Perrhenic acid is the chemical compound with the formula Re2O7(OH2)2. It is obtained by evaporating aqueous solutions of Re2O7. Conventionally, perrhenic acid is considered to have the formula HReO4, and a species of this formula forms when rhenium(VII) oxide sublimes in the presence of water or steam. When a solution of Re2O7 is kept for a period of months, it breaks down and crystals of HReO4·H2O are formed, which contain tetrahedral ReO−4 For most purposes, perrhenic acid and rhenium(VII) oxide are used interchangeably. Rhenium can be dissolved in nitric or concentrated sulfuric acid to produce perrhenic acid.

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