Q & A

  • 1. What is Chlorine dioxide (ClO2)?

    Chlorine dioxide is a selective (a feature of targeted oxidation) oxidant with low oxidation power. Therefore it’s allowed to be present at specific concentrations in drinking water. In addition, due to its strong oxidation capacity (gaining 5 elections) and rich in available chlorine, it is a high efficacy oxidant. Because of these unique properties, Chlorine dioxide becomes a great general purpose disinfection choice against pathogens such as bacteria, viruses, fungi and algae.

    The chlorine dioxide molecule contains an unpaired electron and two reaction centers (chlorine and oxygen), resulting in properties different from those of other oxidants.

    It was first discovered in 1811 by Sir Humphrey Davy.

    It is a disinfectant of high efficacy and safety with broad applications.


    Figure 1: Sir Humphrey Day discovered chlorine dioxide in 1811.

  • 2. What are the properties of chlorine dioxide gas?

    Chlorine dioxide is a synthetic, green-yellowish gas with a chlorine-like, irritating odor at room temperature. It is an unstable gas that dissociates into chlorine gas (Cl2), oxygen gas (O2) and heat. It can be photo-oxidized by sunlight. Chlorine dioxide turns into gas at 11°C (boiling point).

    • Chlorine dioxide is an unstable irritating gas not existing in the nature.
  • 3. Can chlorine dioxide gas be dissolved in water?

    One of the most important properties of chlorine dioxide is its high water solubility, especially in cold water. It is approximately 10 times more soluble in water than chlorine. Chlorine dioxide does not hydrolyze when it enters water.

    • It remains a dissolved gas in solution and retains its oxidative and biocidal properties.
  • 4. How can chlorine dioxide be stored?

    Chlorine dioxide gas cannot be stored for too long because it slowly dissociates into chlorine and oxygen. It is rarely transported because of its explosiveness and instability and usually manufactured on site.

    • In a water solution, chlorine dioxide is very stable if the solution is protected from light and heat.
  • 5. What are the applications of chlorine dioxide?

    Chlorine dioxide has many applications:
    - In the electronics industry to clean circuit boards,
    - In the oil industry to treat sulfides,
    - In pulp and paper industry to bleach paper,
    - In medical industry to sterilize medical equipment, air, surfaces, rooms and tools,
    - In water industry to remove the taste, odor, color, phenol, iron and manganese.

    • As a disinfectant and biocide, it is mainly used in liquid form (water solution).
  • 6. Chlorine dioxide is a selective but weak oxidant

    As an oxidant, chlorine dioxide is targeted where it is needed the most, i.e. “selective”. It quickly attacks the electron-rich centers of organic molecules such as organic sulphides compounds, secondary (R2NH) and tertiary (R3N) amines, phenol, and some other reactive organic substances such as Polycyclic aromatic hydrocarbons (PAHs). It does not break carbon connections, aromatic cores, carboxylic structures, primary (RNH2) amines, urea and other substances.

    Due to its high selectivity, less chlorine dioxide is required to disinfect the pathogens. In comparison to chlorine and ozone, it is an active residual disinfectant.

    Selectivity of ClO2 between humans and bacteria is based not on their different biochemistry, but on their different sizes.

    Figure 2: chlorine dioxide is more selective than other oxidants such as chlorine. While dosing the same concentrations in heavy polluted environment, the residual concentration of chlorine dioxide is much higher than that of chlorine.

    The oxidation strength describes how strongly an oxidant reacts with an oxidizable substance. Ozone has the highest oxidation strength and reacts with every substance that can be oxidized. On the contrary, Chlorine dioxide is significantly weaker on oxidation strength.

    • Weak on oxidation strength and high on selectivity make chlorine dioxide a safe oxidant.
  • 7. Chlorine dioxide is high on oxidation capacity and rich in available chlorine.

    The oxidation capacity means how many electrons are transferred in an oxidation reaction. The chlorine atom in chlorine dioxide has an oxidation number of +4. After receiving 5 electrons, it is reduced to a stable chloride ion (Cl-).

    Chlorine dioxide contains, mole for mole, 276% more available chlorine than sodium chlorite and 263 % more than chlorine, which makes it 2.5 times more oxidation capacity than chlorine.

    • High on oxidation capacity and rich in available chlorine make chlorine dioxide a high oxidative load against pathogens.
  • 8. Does chlorine dioxide produce byproducts?

    Chlorine dioxide, like ozone, the predominant oxidation reaction mechanism is a process known as free radical electrophilic (i.e. electron-attracting) abstraction rather than by oxidative substitution or addition (as in chlorinating agents such as chlorine or hypochlorite). This means that chlorinated organic compounds such as THMs and HAAs are not produced at the end of oxidation reaction.

