.^[1]

Diluted mixtures of chromic and nitric acids can be used to test for silver.

Dichromic acid

Can be obtained by adding excess chromium trioxide to chromic acid. It's generally encountered as a side product in the preparation of chromic acid solutions, rather than a desired compound.

Projects

• Clean glassware
• Jones oxidation
• Make chromyl chloride
• Identify silver (chromonitric)
• Destroy small samples of hazardous organic compounds/alkaloids

Handling

Safety

Chromic acid and its derivatives are extremely corrosive to organic materials and flesh. Contact with bare skin causes an intense heating sensation and, if not washed immediately, the acid will cause severe burns. Hexavalent chromium ions are carcinogenic and proper protection must be worn when handling the product. However, when the chromic acid is neutralized with a base, it immediately converts to the less harmful Cr(III), though traces of Cr(VI) may still be in the final product.

Storage

Chromic acid should be stored in glass bottles with stopper or lid, away from any organic material, with a proper label and a hazard symbol. The acid can be stored for long periods of time without decomposition.

Disposal

Chromic acid should first be diluted in lots of water. After dilution, it can be neutralized with a reducing agent. Sodium/potassium metabisulfite, sodium thiosulfate or sodium sulfite are excellent neutralizing agents.

Sodium bicarbonate will also neutralize the acid to Cr(III), however there might still be traces of Cr(IV) in the product, so this might not be a suitable disposal agent.

References

1. ↑ http://booksonchemistry.com/index.php?id1=3&category=laborotor-tech&author=zaharov-ln&book=1991&page=26

Relevant Sciencemadness threads

• Spill of chromic acid - cleanup
• Does chromic acid react with AgI?