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Chlorine Dioxide

What is chlorine dioxide?

Chlorine dioxide (chemical formula ClO2) is a yellowish-green gas or disinfectant which uses mainly for the treatment or production of drinking water. Chlorine dioxide is a toxic chemical compound that has serious side effects. Commercially, chlorine dioxide is produced by reacting sodium chlorate with sulfur dioxide or hydrochloric acid in a strong sulfuric acid medium.

Chlorine dioxide chemical formula ClO2, structure, production, properties and uses for water treatment and disinfection

The yellow gas condensed to a reddish-brown liquid between 11 °C. Chlorine dioxide is soluble in water to form a yellowish-brown solution. It does not hydrolyze when enters into water. Strong acids can decompose chlorine dioxide into chlorine and oxygen. We used very small quantities of chlorine dioxide for the disinfection of drinking water. When used in large quantities, it should be dangerous for human health.

Chlorine dioxide production

The explosive gas Chlorine dioxide may be prepared by heating potassium chlorate at 90 °C with moist oxalic acid. The liberated carbon dioxide can dilute the explosive gas. It makes advantages for the production of such chemical compounds.

2 KClO3 + 2 H2C2O4 → 2ClO2 + 2 CO2 + K2C2O4 + 2 H2O

How to make chlorine dioxide from sodium chlorite?

Commercially, the gas is manufactured on a massive scale by reacting sodium chlorate with a suitable reducing agent such as methanol, hydrogen peroxide, or sulfur dioxide in a strong acid medium. Hydrochloric acid is also used as a reducing agent but hydrochloric acid contaminates the product with chlorine.
2 NaClO3 + SO2 + H2SO4 → 2 ClO2 + 2 NaHSO4
2 ClO3 + 2 Cl + 4 H+ → 2 ClO2 + Cl2 + 2 H2O

Methanol or hydrogen peroxide is the best reducing agent for ClO2 production because they do not co-produce elemental chlorine.

Pure chlorine dioxide

Pure ClO2 is obtained by the reduction of silver chlorate with dry chlorine gas. The evolved ClO2 gas condensed to a liquid.
2 AgClO3 + Cl2 → 2 AgCl + 2 ClO2 + O2

Structure of ClO2

ClO2 formed bent type structure with O-Cl-O bond angle 117.6°. The O-Cl bond distance in the ClO2 molecule is 149 pm. It is slightly shorter than a single bond. Pauling represents the molecule that contains a resonance hybrid with three electron structures.

Chlorine dioxide chemical formula ClO2, structure

An alternative scheme of the three-electron structures with the odd electron residing on an oxygen orbital may also be worked out. The resonating hybrid is compatible with the sp3d hybridization of the chlorine atom. One orbital has the lone pairs and another makes the coordinate link to one oxygen atom. The third and fourth sp3d orbital makes a sigma and a pi bond with other oxygen atoms. The odd electrons remain located on the fifth sp3d orbitals of the ClO2 molecule.

It has 19 (2 × 6 + 7) odd electrons and a tendency to dimerize. No evidence of dimerization in solid, liquid, or gas phase or in solution.

Properties of chlorine dioxide

Physical properties

  • At standard temperature and pressure, it is a reddish-yellow gas that is soluble in water to form a yellowish-brown solution. It is 10 times more soluble in water than chlorine.
  • It is an unstable irritating gas that decomposes chlorine and oxygen in the presence of strong acids.
  • It is a toxic gas with various types of side effects. Hence we need to maintain safety before exposure to ClO2 in laboratories or production plants.
  • Because of its explosive nature, it is used up at the manufacturing site itself.

The most common properties of chlorine dioxide are given below the table,

IUPAC name Chlorine dioxide
Chemical formula ClO2
Molar mass 67.45 g mol−1
Appearance Yellow to reddish toxic gas
Density 2.757 g dm−3
Melting point −59 °C
Boiling point 11 °C
Solubility soluble in alkaline and sulfuric acid solutions
Solubility in water 8 g/L at 20 °C

Chemical properties

Chlorine dioxide is soluble in water to form dark green solutions up to 8 g/L in dark conditions. In presence of light, ClO2 decomposes solely to form HCl and HClO3. Crystalline clathrate hydrate may be obtained by cooling the solution.