    Unlike the ozonisation of organic substances by ozone, chlorine dioxide does not produce large amounts organic substances such as aldehydes, ketones, ketone acids or other byproducts.

    These are the reasons why chlorine dioxide has many industrial, municipal and residential applications such as sewage water disinfection, cooling tower water disinfection, industrial air treatment, food production and treatment and sterilization of medical equipment.

    • Producing no harmful byproducts makes chlorine dioxide a great disinfectant for many applications.
  • 9. What are the end-products of chlorine dioxide after the oxidation reaction?

    The reaction process of chlorine dioxide with bacteria, viruses and other substances, takes place in two steps.

    First, chlorine dioxide takes up an electron and reduces to chlorite ion:

            ClO2 + e- = ClO2-
    Secondly, the chlorite ion is oxidized and becomes a very stable chloride ion:

            ClO2- + 4H+ + 4e- = Cl- + 2H2O

    • The final end-product of chlorine dioxide oxidation reactions is chloride (Cl-).
  • 10. How does chlorine dioxide destroy bacteria and viruses?

    The reaction of chlorine dioxide with vital amino acids is one of the dominant processes of its action on bacteria and viruses. The oxidizable material within the cells and on the surface of cell membranes reacts with chlorine dioxide, causing cell metabolism to be disrupted. It also reacts directly with the amino acids and the RNA in the cell, which prevents the production of proteins.

    The small pathogens will be killed extremely fast, i.e. the killing time for a bacterium is on the order of milliseconds in a 300 ppm ClO2 solution. A few minutes of contact time is quite enough to kill all bacteria with minimized cytotoxic effects to the living tissues.

    • Chlorine dioxide disrupts the cell metabolism of the bacteria to destroy them, i.e. changing the cell membrane proteins and fat and penetrating the cell wall.
    • Chlorine dioxide eliminates the viruses by reacting with peptone and preventing of protein formation.
  • 11. Can microorganisms become resistant against chlorine dioxide?

    Since the electrophilic abstraction oxidation reaction without oxidative substitution or addition is independent of the reaction time or concentration, the required concentration of chlorine dioxide needed to effectively kill microorganisms is lower than those of non-oxidizing disinfectants.

    Unlike non-oxidizing disinfectants, chlorine dioxide kills microorganisms even when they are inactive. Most microorganisms are unable to build up resistance to it because they are destroyed via the oxidative load placed on their cells by chlorine dioxide.

    • Pathogens cannot built up any resistance against chlorine dioxide.
  • 12. Can chlorine dioxide be used to eliminate bio film?

    Chlorine dioxide in the water solution can penetrate the slime layers of bacteria. It oxidizes the polysaccharide matrix that keeps the bio film together and breaks the bio film, which allows it to act on the bacteria themselves. During this reaction, chlorine dioxide is reduced to chlorite ions (ClO2-). When the bio film starts to grow again, an acid environment is formed and the chlorite ions are transformed into chlorine dioxide, which removes the remaining bio film.

    It is effective against spore-forming bacteria so it can be used against anthrax.

    • Chlorine dioxide eliminates bio film, kills the bacteria associated with the bio film and prevents bio film formation like no other biocides.
  • 13. Can chlorine dioxide be used against protozoans?
    Chlorine dioxide is effective against Giardia Lambia and Cryptosporidium parasites, which are found in drinking water and induce diseases called 'giardiasis' and 'cryptosporidiosis'.

    Chlorine dioxide even kills a variety of parasites.
  • 14. How much chlorine dioxide should be dosed for drinking water treatment?

    For the pre-oxidation and reduction of organic substances, between 0.5 and 2 ppm of chlorine dioxide is required at a contact time between 15 and 30 minutes. Water quality determines the required contact time.

    For post- disinfection, concentrations between 0.2 and 0.4 ppm are applied. The residual byproduct concentration of chlorite is very low and there are no risks for human health.

    • The maximum chlorine dioxide concentration in the drinking water is 0.8ppm by EPA.
  • 15. Can chlorine dioxide be used to disinfect fish pond?

    The more powerful the oxidant, the more harmful for living organisms. Chlorine dioxide is above oxygen but far below ozone in terms of its oxidation power. A fixed low concentration will therefore not result in any problems for fish or other living organisms. The combination of low oxidation strength, high oxidation capacity and no harmful byproducts makes chlorine dioxide safe and easy to use in aquaculture.

    Consistently applying low concentration chlorine dioxide will result in:
    - less bacterial problems
    - less water odor
    - less bio film and algae formed on walls and pipes
    - better water quality
    - more healthy fish

    The maintenance dosage is suggested to be 0.01-0.04 ppm once a week.

