How is combined chlorine calculated?

Combined chlorine is a measurement of the chlorine in water that has already reacted with contaminants, such as organic materials. To calculate the amount of combined chlorine, one must determine the difference between the total chlorine and free chlorine levels in the water.

Total chlorine (TC) is the sum of both free chlorine (FC) and combined chlorine (CC). Therefore, if you subtract the free chlorine from the total chlorine, you isolate the combined chlorine. The correct calculation can be expressed as: combined chlorine equals total chlorine minus free chlorine.

This insight highlights the importance of monitoring both free and total chlorine levels in pools and aquatic facilities to ensure water safety and clarity. Free chlorine is the portion that is active and effective at disinfecting, while combined chlorine is less effective and can contribute to poor water quality, often associated with chloramines, which can cause irritation to the eyes and skin.

The other choices do not accurately represent this relationship. Specifically, total chlorine plus free chlorine does not yield a logical measurement for combined chlorine, as combined chlorine is inherently a subset of total chlorine.