Selenous acid

Selenous acid (also known as selenious acid) is the chemical compound with the formula H2SeO3, perhaps more descriptively written as (HO)2SeO. 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 bonds. In solution it is a diprotic acid:
 * H2SeO3  H+  +  HSeO3-  pKa = 2.62
 * HSeO3-  H+  +  SeO32-  pKa = 8.32

It is moderately oxidizing in nature, but kinetically slow. In 1 M H+:
 * H2SeO3 + 4H+ + 4e&minus; Se + 3H2O    Eo = +0.74 V

In 1 M OH&minus;:
 * SeO32&minus; + 4e&minus; + 3H2O Se + 6OH&minus;   Eo = &minus;0.37 V

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

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.

Congeners

 * S Sulfurous acid
 * Se Selenous acid
 * Te Tellurious acid
 * Po Polonous acid