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Chlorine dioxide

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Chlorine dioxide
Molecular formula ClO2
Molar mass 67.45 g/mol
CAS number [10049-04-4]
EINECS number 233-162-8
Density 3.0 kg/m3 (gas)
Solubility (water) Hydrolysis
Melting point uncertain
Boiling point 10°C
Thermodynamic data
Standard enthalpy
of formation
Δfsolid
+104.60 kJ/mol
Standard molar entropy
solid
257.22 J.K–1.mol–1
Heat capacity Cp 24.12 J.K–1.mol–1
Safety data
EU classification Oxidant (O)
Very toxic (T+)
Dangerous for the
envrionment (N)
R-phrases R6, R8, R24,
R36, R50
S-phrases S1/2, S23, S26,
S28, S36/37/39,
S38, S45, S61
Disclaimer and references

Chlorine dioxide is a reddish-yellow gas, which is one of several known oxides of chlorine. Chlorine dioxide is relatively stable in the gas and liquid states, but can be explode easily. Practically, it is never handled in its pure form.

Uses

Chlorine dioxide is used in the disinfection of water and bleaching of flour and wood pulp. It can also be used for air disinfection, and was the principal agent used in the decontamination of buildings in the United States after the 2001 anthrax attacks. It is effective against viruses, bacteria and protozoa – including the cysts of Giardia and the oocysts of Cryptosporidium. This distinguishes it from chlorine, which has been used for more than a century as the standard water disinfectant. Chlorine dioxide was introduced as a drinking water disinfectant on a large scale when in 1956 Brussels, Belgium, changed from chlorine to chlorine dioxide.

Preparation

Chlorine dioxide can be produced by reducing sodium chlorate in a strong acid solution with a suitable reducing agent (for example, hydrogen peroxide, sulfur dioxide, or hydrochloric acid):

2ClO3- + 2Cl- + 4H+   →   2ClO2 + Cl2 + 2H2O

However, this reaction is of little practical value for disinfection applications. As a disinfectant, chlorine dioxide is almost exclusively produced by either the chlorite - chlorine method:

2 NaClO2 + Cl2   →   2 NaCl + 2 ClO2

or the chlorite - hydrochloric acid method:

5 NaClO2 + 4 HCl   →   5 NaCl + 4 ClO2 + 2 H2O

The latter has gained the reputation of generating an almost chlorine-free product in high yields (nominally >95% with modern generators).

References

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