|Jmol-3D images||Image 1|
|Molar mass||128.97 g/mol|
|Appearance||white hygroscopic crystals|
decomposes at 70°C
|Solubility in water||very soluble|
|Solubility||soluble in ethanol|
|Other anions||selenic acid
|Other cations||sodium selenite|
|Related compounds||sulfurous acid
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Selenous acid (or selenious acid) is the chemical compound with the formula H2SeO3. Structurally, it is more accurately described by (HO)2SeO. It is the principal oxoacid of selenium; the other being selenic acid.
Formation and properties 
Selenous acid is analogous to sulfurous acid, but it is more readily isolated. Selenous acid is easily formed upon the addition of selenium dioxide to water. As a crystalline solid, the compound can be seen as pyramidal molecules that are interconnected with hydrogen bonds. In solution it is a diprotic acid:
- H2SeO3 H+ + HSeO−
3 (pKa = 2.62)
3 H+ + SeO2−
3 (pKa = 8.32)
It is moderately oxidizing in nature, but kinetically slow. In 1 M H+:
- H2SeO3 + 4 H+ + 4 -
e Se + 3 H2O (E
o= +0.74 V)
In 1 M OH−:
3 + 4 -
e + 3 H2O Se + 6 OH− (E
o= −0.37 V)
The major use is in changing the color of steel, especially the steel in guns, the so-called "blueing" process which uses selenous acid, copper(II) nitrate, and nitric acid to change the color of the steel from silver-grey to blue-grey. Some older razor blades were also made of blued steel.
Another use for selenious acid is the chemical darkening and patination of copper, brass and bronze, producing a rich dark brown color that can be further enhanced with mechanical abrasion.
Health effects 
Like many selenium compounds, selenous acid is highly toxic, and ingestion of any significant quantity of selenous acid is usually fatal. Symptoms of selenium poisoning can occur several hours after exposure, and may include stupor, nausea, severe hypotension and death.
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- “Glyoxal Bisulfite”, Organic Syntheses, Collected Volume 3, p.438 (1955).
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