Sulfite is not usually present in surface waters. If sulfite is discharged in effluents or from domestic wastewaters, it readily oxidizes to form sulfate. Sodium sulfite is the most common form of sulfite and is an excellent reducing agent with applications as an oxygen scavenger, in photographic developers, as a bleach in the textile, pulp and paper industries, and as a preservative in foods and beverages.

Sulfite concentrations in boiler and process waters must be monitored routinely to avoid overtreatment. While sulfite removes oxygen, an excess can lower the pH and render the water corrosive. Waste treatment plants which use sulfur dioxide to remove excess chlorine must monitor their effluents for sulfite.

The Titrimetric Method. Sulfite is titrated with iodide-iodate titrant in an acid solution using a starch indicator. Thiosulfate will titrate as sulfite. Results are expressed as ppm (mg/L) SO₃.