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EC number: 233-162-8
CAS number: 10049-04-4
Chlorine dioxide gas is explosive in concentrations in excess of 10% v/v
at atmospheric pressure and will easily be detonated by sunlight or heat.
The international CICAD review and the OECD SIDS report provide a lower
explosion limit (LEL) of 10%v/v for the gas Chlorine dioxide, from the
The ICSC provides information on explosive limits, with a LEL >10%vol in
air for the substance Chlorine dioxide.
Concentrations above 10% can ignite at 130 deg C. Oxidizable organic
dusts can lower decomposition temperature.
The HSDB database provides information on the fire potential of Chlorine
dioxide: the substance can ignite from 130°C if concentration is higher
In Torregrossa's paper (9), experiments were done with ClO2/air
mixtures: detonation was never observed, even up to 35% v/v in air (310
mm Hg), even though Haller & Northgraves (1) did observe it at partial
pressures above 300 mm Hg.
The unknown and presumed dangerous area is with pure ClO2 at
pressures above about 130 mm Hg, and diluted in air at atmospheric
pressure or higher with partial pressure above 300 mm Hg.
In practice, values above 100 mm Hg are carefully avoided.
As an introduction to safety management
in chlorine dioxide plants, this publication summarises observations
from other references about explosive concentration of the substance,
whether pure or diluted in air, and practical maximum recommended limit.
Depending upon its concentration in the vapour phase,
chlorine dioxide can undergo a spontaneous autodecomposition reaction
depending on partial pressure and temperature. At concentrations above
about 4% in the air, a spark can cause a decomposition reaction with
evolution of a small amount of energy [Rosenblatt 1976]. At ambient
temperature, e.g. < 30°C, and concentrations above about 10%, the gas
may undergo a spontaneous decomposition reaction (with a long induction
period) commonly referred to as a "puff" [Gall 1976; Gray and Ip 1972;
Haller and Northgraves 1955; Ip and gray 1972; McHale and von Elbe 1968;
Noack and Doerr 1979; Paillard et al. 1986; Schumacher and Stieger
1930], because its flame propagation if ca 1 m/s, when compared to the
flame propagation wave of an explosion, i.e., ca 300 m/s, is relatively
As the concentration of chlorine dioxide in the vapour
state increases, the violence of this reaction increases. At a
concentration about 17% in the vapour state, the explosion becomes quite
energetic, to the point where rapid, unscheduled disassembly of
equipment can occur. However, a concentration of about 20% chlorine
dioxide has been produced in the vapour space routinely with no problem
[Haller and Northgraves 1955].
Gaseous ClO2 can decompose with a "puff" at
partial pressures at or above 80 mm Hg.
Aqueous solutions of ClO2
Henry's law states that a gas dissolved in solution will
distribute itself into the vapour phase in proportion to its
concentration in solution. Therefore, the safe solubility of ClO2
depends upon temperature and pressure, and thus, by controlling the
concentration of chlorine dioxide in solution, the partial pressure of
chlorine dioxide can be controlled. That is, if the aqueous
concentration of chlorine dioxide is kept below certain well established
limits, chlorine dioxide cannot achieve that concentration in the vapour
phase which can "puff". At 25°C and 30 mm partial pressure, it is safely
soluble to the extent of about 3 g/L. What this means is that a solution
of about 3 g/L concentration can be placed in a sealed container with an
air space, and ClO2 will achieve a concentration in the air
space according to Henry's law (54 atm at 20°C) that will not, at 25°C,
undergo a "puff" (explosive decomposition). Relatively high
concentrations of ClO2 can be stored safely for long periods,
as long as there is no air space for the ClO2 gas to form.
Below about 30°C, if the partial pressure of chlorine
dioxide is kept below about 50 mm Hg, and if organics or other
impurities are not present, an explosive concentration of chlorine
dioxide in the vapour space cannot be achieved [Ingols and Ridenour
1948a; McHale and von Elbe 1968].
Above about 45°C (the lower temperature limit), ClO2
vapour appears to always decompose, regardless of its partial pressure.
book, dedicated to chlorine dioxide, provides summaries of several
studies on explosivity of the substance. For the gas, a lower explosion
limit of 10%v/v is suggested, but can be lowered to 4% in case of
ignition by sparks. For off-gassing of aqueous solution, a conservative
safe upper limit for partial pressure of about 50 mm Hg (6666 Pa) is
generally accepted, which can be controlled from concentration and
temperature. Above 45°C, ClO2 vapor appears to always
Explosive at temperatures >-40 °C  and concentrations in excess of
10% v/v at 1 atm .
