Combined chlorine can be calculated by subtracting the measured value for free chlorine from measured value for what?

The calculation for combined chlorine involves understanding the relationship between free chlorine and total chlorine in a swimming pool. Total chlorine includes both free chlorine, which is the active form that sanitizes the water, and combined chlorine, which is chlorine that has reacted with contaminants such as sweat, urine, and other organic materials.

To determine the amount of combined chlorine present in the pool water, you subtract the measured value of free chlorine from the measured value of total chlorine. This calculation provides the quantity of chlorine that has already been used up in reactions with contaminants, indicating the need for potential shock treatments or increased sanitation measures.

Total alkalinity, hypochlorous acid, and stabilized chlorine do not directly provide the necessary values for calculating combined chlorine. Total alkalinity refers to the buffering capacity of the water, hypochlorous acid is one of the forms of free chlorine but not a measurement itself, and stabilized chlorine pertains to chlorine compounds that contain stabilizers, which affect their effectiveness but do not relate to this specific calculation. Thus, the correct approach to find combined chlorine relies specifically on the relationship between free chlorine and total chlorine.