Overview: Selenous acid, chemical formula H2SeO3, is the hydrated oxoacid of selenium in the +4 oxidation state. It typically appears as a colorless, water-soluble crystalline solid or as an aqueous solution. In solution it acts as a proton donor and as a source of the selenite ion; it is chemically analogous to sulfurous acid (H2SO3).
Chemical structure and behavior
Structurally, H2SeO3 can be pictured with selenium bonded to three oxygens: one double-bonded oxygen and two hydroxyl groups, producing a pyramidal molecular geometry. It is a dibasic acid, able to lose two protons to form hydrogen selenite (HSeO3−) and then selenite (SeO3 2−) ions in water. The species is a mild oxidizing agent and can be reduced to elemental selenium or oxidized further to selenic acid (H2SeO4).
Preparation and reactions
Selenous acid is commonly prepared by dissolving selenium dioxide (SeO2) in water. It reacts with bases to give selenite salts such as sodium selenite and forms coordination compounds with metals. In organic chemistry H2SeO3 and its derivatives may be used for specific oxidative transformations or to introduce selenium functionality into molecules.
Applications and examples
- Precursor to inorganic selenite salts used in glassmaking and pigments.
- Industry and laboratory oxidant for selective reactions and selenium incorporation.
- Analytical reagent in some chemical tests and small-scale synthesis of selenium compounds.
Safety, handling, and distinctions
Selenous acid is toxic and corrosive; selenium compounds are essential trace nutrients at very low levels but become harmful at higher exposures. Proper protective equipment and ventilation are required when handling it, and waste should be managed to prevent environmental release. It is distinct from selenic acid (a stronger +6 acid) and should not be confused with elemental selenium or organic selenium compounds.
For further technical details and handling guidance see published safety data, synthetic notes at preparation sources, structural discussions at chemical references, and comparative information on related acids at acid families.