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EC number: 233-162-8
CAS number: 10049-04-4
May explode on heating, on exposure to sunlight or if
subjected to shock or sparks.
The ICSC provides information relating to explosive potential of the
Chlorine dioxide, when heated, exposed to sunlight, or subjected to
shock or sparks.
Liquid ClO2 is extremely unstable and dangerously explosive.
The slightest shock will initiate detonation and decomposition to
chlorine and oxygen.
Gaseous ClO2 exhibits a very peculiar property, in that it
"auto-decomposes", following an induction period. These properties have
been studied extensively by many workers (ref.1,2,3,4,7,8). The
induction period is related to the surface/volume ratio of the
equipment, and the partial pressure of ClO2 and temperature.
The force of the ensuing explosion is determined by ClO2
concentration, the presence of oxidisables, the location of the
explosion, and the shape and size of the equipment containing it. (...)
As the decomposition products are oxygen and chlorine, the ClO2
decomposition can initiate a second more powerful explosion if reducing
agents such as methanol are present, or with standing flames, the
decomposition can initiate thermite fires in metal equipment. In
addition to these peculiarities, the mechanism of decomposition of
chlorine dioxide is via a branched chain mechanism, so that not only is
there no prior warning of an explosion, but the resulting burning
velocity and detonation forces can be extremely high. It is postulated
that ClO2 is the key intermediate in the burning of ammonium
perchlorate (ref.5,6), and thus contributes to its high velocity and
efficiency as a solid rocket fuel.
As an introduction to safety management in chlorine dioxide plants, this
publication summarises observations from other references about
explosive properties of the substance, in particular parameters that can
influence the induction period, and the gravity of the consequences.
Chlorine dioxide is considerably endothermic (deltaH°f
(g) +103.3 kJ/mol, 1.53 kJ/g) and of limited stability. It is a powerful
oxidant and explodes violently on the slightest provocation as gas or
liquid . It is initiated by contact with several materials, on
heating rapidly to 100°C or on sparking , or by impact as solid at
-100°C . A small sample exploded during vacuum distillation at below
-50°C , and 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
discharge . A kinetic study of the decomposition shows that it is
explosive above 45°C even in absence of light, and subject to long
induction periods due to formation of intermediate dichlorine trioxide.
UV irradiation greatly sensitises the dioxide to explosion . The
solid (a dimer) can be relatively safely handled below -40°C and the gas
at pressures below 50 mbar . A guide on fire and explosion hazards in
industrial use of chlorine dioxide is available , and preparative
precautions have been detailed . An improved and safer method for
continuous production of chlorine dioxide is claimed . A thorough
review has been written, detailing numerous incidents, of hazards
attending industrial preparation and use of chlorine dioxide (now much
used as a low chlorine bleach). Liquid ClO2 can separate from
aqueous solutions >60 g/l, it is exceedingly shock sensitive. A partial
pressure of 130 mbar in air is thought entirely safe, as are aqueous
solutions (but not necessarily the head space above them) . The safe
use of chlorine dioxide and sodium chlorite has been reviewed .
The Bretherick's handbook compiles information on explosive potential of
Chlorine dioxide, from twelve secondary references, showing evidence of
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 (Budavari et al., 1996).
is marketed and transported as a stabilized aqueous solution generally
less than 1% w/v (more concentrated forms are explosive).
The international CICAD review and the OECD SIDS report provide
information relating to explosive potential of the gas Chlorine dioxide,
cited from the Merck Index, when heated or exposed to sunlight. Dilute
aqueous solutions are stable (<1%w/v).
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 ClO2 could 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 ClO2 partial 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 NaClO2 water solution, also were
performed at lower temperatures and lower total pressures than the
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.
Aqueous ClO2 has no explosive properties.
The most commonly promoted characteristic of chlorine
dioxide is that it is an explosive gas.
