Selenous acid (or selenious acid) is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...

with the formula . Structurally, it is more accurately described by . It is the principal oxoacid of

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and telluriu ...

; the other being selenic acid.

Formation and properties

Selenous acid is analogous to

sulfurous acid Sulfurous acid (also sulfuric(IV) acid, sulphurous acid (UK), sulphuric(IV) acid (UK)) is the chemical compound with the formula . There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase. ...

, 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: : (p''K''_a = 2.62) : (p''K''_a = 8.32) It is moderately oxidizing in nature, but kinetically slow. In 1 M : : (''E''^o = +0.74 V) In 1 M : : (''E''^o = −0.37 V) Selenous acid is

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...

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 Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

to change the color of the steel from silver-grey to blue-grey or black. Alternative procedures use copper sulfate and

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solutio ...

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 Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

as an oxidizing agent for the synthesis of 1,2-dicarbonyl compounds, e.g. in laboratory preparation of glyoxal (ethane-1,2-dione) from

acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula CH3 CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is a colorless liquid or gas, boiling near room temperature. It is one of the ...

. Selenious acid is a key component of the

Mecke reagent The Mecke reagent is used as a simple spot-test to presumptively identify alkaloids as well as other compounds. It is composed of a mixture of selenous acid and concentrated sulfuric acid, which is dripped onto the substance being tested. The Unit ...

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 Nausea is a diffuse sensation of unease and discomfort, sometimes perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged and has been described as placing discomfort on the chest, abdomen, or back of the ...

, severe

hypotension Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the systolic blood pressure (the top number) and the dia ...

and death.

References

External links

* {{Portal bar , Medicine Hydrogen compounds Oxidizing acids Oxidizing agents Oxoacids Selenium(IV) compounds X