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Chlorine    Dioxide
Chlorine Dioxide Discover and Time

Chlorine dioxide was discovered in 1814 by Sir Humphrey Davy. He produced the gas by reacting sulphuric acid with potassium chlorate.

Chlorine Dioxide Produced Method

Chlorine dioxide was first produced in 1814 by reacting sulphuric acid with potassium chlorate.

Today, a large number of reaction methodologies are used to produce chlorine dioxide, but these generally involve the reaction of a chlorate (ClO3-) or chlorite (ClO2-) salt with an acid, sometimes in the presence of a Chlorine Donor.

Some typical reaction methodologies for chlorine dioxide generation are shown below. Each approach has advantages and disadvantages depending upon the volume of ClO2 to be produced, practicality and safety considerations. Please do not hesitate to seek our advice if you are unsure.

5NaClO2 + 4 HCl -> 4ClO2 + 5NaCl + 2H2O
2NaClO2 + Cl2 -> 2ClO2 + 2NaCl
2 NaClO2 + Na2S2O8 -> 2ClO2 + 2Na2SO4

Newer technologies, such as Purate generation systems offer the ability to produce ClO2 more efficiently with low Chlorine and Salt, according to the reaction:
NaClO3 + H2O2 + H2SO4     ClO2 + Na2SO4 + O2 + H2O

A number of technologies have been developed to allow ClO2 to be used in lower volume applications, where expensive generation equipment is not required. This can utilise a non-hazardous, low alkalinity liquid (Often referred to as "Stabilised Chlorine Dioxide"), or powder or liquid sachets.

Chlorine Dioxide Reaction When it Oxidises

The predominant oxidation reaction mechanism for chlorine dioxide (and for ozone as well) proceeds through a process known as free radical electrophilic (i.e: electron-attracting) abstraction rather than by oxidative substitution or addition (as in chlorinating agents such as chlorine or hypochlorite).

It has this ability due to unique one-electron exchange mechanisms. One electron is transferred and chlorine dioxide is reduced to chlorite (ClO2- ).

The term "oxidation strength" is used to describe how strongly an oxidizer reacts with an oxidizable substance. Ozone is generally regarded as having the highest oxidation strength and reacts with every substance that can be oxidized. In practical terms, this is often undesirable since a number of side reactions can take place causing undesirable disinfection by-products.

Chlorine dioxide has a lower oxidation strength than ozone, but is more powerful than chlorine. Less chlorine dioxide is normally required to obtain an active residual disinfectant. Unlike ozone, ClO2 can also be used when a large amount of organic matter is present.

Chlorine Dioxide Oxidation Capacity Comparison with other Disinfectants

Oxidant ClO2 H2O2
NaClO2 KMnO4 Cl2 NaClO
Capacity Oxidation 263% 209% 157% 111% 100% 93%

Chlorine Dioxide Reaction with Bacteria and other Biological Material to Disinfect

When bacteria are eliminated, the cell wall is penetrated by chlorine dioxide. Organic substances within cells and on the surface of cell membranes react with chlorine dioxide, causing cell metabolism to be disrupted. Chlorine dioxide also reacts directly with amino acids and the RNA in the cell. This reaction is not dependent on reaction time or concentration. Unlike non-oxidizing disinfectants, chlorine dioxide kills microorganisms even when they are inactive. Microorganisms are unable to build up resistance to chlorine dioxide, in practical terms however, few bacteria live alone, and they are most often found in water and on surfaces in the form of a "biofilm" which is a close association of many millions of bacteria. Many biocides have particular problems in penetrating this biofilm, due to the polysaccharide "glue" that is secreted by the bacteria to hold the biofilm together. Unlike most biocides, chlorine dioxide can effectively penetrate biofilm to provide complete protection.

Chlorine dioxide kills viruses by preventing protein formation. ClO2 reacts with peptone, a water-soluble substance that originates from hydrolisis of proteins to amino acids.

Chlorine dioxide is one of a number of disinfectants that are effective against Giardia Lambia and Cryptosporidium oocysts, which cause diseases suchs as cryptosporidiosis in public drinking water supplies. A number of public water works are now utilising chlorine dioxide generation systems alongside UV systems in order to provide complete protection from Cryptosporidium

The ClO2 Compares with other Disinfectants

Characters ClO2 Chlorhexidine Chlorine / Hypochlorite Phenol Aldehyde NaOH Alcohol
Resistance to Organic Good Ordinary poor General good good General
Activity in Hard-water Yes Yes Yes No Yes Yes Yes
Affect High Temperature Result is best in 26-60 ℃
No Activity decreased below 43 ℃
Activity increased Result is best in 26-60 ℃
No No
PH Range No effect Alkaline Acidic Acidic No effect Alkaline No effect
Anion Soap Compatibility No Yes No Yes yes Yes No
Activity of Residue No Yes No Yes No Yes No
Toxicity or Discomfort No No Yes Yes Yes Yes Yes
Damage to Surface No No Yes No Yes Yes No
Kill the Bacteria Most Part Most Most Yes Most Most
Kill the Spores Yes Part Part No Yes Yes No
Kill the Viruses Yes No Part Part Yes Yes Part

Disinfection By-Products of Chlorine Dioxide

The DBPs of chlorine dioxide reactions are chlorite (ClO2-) and chlorate (ClO3-), and eventually chloride (Cl-).  The fate of any disinfection by-products depends largely on the conditions at the time, such as concentration, temperature and the presence of other molecules.

