Combined chlorine can be calculated by subtracting the measured value for free chlorine from which of the following?

To determine the combined chlorine level in a pool, you need to understand the relationship between free chlorine and total chlorine. Total chlorine includes both free chlorine and combined chlorine. Free chlorine refers to the chlorine that is available to disinfect and sanitize the water, while combined chlorine is the chlorine that has already reacted with contaminants, such as organic matter, and is less effective for sanitation.

When you subtract the measured value for free chlorine from the total chlorine measurement, you arrive at the concentration of combined chlorine. This calculation is essential for pool operators to assess water quality and make necessary adjustments to ensure the water remains safe and clear for swimmers.

This approach is grounded in standard water chemistry practices, and understanding this calculation helps in proper pool maintenance. The other options do not represent a component from which combined chlorine can be derived, emphasizing the importance of knowing these fundamental concepts in pool management.