What is Chlorine Dioxide

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Chlorine dioxide is used primarily (>95%) for bleaching of wood pulp, but is also used for the bleaching of flour and for the disinfection of municipal drinking water. The Niagara Falls, New York water treatment plant first used chlorine dioxide for drinking water treatment in 1944 for phenol destruction. Chlorine dioxide was introduced as a drinking water disinfectant on a large scale in 1956, when Brussels, Belgium, changed from chlorine to chlorine dioxide. Its most common use in water treatment is as a pre-oxidant prior to chlorination of drinking water to destroy natural water impurities that produce trihalomethanes on exposure to free chlorine.

It is more effective as a disinfectant than chlorine in most circumstances against water borne pathogenic microbes such as viruses, bacteria and protozoa – including the cysts of Giardia and the oocysts of Cryptosporidium.

The use of chlorine dioxide in water treatment leads to the formation of the by-product chlorite which is currently limited to a maximum of 1 ppm in drinking water in the USA.4-33 This EPA standard limits the use of chlorine dioxide in the USA to relatively high quality water or water which is to be treated with iron based coagulants (Iron can reduce chlorite to chloride).

It can also be used for air disinfection,and was the principal agent used in the decontamination of buildings in the United States after the 2001 anthrax attacks.

Chlorine dioxide production

  • 1 While there are five principal methods for generating chlorine dioxide, the most common is the Hooker R-2 process, which generates chlorine dioxide from sodium chlorate.

Sodium Chlorate Plus Citric Acid Creates Chlorine Dioxide

sodium chlorate of hhh chemialSodium chlorate is a chemical compound with the chemical formula (NaClO3). When pure, it is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 250 °C to release oxygen and leave sodium chloride.

Sodium chlorate is an industrial salt produced from water and common table salt in a reaction with electricity. Manufacturing: Sodium chlorate is produced from two commonly found raw materials: salt (NaCl, sodium chloride) and water (H2O) along with the use of large amounts of electrical energy. The manufacturing process involves the electrolysis of an acidified sodium chloride solution in a specially designed electrochemical cell.Sodium chlorate crystals are then removed and the remaining liquor containing salt, chloride ions and dichromate are returned to the electrochemical cells. The crystals are washed and dried to form the final product. For some customers, the crystals are dissolved in water and shipped as a liquid solution. Uses: Sodium chlorate is largely used by the pulp & paper industry to produce chlorine dioxide, which is used to bleach wood pulp for the manufacture of higher quality and environmentally friendly white paper products. Pulp mills convert sodium chlorate into chlorine dioxide – an environmentally friendly bleaching agent. The process of using chlorine dioxide to bleach pulp is referred to as “ECF” bleaching meaning “elemental chlorine free”. Sodium chlorate is available in two forms: dry white crystal that looks like table salt and clear solution.

Does chlorine dioxide oxidize in the same way as chlorine?
Contrary to chlorine, chlorine dioxide does not react with ammonia nitrogen (NH3) and hardly reacts with elementary amines. It does oxidize nitrite (N02) to nitrate (NO3). It does not react by breaking carbon connections. No mineralization of organic substances takes place. At neutral pH or at high pH values, sulphuric acid (H2SO3) reduces chlorine dioxide to chlorite ions (ClO2-). Under alkalic circumstances chlorine dioxide is broken down to chlorite and chlorate (ClO3-) :
2ClO2 + 2OH- = H2O + ClO3- + ClO2-

This reaction is catalyzed by hydrogen (H+) ions. The half life of watery solutions of chlorine dioxide decreases at increasing pH values. At low pH, chlorine dioxide is reduced to chloride ions (Cl- ).

Does chlorine dioxide produce byproducts?
Pure chlorine dioxide gas that is applied to water produces less disinfection byproducts than oxidators, such as chlorine. Contrary to ozone (O3), pure chlorine dioxide does not produce bromide (Br-) ions into bromate ions (BrO3-), unless it undergoes photolysis. Additionally chlorine dioxide does not produce large amounts of aldehydes, ketons, keton acids or other disinfection byproducts that originate from the ozonisation of organic substances.

