Selenous acid

Selenous acid[1]
IUPAC name
Selenous acid
7783-00-8 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:26642 YesY
ChemSpider 1060 YesY
ECHA InfoCard 100.029.067
KEGG D05814 YesY
PubChem 1091
Molar mass 128.97 g/mol
Appearance white hygroscopic crystals
Density 3.0 g/cm3
Melting point decomposes at 70°C
very soluble
Solubility soluble in ethanol
Acidity (pKa) 2.46, 7.3[2]
Related compounds
Other anions
selenic acid
hydrogen selenide
Other cations
sodium selenite
Related compounds
sulfurous acid
tellurous acid
polonous acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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

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:[3]

+ HSeO
(pKa = 2.62)
+ SeO2−
(pKa = 8.32)

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

+ 4 H+
+ 4 e Se + 3 H
(Eo = +0.74 V)

In 1 M OH

+ 4 e + 3 H
Se + 6 OH
(Eo = −0.37 V)

It is used in organic synthesis for the synthesis of 1,2-diketones (e.g. glyoxal).[4]


The major use is in protecting and changing the color of steel, especially steel parts on firearms.[5] The so-called cold-bluing process uses selenous acid, copper(II) nitrate, and nitric acid to change the color of the steel from silver-grey to blue-grey or black. Alternative procedures use copper sulfate and phosphoric acid instead. This process deposits a coating of copper selenide and is fundamentally different from other bluing processes which generate black iron oxide. Some older razor blades were also made of blued steel.[5]

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.

It can be use used as an oxidizing agent e.g. in laboratory preparation of glyoxal (ethane-1,2-dione) from glycol.

Selenious acid is a key component of the Mecke reagent used for drug checking.[6]

Health effects

Like many selenium compounds, selenous acid is highly toxic in excessive quantities, and ingestion of any significant quantity of selenous acid is usually fatal, however it is an approved dietary source in proper amounts. Symptoms of selenium poisoning can occur several hours after exposure, and may include stupor, nausea, severe hypotension and death.[7]


  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. Ka and pKa for Polyprotic Acids.
  3. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  4. “Glyoxal Bisulfite”, Organic Syntheses, Collected Volume 3, p.438 (1955).
  5. 1 2 Scarlato, E.A.; Higa, J. (28 June 1990). USES/HIGH RISK CIRCUMSTANCES OF POISONING "SELENIUM" Check |url= value (help). Retrieved 29 December 2010.
  6. "Colour Test Reagents-Kits for Preliminary Identification of Drugs of Abuse" (PDF). National Institute of Justice. 2000-07-01. Retrieved 2012-01-26.
  7. MSDS for "Reagent for Special Opiates (Codeine, Heroin, & Morphine)", Sirchie Finger Print Laboratories, Inc. May 12, 2006. (The page cannot be found)
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