Chlorine dioxide is an active oxidising biocide, that is applied more an more due to the fact that is has less damaging effects to the environment and human health than chlorine. It does not form hydrochlorous acids in water; it exists as dissolved chlorine dioxide, a compound that is a more reactive biocide at higher pH ranges.
Chlorine dioxide is an explosive gas, and therefore it has to be produced or generated on site, by means of the following reactions:
Cl2 + 2 NaClO2 -> 2 NaCl + 2 ClO2
2 HCl + 3 NaOCl + NaClO2 -> 2 ClO2 + 4 NaCl + H2O.