Selenous acid

Selenous acid^[1]
Names
	IUPAC names Selenous acid; Selenic(IV) acid
	Other names Selenious acid
Identifiers
CAS Number	7783-00-8 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:26642 ;
ChemSpider	1060 ;
ECHA InfoCard	100.029.067
KEGG	D05814 ;
PubChem CID	1091;
UNII	F6A27P4Q4R ;
CompTox Dashboard (EPA)	DTXSID9024300 ;
	InChI InChI=1S/H2O3Se/c1-4(2)3/h(H2,1,2,3) Key: MCAHWIHFGHIESP-UHFFFAOYSA-N ; InChI=1/H2O3Se/c1-4(2)3/h(H2,1,2,3) Key: MCAHWIHFGHIESP-UHFFFAOYAW;
	SMILES O=[Se](O)O;
Properties
Chemical 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
Acidity (pKa)	2.46, 7.3
Conjugate base	Hydrogenselenite
Magnetic susceptibility (χ)	−45.4·10−6 cm3/mol
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). verify (what is ?) 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]

H
₂SeO
₃ ⇌ H⁺
+ HSeO⁻
₃ (pK_a = 2.62)

HSeO⁻
₃ ⇌ H⁺
+ SeO²⁻
₃ (pK_a = 8.32)

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

H
₂SeO
₃ + 4 H⁺
+ 4 e⁻ ⇌ Se + 3 H
₂O (E^o = +0.74 V)

In 1 M OH⁻
:

SeO²⁻
₃ + 4 e⁻ + 3 H
₂O ⇌ Se + 6 OH⁻
(E^o = −0.37 V)

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

Uses

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.^{[citation needed]}

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]

References

^ Lide, David R. (1998). Handbook of Chemistry and Physics (87 ed.). Boca Raton, Florida: CRC Press. pp. 4–81. ISBN 0-8493-0594-2.
^ K_a and pK_a for Polyprotic Acids. ucdsb.on.ca
^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
^ “Glyoxal Bisulfite”, Organic Syntheses, Collected Volume 3, p.438 (1955).
^ ^a ^b Scarlato, E.A.; Higa, J. (28 June 1990). USES/HIGH RISK CIRCUMSTANCES OF POISONING "SELENIUM". Retrieved 29 December 2010. {{cite web}}: Check |url= value (help)
^ "Colour Test Reagents-Kits for Preliminary Identification of Drugs of Abuse" (PDF). National Institute of Justice. 2000-07-01. Retrieved 2012-01-26.
^ "Material Safety Data Sheet - Product Name: Reagent for Special Opiates" (PDF). Sirchie Finger Print Laboratories, Inc. May 12, 2006. Archived from the original (PDF) on October 18, 2006. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)

[hand-1] Lide, David R. (1998). Handbook of Chemistry and Physics (87 ed.). Boca Raton, Florida: CRC Press. pp. 4–81. ISBN 0-8493-0594-2.

[2] K_a and pK_a for Polyprotic Acids. ucdsb.on.ca

[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).

[Scarlato1990-5] Scarlato, E.A.; Higa, J. (28 June 1990). USES/HIGH RISK CIRCUMSTANCES OF POISONING "SELENIUM". Retrieved 29 December 2010. {{cite web}}: Check |url= value (help)

[NIJ-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] "Material Safety Data Sheet - Product Name: Reagent for Special Opiates" (PDF). Sirchie Finger Print Laboratories, Inc. May 12, 2006. Archived from the original (PDF) on October 18, 2006. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)

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