Selenous acid

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Selenous acid[1]
Structural formula
Ball-and-stick model
CAS number 7783-00-8 YesY
PubChem 1091
ChemSpider 1060 YesY
KEGG D05814 YesY
ChEBI CHEBI:26642 YesY
Jmol-3D images Image 1
Molecular formula H2SeO3
Molar mass 128.97 g/mol
Appearance white hygroscopic crystals
Density 3.0 g/cm3
Melting point decomposes at 70°C
Solubility in water very soluble
Solubility soluble in ethanol
Related compounds
Other anions selenic acid
hydrogen selenide
Other cations sodium selenite
Related compounds sulfurous acid
tellurous acid
polonous acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
Infobox references

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[edit]

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:[2]

is in equilibrium with H+
+ HSeO
(pKa = 2.62)
is in equilibrium with H+
+ SeO2−
(pKa = 8.32)

It is moderately oxidizing in nature, but kinetically slow. In 1 M H+

+ 4 H+
+ 4 e- is in equilibrium with Se + 3 H
(Eo = +0.74 V)

In 1 M OH

+ 4 e- + 3 H
is in equilibrium with Se + 6 OH
(Eo = −0.37 V)

It is used in organic synthesis for the synthesis of 1,2-diketones (e.g. glyoxal).[3]


The major use is in changing the color of steel, especially the steel in guns, the so-called "bluing" 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.[4]

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.[citation needed]

Selenious acid is a key component of the Mecke reagent used for drug testing.[5]

Health effects[edit]

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.[6]


  1. ^ Lide, David R. (1998). Handbook of Chemistry and Physics (87 ed.). Boca Raton, FL: CRC Press. pp. 4–81. ISBN 0-8493-0594-2 
  2. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  3. ^ “Glyoxal Bisulfite”, Organic Syntheses, Collected Volume 3, p.438 (1955).
  4. ^ Scarlato, E.A.; Higa, J. (28 June 1990). USES/HIGH RISK CIRCUMSTANCES OF POISONING "SELENIUM". Retrieved 29 December 2010. 
  5. ^ "Colour Test Reagents-Kits for Preliminary Identification of Drugs of Abuse". National Institute of Justice. 2000-07-01. Retrieved 2012-01-26. 
  6. ^ MSDS for "Reagent for Special Opiates (Codeine, Heroin, & Morphine)", Sirchie Finger Print Laboratories, Inc. May 12, 2006. (The page cannot be found)