Chlorine - Cl, element 17, atomic mass 35.453, illustrating chlorination at aqueum.comIntroduction to Chlorination

Chlorination is the use of chlorine gas, sodium hypochlorite or calcium hypochlorite to disinfect water.
Similar but distinct disinfectants include chloramine and chlorine dioxide.

Chlorine can be sourced:

  1. As compressed/liquefied Cl2 gas in cylinders or drums
  2. As a liquid aqueous sodium hypochlorite solution, either produced by on site electrolysis of brine, or commercially produced offsite
  3. As solid calcium hypochlorite or sodium dichloroisocyanurate (NaDCC).

Whatever the source, chlorine takes on the following equilibrium once dissolved in water:

HOCl ⇔ H+ + OCl
hypochlorous acid ⇔ hydrogen ion + hypochlorite ion

Hypochlorous acid is, generally speaking, a much stronger disinfectant. So when disinfecting water with chlorine, we aim to shift the above equilibrium to the left. This is practically achieved by lowering the pH. Low temperature and low ionic strength also favour hypochlorous acid, however low temperature also reduces the rate of most reactions, including disinfection.

In order to determine the efficiency of chlorination, it is prudent to estimate the proportion of hypochlorous acid. Please see our hypochlorous acid proportion calculator.