Nitrite (NO₃) is used in closed loop treatment as an oxygen scavenger and metal passivator. Sodium nitrite (Na_2NO₃) combines with oxygen to form sodium nitrate (Na_2NO₄):

                2 Na₂NO₃ +O₂ = 2 Na₂NO₄

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

In collecting a sample for nitrite 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 nitrite level should be tested immediately. If the sample is left open oxygen in the air will combine with the nitrite and result in a lower reading than is actually in the boiler or closed system.

Nitrite may be expressed as nitrite (NO₃) or as sodium nitrite (Na₂NO₃). 1 ppm nitrite (NO₃) equals 1.74 ppm of sodium nitrite (Na₂NO₃).

Nitrite may be removed from water by aeration.

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