Free chlorine refers to chlorine that is present as hypochlorous acid (HOCl) or the hypochlorite (OCl–) ion.

When the chlorine demand of water is satisfied, the remaining free chlorine is available to oxidize contaminants, which is measured by a Free Chlorine sensor. 

Like all halogens, chlorine is a highly reactive element. In solution, chlorine can be found in many different molecular forms. When chlorine is added to water, it reacts with organic materials and metals, forming combined chlorine. Because combined chlorine is not available for disinfection, this effect is called the chlorine demand of the water.