Perchloric acid

Perchloric acid, a mineral acid with the formula HClO₄, is an oxoacid of chlorine. Typically encountered as a colorless aqueous solution, it is a stronger acid than sulfuric, nitric, or hydrochloric acid. While it acts as a powerful oxidizer when heated, aqueous solutions at room temperature containing up to about 70% acid by weight are generally safe, exhibiting only strong acid behavior without oxidizing properties. The acid is primarily used to produce perchlorate salts, notably ammonium perchlorate, a key ingredient in rocket fuel. However, it is dangerously corrosive and can easily form potentially explosive mixtures.

The compound was first synthesized in the mid-1810s by Austrian chemist Friedrich von Stadion, who called it "oxygenated chloric acid" and also produced potassium perchlorate. French pharmacist Georges-Simon Serullas later introduced the modern name and discovered its solid monohydrate, which he mistakenly believed to be the anhydride. Jöns Jacob Berzelius prepared dilute perchloric acid by electrolyzing chloric acid. In the late 1800s, German and Swedish workers commercialized the electrolytic production method.

Industrially, aqueous perchloric acid is made via two routes. The traditional method uses the high solubility of sodium perchlorate in water (209 g per 100 ml at room temperature); treating this solution with hydrochloric acid precipitates solid sodium chloride and yields perchloric acid: NaClO₄ + HCl → NaCl + HClO₄. The concentrated acid can then be purified by distillation. A more direct, salt-free alternative involves anodic oxidation of aqueous chlorine at a platinum electrode. In the lab, small-scale syntheses include distilling perchloric acid from a mixture of potassium perchlorate and sulfuric acid, or treating barium perchlorate with sulfuric acid to precipitate barium sulfate. Another method mixes nitric acid with ammonium perchlorate, then boils the mixture while adding hydrochloric acid; this produces nitrous oxide and perchloric acid, and the remaining nitric and hydrochloric acids can be boiled off to concentrate and purify the product. Anhydrous perchloric acid is made by vacuum-distilling a mixture of azeotropic aqueous perchloric acid and oleum (fuming sulfuric acid).

Anhydrous perchloric acid is an unstable, oily liquid at room temperature. It forms at least five hydrates, several of which have been crystallographically characterized; these solids contain the perchlorate anion linked via hydrogen bonds to H₂O and H₃O⁺ centers, as in hydronium perchlorate. Perchloric acid forms an azeotrope with water at about 72.5% acid, which is commercially available and indefinitely stable. Such solutions are hygroscopic, so if left open to air, concentrated perchloric acid dilutes itself by absorbing moisture. Dehydration with phosphorus pentoxide yields the anhydride dichlorine heptoxide: 2 HClO₄ + P₄O₁₀ → Cl₂O₇ + H₂P₄O₁₁.

Perchloric acid is mainly produced as a precursor to ammonium perchlorate for rocket propellant, and the growth of rocketry has driven increased production—several million kilograms annually. It is also widely used for etching liquid-crystal displays and critical electronics, ore extraction, and in analytical chemistry, as well as for etching chromium. As a superacid, it is one of the strongest Brønsted–Lowry acids; its pKa is lower than −9, evidenced by its monohydrate, which contains discrete hydronium ions and can be isolated as a stable crystalline solid, [H₃O⁺][ClO₄⁻]. The most recent estimate of its aqueous pKa is −15.2 ± 2.0. Perchloric acid provides strong acidity with minimal interference because the perchlorate ion is weakly nucleophilic, explaining its high acidity. Unlike other acids with noncoordinating anions, such as fluoroboric and hexafluorophosphoric acids, perchloric acid is not susceptible to hydrolysis. Despite the explosion hazards of its salts, the acid is often preferred in certain syntheses and is a useful eluent in ion-exchange chromatography. It is also employed in electropolishing or etching aluminum, molybdenum, and other metals. In geochemistry, it aids in digesting silicate mineral samples and completely breaking down organic matter.

Due to its strong oxidizing properties, perchloric acid is heavily regulated; it can react violently with metals and flammable substances like wood, plastics, and oils. Work must be done in fume hoods with wash-down capability to prevent oxidizer buildup in ductwork. On February 20, 1947, in Los Angeles, California, the O'Connor Plating Works disaster killed 17 people and injured 150. A bath containing over 1,000 liters of a mixture of 75% perchloric acid and 25% acetic anhydride, used for electropolishing aluminum furniture, exploded after organic compounds were added when an iron rack was replaced with one coated with cellulose acetobutyrate (Tenit-2 plastic). The explosion destroyed the plant, 25 other buildings, and 40 automobiles, and damaged 250 nearby homes.