Free chlorine residual includes which of the following?

Free chlorine residual refers to the portion of chlorine in the water that remains available to disinfect and provides a measure of the effectiveness of chlorine as a disinfectant. It includes both hypochlorite ion (OCI-) and hypochlorous acid (HOCI).

Hypochlorous acid is a weak acid formed when chlorine is dissolved in water and is considered the most effective form of chlorine for disinfection due to its greater ability to penetrate microbial cells. The hypochlorite ion is the ionized form that exists in water when chlorine is present, especially at higher pH levels.

Both forms work together to contribute to the free chlorine residual, which is crucial for ensuring that the disinfection processes are effective throughout the water distribution system. A measurement of free chlorine residual allows operators to assess how much chlorine is available for disinfection and helps in maintaining adequate levels to prevent microbial growth, especially in long distribution systems.

In contrast, choices that narrow down to only one of the two components or reference chlorine gas do not encompass the complete picture of what constitutes free chlorine residual.