What Is Chlorine Dioxide?

Chlorine dioxide (ClO2) is a synthetic, green-yellowish gas with a chlorine-like, irritating odor.  Chlorine dioxide is very different from elementary chlorine, both in its chemical structure as in its behavior. Chlorine dioxide is a small, volatile and very strong molecule. In diluted, watery solutions chlorine dioxide is a free radical. At high concentrations it reacts strongly with reducing agents. Chlorine dioxide is an unstable gas that dissociates into chlorine gas (Cl2), oxygen gas (O2) and heat. When chlorine dioxide is photo-oxidized by sunlight, it falls apart. The end-products of chlorine dioxide reactions are chloride (Cl), chlorite (ClO) and chlorate (ClO3).

At –59°C, solid chlorine dioxide becomes a reddish liquid. At 11°C chlorine dioxide turns into gas. Chlorine dioxide is 2,4 times denser than air. As a liquid chlorine dioxide has a bigger density than water.

Identifiers Properties
CAS Number 10049-04-4 Molecular formula ClO2
ChEBI CHEBI:29415 Molar mass 67.45 g mol−1
ChemSpider 23251 Appearance Yellow to reddish gas
EC Number 233-162-8 Odor Acrid
Gmelin Reference 1265 Density 2.757 g dm−3
MeSH Chlorine+dioxide Melting Point -59 °C, 214 K, -74 °F
PubChem 24870 Boiling Point 11 °C, 284 K, 52 °F
RTECS number FO3000000 Solublity In Water 8 g dm-3 (at 20 °C)
UNII 8061YMS4RM Vapor pressure 1 atm[2]
Acidity (pKa) (kH) 3.0(5)
Solubility soluble in alkaline and sulfuric acid solutions
Henry’s law constant (kH) 4.01 x 10−2 atm-cu m/mole


Residual Measurement Of ClO2

Determination of residual ClO2 in process or potable water is a tricky situation. As there are several interferences from other oxidants present in water in addition to ClO2 and its disinfection bi-products. The test method for measurement of ClO2 is a normally interfered with its bi-products chlorite & chlorate due to the difficulty in measuring a specific oxidants among other oxidants.

Following are the methods available and selectively used for analytical measurement of ClO2

Amaranth – Amaranth is a colorimetric indicator that is selectively decolorized by mg/L concentration of ClO2. The method does not have interference by chlorite,chlorate,monochloramine,free chlorine & ferric ion. And the minimum measurement that can be performed is up to 5mg/L. Hence the said is recommened only in the lab level and not in field.
Chlorophenol Red ( CPR)
Chlorophenol Red ( CPR). – The method is similar to Amarnath method where in CPR is decolorized by ClO2. The method faces a interference by free chlorine in water. With this method concentration upto 0.1 ppm can be determined. But again the method is operation dependent because of the precise pipetting requirement.

Chlorophenol Red ( CPR)

LGB ( Lissamine Green B) – Like amaranth and CPR,ClO2 selectively decolorizes LGB dye. With this method the minimum residue that can be measured is 2.5 mg/L. but again the method is only recommended at laboratory level and not in field.

DPD method

DPD method –

This is the most commonly used method for testing ClO2 in industrial methods.

DPD forms a pink color & the intensity of the color is proportional to the concentration of ClO2. The problem with the method is chlorine & chlorite ion also reacta with DPD reagents. The system works fairly well in clean water system. But in cases of cooling tower & waste tower there are several interferences from hydrocarbon, SRB etc…

We SVS AQUA TECHNOLOGIES recommend three methods for measuring the residue of ClO2


The photometry is a method of analysis which is often used. Whether it is a public bathhouse, food and beverage industry, laboratories, or industrial disinfection – are everywhere photometer is used for the determination of concentrations in solutions. We are providing the world’s first photometer for the entire visible color spectrum with only one light source and only one sensor. The unique sensor combined with the several –year’s experience in water analysis, made this innovation possible. In addition, the sensor measures with a hitherto unknown devices with photometer accuracy. The Bluetooth technology (select models) and a programmed specifically for the PC software makes it easy to manage measurement and customer data..


