Interferences to aerometric chlorine residual measurement come from nitrite.

• A. calcium
• B. nitrite
• C. dissolved oxygen
• D. organics

Answer: (B)

The main idea is that chlorine residual measurements are based on how much free chlorine remains in solution after it’s exposed to the sample. Substances that quickly react with chlorine can consume it before measurement, causing false low readings. Nitrite is a strong reductant toward chlorine species, so it rapidly reacts with the active chlorine (hypochlorous acid) to form nitrate and chloride. This rapid redox reaction depletes the chlorine available for detection, leading to interference in the aerometric chlorine residual result. Because this reaction happens quickly even at low nitrite concentrations, nitrite is a classic interfering species for this test. Other substances listed don’t produce the same rapid chemical depletion of chlorine in this method, so they don’t cause the same interference.