Chlorine Dioxide Chemistry – Lenntech , MMS
Chlorine Dioxide Chemistry
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.
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.
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.