About Chlorine Dioxide (ClO₂)

ClO₂ history

The discovery of ClO₂^•     Dates back to the early 1800's, when Sir Humphrey Davy created the compound by mixing sulfuric acid with potassium chlorate.

Chemical properties of chlorine dioxide (ClO₂)

ClO₂ is a chemical compound that consists of one chlorine ion in the +4 oxidation state covalently (non-metal to non-metal)bound to two oxygen ions.  its chlorine is in ionic form as part of a compound, such as in sodium chloride (NaCl / common table salt). This is not the same as chlorine in its elemental form. For perspective - chlorine is the most abundant dissolved ion in ocean water.

ClO₂ is a relatively small, volatile, highly energetic molecule and a free radical even while in dilute aqueous solutions.   Although, it is stable in a dilute solution in a closed container in the absence of light. i.e. diluted watery solutions

At high concentrations, it reacts violently with reducing agents.    ClO₂ functions as a highly selective oxidant due to its unique, one-electron transfer mechanism where it is reduced to chlorite ClO₂^-

EPA Guidance Manual 1999    Lennech Tech Water Treatment Solutions

ClO² is a synthetic, green/yellowish gas with a chlorine-like odor.   ClO² is a true gas (like oxygen) and so retains its chemical structure and properties at all times. ClO2 gas is 2.4 times denser than air. At extremely high concentrations (>10%) in air it can be explosive (this is nothing the therapeutic user needs to worry about).

Although it has an irritating chlorine-like odor and contains a chlorine atom in its molecule, ClO₂exhibits physical and chemical properties that are dramatically different from those of chlorine

ClO₂ stability depends on environment:

•    In the absence of light.   ClO₂ is stable in a dilute solution in a closed container

•   In AIR.   ClO₂ is an unstable gas that dissociates into chlorine gas (Cl₂), oxygen gas (O₂) and heat. 

AWWA. 1990. Water Quality and Treatment, fourth edition. McGraw-Hill, Inc., New York, NY.

•   Sunlight (photo-oxidation).    Quickly breaks ClO₂ into Cl₂ and O₂.

ClO₂ can be removed by aeration or carbon dioxide

ClO₂ disinfects by oxidation, not chlorination

Differentiating factors between chlorine dioxide and chlorine

MOST IMPORTANTLY,  ClO₂ behaves differently to chlorine in water

-   ClO₂ is 10 times more soluble than chlorine in water (3.01 g/L at 25°C).    Especially in cold water; when introduced into water or other solvents it does not hydrolyze (i.e. form another compound) and remains a dissolved gas in solution. Because of this fact, it retains its structure and reactive properties

-   In contrast, chlorine in water dissociates to form:

•   Hypochlorous acid.   The main active biocide in solution, which with increasing pH dissociates to form hypochlorite ion, having only 1/20 to 1/300 of the effect of hypochlorous acid at controlling microbes. The efficacy of chlorine as an effective biocide is reduced with increasing pH

•   Hydrochloric acids

-   ClO₂ is highly soluble in organic (hydrocarbon) substances.  E.g. oils, solvents, making its oxidative / biocidal properties potentially useful in other applications

ClO2 reacts with other substances via oxidation, not substitution (i.e. unlike chlorine, ClO2 does not chlorinate.    ClO2 contains no elemental free chlorine)

-   ClO2 is more SELECTIVE in its reactions with other substances than chlorine.   Since ClO2 has lower oxidation strength (Oxidation/Reduction Potential or ORP) than chlorine, but more than twice the oxidative capacity . ClO2 functions as a highly selective oxidant due to its unique, one-electron transfer mechanism, where its reaction end-products depend on pH

In drinking water, chlorite (ClO₂^-) is the predominant reaction end-product, with approximately 50-70% of the chlorine dioxide converted to chlorite and 30% to chlorate (ClO₃^-) and chloride (Cl)

Werdehoff, K.S, and P.C. Singer. 1987. "Chlorine Dioxide Effects on THMFP, TOXFP and the Formation of Inorganic By-Products."J. AWWA. 79(9):107.

Comparing CL₂ and CLO₂ oxidative strenghth

•   The powerful oxidizer chlorine (Cl₂) tends to produce toxic by-products by reacting with (and thus limiting its effect as a biocide):

(a)  Hydrocarbons (C-H) to produce toxic organic chlorides (C-Cl) E.g trihalomethanes (THMs)

and (b) Various amines (C-NH2) to produce toxic chloramines (C-NH-Cl) -amines are derivatives of ammonia (NH3)

•   In contrast, the weaker oxidizer ClO₂ does not react with ammonia and most organic substances and does not generate any significant quantities of undesirable/toxic organochlorine compounds.

 Typically, ClO₂ ONLY (preferentially) reacts with:

(a) Compounds with activated carbonbonds -E.g. phenols (ArOH); does not break carbon bond; this property to destroy phenols is useful in water disinfection, since phenols can be responsible for an odor or taste

(b) Reduced metals -ferrous (Fe++), manganous (Mn++)

(c) Thiols (RSH)

(d) Aldehydes (RCHO)

(e) In aqueous solutions, tertiary amines (RNR'R") (producing secondary amine and an aldehyde) and very slowly or not at all with primary and secondary amines (RNHR') and other active compounds.   E.g. sulfides, cyanides

Hull LA, Davis GT, Rosenblatt DM, Williams HKR, Wegein RC, Oxidation of amines, duality of mechanism in the reaction of amines and chlorine dioxide. 1967. J. Am. Chem. Soc. 89, 1163,

(f) Other active compounds -E.g. sulfides, cyanides

Comparing CL₂ and CLO₂ oxidative capacity

Having an oxidation number of +4, the chlorine atom in the ClO2 molecule accepts 5 electrons when reduced to chloride ion.   The oxidation capacity represents the "available chlorine"in the molecule, so you will notice in the chartabove that ClO2contains 263% "available chlorine"compared to 100% for chlorine i.e. > 2.5 times the oxidation capacity of chlorine. Having greater oxidative capacity compared to other oxidizers, such as ozone or chlorine, means that less chlorine dioxide is required to obtain an active residual concentration.