Note/Disclaimer:
The chemical formulas shown on these pages are a "chemical shorthand" developed by chemists to represent our understanding of what is actually going on in water.
The actual processes are more complicated and involve compounds other than distilled water and pure elements and compounds. Subscribing, superscripting and some symbols are also difficult to display in HTML and are therefore not entirely correct on this page.
Free chlorine and total chlorine:
Chlorine in water can be present in two forms, free and bound.
Free chlorine does the hard work of killing bacteria and oxidizing contaminants.
When you add chlorine to water, you are actually adding free chlorine.
When the free chlorine combines with impurities, it becomes combined chlorine or chloramines.
In water, this form of chlorine has only a very low disinfectant effect and no oxidizing effect.
Total chlorine is just the sum of combined chlorine and free chlorine.
Differences between Hypochlorous Acid (HOCl) & Hypochlorite Ion (OCl-)
Hypochlorite ion (OCl-)
The chlorine species, which are produced by the dissociation (splitting) of hypochlorous acid (HOCl) into its components H+ and OCl (hypochlorite ion) is created.
This happens when the pH is too high – if it is too low, the hypochlorous acid dissociates into molecular chlorine (Cl2).
Hypochlorite ion is a poor disinfectant, since the negative charge represents an obstacle to penetration into the cell wall.
Hypochlorous acid kills a microorganism 100 times faster than hypochlorite ion.
Hypochlorous acid:
HOCl = Free Chlorine = FAC= Free active/available chlorine
Also known as free chlorine. It arises, when calcium hypochlorite, dichlor, trichlor or chlorine gas are mixed with water and dissociate.
This is the main bath water and another type of water disinfectant.
Hypochlorous acid acts as:
- a disinfectant, which kills potentially harmful bacteria and microorganisms. It can penetrate the cell wall and disrupt a cell's protein and enzyme function.
- a oxidizing agent, which removes organic and inorganic contaminants through a process similar to incineration, for example by incinerating contaminants brought in 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 Stabilizer level is not too high
- is pH value is within certain limits, see pH values and concentrations in the table below:
| pHValue | 8,0 | 7,8 | 7,5 | 7,2 | 7,0 | 6,0 | 5,0 |
| % as HOCL: | 22 | 33 | 48 | 66 | 72 | 96 | 100 |
| % as OCL-: | 78 | 67 | 52 | 34 | 28 | 4 | 0 |
The Hypochlorite ion has a higher oxidation potential than hypochlorous acid, yet hypochlorous acid is a better disinfectant.
The fact, The fact that hypochlorous acid has no charge allows it to penetrate microbial cell walls more easily.
Therefore: the lower the pH, the better the disinfectant power of a chlorine solution due to the formation of hypochlorous acid. The hypochlorite ion is overall more reactive because it has a harder effect on membranes and other construction materials.
HOCl/OCl chemistry
When pure molecular chlorine is added to water, it forms hypochlorous acid and hydrochloric acid:
Cl2 + H2O –> HOCl + HCl
- Hypochlorous acid (HOCl) is the stronger form of free chlorine,
- and Hydrochloric acid (HCl) lowers pH and alkalinity.
Hypochlorous acid further dissociates to the hypochlorite ion OCl-, the weaker form of free chlorine, and to free hydrogen (H+):
HOCl –> OCl– + H+
This dissociation is reversible and pH driven.
When HOCl is used to kill algae/microorganisms or when it evaporates, OCl shifts back to HOCl to maintain pH mandated balance.
Representative pH values and their corresponding percentages of HOCl and OCl- are:
| pHValue | 8,0 | 7,8 | 7,5 | 7,2 | 7,0 | 6,0 | 5,0 |
| % as HOCL: | 22 | 33 | 48 | 66 | 72 | 96 | 100 |
| % as OCL-: | 78 | 67 | 52 | 34 | 28 | 4 | 0 |
The complete equation can be represented as follows:
Cl2 + H2O –> HOCl + HCl
HOCl –> OCl– + H+
- HOCl is of course the “active ingredient”.
- The OCl- is a bank or reservoir with less active chlorine.
