Differences between Free Chlorine and Total Chlorine

Note / Disclaimer:

The chemical formulas shown in these pages are a "chemical shorthand" developed by chemists to represent our understanding of what is actually going on in the water.
The actual processes are more complicated, involving other compounds than distilled water, and pure elements and compounds. Also, subscripting, superscripting, and some symbols are difficult to render in HTML and are therefore not entirely accurate on this page.

Differences between Hypochlorous Acid (HOCl) & Hypochlorite Ion (OCl-)

Hypochlorite Ion (OCl-)

The species of chlorine resulting from dissociation (splitting up) of hypochlorous acid (HOCl) into its constituent parts H+ and OCl- (Hypochlorite Ion).

This happens if the pH is too high - if it is too low the hypochlorous acid dissociates into molecular chlorine (Cl₂).

Hypochlorite ion is a poor disinfectant because the negative charge creates an obstacle to penetrating the wall of the cell.

Hypochlorous acid is 100 times faster than hypochlorite ion in killing a microorganism.

Hypochlorous Acid:

HOCl = Free Chlorine = FAC= Free active/available Chlorine

Also known as free chlorine. It is formed when calcium hypochlorite, dichlor, trichlor or chlorine gas are mixed with water and dissociate.

This is the main pool water and other type of water disinfectant.

Hypochlorous acid acts as:

• a sanitiser killing potentially harmful bacteria and microorganisms. It can enter a cell's wall and upset its protein and enzyme function.
• an oxidising agent eliminating organic and inorganic impurities by a process similar to combustion, e.g it burns out pollution introduced by bathers such as sweat and urine for example in the swimming pool water area.

Useful amounts of hypochlorous acid can only be obtained if:

• the stabiliser level is not too high
• the pH-value is within certain limits, see pH-values and concentrations in the table below:

The hypochlorite ion has a higher oxidation potential than hypochlorous acid, yet hypochlorous acid is a better disinfectant.
The fact that hypochlorous acid has no charge allows it to penetrate microbial cell walls easier.
Therefore, the lower the pH, the better disinfecting power of a chlorine solution due to hypochlorous acid formation. The hypochlorite ion is overall more reactive, being harder on membranes and other materials of construction.

HOCl / OCl- Chemistry

When pure molecular chlorine is added to water, it forms hypochlorous acid and hydrochloric acid:

Cl2 + H2O --> HOCl + HCl

1. Hypochlorous acid (HOCl) is the stronger form of free chlorine,
2. and hydrochloric acid (HCl) lowers pH and alkalinity.

Hypochlorous acid further dissociates to hypochlorite ion OCl–, the weaker form of free chlorine)and free hydrogen (H+):

HOCl --> OCl– + H+

This dissociation is reversable, and pH driven.

As HOCl is used to kill algae / microrganisms, or as it evaporates, OCl– shifts back to HOCl to maintain the pH–mandated equilibrium.

Representative pH levels and their corresponding percentages of HOCl and OCl– are:

The full equation may be represented like this:

Cl2 + H2O --> HOCl + HCl

 

HOCl --> OCl– + H+

1. HOCl is, of course, the “active ingredient”.
2. The OCl– is a bank, or reservoir of less active chlorine.

Here you can find some ECA Water Applications