During HAA5 analysis, which chemical is used to quench further HAA5 formation?

During HAA5 analysis, which chemical is used to quench further HAA5 formation?

• A. HNO3
• B. NO2
• C. H2SO4
• D. NH4Cl

Answer: (D)

Stopping new formation of haloacetic acids after the sample is collected is essential so the measurement reflects what was present at collection, not what forms during storage. Ammonium chloride does this job by reacting with any residual free chlorine (like hypochlorous acid) to form chloramines, which are much weaker oxidants. This dramatically reduces the oxidizing conditions that would drive further HAA formation, so the sample’s HAA5 concentration stays effectively fixed until analysis. The other options don’t provide the same quenching effect. Strong acids might lower pH but don’t neutralize free chlorine effectively and can introduce other interferences; nitrogen dioxide isn’t a practical quenching agent for this purpose. Ammonium chloride is preferred because it reliably suppresses ongoing formation without introducing interfering species that would affect the analysis.

Stopping new formation of haloacetic acids after the sample is collected is essential so the measurement reflects what was present at collection, not what forms during storage. Ammonium chloride does this job by reacting with any residual free chlorine (like hypochlorous acid) to form chloramines, which are much weaker oxidants. This dramatically reduces the oxidizing conditions that would drive further HAA formation, so the sample’s HAA5 concentration stays effectively fixed until analysis.

The other options don’t provide the same quenching effect. Strong acids might lower pH but don’t neutralize free chlorine effectively and can introduce other interferences; nitrogen dioxide isn’t a practical quenching agent for this purpose. Ammonium chloride is preferred because it reliably suppresses ongoing formation without introducing interfering species that would affect the analysis.