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Chlorine Dioxide (ClO2) Chemistry

Historical Background

The discovery of chlorine dioxide has largely been credited to Sir Humphrey Davy, who, in 1814, created the compound by mixing sulfuric acid with potassium chlorate. Since its discovery, researchers have found that chlorine dioxide shares some common characteristics with chlorine. Specifically, chlorine dioxide is a greenish-yellowish gas with a chlorine-like odor that is irritating to the eyes, nose, and throat. Apart from these very limited similarities, however, it has been learned that chlorine dioxide exhibits physical and chemical properties that are dramatically different from those of chlorine, even though it contains a chlorine atom in its molecular structure.

Differentiating Factors
One of the most important properties of chlorine dioxide that sets it apart from chlorine is its behavior when placed in water. Not only is chlorine dioxide 10 times more soluble in water than chlorine (3.01 grams/liter at 25 degrees C), it doesn't hydrolyze when placed in solution. It remains as a "true" dissolved gas that retains its useful oxidative and biocidal properties throughout the entire 2 to 10 pH range. By way of contrast, chlorine dissociates when placed in water to form hypochlorous and hydrochloric acids. Hypochlorous acid is the primary biocide in solution, which dissociates to form hypochlorite ion with increasing pH. Hypochlorite ion is only from 1/20 to 1/300 as effective in controlling microbes as hypochlorous acid. Thus, chlorine can only be an effective biocide in systems with low pH. The high degree of solubility exhibited by chlorine dioxide in water has also been observed in a variety of organic materials, such as oils and solvents, thereby allowing for utilization of its unique oxidative and biocidal properties in a wide range of potential applications.

Molecular Properties & Oxidation

Chlorine dioxide is a small, volatile, and very strong molecule that reacts with other substances by way of oxidation rather than by substitution (i.e., chlorination). chlorine dioxide has lower oxidation strength than chlorine, but more than twice the oxidative capacity. Oxidation strength describes how strongly an oxidizer will react with an "oxidizable" substance. The higher its oxidation strength, the more substances the oxidant compound will react with. chlorine dioxide is comparatively weak, and has a lower oxidation potential than ozone, chlorine or even hypochlorous acid. Oxidation capacity refers to the number of electrons transferred during an oxidation or reduction reaction. The chlorine atom in the ClO2 molecule has an oxidation number of +4. For this reason ClO2 accepts 5 electrons when reduced to chloride ion. By way of comparison, ClO2 contains 263 percent 'available chlorine,' which is more than 2.5 times the oxidation capacity of chlorine.

Because chlorine dioxide has lower oxidation strength, it is more selective in its reactions. Typically, chlorine dioxide will only react with compounds that have activated carbon bonds such as phenols, or with other active compounds like sulfides, cyanides, and reduced iron and manganese compounds. Chlorine is a more powerful oxidizer than chlorine dioxide, and will react with a wider variety of chemicals, including ammonia. This property limits its overall effectiveness as a biocide. Conversely, because chlorine dioxide has more oxidative capacity compared to ozone or chlorine, less chlorine dioxide is required to obtain an active residual concentration of the material when used as a disinfectant.

An Effective Biocide

The propensity of chlorine dioxide to react by oxidation rather than substitution makes it a useful alternative to chlorine in drinking water disinfection applications where the formation of potentially carcinogenic halogenated disinfection byproducts, such as trihalomethanes and halogenated acidic acids, is of concern. Additionally, chlorine dioxide does not produce significant amounts of aldehydes, ketons, keton acids, or other disinfection byproducts that originate from ozonation of water containing organic substances.

The reaction of ClO2 with microorganisms or other oxidizable substances takes place in two steps. In the first stage of the reaction, the ClO2 molecule accepts an electron and chlorite ion is formed (ClO2-). In the second stage, ClO2 accepts 4 electrons and chloride ion (Cl-) is formed.

The mechanism of action by which chlorine dioxide inactivates microorganisms is not entirely well understood. As a general matter, however, it is known that chlorine dioxide destroys microbes by attacking their cell walls (or viral envelopes) and interfering with essential protein formation. It is also known that chlorine dioxide is more effective against viruses than either chlorine or ozone. Furthermore, chlorine dioxide is known to be effective against hearty waterborne protozoans such as Giardia Lambia and Cryptosporidium, the causative agents of giardiasis and cryptosporidiosis, respectively. Since chlorine dioxide is an oxidative biocide, microorganisms cannot build up a resistance to it.

Many Applications
Because chlorine dioxide always exists as a true gas under standard conditions of temperature and pressure, whether in open air or dissolved in solution, its antimicrobial properties can be harnessed for either liquid or gaseous application. The "free radical" property of chlorine dioxide makes it particularly useful for addressing structural microbial contamination problems. Liquid chlorine dioxide solution can be applied directly to known areas of microbial contamination, or entire contaminated structures can be fumigated with the gas by simply stripping it back out of solution at the point of application. Once applied, chlorine dioxide quickly decays on its own to invisible, harmless concentrations of various sodium salts including chlorite, chlorate, and chloride ion.