Chlorine Compounds As A Means              

Of Disinfection.                                                       

N.E.M Business Solutions 01823 680119                                                                 Chlorine Gas


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 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.


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. Chlorine gas is highly toxic and corrosive.


NOTE the process for making “Electrolysed water” is effectively the same.

Electrolysed water is produced by passing a current of electricity through a dilute saltwater solution.  One product of the reaction is sodium hydroxide (NaOH) and the other is Hypochlorous acid, which has a low pH, contains active chlorine, and has a strong oxidation-reduction potential similar to that of ozone. The properties of Electrolysed water  can be optimised by increasing the voltage and increasing the salt concentration which results in a more acidic solution and higher residual chlorine level.  Three forms of the solution can be produced, an acidic form, a neutral pH form and an alkaline form.  The electrolysis unit produces the solutions in a concentrated form which is then diluted through an automatic dosing system to the required concentration.





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. As a bleaching agent for domestic use it usually contains about 5% sodium hypochlorite (with a pH of around 11, it is irritating).

A more concentrated solution, containing around 10-15%  of sodium hypochlorite (with a pH of around 13, causes skin burns and is corrosive). Sodium hypochlorite is unstable. Chlorine evaporates at a rate of approximately 0.75-gram active chlorine per day from the solution.

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:
  acids, alkanes, alkynes, alcohols, aldehydes, aliphatic amines, ammonia, azole, carbohydrates, ethers, fats, glycol, ketones, methanol, polysaccharides, saccharides, unsaturated fatty acids and unsubstituted aromatics, among others.

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).






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.





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