Definition - What does Breakpoint Chlorination mean?
Breakpoint chlorination is defined as the point where enough chlorine has been added to a quantity of water to satisfy its disinfecting demand. In other words, it is the point where all undesirable contaminants have been removed from the water. At breakpoint chlorination, all the added chlorine is consumed by chemical reactions with the contaminants, resulting in no free available chlorine (FAC) in the water.
Corrosionpedia explains Breakpoint Chlorination
Chlorine is added to water to react with and break down contaminating compounds. As chlorine is added, it reacts with ammonia and nitrogen in the water, oxidizing them to create disinfecting byproducts known as chloramines. Here the added chlorine is being "used up" or combined with contaminants to sanitize the water. During this process, the overall chlorine residual increases.
However, as additional chlorine is added, it begins to react with the chloramines present in the water. The chlorine and chloramines cancel each other out, reducing the chlorine residual concentration. When the added chlorine can undergo no further reactions (or when the chlorine demand is satisfied) breakpoint chlorination is said to be achieved. The addition of chlorine beyond the breakpoint creates a presence of free available chlorine (FAC), i.e., uncombined chlorine that can act as a disinfecting agent.
The presence of FAC is correlated with the absence of unsafe, disease-causing organisms and therefore is used as a measure of water potability.