c. 1280

Aqua Regia

Smithson Tennant (1761–1815), George Charles de Hevesy (1885−1966), Max von Laue (1879−1960), James Franck (1882−1964)

There aren’t all that many mixtures that are still referred to by the same term used in the Middle Ages, but aqua regia (Latin for “royal water”) is a memorable liquid. The corrosive mixture—one of the few reagents that can dissolve gold—was first mentioned by an anonymous European alchemist known as Pseudo-Geber (and identified by some scholars as the Franciscan Paul of Taranto) in his late-thirteenth-century discussions on transmutation of the elements and related subjects.

You can’t order aqua regia from a chemical supply house; it has to be prepared fresh because it decomposes on standing. The classic recipe is one part concentrated nitric acid to three parts concentrated hydrochloric acid. The two acids react to form a vicious brew whose least toxic component is probably plenty of dissolved chlorine gas, which will be noticeable immediately if you are so unwise as to prepare this solution outside of a good fume hood. Gold is attacked fairly readily, and platinum more slowly, but some related metals aren’t touched at all. In fact, the elements iridium and osmium—which occur in the same ores as platinum—were first discovered by the British chemist Smithson Tennant in 1803 after he found dark residues in the bottom of a flask after the aqua regia had eaten away the platinum.

Aqua regia is still used in some high-level gold refining processes, but its most famous use came early in World War II. Germany had invaded Denmark in 1940, and at the Niels Bohr Institute in Copenhagen, the Hungarian radiochemist George Charles de Hevesy dissolved the gold Nobel Prize medals of his colleagues, the German-Jewish physicists Max von Laue and James Franck, to prevent their confiscation by the Nazis. He left the acid solution in a storeroom and returned after the war to find it undisturbed. He then precipitated the gold back out through a reduction reaction and sent the powder to Stockholm, where it was recast into medals and presented again to the Nobel laureates.

A sample of gold dissolving in aqua regia—a sight that few chemists have witnessed in person.