Sodium hypochlorite (NaOCl) is a compound that can be effectively used for water purification. It is used on a large scale for surface purification, bleaching, odor removal and water disinfection.
When was sodium hypochlorite discovered?
Sodium hypochlorite has a long history. Around 1785 the Frenchman Berthollet developed liquid bleaching agents based on sodium hypochlorite. The Javel company introduced this product and called it 'liqueur de Javel'. At first, it was used to bleach cotton. Because of its specific characteristics it soon became a popular compound. Hypochlorite can remove stains from clothes at room temperature. In France, sodium hypochlorite is still known as 'eau de Javel'.
What are the characteristics of sodium hypochlorite?
Sodium hypochlorite is a clear, slightly yellowish solution with a characteristic odor.
Sodium hypochlorite has a relative density of is 1,1 (5,5% watery solution).
As a bleaching agent for domestic use it usually contains 5% sodium hypochlorite (with a pH of around 11, it is irritating). If it is more concentrated, it contains a concentration 10-15% sodium hypochlorite (with a pH of around 13, it burns and is corrosive).
Sodium hypochlorite is unstable. Chlorine evaporates at a rate of 0,75 gram active chlorine per day from the solution. Then heated sodium hypochlorite disintegrates. This also happens when sodium hypochlorite comes in contact with acids, sunlight, certain metals and poisonous and corrosive gasses, including chlorine gas. Sodium hypochlorite is a strong oxidator and reacts with flammable compounds and reductors. Sodium hypochlorite solution is a weak base that is inflammable.
These characteristics must be kept in mind during transport, storage and use of sodium hypochlorite.
What happens to the pH value when sodium hypochlorite is added to water?
Due to the presence of caustic soda in sodium hypo chlorite, the pH of the water is increased. When sodium hypo chlorite dissolves in water, two substances form, which play a role in for oxidation and disinfection. These are hypochlorous acid (HOCl) and the less active hypochlorite ion (OCl-). The pH of the water determines how much hypochlorous acid is formed. While sodium hypochlorite is used, hydrochloric acid (HCl) is used to lower the pH. Sulfuric acid (H2SO4) can be used as an alternative for acetic acid. Less harmful gasses are produced when sulfuric acid is used. Sulfuric acid is a strong acid that strongly reacts with bases and that is very corrosive.
How can sodium hypochlorite be produced?
Sodium hypochlorite can be produced in two ways:
- By dissolving salt in softened water, which results in a concentrated brine solution. The solution is electrolyzed and forms a sodium hypochlorite solution in water. This solution contains 150 g active chlorine (Cl2) per liter. During this reaction the explosive hydrogen gas is also formed.
- By adding chlorine gas (Cl2) to caustic soda (NaOH). When this is done, sodium hypochlorite, water (H2O) and salt (NaCl) are produced according to the following reaction:
Cl2 + 2NaOH + → NaOCl + NaCl + H2O
What are the applications of sodium hypochlorite?
Sodium hypochlorite is used on a large scale. For example in agriculture, chemical industries, paint- and lime industries, food industries, glass industries, paper industries, pharmaceutical industries, synthetics industries and waste disposal industries. In the textile industry sodium hypochlorite is used to bleach textile. It is sometimes added to industrial waste water. This is done to reduce odors. Hypochlorite neutralizes sulphur hydrogen gas (SH) and ammonia (NH3). It is also used to detoxify cyanide baths in metal industries. Hypochlorite can be used to prevent algae and shellfish growth in cooling towers. In water treatment, hypochlorite is used to disinfect water. In households, hypochlorite is used frequently for the purification and disinfection of the house.
How does sodium hypochlorite disinfection work?
By adding hypochlorite to water, hypochlorous acid (HOCl) is formed:
NaOCl + H2O → HOCl + NaOH-
Hypochlorous acid is divided into hydrochloric acid (HCl) and oxygen (O). The oxygen atom is a very strong oxidator.
Sodium hypochlorite is effective against bacteria, viruses and fungi. Sodium hypochlorite disinfects the same way as chlorine does.
How is sodium hypochlorite applied in swimming pools?
