aqua regia

Aqua regia — 'royal water' or 'kingly water' in Medieval Latin — is a mixture of concentrated nitric acid and hydrochloric acid, typically in a ratio of one part nitric to three parts hydrochloric. The name was bestowed because the mixture dissolves gold, which was called the king of metals both for its color and its extraordinary chemical stability. The earliest known description of aqua regia is found in writings attributed to the Arab alchemist Jabir ibn Hayyan, though some historians attribute it to his near-contemporary school. European alchemists encountered it through the Latin translations of Arabic works and were immediately transfixed: here was a liquid that could do what nothing else could — dissolve the noblest, most resistant metal in the alchemist's hierarchy.

The chemistry of aqua regia was not understood until the modern era. When nitric and hydrochloric acids are mixed, they react to form nitrosyl chloride and chlorine gas, both of which are powerful oxidizing and complexing agents. Gold, which resists attack by most single acids because it does not easily form ions, is dissolved because aqua regia's components work cooperatively: the nitric acid oxidizes the gold atoms while the chloride ions from the hydrochloric acid form stable gold chloride complexes that dissolve readily. Platinum is similarly dissolved. The medieval alchemists observed the result — the glorious purple-red 'tincture of gold' or purple of Cassius that formed when aqua regia dissolved gold — without understanding the mechanism, but they were right that they had made something uniquely powerful.

The most famous use of aqua regia outside the laboratory occurred during the Second World War. In 1940, when German forces occupied Denmark and the Nobel Prizes of two refugee physicists — Max von Laue and James Franck — were being held at the Niels Bohr Institute in Copenhagen, the Hungarian chemist George de Hevesy dissolved the gold medals in aqua regia rather than allow them to be confiscated by the Nazis (who had forbidden Germans from accepting the prize). He placed the orange solution on a shelf in the laboratory. After the war, the solution was retrieved, the gold precipitated out, and the medals were recast. The Nobel Foundation presented them to their owners. Aqua regia had preserved the gold by dissolving it.

The phrase aqua regia entered common European scientific vocabulary in the Middle Ages and remained a standard term as chemistry developed. When Lavoisier and others reorganized chemical nomenclature in the late eighteenth century, they named the components of aqua regia systematically — nitric acid and hydrochloric acid — but the mixture itself retained its medieval name, which had too much history to be easily replaced. Today aqua regia is used in gold refining, in the preparation of platinum-group catalysts, in etching, and in analytical chemistry for dissolving noble metals. It remains royal in its function: the only common reagent capable of dissolving gold.