Perchloric acid

Perchloric acid is a mineral acid with the formula HClO4. Usually found as an aqueous solution, this colorless compound is a stronger acid than [sulfuric] and [nitric acid]. It is a powerful oxidizer when hot, but its aqueous solutions up to approximately 70% by weight at room temperature are generally safe, only showing strong acid features and no oxidizing properties. Perchloric acid is useful for preparing [perchlorate salts], especially [ammonium perchlorate], an important rocket fuel component. Perchloric acid is dangerously corrosive and readily forms potentially explosive mixtures.''

Properties
Anhydrous perchloric acid is an unstable oily liquid at room temperature. It forms at least five hydrates, several of which have been characterized crystallographically. These solids consist of the perchlorate anion linked via hydrogen bonds to H2O and H3O+ centers[7] Perchloric acid forms an azeotrope with water, consisting of about 72.5% perchloric acid. This form of the acid is stable indefinitely and is commercially available. Such solutions are [hygroscopic]. Thus, if left open to the air, concentrated perchloric acid dilutes itself by absorbing water from the air.Dehydration of perchloric acid gives the anhydride dichlorine heptoxide:[8] 2 HClO4 + P4O10 → Cl2O7 + H2P4O11

Uses
Perchloric acid is used primarily used as a rocket fuel. "Perchloric acid is one of the most proven materials for etching of liquid crystal displays and critical electronics applications as well as ore extraction and has unique properties in analytical chemistry"

Safety
Wear appropriate Personal Protective Equipment (lab coat, safety glasses, and acid-resistant gloves) when handling perchloric acid. Do not handle perchloric acid on a wooden surface, and do not let it come into contact with oxidizable materials such as cloths, paper towels, or grease. Such materials can become highly flammable and may ignite spontaneously or even explode after absorbing perchloric acid liquid or vapor. Do not subject perchloric acid to strong dehydrating conditions. Dilute by adding perchloric acid to water, not by adding water to acid. If solutions containing perchloric acid are filtered through a paper filter, the filter (and precipitate) should be washed thoroughly with water to remove all perchlorate before being allowed to dry. Do NOT mix concentrated perchloric acid (>72%) with organic chemicals if temperatures could rise above ambient levels. Perchloric acid digestions and other uses at elevated temperatures require that the procedures be conducted in a specially designed fume hood with a water wash-down system. This system is required to prevent the buildup of explosive perchlorates in the ductwork. Do not heat perchloric acid in an oil bath. Use a sand bath, a heating mantle, or a hot plate.

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