It dissolves in sodium hydroxide to form chlorite and chlorate.
2 ClO2 + 2 NaOH → NaClO2 + NaClO3 + H2O

It is a powerful oxidizing and chlorinating agent. Aqueous suspension of powdered metals (Mg, Zn, Ni, Cd, and Al) are converted to their chlorites by ClO2 at ordinary temperature and pressure.
Mg + 2 ClO2 → Mg(ClO2)2
Al + 3 ClO2 → Mg(ClO2)3

Ozone can be oxidizes ClO2 to Cl2O6.
2 ClO2 + O3 → Cl2O6 + 2 O2

Alkaline hydrogen peroxide should be reacted with ClO2 to give sodium chlorite. It is used for belching textiles and paper pulp.
2 ClO2 + 2 NaOH + H2O2 → 2 NaClO2 + O2 + 2 H2O

Uses of chlorine dioxide

Chlorine dioxide is a powerful oxidant and disinfectant used commonly for the disinfection of municipal drinking water. Huge quantities of ClO2 are produced for bleaching wood pulp, cellulose, and textiles products. It is also used for making NaClO2. It can be used in manufacturing sites due to its explosive nature.

For Cleaning

Chlorine dioxide has several cleaning applications.

  • It is used in the electronics industry to clean circuit boards.
  • Chlorine dioxide gas and solution are used to sterilize and clean medical and laboratory equipment, surfaces, rooms, and tools.
  • Mouthwash containing small quantities of chlorine dioxide can improve bad breath. When we take it in the mouth, it is unsafe for our health. But it is possibly safe when we used very small quantities of ClO2 in mouthwash.


Present-day, chlorine dioxide is widely used for bleaching paper and textile industries. It produced cleaner and stronger fiber than chlorine. During bleaching, it does not produce harmful byproducts like organochlorine compounds.

Disinfection with chlorine dioxide

Chlorine dioxide should be used widely as a disinfectant or oxidant. It is a very strong oxidizer and effectively worked in many disinfection processes. It kills pathogenic microorganisms such as fungi, bacteria, and viruses. We use ClO2 instead of chlorine because it prevents the formation of harmful halogenated disinfection byproducts like trihalomethanes and halogenated acidic acids.

  • Drinking water treatment or disinfection is the main application of chlorine dioxide.
  • It is effectively sanitized molds and yeast from several fruits like blueberries, raspberries, and strawberries
  • It has the disinfecting ability to fight against Giardia Lambia and Cryptosporidium parasites.
  • The spray of ClO2 may be used to disinfect poultry farms.
  • For swimming pool disinfection, we used a combination of chlorine and ClO2.
  • It is used to disinfect the water that flows through cooling towers.

Chlorine dioxide for water treatment

It is less corrosive than chlorine and significantly controls several types of bacteria in the water treatment process. It is a very good water disinfectant than other secondary disinfectants. Therefore, chlorine dioxide is used in many water treatment plants such as cooling towers, water, and food processing plant.

In 1956, chlorine dioxide was introduced on a large scale for disinfection of drinking water. It was primarily used for removing inorganic components such as manganese and iron. It has no taste or odor in water and destroys taste or odor-producing phenolic compounds. In drinking water treatment, chlorine dioxide acts as a disinfectant and oxidizing agent.

  • It can be used for treating water in pre-oxidation and post-oxidation steps. In all the stages, chlorine dioxide should be chlorination in drinking water to destroy natural water impurities or prevent the growth of algae and bacteria.
  • It is a powerful disinfectant for several types of bacteria and viruses. It helps to prevent the growth of bacteria in municipal water supply systems.
  • It should be active against biofilm in the water distribution system. Biofilm is a protective layer of pathogenic microorganisms which does not defeat by most common water disinfectants. It has the ability to destroy or kill pathogenic microorganisms layer.

Side effects

The supplements which contain chlorine dioxide are likely to be unsafe for human health. We used a very small amount of ClO2 for the disinfection of drinking water used in the public sector. Acute ClO2, exposure can form chlorine gas which causes irritations and burns skin and eyes. It can be absorbed by the skin and damage the tissues and blood cells of our skin.

Inhalation of chlorine dioxide causes serious toxic side effects such as coughing, a sore throat, severe headaches, vomiting and diarrhea, liver failure, and death.