What is the difference between free and combined chlorine?

Pure chlorine exists as a molecule as a gas (Cl₂ ) and when dissolved in water it will react with water to form mostly hypochlorous acid (HOCl):

Cl₂ + H₂O ↔ HCl + HOCl

Depending on the pH of the water (see image right), the hypochlorous acid (HOCl) will partially dissociate to the hypochlorite ion (OCl^–). Both hypochlorous acid and hypochlorite disinfect water but hypochlorous is acid is a more effective disinfectant.

By definition, free chlorine refers to all chlorine present in the water as Cl₂(g), HOCl(aq) and OCl^–(aq).

Portable water testing methods such as DPD do not distinguish between these three species, and they are all termed free chlorine, with the capability to disinfect microorganisms in the same way as Cl₂. Free chlorine will often be denoted as Cl₂ in water treatment applications and literature, but can refer to any of the three forms.

When free chlorine is initially added to water it can undergo a very quick reaction with other contaminants in the water, mostly ammonia, NH3. This will result in the formation of chloramines, and the ‘free’ chlorine is converted to ‘combined’ chlorine:

NH₃ → NH₂Cl → NHCl₂ → NCl₃

That is; ammonia → monochloramine → dichloramine → nitrogen trichloride (trichloramine)

How to test free and total chlorine?

For the most effective monitoring of free and total chlorine, we recommend Kemio Disinfection. The latest multiparameter platform from Palintest, the Kemio Disinfection tests for bromine, free chlorine, total chlorine, chlorine dioxide, chlorite and peracetic acid (PAA).

Giving repeatable and reliable results for free chlorine in 60 seconds, Kemio guides the user and prompts action when concentration levels fall outside of your defined limits.