Differences with other oxidants

Although chlorine and chlorine dioxide are both strong oxidizing agents, they differ in reactions with various organic and inorganic compounds. ClO₂ for example, does not combine with ammonia as does Cl2. Chlorine dioxide is a better disinfectant in the presence of organic matter, and bacterial kill is not appreciably changed with change in pH. Hypochlorite has a higher oxidation potential and is an indiscriminate “chlorinator”, adding a permanent chlorine atom to organic molecules. This unfortunately, produces a number of unwanted chlorinated hydrocarbons such as chloroform and chlorophenol. Chemicals found in industrial waste discharges for example, all react to produce chlorinated by-products that are hazardous to health.  ClO , on the other hand, oxidizes (removes electrons) without adding an atom of its own to the oxidized product. The pKa for the chlorite ion, chlorous acid equilibrium, is extremely low at pH 1.8. This is different from the hypochlorous acid/hyopochlorite base ion pair equilibrium found near neutrality, and indicates the chlorite ion will exist as the dominant species in drinking water and in the human body.

 When purifying water supplies, ClO₂ combines with phenols particularly fast by attacking the benzene ring. Odorless, tasteless products are formed directly, without intermediate compounds, as is the case with chlorine.40  ClO may be more effective than copper sulfate in controlling algae; it is believed to attack the pyrrole ring of the chlorophyll which cleaves the ring and leaves the chlorophyll inactive. The reaction of ClO₂ with algae, again, forms tasteless, odorless products.

Olefins react much more rapidly with permanganate than with chlorine dioxide, whereas, triethylamine is thousands of times more reactive with chlorine dioxide than with permanganate.

Unlike most other oxidizing compounds, ClO₂, and its reduction product ClO₂¯ , can act either as oxidizing or reducing agents (NCASI No. 673). Under acid conditions hydrogen peroxide will reduce ClO₂ to form chlorous acid, but ClO₂ also can be oxidized by chlorine to produce chlorate, and by ozone to produce Cl2O6. ClO₂¯  similarly can oxidize iodide to form iodine, or be oxidized by hypochlorite ion to form chlorate. Combining ClO₂ with blood causes methemoglobin by oxidizing Fe3 to Fe2 in the red blood cell. Breathing ClO₂ can have this effect.

When ClO₂ oxidizes organics, it usually takes in one electron and reduces to ClO₂¯ . ClO₂ can oxidize some inorganics, like ferric oxide, remove 5 electrons rather than one, and reduce all the way to chloride. The amount of electron exchange is the oxidizing capacity, not the redox potential or driving force of the reaction.

ClO₂ (aq) + e^– = ClO₂^– (E0 = 0.95V)

Get stronger with some compounds

ClO₂ + 5e^– = Cl + 2O₂^– (about 1.5V)

When oxidizing organic molecules, there is no chlorine atom exchange to produce chlorinated hydrocarbons.