The peer reviewed ATSDR profile for the substance Chlorine dioxide
provides information relating to its explosive limits.
The experiments and their results are summarized in the
following tables, showing the dependence of tinit, and tPmax
and f on the partial pressure of ClO2 and water.
Water inj (µL)
Pressure (mbar) registered
Pressure (mbar) corrected
Explosion times (ms)
Explosion pressures (mbar)
Decomposition of the ClO2could not be avoided.
Even with extremely low partial pressures (10 mbar and less), pressure
increases of 50% (f ca 1.5) were registered. The pressure factor f rises
with increasing partial pressure of ClO2. Time of initiation,
tinit, and time for pressure increase tPmax, are
strongly dependent on the ClO2partial pressure. It can also
be stated that water is a very good inhibitor for the decomposition, a
fact which is also mentioned in the literature. It should be pointed out
that experiments N°12-17 with only 9 mbar, all of which emerging from
NaClO2water solution, also were performed at lower
temperatures and lower total pressures than the others.
As decomposition are taking place at partial pressures of
ClO2 which are thought to be completely safe (based on literature), the
equipment was controlled, the pressure transducers have been calibrated,
no decomposition is registered on the oscilloscope with pure air in the
explosion vessel. The conclusion has been that the ignition energy is
too high. It is also well-known that an electric arc is producing a
rather high amount of UV-light, which in the literature (McHale 1968,
Crawford 1968) has been mentioned as a very potent decomposition source.
According to the characteristics of the AC arc (see above), it is
difficult to both calculate and regulate (decrease) the energy
effectively. With an ignition time of 0.35 second, the nergy can be
calculated to 2000 V x 20 mA x 0.35 sec = 14 J.
Explosion trials with ClO2, conducted according to a
non-standardised method, indicate that this gas is very sensitive to an
(AC) electric arc as ignition source. Decomposition was recorded with
only 10 mbar of ClO2 at a total of 170-180 mbar, the rest
being mostly water vapour. This is far below what is normally said to be
the lower explosion limit (ca 80 mbar). This was supposed to be due to
the high energy and/or UV release of the ignition source.
The initiation period before explosion and the time to reach maximum
pressure are longer, the lower the ClO2 pressure is. The
pressure rise factor (Pmax/Pstart) decreases linearly with the ClO2 pressure.
Water was confirmed to be a good inhibitor.
Concentration of ClO2 vapour at about 11% in air may give
mild explosions or "puffs", and concentrations of over 4% in air may be
set off by an electric spark to sustain a decomposition wave.
This publication cites, as a secondary source, data on the explosive
portential for the gas chlorine dioxide: a general limit of 11% is set,
which can be lowered to 4% in case of ignition by electric sparks.
(...) it was stated that decomposition by sparking begins to become
hazardous at concentrations of 7–8% in air , and that at 10%
concentration in air (0.1 bar partial pressure) explosion may occur from
any source of initiation energy, such as sunlight, heat or electrostatic
The Bretherick's handbook compiles information on explosive potential of
Chlorine dioxide, including data on LEL = 10% in air for any source of
initiation, and even 7-8% if sparks.
Chlorine dioxide gas: LEL = 4 (sparks) - 10 (any source of ignition) %v/v in air.
An experimental, non-standard, study is available, as well as numerous literature data regarding explosive properties of chlorine dioxide gas and safe management. A Lower Explosion Limit of 10 %v/v in air is generally recognised, except in case of ignition by sparks, where lower values, between 4 and 8%, are set. For safety reasons, the lowest concentration will be retained in the present dossier.
Flammability if contact with water is of no concern, based on the weight of evidence that the substance is generated as aqueous solution.
Solution of chlorine dioxine generated in-situ at less than 2% in water (as registered): This endpoint does not apply to the substance as registered (aqueous solution generated in-situ), as liquid.
Other information: The gas has a Harmonised Classification as Oxidising, therefore it does not need to be classified in any other hazard class.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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