PERCHLORIC ACID

'Perchloric acid'
Perchloric acid Perchloric acid
General
Chemical name	Perchloric acid Chloric(VII) acid
Chemical formula	HClO4
SMILES	OCl(=O)(=O)=O
Molar mass	100.46 g/mol
Appearance	colorless solution
CAS number	7601-90-3
Properties
Density of 72.5% solution	1.67 g cm−3
Solubility in water	Miscible
Melting point (anhydrous)	−17 °C
Boiling point	203 °C
Acid dissociation constant p''K''a	−10.0
Hazards Perchloric acid (70% in water)
MSDS	External MSDS
EU classification	Oxidant ('O') Corrosive ('C')
NFPA 704
R-phrases	, ,
S-phrases	, , , ,
RTECS number	SC7500000
Related compounds
Related halogen oxoacids	Hypochlorous acid (HClO) Chlorous acid (HClO2) Chloric acid (HClO3) Perbromic acid (HBrO4) Metaperiodic acid (HIO4)
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa)

'Perchloric acid', HClO₄, is an oxoacid of chlorine and is a colorless liquid soluble in water. It is a strong acid comparable in strength to sulfuric acid or nitric acid. It is a superacid, but it is not the strongest Brønsted-Lowry acid (which is fluoroantimonic acid, HFSbF₅). Its pK_a is −10.^[1]
Pure ''anhydrous'' perchloric acid is an oily liquid; it can explode, and it slowly decomposes at normal temperature. Perchloric acid monohydrate, a crystalline substance, is more stable, but can also explode. Neither of these is commonly used or prepared; neither can be made by distilling the concentrated acid, since it forms an azeotrope with water, about 72.5% perchloric acid. This form of the acid is stable indefinitely and is commercially available. Such solutions are hygroscopic; if left unsealed, concentrated acid dilutes itself by absorbing water from the air.
A 0.100 molar solution in glacial acetic acid is used as an analytical reagent. Titration of weak bases is made easier if the usual medium, water, is replaced by glacial acetic acid. Glacial acetic acid is a much weaker base than water, so the base being titrated appears to be stronger. As a counterpart, the strength of acids is reduced. This shows the difference in strength among the strong acids.

Contents

Laboratory preparation

Safety

External links

References

Laboratory preparation

The diluted acid can be prepared by distillation of a solution of sodium perchlorate in concentrated sulfuric acid.
:NaClO₄ + H₂SO₄ → NaHSO₄ + HClO₄
In a related method, barium perchlorate reacts with dilute sulfuric acid to precipitates barium sulfate, leaving only perchloric acid in solution. It also can 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..

Safety

According to the CRC "Handbook of Laboratory Safety", perchloric acid is extremely hazardous. It is very corrosive to skin and eyes and should be handled with the utmost care. It can also ignite or explode upon contact with organic material such as cloth or wood. The salts of perchloric acid are powerful oxidizers that are often used in explosive compositions. Perchlorate salts tend to be less reactive and more stable than their chlorate counterparts, which has led to their increased use in pyrotechnic compositions due to safety concerns.

External links

★ International Chemical Safety Card 1006

References

1. dissociation constants