Much work has been done to investigate its explosive
properties [Schumacher and Stieger 1930; Haller and Northgraves 1955;
McHale and von Elbe 1967; Crawford and Dewitt 1968; McHale and von Elbe
1968; Gray and Ip 1972; Ip and gray 1972; Torregrossa et al. 1976;
Paillard et al. 1986; Lopez et al. 1994]
When ClO2 is placed in a closed vessel, under
some circumstances the gas can decompose energetically. The energy of
the decomposition reaction increases with concentration of chlorine
dioxide in the gaseous phase. Depending on a number of factors, there is
an induction period where no temperature increase is noted, prior to
explosive decomposition. Several factors influence the induction period.
These include the surface/volume ratio of the container, the partial
pressure of ClO2, and the temperature. The force of the
ensuing decomposition is determined by the ClO2
concentration, the presence of oxidizable material, and the shape and
size of the equipment containing the gaseous ClO2. The
velocity of the decomposition wave is about 1 m/sec at 130 mm Hg
pressure and about 3°C; at 207 mm Hg, the velocity of the wave is about
2 m/sec [Haller and Northgraves 1955]. As partial pressure increases,
the violence of the reaction increases so that a detonation occurs at
300 mm Hg.
Uncontaminated gaseous ClO2 only explodes above a
temperature of 88°C; below that temperature, only an exotherm is
observed [Gilmont 1968]. Water has a strong inhibiting effect on the
decomposition of gaseous ClO2 up to 90°C [Crawford and Dewitt 1968].
Due to its sensitivity to temperature, pressure and
shock, it is impossible to store or ship as a pure liquid or a
Above melting temperature (-59°C), ClO2
forms a red, unstable liquid which is very explosive at temperatures
above -40°C. Liquid ClO2 will also separate from aqueous concentrated
solutions (for example above 60 g/L at normal temperatures). Pure liquid
ClO2 is sensitive to shock and light, and decomposes with
about 1/3 of the force of TNT [Sattelberger et al. 2002].
The Simpson's book, dedicated to chlorine dioxide,
summarizes several studies on its explosive properties.
Gaseous ClO2 can decompose explosively as a
function of temperature and pressure. The decomposition can be triggered
by a spark. The induction time can be substantially decreased by the
presence of impurities, which include gaseous organics, or oxidizable
materials, which include rubber, cork, or sulfur. At a constant
temperature, as the partial pressure of ClO2 increases, the
violence of the reaction increases.
Aqueous ClO2 has no explosive properties.
Explodes when heated or by reaction with organics. 
In concentrations in excess of 10%, 1 atm, easily detonated by sunlight,
heat, contact with mercury or carbon monoxide. 
Explosive decomposition at 100 deg C. Explosive hazard by heating,
exposing to sunlight, contacting mercury or carbon monoxide. 
The HSDB database provides information, from several sources, relating
to explosive potential of the substance Chlorine dioxide, when heated or
exposed to sunlight.
Solution of chlorine dioxine generated in-situ at less than 2% in water (as registered): not considered to be explosive, but potential hazard if off-gassing.
The properties of gaseous chlorine dioxide are discussed because, although the substance as registered is in-situ generated aqueous solution, under certain conditions off-gassing may occur. Thus, the properties of gaseous chlorine dioxide, as well as the properties of the aqueous solution are important.
An experimental, non-standard, study is available, as well as numerous literature data regarding explosive properties of chlorine dioxide gas and safe management. The gas is well-known to be sensitive to various sources of ignition (heat, light, shock, and in particular sparks), and is characterised by an induction period.
Water was shown to be a good inhibitor, and diluted aqueous solutions are considered to be stable. Partial pressure of ClO2 over the solution, and therefore potential hazard, is a function of concentration and temperature (please refer to points 4.6 and 4.13).
The gas has a Harmonised Classification as Oxidising, therefore it does
not need to be classified in any other hazard class.
For chlorine dioxide in aqueous solution, no physical hazard is
established in the harmonised entry. However, based on the potential
hazard if off-gassing, the EUH018 phrase will be proposed.
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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