What is breakpoint?
Chlorine is added to the water sample and dissolves to form hypochlorous acid and the hypochlorite ion:
Cl2 + H2O à HOCl + –OCl
On addition of the chlorine, the water will consume some of this chlorine. The amount it consumes is known as the chlorine demand of the water.
After this initial period of consumption of chlorine by ammonia, the chlorine reacts with the free ammonia and forms monochloramines and a small amount of organic chlorine compounds:
HOCl + NH3 ↔ NH2Cl + H2O
Once all the ammonia in solution has been consumed to form chloramines, dichloramines and nitrogen trichloride begin to form. The amount of monochloramine and organic chlorine compounds are reduced:
HOCl + NH2Cl ↔ NHCl2 + H2O
HOCl + NHCl2↔ NCl3 + H2O
As more and more chlorine is added, the free chlorine begins to oxidise the chloramines.
The point at which all the dichloramine/nitrogen trichloride is oxidised into nitrogen is the ‘breakpoint’ and is the point most super-chlorination techniques are seeking to achieve as from this point, any additional chlorine added exists as free chlorine and therefore has high germicidal properties.
In general, the ratio of Chlorine:Nitrogen ratio required to achieve breakpoint is 7:6 and in some applications such as fresh produce manufacturing, may be higher if there are high concentrations of organic nitrogen.
This description is a simplistic representation to explain the theory of breakpoint chlorination. In reality, there are many complicated side reactions that occur depending on various conditions such as pH, and it is hard to predict exactly what is formed and at what levels as the chlorine is dosed. Likewise, the nuisance residual can be unpredictable and differ between water samples.