Combined chlorine residual refers to the presence of which species?

• A. chloramines (chlorine that has reacted with ammonia or other organic compounds)
• B. free chlorine
• C. total chlorine
• D. monochloramine

Answer: (A)

The idea being tested is how chlorine residuals exist in water. When chlorine is added to water, some of it remains as free chlorine, and some reacts with nitrogen-containing compounds like ammonia or organic nitrogen to form chloramines. These chloramines, which include monochloramine, dichloramine, and trichloramine, stay in the water as a residual disinfectant and are collectively called the combined chlorine residual.

So, the presence of a combined chlorine residual specifically points to chloramines being present, not free chlorine or total chlorine. Monochloramine is one type of chloramine, but the term combined chlorine refers to the group as a whole.