Chromium trioxide

Chromium trioxide

Impure chromium trioxide
Names
IUPAC name Chromium trioxide
Other names Chromic anhydride Chromic acid anhydride Chromium(VI) oxide
Identifiers
Jmol-3D images	Image
SMILES O=[Cr](=O)=O
Properties
Chemical formula	CrO3
Molar mass	99.99 g/mol
Appearance	Dark red solid
Odor	Odorless
Density	2.7 g/cm3 (at 20 °C)
Melting point	197 °C (387 °F; 470 K)
Boiling point	250 °C (482 °F; 523 K) (decomposes)
Solubility in water	164.8 g/100 mL (0 °C) 169 g/100 mL (25 °C) 172.6 g/100 mL (40 °C) 198.1 g/100 mL (100 °C)
Solubility	Soluble in acetic acid, acetone, diethyl ether, nitric acid, sulfuric acid
Vapor pressure	~0 mmHg
Thermochemistry
Std molar entropy (So298)	73.2 J/mol·K
Std enthalpy of formation (ΔfHo298)	−589.3 kJ/mol
Hazards
Safety data sheet	ScienceLab
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	80 mg/kg (rats, oral)
Related compounds
Related compounds	Chromium(III) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Chromium trioxide is an inorganic compound with the formula CrO₃. It is a dark red/crimson solid, widely used in synthesis and industry. It is the acidic anhydride of chromic acid.

Properties

Chemical

Chromium trioxide decomposes above 197 °C, yielding chromium(III) oxide and liberating oxygen:

4 CrO₃ → 2 Cr₂O₃ + 3 O₂

It is used in organic synthesis as an oxidant, often as a solution in acetic acid, or acetone in the case of the Jones oxidation. In these oxidations, the Cr(VI) converts primary alcohols to the corresponding carboxylic acids and secondary alcohols to ketones. The reactions are shown below:

Primary alcohols to carboxylic acids:

4 CrO₃ + 3 RCH₂OH + 12 H⁺ → 3 RCOOH + 4 Cr³⁺ + 9 H₂O

Secondary alcohols to ketones:

2 CrO₃ + 3 R₂CHOH + 6 H⁺ → 3 R₂C=O + 2 Cr³⁺ + 6 H₂O

Dry chromium trioxide reacts violently with ethanol and other organic compounds, causing them to combust on contact.

Physical

Chromium trioxide is a dard red/crimson solid in its anhydrous form, and red-orange in aqueous solution. It is very soluble in water and soluble in some organic solvents, such as acetone or diethyl ether, as well as sulfuric acid.

Availability

Chromium trioxide is sold by various chemical suppliers, though due to its hazards it's not easy to get hold of.

Preparation

Chromium trioxide can be readily prepared by reacting a Cr(VI) containing compound, such as sodium chromate/dichromate, or potassium chromate/dichromate with concentrated sulfuric acid, albeit its purification is complicated.

H₂SO₄ + Na/K₂CrO₄ → CrO₃ + Na/K₂SO₄ + H₂O

H₂SO₄ + Na/K₂Cr₂O₇ → 2 CrO₃ + Na/K₂SO₄ + H₂O

Potassium dichromate is generally used, as it's cheap and more readily available.

Chromium trioxide can be obtained as such; in a beaker, 100 g of potassium dichromate is added. 200 ml of distilled water is added, and the solution is heated on a hotplate with a stirrer, until all the salt dissolves in the hot water. Then, 400 ml of conc. (98%) sulfuric acid is slowly added to the hot solution of dichromate, and this must be done carefully to prevent the solution from boiling, and all of this must be done outside or in a fume hood. After the H₂SO₄ is added, the solution turns dark, and then another 200 ml of conc. sulfuric acid is added, while solution is cooled. This causes the chromium trioxide to precipitate out of the solution. Cooling the solution will cause more CrO₃ to precipitate, though the solution should not be cooled too much, as potassium bisulfate might precipitate out of the solution. The resulting thick suspension of chromium trioxide is filtered using a fritted glass funnel under vacuum. The crude filtrate still contains lots of sulfuric acid and leftover potassium sulfate salts, and needs washing. But because CrO₃ will react with water and most solvents, only conc. nitric acid (>65%) is suitable for washing this compound. The filtrate is washed using conc. nitric acid, and then it is dried using an oven and under vacuum. The final product may still contain traces of acids, which can be removed by further washing and vacuum drying.

Projects

• Chrome plating
• Instantaneous ignition of alcohols

Handling

Safety

Chromium trioxide is a powerful oxidizer, highly toxic, corrosive, and carcinogenic. Avoid contact with alcohols.

Storage

Chromium trioxide should be kept in plastic or glass containers, with a proper label and a hazard symbol, in a dry place. If you're using glass, a good idea would be to keep the glass container inside a plastic container.

Since the compound ignites alcohols on contact, keep it away from any source of volatile solvents or products containing alcohols.

Disposal

A reducing agent, such as sodium metabisulfite, sodium sulfite or sodium thiosulfate can be used to turn CrO₃ into the less toxic Cr(III) oxide. Glucose may also be used, albeit in a diluted solution. The neutralization must be done outside or in a fumehood, as the process will release Cr(VI) aerosoles, which are harmful and carcinogenic.

References

Relevant Sciencemadness threads

• CrO3