Chemistry

Chlorine Dioxide Chemistry

Historical Background

The discovery of ClO₂ 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 ClO₂ shares some common characteristics with chlorine. Specifically, ClO₂ 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 ClO₂ 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 ClO₂ that sets it apart from chlorine is its behavior when placed in water. Not only is ClO₂ 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 ClO₂ 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

ClO₂ is a small, volatile, and very strong molecule that reacts with other substances by way of oxidation rather than by substitution (i.e. chlorination). ClO₂ 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. ClO₂ 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 ClO₂ has lower oxidation strength, it is more selective in its reactions. Typically, ClO₂ 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 that ClO₂, and will react with a wider variety of chemicals, including ammonia. This property limits its overall effectiveness as a biocide. Conversely, because ClO₂ has more oxidative capacity compared to ozone or chlorine, less ClO₂ is required to obtain an active residual concentration of the material when used as a disinfectant.

An Effective Biocide

The propensity of ClO₂ 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 triholomethanes and halogenated acidic acids, is of concern. Additionally, ClO₂ 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 ClO₂ inactivates microorganisms is not entirely well understood. As a general matter, however, it is known that ClO₂ destroys microbes by attacking their cell walls (or viral envelopes) and interfering with essential protein formation. It is also known that ClO₂ is more effective against viruses than either chlorine or ozone. Furthermore, ClO₂ is known to be effective against hearty waterborne protozoans such as Giardia Lambia and Cryptosporidium, the causative agents of giardiasis and cryptosporidiosis, respectively. Since ClO₂ is an oxidative biocide, microorganisms cannot build up a resistance to it.

Many Applications

Because ClO₂ 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 ClO₂ makes it particularly useful for addressing structural microbial contamination problems. Liquid ClO₂ 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, ClO₂ quickly decays on its own to invisible, harmless concentrations of various sodium salts including chlorite, chlorate, and chloride ion.