How to determine free chlorine
Free Chlorine, DPD Photometric Method (0-4 ppm)
The test for chlorine is a test for the hydrolysis products of chlorine gas in water. Chlorine gas is soluble in water, forming hypochlorite ion and hypochlorous acid. It is in the latter form that chlorine exerts its disinfecting and oxidizing properties.
In industrial water conditioning, chlorination is used primarily for the control of slime and algae, although chlorine may also be used to assist in coagulation, taste odor, color and iron removal problems.