The chlorine used to reach breakpoint is referred to as what?

The chlorine used to reach breakpoint is referred to as "the demand." In water treatment, particularly in the disinfection process, the term "demand" signifies the amount of chlorine that is needed to react with contaminants in the water, such as organic materials, ammonia, and other substances. Only after this demand is satisfied can additional chlorine be effectively used for disinfection.

At the breakpoint, all of the combined chlorine (chloramines) has been broken down, and free chlorine is present, which is the effective form of chlorine for disinfection. Therefore, understanding the concept of demand is crucial for determining how much chlorine must be added to ensure effective water treatment. The other terms like "residual," "excess," and "active chlorine" denote different aspects of chlorine usage and effects in water treatment processes, but they do not specifically refer to the chlorine required to reach that pivotal disinfection index known as breakpoint.