Chromium trioxide

Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO₃. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.
This compound is a dark-purple solid under anhydrous conditions, bright orange when wet and which dissolves in water concomitant with hydrolysis. Millions of kilograms are produced annually, mainly for electroplating. Chromium trioxide is a powerful oxidiser and a carcinogen.

Production, structure, and basic reactions

Chromium trioxide is generated by treating sodium dichromate with sulfuric acid:
:H₂SO₄ + Na₂Cr₂O₇ → 2 CrO₃ + Na₂SO₄ + H₂O

Approximately 100,000 tonnes are produced annually by this or similar routes.

The solid consists of chains of tetrahedrally coordinated chromium atoms that share vertices. Each chromium center therefore shares two oxygen centers with neighbors. Two oxygen atoms are not shared, giving an overall stoichiometry of 1:3.

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The structure of monomeric CrO₃ has been calculated using density functional theory, and is predicted to be pyramidal (point group C_3v) rather than planar (point group D_3h).

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Chromium trioxide decomposes above 197 °C, liberating oxygen and eventually giving Cr₂O₃:
: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

Applications

Chromium trioxide is mainly used in chrome plating. It is typically employed with additives that affect the plating process but do not react with the trioxide. The trioxide reacts with cadmium, zinc, and other metals to generate passivating chromate films that resist corrosion. It is also used in the production of synthetic rubies. Chromic acid solution is also used in applying types of anodic coating to aluminium, which are primarily used in aerospace applications. On the International Space Station, it is used to control bacteria growth in the wastewater storage tank. A chromic acid/phosphoric acid solution is also the preferred stripping agent of anodic coatings of all types.

Safety

Chromium trioxide is highly toxic, corrosive, and carcinogenic. It is the main example of hexavalent chromium, an environmental hazard.The environmental impact of hexavalent chromium inspired the 2000 biographical Hollywood movie ''Erin Brockovich''. The related chromium(III) derivatives are not particularly dangerous; thus, reductants are used to destroy chromium(VI) samples.

Chromium trioxide, being a powerful oxidizer, will ignite organic materials such as alcohols on contact.

Images

Reaction between potassium dichromate and sulfuric acid (1).jpg, A concentrated solution of potassium dichromate in water.
Reaction between potassium dichromate and sulfuric acid (2).jpg, Addition of sulfuric acid to the solution.
Reaction between potassium dichromate and sulfuric acid (3).jpg, Crystallization of chromium trioxide from the reaction.


Reaction between chromium(VI) oxide and ethanol (1).JPG, Reaction between chromium trioxide and ethanol
Reaction between chromium(VI) oxide and ethanol (2).JPG
Reaction between chromium(VI) oxide and ethanol (3).JPG

References

External links

ATSDR Case Studies in Environmental Medicine: Chromium Toxicity
U.S. Department of Health and Human Services

Chromium Trioxide
at ''The Periodic Table of Videos'' (University of Nottingham)

Reactions with Chromium Trioxide as Oxidizing Agent

{{oxygen compounds

Acidic oxides
Chromium(VI) compounds