Ammonium sulfite, with the chemical formula (NH4)2SO3, is a chemical compound composed of ammonium ions (NH4+) and sulfite ions (SO3^2-). It is a white to yellowish crystalline solid that is highly soluble in water.

Ammonium sulfite is not a stable compound and is mainly used as an aqueous solution rather than a solid. It is a reagent commonly used in laboratory settings and certain industrial processes. Some key properties and uses of ammonium sulfite include:

1. Chemical Reducing Agent: Ammonium sulfite is a mild reducing agent and is used in various chemical reactions to reduce certain substances.
2. Photographic Fixer: Historically, ammonium sulfite was used in photography as a photographic fixer to remove unexposed silver halide from photographic films or papers.
3. Water Treatment: Ammonium sulfite has been used in water treatment processes to remove chlorine and other oxidizing agents from water.
4. Analytical Chemistry: It is used in analytical chemistry, particularly in qualitative analysis, to detect and identify certain metal ions.

It is important to note that ammonium sulfite can rapidly react with oxygen in the air to form ammonium sulfate [(NH4)2SO4], which is a stable compound. As a result, ammonium sulfite solutions are not stable and should be prepared fresh when needed for specific reactions or applications.

Ammonium sulfite is considered a hazardous chemical, and proper safety precautions, such as good ventilation and the use of personal protective equipment, should be followed when working with ammonium sulfite solutions in the laboratory or industrial settings.