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The anhydride of perchloric acid, Cl2O7, was discovered by Michael and Conn in 1900. 10 gm. of phosphorus pentoxide are placed in a small stoppered retort connected with a phosphorus pentoxide drying-tube and a receiver cooled in ice and salt. Pure perchloric acid is added, in quantities of 10 drops at a time, and allowed to trickle down the sides of the retort on to the P2O5: an interval of ten minutes is allowed to elapse after each addition, and the retort is kept at a temperature of -10° in a freezing mixture. After allowing to stand twenty-four hours in the freezing mixture, the retort is warmed to 85°, and a colourless oily liquid distils over, boiling at 82°. This is perchloric anhydride: 2HCIO4 + P2O5 = 2HPO3 + Cl2O7. Violent explosions may occur in its preparation, although Cl2O7 is more stable than Cl2O or ClO2, and may be poured on paper, wood, sulphur, or phosphorus without explosion. It explodes when heated or struck, and decomposes on standing for a few days. It sinks in water, and slowly forms HClO4: Cl2O7 + H2O = 2HClO4.
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