Perchloric acid

Perchloric acid

Names
Other names Hyperchloric acid[1]
Identifiers
CAS Registry Number	7601-90-3 Y
ChEBI	CHEBI:29221 Y
ChEMBL	ChEMBL1161634 Y
ChemSpider	22669 Y
EC number	231-512-4
InChI InChI=1S/ClHO4/c2-1(3,4)5/h(H,2,3,4,5) Y Key: VLTRZXGMWDSKGL-UHFFFAOYSA-N Y InChI=1/ClHO4/c2-1(3,4)5/h(H,2,3,4,5) Key: VLTRZXGMWDSKGL-UHFFFAOYAD
Jmol-3D images	Image
PubChem	24247
RTECS number	SC7500000
SMILES OCl(=O)(=O)=O
UN number	1873
Properties
Chemical formula	HClO4
Molar mass	100.46 g/mol
Appearance	colorless liquid
Density	1.67 g/cm3
Melting point	−17 °C (1 °F; 256 K) (azeotrope)[2] -112 °C (anhydrous)
Boiling point	203 °C (397 °F; 476 K) (azeotrope)[3]
Solubility in water	miscible
Acidity (pKa)	≈ −10.[4]
Hazards
MSDS	ICSC 1006
EU Index	017-006-00-4
EU classification	Oxidant (O) Corrosive (C)
R-phrases	R5, R8, R35
S-phrases	(S1/2), S23, S26, S36, S45
NFPA 704	0 3 3 OX
Related compounds
Related compounds	Hydrochloric acid Hypochlorous acid Chlorous acid Chloric acid
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Perchloric acid is an inorganic compound with the formula HClO₄. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric and nitric acid. It is a powerful oxidizer when hot, but its aqueous solutions up to approximately 70% and room temperature are generally safe, only showing strong acid features and no oxidizing properties. Perchloric acid is useful for preparing perchlorate salts, especially ammonium perchlorate, an important rocket fuel. Overall, perchloric acid is dangerously corrosive and readily forms potentially explosive mixtures.

Production

Perchloric acid is produced industrially by two routes. The traditional method exploits the high aqueous solubility of sodium perchlorate (209 g/100 mL of water at room temperature). Treatment of such solutions with hydrochloric acid gives perchloric acid, precipitating solid sodium chloride:

NaClO₄ + HCl → NaCl + HClO₄

The concentrated acid can be purified by distillation. The alternative route, which is more direct and avoids salts, entails anodic oxidation of aqueous chlorine at a platinum electrode.^[5][6]

Laboratory preparations

Treatment of barium perchlorate with sulfuric acid precipitates barium sulfate, leaving perchloric acid. It also be made by mixing nitric acid with ammonium perchlorate. The reaction gives nitrous oxide and perchloric acid due to a concurrent reaction involving the ammonium ion.

Properties

Anhydrous perchloric acid is an unstable oily liquid at room temperature. It forms at least five hydrates, several of which have been characterized crystallographically. These solids consist of the perchlorate anion linked via hydrogen bonds to H₂O and H₃O⁺ centers^[7] Perchloric acid forms an azeotrope with water, consisting of about 72.5% perchloric acid. This form of the acid is stable indefinitely and is commercially available. Such solutions are hygroscopic. Thus, if left open to the air, concentrated perchloric acid dilutes itself by absorbing water from the air.

Dehydration of perchloric acid gives the anhydride dichlorine heptoxide, which is even more dangerous:^[8]

2 HClO₄ + P₄O₁₀ → Cl₂O₇ + "H₂P₄O₁₁"

Uses

Perchloric acid is mainly produced as a precursor to ammonium perchlorate, which is used in rocket fuel. The growth in rocketry has led to increased production of perchloric acid. Several million kilograms are produced annually.^[5] Perchloric acid is one of the most proven materials for etching of liquid crystal displays and critical electronics applications as well as ore extraction and has unique properties in analytical chemistry.^[9] Additionally it is a useful component in etching of chrome^[10]

As an acid

Perchloric acid, a superacid, is one of the strongest Brønsted-Lowry acids. Its pK_a is −10.^[4] It provides strong acidity with minimal interference because perchlorate is weakly nucleophilic (explaining the high acidity of HClO₄). Other acids of noncoordinating anions, such as fluoroboric acid and hexafluorophosphoric acid are susceptible to hydrolysis, whereas perchloric acid is not. Despite hazards associated with the explosiveness of its salts, the acid is often preferred in certain syntheses.^[11] For similar reasons, it is a useful eluent in ion-exchange chromatography.

It is also used for electropolishing/etching of aluminum, molybdenum, and other metals.

Safety

Given its strong oxidizing properties, perchloric acid is subject to extensive regulations.^[12] It is highly reactive with metals (e.g., aluminium) and organic matter (wood, plastics). On February 20, 1947, in Los Angeles California, 17 people were killed and 150 injured when a bath, consisting of over 1000 litres of 75% perchloric acid and 25% acetic anhydride by volume, exploded. The plant, 25 other buildings and 40 automobiles were obliterated and 250 nearby homes were damaged. The bath was being used to electro-polish aluminum furniture. In addition, organic compounds were added to the overheating bath when an iron rack was replaced with one coated with cellulose acetobutyrate (Tenit-2 plastic). A few minutes later the bath exploded.^[13][14]

Work conducted with perchloric acid must be conducted in fume hoods with a wash-down capability to prevent accumulation of oxidisers in the ductwork.

See also

• Chloric acid
• Oxidizing acid

References

External links

• International Chemical Safety Card 1006