What must the pH be for a sample if free chlorine is to be effectively tested?

To effectively test free chlorine in water, the pH level should typically be around 7.0. At this neutral pH, chlorine exists primarily in its free form, which is the hypochlorous acid (HOCl). This form is more effective as a disinfectant compared to its ionic counterpart, hypochlorite ion (OCl-), which predominates at higher pH levels.

When pH levels rise above 7.0, the proportion of free chlorine that remains in the hypochlorous acid form decreases, reducing its efficacy. Therefore, maintaining a pH around neutral (7.0) is crucial for accurate testing and assessment of free chlorine levels in drinking water. Testing at this pH allows for the most reliable results regarding the disinfecting capacity of chlorine in the water.

This understanding is important in water quality monitoring and treatment processes, highlighting the need for regular pH testing in conjunction with free chlorine analysis to ensure the safety and effectiveness of drinking water.