Chloramination chemistry and pH control During disinfection by chloramination, maintaining the pH slightly below 7 favors the formation of which chloramine species?

Introduction / Context:
Chloramination forms combined chlorine species by reacting free chlorine with ammonia. The distribution among monochloramine, dichloramine, and nitrogen trichloride depends strongly on pH, chlorine-to-ammonia ratio, temperature, and contact time. Utilities set pH targets to favour desired species and minimize taste/odour complaints.

Given Data / Assumptions:

• Objective: determine which species is favoured when pH is slightly below 7.
• Ammonia is present and chlorine is dosed appropriately.

Concept / Approach:
General speciation trend with decreasing pH is: monochloramine (dominant around pH 7.5–8.5) → dichloramine (favoured as pH drops below ~7) → nitrogen trichloride (favoured at still lower pH, typically <4.5), which causes strong odour issues. Therefore, maintaining pH slightly below 7 shifts equilibrium toward dichloramine.

Step-by-Step Solution:

Verification / Alternative check:
Operational guidance notes recommend pH control (often 7.2–8.5) to prioritise monochloramine and avoid dichloramine/trichloramine taste/odour.

Why Other Options Are Wrong:

• Monochloramine (NH2Cl): favoured at higher pH, not slightly below 7.
• Nitrogen trichloride (NCl3): requires much lower pH.
• “No chloramines” and “free chlorine only”: incorrect when ammonia is present under typical chloramination conditions.

Common Pitfalls:
Using incorrect formulas for chloramines; the correct species are NH2Cl (mono), NHCl2 (di), and NCl3 (tri).

Final Answer:
Dichloramine (NHCl2)