Free chlorine refers to the concentration of chlorine atoms that are not chemically bound to other elements or compounds in the water. When sodium hypochlorite is added to water, it dissociates and forms hypochlorous acid (HOCl), which is the primary disinfectant species responsible for the sanitizing effects of chlorine. HOCl can further dissociate into hydrogen ions (H+) and hypochlorite ions (OCl-), with the relative proportions of each species depending on the pH of the water.
In the context of water disinfection, free chlorine specifically refers to the concentration of HOCl and OCl- ions present in the water. The presence of free chlorine ensures the disinfection efficacy and residual protection against microbial contamination. Therefore, sodium hypochlorite can be considered a source of free chlorine when it is added to water and dissociates to form HOCl.
However, it's important to note that the term "free chlorine" can also encompass other chlorine compounds, such as chlorine gas (Cl2) and chloramines, which are formed when chlorine reacts with nitrogenous compounds in water. Therefore, the specific definition and composition of free chlorine may vary depending on the context and application.
