Most methods of measuring chlorine in water refer to free and total chlorine, but what exactly do these mean and what is the difference?

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)

Most test methods available will give the result of both the free and total chlorine present, such as the Chlorometer Duo or the Pocket Colorimeter for Chlorine