Aqua regia (Latin for "royal water") is a highly corrosive, fuming yellow liquid, formed by a fresh mixture of concentrated nitric acid (otherwise known as aqua fortis) and concentrated hydrochloric acid, usually in the ratio of one to three. It is one of the few reagents able to dissolve gold and platinum. It was so named because it can dissolve the so-called royal, or noble metals, although tantalum and a few other extremely passive metals are able to withstand it. Aqua regia is used in etching and in certain analytic procedures. Aqua regia does not last very long, thus it has to be mixed immediately before use.
Hydrochloric acid was first discovered around the year 800 by Persian alchemist Jabir Ibn Hayyan (Gaber), by mixing common salt with vitriol (sulfuric acid). Jabir's invention of gold dissolving aqua regia, consisting of hydrochloric acid and nitric acid, contributed to the effort of alchemists to find the philosopher's stone.
How it works
Aqua regia works to dissolve gold, even though neither constituent acid will do so alone because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a tiny (virtually undetectable) amount of gold, forming gold ions. The hydrochloric acid provides a ready supply of chloride ions, which react with the latter, thus taking the gold out of the solution. This allows further oxidation of gold to take place, and so the gold is dissolved. Appropriate equations are:
- Au + 3NO3- + 6H+ → Au3+ + 3NO2↑ + 3H2O
- Au3+ + 4Cl- → AuCl4-
When Germany invaded Denmark, the Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of Max von Laue and James Franck into aqua regia and placed this reagent on a shelf in his laboratory at the Niels Bohr Institute. After the war, he returned to find the solution undisturbed and precipitated the gold out of the acid.