Free chlorine is a component of total residual chlorine, the portion of dissolved chlorine gas that is not bonded to any other reactants in water. The other portion is known as combined chlorine, which has bonded with chemicals, typically nitrates, in the water. These are terms used by water treatment professionals examining the safety of drinking water.
Chlorine is introduced to water supplies because of its ability to alter pollutants and kill harmful bacteria and other pathogens. The amount of contaminants in the water, whether they are metals or organic substances, determines the chlorine demand of the water. This is the amount of free chlorine that is consumed by reactions in the water. This reacted chlorine becomes combined chlorine, while any excess remains free chlorine. In perfectly clean water, chlorine demand is zero.
In certain cases, combined chlorine is introduced to the water or left in it as a disinfecting agent itself, preventing the growth of algae, aiding in the coagulation of organic substances and reducing odor. In either case, both free chlorine and combined chlorine are part of total residual chlorine. Small amounts of residual chlorine of both types are frequently left in the water sent out for residential and commercial use, as this helps keep the lines disinfected as well.