Hydrochloric acid

Hydrochloric acid—also called muriatic acid or spirits of salt—is hydrogen chloride gas dissolved in water. The liquid has no color but gives off a sharp, recognizable smell. Chemists classify it as a strong acid. It naturally occurs in the gastric acid found in the digestive systems of most animals, including humans. The substance also serves as a key chemical in laboratories and in industry.

The name “spirits of salt” and the Latin *acidum salis* (salt acid) come from early European alchemists, who first made it from rock salt following methods developed by Johann Rudolph Glauber. These older names persist in some languages, such as German *Salzsäure*. Gaseous hydrogen chloride was once called marine acid air. “Muriatic acid” also traces back to the same root—*muriatic* means “related to brine or salt,” and *muriate* means hydrochloride—and this term is still used occasionally. The modern name “hydrochloric acid” was introduced in 1814 by French chemist Joseph Louis Gay-Lussac.

In the early 900s, Persian physician and alchemist Abu Bakr al-Razi (known in Latin as Rhazes) distilled ammonium chloride with hydrated metal sulfates, producing hydrogen chloride gas. This process may have been an early, crude way to make hydrochloric acid. One of his recipes called for mixing equal parts of several salts with crystallized sal ammoniac, then distilling the mixture to yield a “strong water” that could instantly split stone. However, al-Razi mostly focused on color changes in the residue and ignored the gaseous products. Historian Robert P. Multhauf notes that hydrogen chloride was generated many times without anyone realizing that dissolving it in water would produce hydrochloric acid.

Drawing on al-Razi’s work, an 11th- or 12th-century Arabic text translated into Latin by Gerard of Cremona described heating metals with various salts. When mercury was used, the process formed mercury(II) chloride (corrosive sublimate). In that reaction, hydrochloric acid began to form but immediately reacted with the mercury. Thirteenth-century Latin alchemists, who relied heavily on that text, became fascinated by corrosive sublimate’s chlorinating properties. They soon found that by removing metals from the heating of vitriols, alums, and salts, strong mineral acids could be distilled directly.

A major outcome of discovering mineral acids was aqua regia—a 1:3 mixture of nitric acid and hydrochloric acid that dissolves gold. This mixture was first described around 1300 in a pseudo-Geber work, where it was made by adding ammonium chloride to nitric acid. The fact that aqua regia is defined as a mix of nitric and hydrochloric acids does not mean hydrochloric acid was isolated before or alongside it; that isolation came about 300 years later. Producing hydrochloric acid as a separate substance required more efficient cooling apparatus, which was developed only in later centuries.

In Western chemistry, hydrochloric acid was the last of the three well-known mineral acids to have its production method appear in print. Recipes began surfacing in the late 1500s. The earliest are found in Giovanni Battista Della Porta’s *Magiae naturalis* and in works by Andreas Libavius, Jean Beguin, and Oswald Croll. Historians who have examined these include Hermann Franz Moritz Kopp, Edmund Oscar von Lippmann, Herbert and Lou Henry Hoover, Robert Jacobus Forbes, Mary Elvira Weeks, F. Sherwood Taylor, and J. R. Partington. Italian chemist Ladislao Reti summarized their findings: the first clear preparation of hydrochloric acid appears in Della Porta (1589 and 1608), Libavius (1597), pseudo-Basil (1604), van Helmont (1646), and Glauber (1648). Less convincing earlier references exist in Rosetti (1540) and Agricola (1558). For the first practical method using vitriol and common salt, pseudo-Basil precedes Glauber, but Glauber deserves credit for showing how to produce the acid in a way later adopted by industry on a large scale.

Knowledge of mineral acids like hydrochloric acid proved crucial to 17th-century chemists such as Daniel Sennert and Robert Boyle. They used the acids’ ability to quickly dissolve metals to demonstrate that bodies are composed of multiple parts.

During the Industrial Revolution, further developments followed.