Chlorine Dioxide

Chlorine dioxide in aqueous solution

To study the behavior of a compound, it is essential to consider the context in which it is found: its physical state, concentration, temperature and the presence of other compounds with which it may interact.

In its pure state, chlorine dioxide (ClO2) is a reddish yellow gas at ambient conditions. Its smell is similar to chlorine, reminiscent of nitric acid. Its solubility in water at atmospheric pressure and 25 ° C is close to 3 g / L (≈ 3000 ppm). Although the chemistry of chlorine dioxide in aqueous solution has not been fully studied, it has been enough that health authorities around the world have included it as one of the oxidizing compounds allowed to disinfect water for purification purposes.

The advantage of chlorine dioxide over other compounds used to disinfect water lies in the fact that its oxidation potential is lower. It makes it a less aggressive oxidant, which decreases the formation of disinfection byproducts, such as chloramines and chlorinated organic compounds. Some of these, such as trihalomethanes, are characterized by high carcinogenicity.

Oxidation potential (volts)
Hydroxyl radicals (OH)
Molecular chlorine
Hypochlorous acid (so-called "free chlorine")
Hydrogen peroxide
Chlorite Ion
Chlorine Dioxide

Pure chlorine dioxide

It is highly oxidizing and reacts violently with organic materials. In its gaseous state, at concentrations above 10% in air at atmospheric pressure, it detonates easily in the presence of sunlight, heat or in contact with mercury, phosphorus, potassium hydroxide, sulfur, mercury or carbon monoxide.

Basic physical properties:

Molecular weight: 66.96 g / gmol

Melting point: -59 ° C

Boiling point: 11 ° C

Density in liquid state at 0 ° C: 1.642 g / mL

Potential health effects:

Inhalation: severe respiratory irritant. May cause bronchospasm and pulmonary edema, although not immediately. May also cause severe headache. All symptoms may be delayed and persistent. Long-term exposure may cause chronic bronchitis.

Ingestion: There are no reported cases of significant health effects from ingestion.

Oral LD50, rat: 94 to over 10,000 mg / kg, depending on the source.

Exposure limits set by the American Conference of Governmental Industrial Hygienists (ACGIH) 1992-93:

  • Time-Weighted Average (TWA) (recommended exposure limit in air as an average during an 8-hour work shift): 0.1 ppm (2.76 mg / m3).
  • Short-Term Exposure Limit (STEL) (exposure limit in air -15 minutes): 0.3 ppm (8.3 mg / m3).

Carcinogenicity: not listed by IARC (International Agency for Research on Cancer) or ACGIH (American Conference of Governmental Industrial Hygienists).

Mutagenicity:information not available.

Reproductive effects:information not available.

Teratogenicity and fetotoxicity:information not available.

Synergistic materials:May have synergistic effects in conjunction with chlorine, other chlorine oxides and chlorine fluoride compounds.

Chlorine dioxide in aqueous solution

When chlorine dioxide dissolves in water, most of it does not hydrolyze: it remains as a dissolved gas in solution. Another part does, and forms as much chlorite ion (ClO2) as chlorate ion (ClO3).

As mentioned, the solubility of chlorine dioxide in water at sea level and at 25 ° C is close to 3 g / L (≈ 3000 ppm). Its solubility increases at lower temperatures. Therefore, when its concentration is greater than 3 grams per liter, it is common to store it at temperatures close to 5 ° C.

Dissolved in pure water, in a hermetically sealed container, in the absence of light, and at low temperature, it is quite stable although it slowly decomposes into chlorine and oxygen. Chlorides catalyze its decomposition.

Chlorite (ClO2)

From animal studies, NAS arrived at a calculated dose for non-adverse effect doses of 0.21 mg / L with an uncertainty factor of 100. The oral LD50 in rats of sodium chlorite that is marketed at a concentration of 80% (and where the remainder is primarily sodium chloride) is 165 mg / kg.

Chlorate (ClO3)

Chlorate is a minor by-product of the chlorine dioxide disinfection treatment of drinking water. The oral LD50 in rats is between 1200 and 7000 mg / kg. The effect of dilution has not been determined, and no mutagenicity, teratogenicity, or carcinogenicity data are available.

Chlorine dioxide as a biocide

Chlorine dioxide disinfects by oxidation. Of the oxidizing biocides, it is the most selective. It is effective at pH values between 4 and 10. Its low oxidation potential does not allow it to oxidize organic compounds; that is, it does not break C-C or C-H bonds. It only reacts when it encounters reduced atoms, such as protons, which take away its oxygen. Unlike free chlorine (hypochlorous acid or hypochlorite ion), chlorine dioxide does not react with ammonia (NH3) or ammonium hydroxide (NH4OH) and hardly reacts with elemental amines.

Mechanism by which chlorine dioxide oxidizes

An oxidizing compound is capable of accepting electrons. The chlorine atom that is part of the chlorine dioxide molecule has an oxidation number of +4. In its maximum state of oxidation, the chlorine can be left with an oxidation number of -1. This means, that it is able to receive up to five electrons. In a first step, the chlorine dioxide receives an electron that reduces it to chlorite ion:

ClO2 + e- ⟶ ClO2

As a second step, the chlorite ion is oxidized to chloride ion; which explains why no chlorinated substances are formed:

ClO2  +  4H+ +  4e ⟶ Cl  +  2H2O

The reaction shows that each oxygen atom that was part of the chlorite ion neutralizes a proton and forms a water molecule.

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