Perchloric acid

Production

Aqueous perchloric acid is produced industrially by two routes. The traditional method exploits the high aqueous solubility of sodium perchlorate (209 g/100 ml of water at room temperature). Treatment of such solutions with hydrochloric acid gives perchloric acid, precipitating solid sodium chloride:

NaClO₄ + HCl → NaCl + HClO₄

The concentrated acid can be purified by distillation. The alternative route, which is more direct and avoids salts, entails anodic oxidation of aqueous chlorine at a platinum electrode.

In the laboratory for small-scale syntheses, it can be distilled from a solution of potassium perchlorate in sulfuric acid. It can also be synthesized by the treatment of barium perchlorate with sulfuric acid precipitating barium sulfate, leaving perchloric acid. It can also be made by mixing nitric acid with ammonium perchlorate and boiling while adding hydrochloric acid. The reaction gives nitrous oxide and perchloric acid due to a concurrent reaction involving the ammonium ion, and can be concentrated and purified significantly by boiling off the remaining nitric and hydrochloric acids.

Anhydrous perchloric acid is synthesized by the vacuum distillation of a mixture of azeotropic aqueous perchloric acid and oleum (fuming sulfuric acid).

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. An example is hydronium perchlorate. 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:

2 HClO₄ + P₄O₁₀ → Cl₂O₇ + H₂P₄O₁₁

Uses

Perchloric acid is mainly produced as a precursor to ammonium perchlorate, which is used in rocket propellant. The growth in rocketry has led to increased production of perchloric acid. Several million kilograms are produced annually. 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. Additionally it is a useful component in etching of chromium.

As an acid

Perchloric acid, a superacid, is one of the strongest Brønsted–Lowry acids. That its pK_a is lower than −9 is evidenced by the fact that its monohydrate contains discrete hydronium ions and can be isolated as a stable, crystalline solid, formulated as [H₃O⁺][ClO^–
₄]. The most recent estimate of its aqueous pK_a is −15.2±2.0. It provides strong acidity with minimal interference because perchlorate is weakly nucleophilic (explaining the high acidity of HClO₄). Other acids of noncoordinating anions, such as fluoroboric acid and hexafluorophosphoric acid are susceptible to hydrolysis, whereas perchloric acid is not. Despite hazards associated with the explosiveness of its salts, the acid is often preferred in certain syntheses. For similar reasons, it is a useful eluent in ion-exchange chromatography. It is also used in electropolishing or the etching of aluminium, molybdenum, and other metals.

In geochemistry, perchloric acid aids in the digestion of silicate mineral samples for analysis, and also for complete digestion of organic matter.

Safety

Given its strong oxidizing properties, perchloric acid is subject to extensive regulations because it can react violently with metals and flammable substances such as wood, plastics, and oils. Work conducted with perchloric acid must be conducted in fume hoods with a wash-down capability to prevent accumulation of oxidisers in the ductwork.

On February 20, 1947 in Los Angeles, California, 17 people were killed and 150 injured in the O'Connor Plating Works disaster. A bath, consisting of over 1000 litres of 75% perchloric acid and 25% acetic anhydride by volume which was being used to electro-polish aluminium furniture, exploded. Organic compounds were added to the overheating bath when an iron rack was replaced with one coated with cellulose acetobutyrate (Tenit-2 plastic). A few minutes later the bath exploded. The O'Connor Electro-Plating plant, 25 other buildings, and 40 automobiles were destroyed, and 250 nearby homes were damaged.