What defines free chlorine in a water treatment context?

Free chlorine refers to the amount of chlorine that remains in the water after its initial demand has been satisfied, meaning it is available for disinfection purposes. It primarily includes hypochlorous acid and hypochlorite ion, which are the active forms of chlorine that effectively kill pathogens in the water.

When treatment occurs, chlorine is added to the water to eliminate bacteria and other microorganisms. However, some of the chlorine is consumed in reacting with contaminants, such as organic matter or ammonia. Once this initial demand is met, the residual chlorine that persists is labeled as free chlorine. This measurement is critical in water treatment because it indicates how much disinfectant is still active and available to maintain water quality and safety.

The other options don’t accurately capture the concept of free chlorine. Chemically reacted chlorine would pertain to chlorine that has already engaged with substances in the water. The total amount of chlorine present would encompass both free and combined chlorine, while chlorine combined with ammonia refers specifically to chloramines, which are less effective in disinfection compared to free chlorine. Therefore, the choice that highlights the chlorine remaining after consumption by pollutants is the correct representation of free chlorine in water treatment.