Sulfite (SO₃) is used in boiler and closed loop treatment as an oxygen scavenger and metal passivator. Sodium sulfite (Na₂SO₃) combines with oxygen to form sodium sulfate (Na₂SO₄):

2 Na₂SO₃ +O₂ = 2 Na₂SO₄

Sulfite is a reducing agent, and will reduce iron oxide to magnetite.

In collecting a sample for sulfite testing, it is important that air not be introduced into the sample. The sample collection bottle should be filled completely to the top and then cooled prior to testing. Once the sample is opened the sulfite level should be tested immediately. If the sample is left open oxygen in the air will combine with the sulfite and result in a lower reading than is actually in the boiler or closed system.

Sulfite may be expressed as sulfite (SO₃) or as sodium sulfite (Na₂SO₃). 1 ppm sulfite (SO₃) equals 1.575 ppm of sodium sulfite (Na₂SO₃).

Sulfite may be removed from water by aeration.

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