Question 1

Why a 4:1 ratio?

Accepted Answer

At weight ratios between 3:1 and 5:1 (Cl₂:N), monochloramine (NH₂Cl) is the dominant species and is stable for days in distribution. Outside that band, less stable di- and trichloramines form.

Question 2

What about breakpoint chlorination?

Accepted Answer

Breakpoint occurs near ~7.6:1 Cl₂:N when all ammonia has been oxidized. Past breakpoint you have free chlorine, not chloramines.

Question 3

Does NH₃ vs NH₃-N matter?

Accepted Answer

Yes. NH₃-N counts only the nitrogen atom (14 g/mol). NH₃ counts the whole molecule (17 g/mol). Multiply NH₃-N by 1.22 to get NH₃ mass.

Question 4

How do I prevent nitrification?

Accepted Answer

Maintain chloramine residual ≥ 1.0 mg/L, free ammonia <0.10 mg/L, and exercise dead-end mains. Many systems shock-chlorinate quarterly.

Chloramine Cl₂:NH₃-N Ratio Calculator