When sodium hypochlorite is added to water, it forms sodium hydroxide (NaOH) and hypochlorous acid (HOCl). All three forms of chlorine produce hypochlorous acid (also called available chlorine or active chlorine). Hypochlorous acid is what kills pathogens. In high pH solutions, most of the hypochlorous acid disassociates to form hypochlorite ion (OCl-), which is not an effective sanitizer. Testing kits for free chlorine measure both hypochlorous acid and hypochlorite ion and alone do not indicate the quantity of available chlorine that kills pathogens. Chlorine solutions with pH above 8 are relatively ineffective against pathogens. Below pH 6, chlorine is more corrosive to equipment and activity is rapidly lost. A pH of around 7 will maintain about 80% of the chlorine in the available (hypochlorous acid) form with very little gas formed (Figure 1). Thus, in order to know the sanitizing strength of ones chlorine solution, both pH and free chlorine must be measured.