Active Chlorine

Chlorine’s effectiveness

Chlorine eliminates 99% of bacteria and protects water against recontamination* when residual chlorine is left in the water.

Chlorine is by far the most commonly used disinfectant in the world, states the World Chlorine Council. Where it is widely adopted, chlorine has helped to virtually eliminate water-borne diseases such as cholera, typhoid and dysentery. Chlorine also eliminates slime bacteria, moulds and algae that commonly grow on the walls of water pipes and water storage tanks.

It should be understood that even if bacteriological contamination of water is responsible for the great majority of water-borne diseases, active chlorine (as well as active chlorine produced by WATA® devices) only treats bacteriological contamination of water. Active chlorine does not take away the different sediments and materials that make water cloudy. Active chlorine also does not treat chemical contaminations of water (e.g. by heavy metals, arsenic, fluorine, etc.).

* (WHO, 2004, Water treatment and pathogen control, IWA publishing, p 44-50)

The WATA ® device produces a chlorinated compound: sodium hypochlorite

Today, sodium hypochlorite is produced in 85 countries. Production has risen from 35,000 tonnes at the end of the 19th century to 44 million tonnes today. It is still used in most water supply networks in northern Europe and North America to guarantee that drinking water is perfectly hygienic.

While some other chlorinated compounds used for water treatment are dangerous, the sodium hypochlorite produced by WATA® is less corrosive. Ingestion of sodium hypochlorite is unpleasant but not dangerous. The chlorine solution produced with WATA®:

  1. Has a chlorine concentration that is four times lower than bleach.
  2. Has a lower pH than bleach, making it more active and more efficient as a disinfecting agent.

For use in bottles, we recommend that chlorine is stabilised by adding caustic soda.

Conservation & stabilisation of sodium hypochlorite

Proper storage is needed to guarantee its stability over time.

The active chlorine concentrate must be stored in an opaque, non-metallic and properly closed receptacle. Store the receptacle in a cool room, away from light. A high storage temperature reduces the stability of the solution, thereby causing a decrease in concentration and faster decomposition of the active chlorine into chlorates. Note that a full receptacle will keep longer than a half-empty one, where the air will oxidise the concentrate.

The presence of metallic impurities in the raw materials used to produce it (water and salt) needs to be avoided to ensure that the sodium hypochlorite solution lasts for longer.

Storage life of the solution produced by WATA®.

The active chlorine concentration of the sodium hypochlorite produced by WATA® is not stable over time. Antenna recommends that the solution is used within 24 hours of production.

If you would like to use your chorine solution for a longer period of time, you must stabilise the solution following the method described in the following document.

Stabilising a solution of sodium hypochlorite 

The pH of the sodium hypochlorite solution needs to be increased to a value between 11.8 and 12 by adding a solution of caustic soda 5M. This process requires the use of a pH-meter or pH-indicator strips. It also requires laboratory skills but ensures a good-quality product with a shelf life of at least six months. Current legislation in the country where you are operating must be respected. A quality control procedure validated by the local authorities must be set up.

Operating method for using caustic soda to stabilise sodium hypochlorite

Checking the active chlorine concentration of stabilised sodium hypochlorite

Once stabilised, the concentration of the sodium hypochlorite solution cannot be measured directly using the WataTest® reagent. For the WataTest® to be used on a stabilised solution, the sample to be tested has to be acidified using citric acid, following the operating method described in this document: (draft pending). An alternative method, proposed by Antenna, to measure the active chlorine concentration consists of titrating the sample with sodium thiosulfate, following the instructions described in this document.

When distributing chlorine flasks on a large scale, the concentration of the stabilised sodium hypochlorite solution must be checked for each batch, using the thiosulfate titration method, which offers a more accurate measurement.

For more information on the studies carried out by Antenna on stabilising sodium hypochlorite, visit the “Research and development” page.