HSeO3

HSeO3 is the chemical formula for selenous acid. Selenous acid is an inorganic compound with the formula H2SeO3. It is an unstable weak acid that exists in aqueous solution. In its solid state, it is a white crystalline substance. Selenous acid is a selenium oxoacid. It is the neutral form of the selenite ion, SeO3^2-.

Selenous acid can be prepared by dissolving selenium dioxide (SeO2) in water. The reaction is: SeO2 + H2O -> H2SeO3. It can also be formed by the reaction of barium selenite with sulfuric acid: BaSeO3 + H2SO4 -> H2SeO3 + BaSO4.

Selenous acid is a moderately strong reducing agent. It can be oxidized to selenic acid (H2SeO4). It can also reduce certain oxidizing agents, such as halogens. For example, it reacts with bromine to form selenium and hydrobromic acid: H2SeO3 + Br2 + H2O -> H2SeO4 + 2HBr. However, under certain conditions, it can be oxidized by bromine to selenic acid.

The compound is corrosive and toxic. Exposure can cause irritation to the skin, eyes, and respiratory tract. Ingestion can lead to nausea, vomiting, and diarrhea. Selenium compounds, in general, are toxic if ingested in large amounts.

Selenous acid and its salts, the selenites, are used in various applications, including as reducing agents in chemical synthesis, as glass decolorizing agents, and in the production of some pigments. Historically, selenites have also been used in medicine, though their use is now limited due to toxicity concerns.