Chlorine
Compounds As A Means
Of Disinfection.
N.E.M Business
Solutions 01823 680119
The chemistry of Chlorine
in many of its forms and
general information
:
Chlorine:
pale green gas, highly poisonous, chemical formula Cl
Hypochlorous acid:
unstable compound only found in solution, effective bactericide, chemical
formula HOCl
Hypochlorite:
Usually the Sodium or Calcium salt of Hypochlorous acid, the common component of
bleach, chemical formula
(NaOCl or Ca(OCl)2 )
Chlorine dioxide:
yellow to brown coloured gas, chemical formula ClO2
Chlorine:
Chlorine is a poisonous gas and
as a general rule is not used in this form in the food and beverage industry.
However it is still used to Chlorinate water supplies and consequently you may
come across it in some sites. Due to the potential dangers it has generally been
replaced with one of its less dangerous compounds.
Chlorine dissolved in water forms both hypochlorous acid and
hypochlorite; these are the components that actually give Chlorine its
bactericidal effects.
Chlorine (Cl) is among the
most abundant of nature's elements, and combines with other elements to sustain
life and the natural
processes that make up our environment. Chlorine is found in the Earth and, in
the oceans. Chlorine is highly reactive and, as such, is typically found in
nature bonded to other elements like
sodium, potassium, and magnesium. When isolated as a free element,
chlorine is a greenish yellow gas, which is 2.5 times heavier than air. It turns
to a liquid at -34°C (-29°F), and becomes a yellowish crystalline solid at -103°C
(-153°F).
Hypochlorous
Acid: 
A weak, unstable acid occurring
only in solution and used as a bleach, oxidiser, deodorant, and disinfectant.
Chemical formula: HOCl.
Hypochlorous acid is a weak acid,
which can only exist in solution, because it is highly unstable when isolated in
a pure form. There are a number of uses for this chemical, most of which take
advantage of its strong oxidising properties, which allow it to be used as a
bleach and disinfectant, among many other things.
Hypochlorous acid is the most
effective of all the chlorine forms, similar in structure to water.
The germicidal efficiency of HOCl is due to the relative ease with which
it can penetrate cell walls. This penetration is comparable to that of water,
and can be attributed to both its modest size and to its electrical neutrality.
Formation.
Addition of chlorine to water
gives both hydrochloric acid (HCl) and hypochlorous acid:
Cl2 + H2O «
HClO + HCl
In organic synthesis, HOCl
converts alkenes to chlorohydrins.
In biology, hypochlorous acid is
generated in activated neutrophils by myeloperoxidase-mediated peroxidation of
chloride ions, and contributes to the destruction of bacteria and this is used
in water treatment such as the acid being the active sanitizer in
hypochlorite-based swimming pool products.
Production Using Electrolysis:
Solutions of hypochlorites can be
produced by electrolysis of an aqueous chloride solution. Chlorine gas is
produced at the anode, while hydrogen forms at the cathode. Some of the chlorine
gas produced will dissolve forming hypochlorite ions through the above reaction.
The geometry of the cell is critical to ensure that as much of the chlorine as
possible dissolves, rather than simply bubbling out of the cell.
At
the anode: 2 Cl- ---> Cl2 (g) + 2e- . This technique is often used to produce what is called “electrolysed
water”.
At
the cathode: 2H+ + 2e- ---> H2 (g)
It
can be seen that over time, the electrolyte will become increasingly basic.
There
are a number of potential hazards and challenges associated with this process.
Untrained operators should not attempt it.
The
electrochemical environment of the cell is highly corrosive, particularly at the
anode. Few materials are suitable as an anode electrolyte. Graphite can be used,
but will degrade quickly (which also results in contamination of the cell with
finely divided carbon particles). Graphite supported lead dioxide electrodes
have been reported to be more effective.
If
the reaction conditions are not controlled, the produced hypochlorite can react
with the hydroxide ions to form chlorate ions. These can additionally be
electrochemically oxidized to perchlorate ions (within the same cell).
Hypochlorite
is a powerful oxidizing agent, and will attack the dyes used in pH paper and
damage pH sensors, making measurement and control of the conditions difficult.
Hydrogen
gas is highly flammable, and can form explosive mixtures with both air and
chlorine over a wide range of concentrations.
NOTE
the process for making “Electrolysed water” is effectively the same.
Electrolysed water is produced by passing a current of electricity through
Hypochlorite:
Usually the Sodium or Calcium
salt of hypochlorous acid, the common component of bleach, chemical formula (NaOCl
or Ca(OCL)2 )
Sodium Hypochlorite is the most
common form found in the food and beverage industry, it is often referred to as
“Hypo” however there are numerous trade names.
Sodium hypochlorite is by no
means a modern product. In about 1785 Berthollet, a Frenchman 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. In France, sodium
hypochlorite is still sometimes referred to as 'eau de Javel'.
