The element Chlorine exists as a gas Cl2, when this is mixed with water it forms hypochlorous acid (HOCl) which is the disinfecting agent;
Cl2 + H2O HCl + HOCl
This HOCl can partly disassociate to form the hypochlorite ion OCl-;
HOCl H+ + OCl-
All of the hypochlorous acid (HOCl) and hypochlorite ion (OCl-) species are referred to as free chlorine.
These species can then react with nitrogen species in the water, usually ammonia (NH3), to form chloramines such as;
Monochloramine; NH2Cl / Dichloramine; NHCl2 / Trichloramine or Nitrogen Trichloride; NCl3
These chloramine species are referred to as combined chlorine.
Free Chlorine measures the amount of free chlorine only – hypochlorous acid (HOCl) and hypochlorite ions (OCl-) in the water.
Total Chlorine measures the free and combined chlorine – hypochlorous acid (HOCl), hypochlorite ion (OCl-) plus the chloramines (NHCl species)