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Sodium hypochlorite
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?
Exposure
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?
Advantages
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.
Disadvantages
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.
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