Selenous acid

Template:Chembox image cellTemplate:Chembox image cellTemplate:Chembox image cellTemplate:Chembox AllOtherNamesTemplate:Chembox headerbarTemplate:Chembox IndexlistTemplate:Chembox JmolTemplate:Chembox ChEMBLTemplate:Chembox ECHATemplate:Chembox E numberTemplate:Chembox IUPHAR ligandTemplate:Chembox UNIITemplate:Chembox CompToxTemplate:Chembox headerbarTemplate:Chembox SolubilityInWaterTemplate:Chembox ConjugateAcidBaseTemplate:Chembox headerbarTemplate:Chembox headerbarTemplate:Chembox GHS (set)Template:Chembox headerbarTemplate:Chembox Datapage checkTemplate:Chembox Footer

Selenous acid

Template:Longitem	Template:Unbulleted list
ChEBI	Template:Unbulleted list
ChemSpider	Template:Unbulleted list
DrugBank	Template:Unbulleted list
EC Number	Template:Unbulleted list
KEGG	Template:Unbulleted list
Template:Longitem	Template:Unbulleted list
RTECS number	Template:Unbulleted list
UN number	3283 2630
Template:Longitem	Template:Chem2
Molar mass	Template:Chem molar mass
Appearance	white hygroscopic crystals
Density	3.0 g/cm3
Melting point	Template:Chembox CalcTemperatures
Solubility	soluble in ethanol
Acidity (pKa)	pKa1 = 2.46 pKa2 = 7.3
Template:Longitem	−45.4·10−6 cm3/mol
Template:Longitem	Intravenous infusion
Legal status	Template:Chembox Legal status/format
Template:Longitem	Selenic acid Hydrogen selenide
Template:Longitem	Sodium selenite
Template:Longitem	Template:Ubl

Selenous acid (or selenious acid) is the chemical compound with the formula Template:Chem2. Structurally, it is more accurately described by Template:Chem2. 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:

Template:Chem2  (pK_a = 2.62)

Template:Chem2  (pK_a = 8.32)

It is moderately oxidizing in nature, but kinetically slow. In 1 M Template:Chem2:

Template:Chem2  (E^o = +0.74 V)

In 1 M Template:Chem2:

Template:Chem2  (E^o = −0.37 V)

Selenous acid is hygroscopic.

Uses

The major use is in protecting and changing the color of steel, especially steel parts on firearms. 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.

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 is used in organic synthesis as an oxidizing agent for the synthesis of 1,2-dicarbonyl compounds, e.g. in laboratory preparation of glyoxal (oxaldehyde) from acetaldehyde.

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

Medical

Selenous acid can supply the trace element indicated in people as a source of selenium.

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.