Chlorine Reactions in Water:

If ammonia is present in the water, the reaction is:

NH₃ + HClO → NH₂Cl + H₂O (at pH 4.5 - 8.5 and alone at pH above 8.5) Monochloramine

NH₂Cl + HClO → NH₁Cl₂ + H₂O/ dichloramine (at pH 4.5 - 8.5)

NH₂Cl + HClO → NH₃ + H₂O/trichloramine (alone at pH less than 4.4)

The chloroamines are the combined residuals. They are more stable than the free residuals but less effective as disinfectants. For a given kill rate with constant residual, the combined form required over hundred times the contact time required by the free residual. Alternatively for a given contact time, the combined residual concentration must be twenty five times the free residual concentration to give the desired killing.

Trace concentrations of ammonia are desirable in water destined for chlorination in order to ensure presence of residuals in the distribution system. In the presence of ammonia, the continued addition of chlorine produces the characteristic curve shown in Figure 6. Theoretically three moles of chlorine react with two moles of ammonia to give off nitrogen gas to reduce chlorine to the chloride ion as follows

2NH₃ + 3Cl₂   → N₂ + 6HCl

Thus the breakpoint, which is the point at which the concentrations of the combined residuals have declined to a minimum should occur at chlorine to ammonia ratio 3 : 2, but in practice the ratio is nearer to 10 : 1 because of pH and other factors. Past the breakpoint, the free residual (HClO) is proportional to the dose. The actual chlorine demand is the difference between the applied chlorine dose and the residual concentration at the breakpoint.