Dichlorine heptoxide is the chemical compound with the formula Cl2O7. This chlorine oxide is the anhydride of perchloric acid. It is produced by the careful distillation of perchloric acid in the presence of the dehydrating agent phosphorus pentoxide:
- 2 HClO4 + P4O10 → Cl2O7 + H2P4O11
The chlorine(VII) oxide can be distilled off from the mixture.
It may also be formed by illumination on mixtures of chlorine and ozone. It slowly hydrolyzes back to perchloric acid, which is also hazardous when anhydrous.
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