Selenous acid
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'''Selenous acid''' (also known as '''selenious acid''') is the [[chemical compound]] with the [[chemical formula|formula]] H<sub>2</sub>SeO<sub>3</sub>, perhaps more descriptively written as (HO)<sub>2</sub>SeO. Selenous acid is the principal [[oxoacid]] of [[selenium]].
==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 bond]]s. In solution it is a diprotic acid:<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref>
:H<sub>2</sub>SeO<sub>3</sub> {{unicode|⇌}} H<sup>+</sup> + HSeO<sub>3</sub><sup>-</sup> pK<sub>a</sub> = 2.62
:HSeO<sub>3</sub><sup>-</sup> {{unicode|⇌}} H<sup>+</sup> + [[selenite|SeO<sub>3</sub><sup>2-</sup>]] pK<sub>a</sub> = 8.32
It is moderately oxidizing in nature, but kinetically slow. In 1 M H<sup>+</sup>:
:H<sub>2</sub>SeO<sub>3</sub> + 4H<sup>+</sup> + 4e<sup>−</sup> {{unicode|⇌}} Se + 3H<sub>2</sub>O ''E''<sup><s>o</s></sup> = +0.74 V
In 1 M OH<sup>−</sup>:
:SeO<sub>3</sub><sup>2−</sup> + 4e<sup>−</sup> + 3H<sub>2</sub>O {{unicode|⇌}} Se + 6OH<sup>−</sup> ''E''<sup><s>o</s></sup> = −0.37 V
It is used in [[organic synthesis]] for the synthesis of 1,2-di[[ketone]]s (e.g. [[glyoxal]]). <ref>[http://www.orgsyn.org/orgsyn/pdfs/CV3P0438.pdf “Glyoxal Bisulfite”], Organic Syntheses, Collected Volume 3, p.438 (1955).</ref>
==Health effects==
Like many compounds of selenium, 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.<ref>[http://www.sirchie.com/msds/NARK20010.pdf MSDS for "Reagent for Special Opiates (Codeine, Heroin, & Morphine)"], Sirchie Finger Print Laboratories, Inc. May 12, 2006.</ref>
==Congeners==
*S [[Sulfurous acid]]
*Se [[Selenous acid]]
*Te [[Tellurious acid]]
*Po [[Polonous acid]]
==References==
<references/>
==External links==
[[Category:Selenites]]
[[Category:Acids]]
{{inorganic-compound-stub}}
[[it:Acido selenioso]]
[[ja:亜セレン酸]]