    You can dose 10ml - 30ml of 12,000 ppm (i.e. 0.012 – 0.036 ppm) or 40ml - 120 ml of 3,000 ppm stock solution per 10,000 liter or 2,600 Gal of water.

    First-time-use dosage is suggested to be 0.1 - 0.15 ppm once every other day for 5 times in succession due to high level of bacteria and dissolved organic matters (DOM) in water. Afterwards, switch to maintenance dosage.

    You can dose 100ml of 12,000 ppm (i.e. 0.12 ppm) or 400ml of 3,000 ppm stock solution per 10,000 liter or 2,600 Gal water.

    • Using chlorine dioxide to purify the fish pond water will maintain high water quality and active and healthy fish.
  • 16. Can chlorine dioxide be used to disinfect swimming pool?

    For swimming pool, chlorine dioxide will be a very good choice of disinfectant. It kills the pathogens including bacteria, viruses and fungi in the water and stop algae from growing on the wall. It protects women from gynecologic diseases caused by different types of bacterial infections. For babies or persons with sensitive skin, there will be much less irritation from chlorine, especially in the public swimmy pool. Some people will have pink eye (i.e. conjunctivitis), which is a viruses disease induced by bacteria infection. It protects swimmers from exposing to toxic nitrogenous (chloramines) or carcinogenic organic residuals (trihalomethanes)and HAAs when chlorine dioxide replaces chlorine as the water purifier.

    The maintenance dosage of chlorine dioxide is suggested at: 0.1 - 0.2 ppm once a week.

    You can dose 100ml of 12,000 ppm (i.e. 0.12 ppm) or 400ml of 3,000 ppm stock solution per 10,000 liter or 2,600 Gal water.

    • Using chlorine dioxide in swimming pool disinfection generates no harmful byproducts and protect swimmers from many diseases.
  • 17. Can chlorine dioxide help remove the bad breath and other oral diseases?

    Oral malodor, common on awakening, can affect people of all ages. Longstanding oral malodor is usually caused by oral diseases. The most likely causes are the accumulation of food debris and dental bacterial plaque on the teeth and tongue from poor oral hygiene, gingivitis (gum inflammation) and periodontitis (gum disease). Adult periodontitis by gradual plaque related loss of periodontal attachment can cause variable degrees of oral malodor. Aggressive periodontitis by rapid loss of periodontal bone and resultant tooth mobility can cause intense oral malodor. Xerostomia (dryness of the mouth) and wearing dentures may also cause or enhance malodor.

    For oral cleansing, it is suggested to gargle for 30 seconds with 1 ppm solution once a day.

    You can use 10ml of 100 ppm (i.e. 1 ppm) or 3ml of 300 ppm stock solution per 1 liter of water.

    • Chlorine dioxide can be used as a mouthwash to minimize the oral bacterial and help prevent bad breath, dental plague, decayed tooth, gum disease and ulcer.
  • 18. Can chlorine dioxide help onychomycosis and athlete's foot?

    Onychomycosis is a fungus infection of the finger or toe nails which is very difficult to cure. A person usually has had athlete's foot for a long time prior to the development of onychomycosis. Human bodies allow the fungus to become established without an immune response to suppress the fungus. Antifungal compounds usually cannot penetrate the nail bed to kill the fungus at its source.

    Soaking the fungal infected nails in the chlorine dioxide solution, allowing it to penetrate the nail plate and inactivates pathogens residing in the nailbed, for a period of time helps improve the appearance of the nail, destroy the fungal infection and promote healthy nail growth.

    For eliminating the fungal infection, it is suggested to soak the infected nails in the solution of 100 ppm for 30-60 minutes once a day for a week.

    • Chlorine dioxide can kill fungi and help improve the fungus infected nails and athlete’s foot.
  • 19. Can chlorine dioxide be used to disinfect cooling towers?

    It can disinfect the water, remove bio film and prevents bio film formation in cooling towers. The removal of bio film prevents damage to and corrosion of equipment and piping. It is effective in removing Legionella in cooling towers, an ideal environment for Legionella growth.

    • Chlorine dioxide is ideal to kill the fatal Legionella in the high humidity environments such as cooling tower, spa and warm spring.
  • 20. Does the pH value affect chlorine dioxide disinfection efficacy?

    Contrary to chlorine, chlorine dioxide is effective at a pH of between 3 and 10. Chlorine dioxide does not hydrolyze, so pH value does not affect its oxidation and disinfection capability. At the concentrations required for disinfection, chlorine dioxide is non-corrosive and non-irritating.

    • Chlorine dioxide solution is very effective but non-irritating or non-corrosive at low concentration.