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Perchloric acid

physical science Maturity 11-13 Next article →

Perchloric acid is a powerful mineral acid with the chemical formula HClO4. It belongs to a group of substances called oxoacids of chlorine. This colorless compound is a superacid, which means it is exceptionally strong. In fact, it is stronger than sulfuric acid, nitric acid, and hydrochloric acid. Because of its extreme strength, it is used in many high-tech industries. It is also a potent oxidizer, especially when it is heated.

Anhydrous perchloric acid
Anhydrous perchloric acid

The acid works through its unique chemical structure. It is a Brønsted–Lowry acid, which is a way of describing how substances donate protons. Perchloric acid has a very low pKa value, which is a measurement of acid strength. Its pKa is lower than −9. This high acidity is due to the perchlorate anion. This anion is weakly nucleophilic, meaning it does not interfere much with other reactions. This allows the acid to provide strong acidity with very little interference. This makes it much more stable during certain reactions than other acids.

There are several ways to produce perchloric acid on a large scale. One common industrial method uses sodium perchlorate. This salt is highly soluble in water, with 209 grams dissolving in every 100 ml at room temperature. When this solution is treated with hydrochloric acid, perchloric acid is created. This reaction also produces solid sodium chloride as a byproduct. Another direct method involves anodic oxidation. In this process, aqueous chlorine is oxidized at a platinum electrode. This method is useful because it avoids the use of salts.

Scientists use different methods to make small amounts in a laboratory. One way is to distill the acid from a solution of potassium perchlorate in sulfuric acid. Another method involves treating barium perchlorate with sulfuric acid. This reaction leaves perchloric acid and creates a precipitate of barium sulfate. A third laboratory method involves mixing nitric acid with ammonium perchlorate. This mixture is boiled while adding hydrochloric acid. This specific reaction produces nitrous oxide and perchloric acid. The acid can then be concentrated by boiling off the remaining liquids.

The history of this acid involves several important discoveries. In the mid-1810s, an Austrian chemist first synthesized it. He produced it alongside potassium perchlorate and called it "oxygenated chloric acid." Later, a French pharmacist named Georges-Simon Serullas gave it its modern name. He also discovered its solid monohydrate form. He mistakenly believed this solid was an anhydride. Another scientist, Berzelius, produced a dilute version of the acid. He did this through the electrolysis of chloric acid. By the late 1800s, workers in Germany and Sweden began commercializing the electrolysis method.

Perchloric acid is essential for modern technology and science. It is mainly used as a precursor to ammonium perchlorate. This substance is a vital component in rocket propellant. Because of the growth in rocketry, several million kilograms of perchloric acid are produced every year. The acid is also used to etch liquid crystal displays and critical electronics. It is helpful for etching metals like aluminum and molybdenum. In the field of geochemistry, it helps scientists digest silicate mineral samples. It is also used for the complete digestion of organic matter.

However, the acid can be very dangerous if handled incorrectly. At room temperature, aqueous solutions up to 70% by weight are generally safe. However, it becomes a powerful oxidizer when it is hot. It can react violently with metals and flammable materials like wood, plastics, or oils. In 1947, a major disaster occurred at the O'Connor Plating Works in Los Angeles. A large bath containing over 1000 liters of 75% perchloric acid and 25% acetic anhydride exploded. This explosion killed 17 people and injured 150 others. It also destroyed many buildings and homes in the area.

Anhydrous perchloric acid
Anhydrous perchloric acid
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