Sodium hypochlorite is a clear,
slightly yellowish solution with a characteristic odour.
A more concentrated solution,
containing around 10-15% of sodium
hypochlorite (with a pH of around 13, causes skin burns and is corrosive).
When heated sodium hypochlorite
disintegrates. This also happens when sodium hypochlorite comes in contact with
acids, sunlight, and certain metals; giving off poisonous and corrosive gasses,
including chlorine gas.
Due to the presence of sodium
hydroxide in hypochlorite dissolves in water, two substances form, which play a
role in 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. The lower the pH the greater the level of Hypochlorous acid.
Chlorine
dioxide: 
Chlorine dioxide was first produced from the
reaction of potassium chlorate and hydrochloric acid by Davy in 1811. 
Not until the industrial-scale preparation of
sodium chlorite, from which chlorine dioxide may more readily be generated
however, did its widespread use occur
The most common traditional methods of generating
chlorine dioxide involve mixing sodium hypochlorite (NaClO), acid (HCl2) and
sodium chlorite (NaClO2) or chlorine gas (Cl2) and sodium
chlorite.
NaOCl + HCl + 2NaClO2
2ClO2 + 2NaCl + NaOH
Cl2 + 2NaClO2
2ClO2 + 2NaCl
New technology has dramatically improved the
onsite generation of chlorine dioxide. This new technology generates chlorine
dioxide by electro catalytic and electrochemical techniques. These generators
typically use only one precursor, sodium chlorite. The elimination of both
chlorine and acid in the process has resulted in a much safer and simpler
generation process. Some of these new generators produce chlorine dioxide
directly in an aqueous solution with a concentration below 1000 mg/L. This low
concentration in an aqueous solution dramatically enhances the safety of the
process.
Chlorine Dioxide has the chemical formula ClO2
and is a yellow to brown coloured gas at room temperature and pressure. It is a
highly reactive oxidant and for all practical areas of water disinfection, it
must be generated on site using proprietary reaction and dosing equipment
By comparison: At room temperature, chlorine is a
greenish-yellow poisonous gas. When added to water, however, chlorine combines
with water to form hypochlorous acid that then ionises to form hypochlorite ion
- 'bleach'
In general, chlorine dioxide has been found to
produce fewer organic by-products with naturally occurring dissolved organic
material. Chlorine dioxide is an explosive gas, but is stable in water in the
absence of light and elevated temperatures. ClO2 is capable of
oxidizing iron and manganese, removing colour, and lowering THM (Trihalomethanes)
formation potential. It also oxidizes many organic and sulphurous compounds that
cause off-tastes and odours.
The physiological mode of inactivation of bacteria by chlorine
dioxide has been attributed to a disruption of protein synthesis.
In the case of viruses, chlorine dioxide preferentially inactivates the
outer protein layers, rather than nucleic acids.
It is well known that ClO2 does not
react with ammonia; however, this is only one of many chemicals not affected by
“The Selective Oxidiser.”
ClO2 does not react with:
Organic contaminants, such as those mentioned
above, are regularly found in cooling and process water systems. The cooling
system contaminants could be ammonia in a semiconductor plant, glycol in a
process heat exchanger, oil in a textile air washer or a steel mill, food from a
cooker or methanol from a chemical plant. Because ClO2 does not react with these
contaminants, its demand is based on the microbiological loading in the water
only. This demand impact is a major advantage in the areas of cost and
performance.
As regards to the cost of ClO2 it is
certainly more expensive on a pound for pound basis, than chlorine gas (as much
as 1OX), or hypochlorite (4X). It is only when the consumption of the chlorine
by the systems demand is factored in, can the economics favour the ClO2
– cases have been witnessed in heavily contaminated systems where a 1:25
replacement by ClO2 for chlorine, resulted in satisfactory treatment
program where one did not exist prior. (John Murphy, Microcide Consultants).
TERMS:
Free Available Chlorine:
Refers to the hypochlorous acid (HOCl) form of
chlorine only. It is the free, un-combined form of chlorine that is effective
for disinfection.
Total Free Chlorine:
Refers to the sum of hypochlorous acid (HOCl) and
hypochlorite ion (OCl-). The hypochlorite ion is not effective for
disinfection, but it is in a free form. All
of the total free chlorine would be in the form of hypochlorous acid if the pH
were low enough.
Combined Chlorine:
Refers to chlorine, which is not readily
available. For example, chlorine combined as chloramines or organic nitrogen is
not an effective disinfectant and will not readily convert to hypochlorous acid
or hypochlorite ion.
Total Residual Chlorine:
Refers to the sum of total free chlorine and
combined chlorine. Low total residual chlorine is of particular interest to
ensure there are no downstream consequences for aquatic life.
©2009
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