Aqua regia

In the 8th century AD, an alchemist by the name of Jabir ibn Al-Hayyan prepared the first recorded batch of aqua regia, a mixture of nitric and hydrochloric acid in a 1:3 ratio.

Named after the Latin for ‘royal water’, this volatile mixture turns from colourless to a fiery yellow–orange within a few seconds of preparation. For further dramatic effect, it fumes vigorously. Because its components are so volatile, it is usually only mixed immediately before use. If left for a period of time, the concentrated nitric and hydrochloric acid react together to form products such as nitrosyl chloride and chlorine – neither of which you want hanging about.

As you would expect from two strong and concentrated acids, aqua regia is incredibly corrosive. Indeed, it has been the culprit behind several laboratory explosions – usually due to chemical mishandling. In 2015, a glass waste bottle exploded in a Texas Tech University teaching laboratory – a student had accidentally poured nitric acid waste into a bottle already containing hydrochloric acid, methanol and dimethylglyoxine. The ensuing reactions caused a build-up of pressure, and a minor explosion. Luckily, no-one was seriously hurt.

The primary use of aqua regia is for the production of chloroauric acid, the electrolyte in the Wohlwill process for refining gold. Although gold is typically an inert metal, it will dissolve in aqua regia because of the unique action of nitric and hydrochloric acid. Nitric acid is a powerful oxidising agent, capable of converting small amounts of gold to its ionic form, Au³⁺. Once this ionic form is present in the solution, the hydrochloric acid provides a source of chlorine anions which react with the gold cations to form tetrachloroaurate(III) anions. As the reaction with hydrochloric acid is an equilibrium reaction favouring the formation of chloroaurate anions (AuCl₄^-), the gold ions are removed from solution making room for more oxidation to occur. And as the solution is so acidic, the chloroaurate anions are swiftly protonated to form chloroauric acid. Using this method, it is possible to produce gold with a purity of 99.999%. Its reactivity and strength means aqua regia can also dissolve platinum in a similar way.

Aqua regia is also handy for deep cleaning the tubes used in nuclear magnetic resonance, or NMR, spectroscopy. It is very efficient at this as it can remove all traces of the paramagnetic element chromium, which can contaminate spectra and ruin research.

Perhaps aqua regia’s most famous use was in 1940 at the Niels Bohr Institute in Denmark. Hitler had just invaded, and his forces were rapidly approaching. The chemist George de Hevesy had a problem. In his lab he had two Nobel prize medals, smuggled out of Germany on behalf of Max von Laue and James Franck. If found, they would all face severe punishment. Thinking fast, de Hevesy placed the medals into a solution of aqua regia. When the Nazis entered the lab, they bypassed the beaker thinking it of no importance. Amazingly, when de Hevesy returned after the war he found the beaker undisturbed. He precipitated the dissolved gold and returned it to the Nobel society, who then recast the medals in their original gold.