Chlorine dioxide as an oxidizer
As an oxidizer chlorine dioxide is very selective. It has this ability due to unique one-electron exchange mechanisms. Chlorine dioxide attacks the electron-rich centers of organic molecules. One electron is transferred and chlorine dioxide is reduced to chlorite (ClO2- ).

Figure 2: chlorine dioxide is more selective as an oxidizer than chlorine. While dosing the same concentrations, the residual concentration of chlorine dioxide is much higher with heavy pollution than the residual concentration of chlorine.

By comparing the oxidation strength and oxidation capacity of different disinfectants, one can conclude that chlorine dioxide is effective at low concentrations. Chlorine dioxide is not as reactive as ozone or chlorine and it only reacts with sulphuric substances, amines and some other reactive organic substances. In comparison to chlorine and ozone, less chlorine dioxide is required to obtain an active residual disinfectant. It can also be used when a large amount of organic matter is present.


What are the disinfection applications of chlorine dioxide?

Drinking water treatment is the main application of disinfection by chlorine dioxide. Thanks to its adequate biocidal abilities, chlorine dioxide is also used in other branches of industry today. Example are sewage water disinfection, industrial process water treatment, cooling tower water disinfection, industrial air treatment, mussel control, foodstuffs production and treatment, industrial waste oxidation and gas sterilization of medical equipment.

How does chlorine dioxide disinfect?

Chlorine dioxide disinfects through oxidation. It is the only biocide that is a molecular free radical. It has 19 electrons and has a preference for substances that give off or take up an electron.

Chlorine dioxide

Its main action consists in the oxidation of cellular constituents. Chlorine dioxide has a direct action on cell membranes, either altering (at high concentrations) or disrupting their permeability (at low concentrations) (USDA, 2002a) and then penetrating into the cell and disrupting the protein synthesis. At a pH of 8.5, chlorine dioxide was reported as 20 times more effective than chlorine at killing E. coli (Benarde et al., 1965).

Approved Legal Uses of the Substance :

Sodium chlorite is made by the reduction of chlorine dioxide, which is, in turn, from the reduction of sodium chlorate in the presence of sulfuric and hydrogen peroxide or sulfuric acid and sodium chloride.

1. The U.S. Food and Drug Administration (FDA) approved

  1. ASC solution as an antimicrobial agent (21 CFR §173.325) under the Specific Usage Additives section of the Secondary Direct Food Additives Permitted in Food for Human Consumption. ASC solution is produced by mixing an aqueous solution of sodium chlorite with any generally recognized as safe (GRAS) acid, in accordance with current industry standards of good manufacturing practice, for use in red meat, poultry, seafood, and raw agricultural commodities.
  2. as a sanitizing solution (21 CFR §178.1010 (b) (46)), which is an aqueous solution of oxy-chloro species generated by acidification of sodium chlorite, listed under the Substances Utilized to Control the Growth of Microorganisms of Indirect Food Additives: Adjuvants, Production Aids, and Sanitizers. In addition to use on food-processing equipment and utensils, this solution may be used on dairy-processing equipment.

The USDA Food Safety and Insection Service (FSIS) identified the ASC for use in red meat and poultry products as processing aids (SIS Directive 120.1, Attachment 1). For poultry products, the pH and concentration levels of the solutions have been referred to 21 CFR §173.325. For red meat products, ASC solution applied as a spray or dip has the pH 5.0 – 7.5, in addition, the concentrations of sodium chlorite and chlorine dioxide are not exceed 1200 and 30 ppm, respectively.

  1. The U.S. Environmental Protection Agency (EPA) approved oxy-chloro species (40 CFR §180.940) generated by acidification of an aqueous solution of sodium chlorite as a food-contact surface sanitizing solutions. The end-use concentration is not to exceed 200 ppm of chlorine dioxide.


  1. The Canada Food Inspection Agency, Meat Hygiene Directive listed ASC as an approved microbial control agent, in a range of 500-1200 ppm at pH 2.5-2.9, for use on poultry. (May 2001)
  1. The Food Standards Australia New Zealand (FSANZ) approved ASC as a food processing aid for antimicrobial use. (October 2003)

3. The European Food Safety Authority (EFSA) endorsed ASC to clean chicken carcasses. (January 2006)

  1. Codex Committee on Food Additives recommended that they consider adding ASC to the inventory of processing aids (IPA). (April 2008)


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