Reagent tablets offer the highest level of assurance that always the exact amount of reagent needed will be used. Most marketed tablet types, such as DPD for determining Chlorine in water, are regulated by strict international standards aimed at guaranteeing that all tablets with that designation (e.g. DPD 1) can also be used in all designated measurement devices made by any manufacturer. Our devices and reagents can therefore be combined with other manufacturer’s reagents, and this is also true the other way around. One of the questions discussed often is the difference between RAPID and Photometer tablets. Basically both detect the same content but are different in one important point: RAPID tablets contain a material which “bursts” the tablet, e.g. chemical pulp that expands upon contact with water. This “burst material” would, however, disrupt the light ray of the photometer (suspended particles) which is why RAPID tablets should never be used with electronic devices such as photometers.

Other Related Products

CDD 5000
A two component powder system. Generates 5000 ppm ( 5 gm/liter) chlorine dioxide. Solution stable for 15 days at least and formulated to avoid chlorite formation. Suitable for all process water applications. ClO2 generates is in aqueous solution form. Read More
CDL 7500
Two component Liquid System. Generates 7500 ppm (7.5 gm/liter) chlorine dioxide. Solution stable for 30 days at least and formulated to avoid chlorite formation and to suppress byproduct formation. ClO2 generates is in aqueous solution form. Read More
Tablets which generates ClO2 after adding in water. The solution is ready to use instantaneously. Recommended for use in poultry & emergency situation. Read More
Chlorone Read More
Gelchor Read More


Other  Related Information

History Of ClO2
About a decade after the early work, H. Davy gave the name of “euchlorine” to a mixture of gases arising from a reaction of various chemicals, including potassium chlorate and hydrochloric acid [Davy 1811].

Properties Of Cl02
One of the most important qualities of chlorine dioxide is its high water solubility, especially in cold water. Chlorine dioxide does not hydrolyze when it enters water; it remains a dissolved gas in solution.

Applications Of ClO2

Safeguard your live stock
Correct treatment to your livestock has direct impact on feed conversion, mortality, morbidity, consumption of antibiotics and health. And we at SVS Aqua provides water treatment solution to all the livestock industry segments. Read More
Hygienic and quality food processing
Keeping the product hygienic & free from microbial infection is the single most important requirement. And we at SVS Aqua provides various water treatment solutions to various industry like Poultry , Meat processing , Sea food processing, Fruits & vegetable washing, Dairy Products. Let it be biofilm, molds, pseudomonas or any other bacteria, virus or fungi we have a solution.
Water Treatments For Clean Water
Potable water is a prime need for health. SVS Aqua provides viable water treatment systems to ensure eco-friendly and reliable water treatment processes for Potable water treatment and Waste water treatment.
Water Treatments For Power Plants
Power plant is one of the areas where water treatment is essential to avoid pollution. We provide waste water management solutions for Cooling tower and DM plant. Read More
Potable Water For Pharma & Biotech Industries
Pharmaceutical or biotechnology industries have to assure use of potable water according to WHO standards. We provide reliable water purification systems for pharmaceutical industries. Read More
Reliable Water Treatments For RO & UF Membrane
To prevent Biofouling and to promote the death of the microorganisms and the oxidation of the organic matter, sound water treatment system is important. We provide solid water treatment systems for RO & UF membrane treatments. Read More
Hospitals & Research Centers
In 21th century, where the world is well aware of Green Earth, the environment friendly technologies have great importance in almost all industries. Read More
Go Green At Horticulture & Green House Systems
Horticulture and green house are eco-friendly systems. We provide green water treatment processes to increase the efficiency. Read More
ClO2 has been introduced to sterilize containers in the food industry. Bacteria, yeast, molds and spoilage microorganisms are the enemies of fine wine product. ClO2 is recommended for use in beverages treatment. ClO2 generated by SVS Aqua does not have free chlorine hence the benefits of ClO2 can be taken without disadvantages of ClO2 generated with chlorine contamination.Read More

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