Difference between revisions of "Chromic acid"
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Chromic acid is a dark red to red-brownish viscous liquid, that decomposes on boiling.
Chromic acid is a dark red to red-brownish viscous liquid , that decomposes on boiling.
Latest revision as of 22:01, 20 January 2020
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| IUPAC name
| Systematic IUPAC name
| Other names
|Molar mass||118.01 g/mol|
|Appearance||Dark red/reddish-brown viscous liquid|
|Melting point||197 °C (387 °F; 470 K)|
|Boiling point||250 °C (482 °F; 523 K)|
|169 g/100 ml|
|Solubility|| Reacts with alcohols|
Soluble in acetone
|Vapor pressure||~0 mmHg|
|Acidity (pKa)||-0.8 to 1.6|
|Safety data sheet||ScienceLab|
|Lethal dose or concentration (LD, LC):|
LD50 (Median dose)
|51.9 mg/kg (H2CrO4·2Na, rat, oral)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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.
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).
Chromic acid is a dark red to red-brownish viscous liquid or solid, that decomposes on boiling.
Chromic acid is generally not sold, and has to be prepared in situ.
Chromic acid can be prepared by adding concentrated sulfuric acid to a cooled aqueous solution of a dichromate salt:
- H2SO4 + K2Cr2O7 → H2CrO4 + K2SO4
- CrO3 + H2O ⇋ H2CrO4
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.
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.
Diluted mixtures of chromic and nitric acids can be used to test for silver.
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.
- Clean glassware
- Jones oxidation
- Make chromyl chloride
- Identify silver (chromonitric)
- Destroy small samples of hazardous organic compounds/alkaloids
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.
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.
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.