To what does the term "combined chlorine" refer?

The term "combined chlorine" specifically refers to chloramines, which are formed when ammonia reacts with chlorine in water. This interaction occurs when chlorine is used for disinfection purposes, particularly in water treatment processes. Combined chlorine includes various chloramine species, such as monochloramine, dichloramine, and nitrogen trichloride.

Chloramines are significant because, while they are effective at disinfecting water, they are less potent as oxidants compared to free chlorine. Their presence also indicates that some chlorine is still available for disinfection, but in a form that is combined or bonded with other substances like ammonia, rather than freely available in the water. Understanding combined chlorine is crucial for water operators as it directly impacts water quality and the effectiveness of disinfection processes.

The other options do not accurately describe combined chlorine: free chlorine refers solely to the chlorine available in its molecular form, not combined; the chlorine part of chlorinated compounds encompasses more than just combined forms; and microbiological agents in water are not related to the specific chemical interactions that define combined chlorine.