Sodium hypochlorite is applied in swimming pools for water disinfection and oxidation. It has the advantage that microorganisms cannot build up any resistance to it. Sodium hypochlorite is effective against Legionella bacteria and bio film, in which Legionella bacteria can multiply.
Hypochlorous acid is produced by the reaction of sodium hydroxide with chlorine gas. In water, the so-called 'active chlorine' is formed.
There are various ways to use sodium hypochlorite. For on-site salt electrolysis, a solution of salt (NaCl) in water is applied. Sodium (Na+) and chloride (Cl-) ions are produced.
4NaCl- → 4Na+ + 4Cl-
By leading the salty solution over an electrolysis cell, the following reactions take place at the electrodes:
2Cl- → Cl2 + 2e- 2H2O + 2e- → H2 + 20H-
2H20 → O2 + 4H++ 4e-
Subsequently, chlorine and hydroxide react to form hypochlorite:
OH- + Cl2 → HOCl + Cl-
The advantage of the salt electrolysis system is that no transport or storage of sodium hypochlorite is required. When sodium hypochlorite is stored for a long time, it becomes inactive. Another advantage of the on site process is that chlorine lowers the pH and no other acid is required to lower pH. The hydrogen gas that is produced is explosive and as a result ventilation is required for expolsion prevention. This system is slow and a buffer of extra hypochlorous acid needs to be used. The maintenance and purchase of the electrolysis system is much more expensive than sodium hypochlorite.
When sodium hypochlorite is used, acetic or sulphuric acid are added to the water. An overdose can produce poisonous gasses. If the dosage is too low, the pH becomes to high and can irritate the eyes.
Because sodium hypochlorite is used both to oxidize pollutions (urine, sweat, cosmetics) and to remove pathogenic microorganisms, the required concentration of sodium hypochlorite depends on the concentrations of these pollutions. Especially the amount of organic pollution determines the required concentration. If the water is filtered before sodium hypochlorite is applied, less sodium hypochlorite is needed.
What are the health effects of sodium hypochlorite?
There is no threshold value for to sodium hypochlorite exposure. Various health effects occur after exposure to sodium hypochlorite. People are exposed to sodium hypochlorite by inhalation of aerosols. This causes coughing and a sore throat. After swallowing sodium hypochlorite the effects are stomach ache, a burning sensation, coughing, diarrhea, a sore throat and vomiting. Sodium hypochlorite on skin or eyes causes redness and pain. After prolonged exposure, the skin can become sensitive. Sodium hypochlorite is poisonous for water organisms. It is mutagenic and very toxic when it comes in contact with ammonium salts.
Sodium hypochlorite in swimming pools
The concentration of sodium hypochlorite that is found in swimming pools is generally not harmful to people. When there is too much chlorine in the water, this burns the body tissues, which causes damage to air tracts, the stomach and the intestines, the eyes and the skin. When sodium hypochlorite is used in swimming pools, it sometimes causes red eyes and it gives off a typical chlorine odor. When there is a lot of ureum (a mixture of urine and sweat) present, hypochlorous acid and ureum react to form chloramines. These chloramines irritate mucous membranes and cause the so-called ' chlorine smell'. In most swimming pools, these problems are prevented by water purification and ventilation. Eyes irritation disappears after a while.
What are the advantages and disadvantages of sodium hypochlorite use?
Sodium hypochlorite as a disinfectant has the following advantages:
It can easily and be stored and transported when it is produced on-site. Dosage is simple. Transport and storage of sodium hypochlorite are safe. Sodium hypochlorite is as effective as chlorine gas for disinfection. Sodium hypochlorite produces residual disinfectant.
Sodium hypochlorite is a dangerous and corrosive substance. While working with sodium hypochlorite, safety measures have to be taken to protect workers and the environment. Sodium hypochlorite should not come in contact with air, because that will cause it to disintegrate. Both sodium hypochlorite and chlorine do not deactivate Giardia Lambia and Cryptosporidium.
What is the legislation for sodium hypochlorite?
The regulation for sodium hypochlorite is the same as the regulation considering chlorine.