Generally, it is the concentration of chlorite residuals that is the "monitored" DBP of chlorine dioxide. Modern generation systems are able to monitor the downstream residual DBP and adjust the dose rate to ensure that environmental limits are not breached. In special cases, downstream reactions can be used to remove excess chlorite residual from the water stream.

It is important to note that the disinfection by-products of chlorine dioxide are easily managed with the correct experience and advice, and do not present nearly the same scale of problem as found with other biocides. Unlike ozone (O3), chlorine dioxide does not oxidise bromide (Br-) ions into bromate ions (BrO3-). Additionally, chlorine dioxide does not produce large amounts of aldehydes, ketones, or other disinfection by-products that originate from the ozonisation of organic substances.

Chlorine Dioxide be Used in Combination with other Disinfectants

Chlorine dioxide is often used in combination with chlorine in municipal drinking water plants in order to reduce the amount of Trihalomethanes and HAAs that would be formed if chlorine were used alone. Chlorine dioxide is added as the primary disinfectant in order to remove a number of oxidisable compounds without forming chlorinated DBPs, while chlorine is added at low levels in order provide a residual biocide for use in the disinfection system.

Recent research indicates that applying chlorine dioxide and chlorine within the same mixing zone can exhibit some synergistic effects (The combined effect being greater than the sum of the two parts).

Chlorine Dioxide Compares with Chlorine

While chlorine dioxide has “chlorine” in its name, its chemistry is radically different from that of chlorine. As we all learned in high school chemistry, we can mix two compounds and create a third that bears little resemblance to its parents. For instance, by mixing two parts of hydrogen gas with one of oxygen - liquid water is the formed. We should not be misled by the fact that chlorine and chlorine dioxide share a word in common. The chemistries of the two compounds are completely different.

Chlorine has been used as a water disinfectant for many decades, and many people are familiar with its use in water disinfection systems-so why change

We list below a number of advantages that chlorine dioxide treatments have over chlorine based systems:

Chlorine Chlorine Dioxide
Does not remove biofilm Will remove biofilm and thus clean tanks and pipes
Produces unwanted by-products including carcinogens Does not form chlorinated by-products
Is corrosive and unpleasant to handle Is much less corrosive than chlorine. Does not hydrolyse to form an acid
Already Banned in certain parts of Europe and the USA Is rapidly replacing chlorine in many of these areas
Is pH dependent and very ineffective above pH 7 Is not pH dependent (<pH 11)
Is ineffective against complex organisms (e.g: Cysts & Protozoa) A very broad spectrum kill*
Limited oxidative effect against various chemical contaminants. Forms chlorinated phenols Destroys phenols (without forming chlorinated phenols) specific destruction of Hydrogen Sulphides. Destruction of a wide range of chemical contaminants#
Neutralisation required before dumping to the foul drain Because no unwanted by-products are formed, and will have a lower residual after use, no neutralization normally required
Can not be used at temperatures above 40oC due to the release of chlorine gas Effective at higher temperatures-does not disassociate as rapidly as chlorine
Increased disinfection time and more service work required to combat high bug counts Cost savings in labour and use efficiency outweighs the additional chemical costs

*Includes aerobic, non-aerobic, gram positive & gram negative bacteria, spores, viruses, fungi, cysts and protozoa
# Includes iron, manganese and other metallics, phenols, trichlorophenols, Hydrogen Sulphides and Sulphides. Refer to Scotmas`s ClO2 reactivity booklet for further information and for specific reactivity rates for particular contaminants

Chlorine dioxide is generally accepted to be, more powerful, easier to use, and more environmentally friendly than equivalent chlorine treatments. It is a more expensive treatment, but its superior environmental performance meants that it is rapdly replacing chlorine in a number of applications.

Chlorine and chlorine dioxide are both oxidising agents (electron receivers). However, chlorine has the capacity to take in two electrons, whereas chlorine dioxide can absorb five. This means that, mole for mole, ClO2 is 2.5 times more effective than chlorine.

If equal, if not greater importance is the fact that chlorine dioxide will not react with many organic compounds, and as a result ClO2 does not produce environmentally dangerous chlorinated organics. For example, aromatic compounds have carbon atoms arranged in rings and they may have other atoms, such as chlorine, attached to these rings, to form a chlorinated aromatic - a highly toxic compound that persists in the environment long after it is produced.  

Chlorine dioxide's behaviour as an oxidising agent is quite dissimilar. Instead of combining with the aromatic rings, chlorine dioxide breaks these rings apart. In addition, as the use of chlorine dioxide increases, the generation of chlorinated organics falls dramatically.

Chlorine Dioxide Characters

Chlorine Dioxide is a small, volatile and very strong molecule consisting of 1 Chlorine atom and 2 oxygen atoms. Abbreviated to ClO2, chlorine dioxide exists as a free radical in dilute solutions

It has a molecular weight of 67.45.
It is a gas at normal temperatures and pressures.
It has a melting point of -59oC.
It has a boiling point of  11oC.
It is yellowish/green and has an odour similar to that of Chlorine.
It is denser than air and is water soluble at standard temperatures and pressures up to 2500ppm.
It is explosive in air at concentrations > 10%
It is prohibited from road and sea transport in its "free" form, and is normally generated at the point of application using 2 precursor chemicals.
It will decompose in the presence of UV, high temperatures, and high alkalinity(>pH12)
Notice: The information from Shareclean and Scotmas.