Science · Chemistry

Aqua regia

Chemistry — acids and reagents

Aqua regia — Latin for "royal water" — is a fuming orange-yellow mixture of concentrated hydrochloric and nitric acids, traditionally three parts to one, and one of the very few reagents that will dissolve gold and platinum. Neither acid can do it alone. Known to the alchemists from at least the fourteenth century, it has a celebrated modern moment: in April 1940, as German troops entered Copenhagen, George de Hevesy dissolved the gold Nobel medals of Max von Laue and James Franck in a flask of it at Niels Bohr's institute. The solution sat unremarked on a shelf for the whole war, and the recovered gold was recast into new medals in 1952.

How to spot them

▸Latin for 'royal water', because it attacks the royal metals
▸Three parts concentrated hydrochloric acid to one part nitric
▸Fumes orange-yellow and dissolves gold and platinum, which no single common acid will
▸Two Nobel medals were hidden in a flask of it in Copenhagen through the Second World War

Key facts

Its Latin name means 'royal water', for the metals it can attack

Gold and platinum were the noble or royal metals

Three parts concentrated hydrochloric acid to one part concentrated nitric

Freshly mixed, it fumes orange-yellow with nitrosyl chloride and chlorine

In 1940 George de Hevesy used it to hide two Nobel medals from the Nazis (1940)

The gold medals of Max von Laue and James Franck, dissolved at Bohr's institute in Copenhagen

One of the very few reagents that will dissolve both gold and platinum

Neither of its two component acids can do so on its own

Described by the alchemical writer known as Pseudo-Geber in the fourteenth century

The alchemists' great prize was a solvent for the noble metals

Nitric acid oxidises the metal while chloride ions cage it as a tetrachloro complex

The [AuCl4]− ion drives the reaction forward by removing gold ions from solution

Key ideas

Alone, nitric acid oxidises gold only to a negligible extent because the equilibrium lies far to the left; the chloride from hydrochloric acid removes the gold ions as chloroauric acid, HAuCl4, letting the oxidation run to completion. It is a textbook case of two reagents doing together what neither can do alone.
It decomposes quickly once mixed, so it must be made up fresh; the fumes are nitrosyl chloride and chlorine, which is where the orange colour comes from

Quick-fire facts

•A mixture of concentrated nitric and hydrochloric acids, classically 1:3 by volume; the reverse ratio is called inverse aqua regia
•Its name is Latin for 'royal water', because it dissolves the noble or royal metals, gold and platinum
•It is first described in the works attributed to Pseudo-Geber, around the fourteenth century
•Gold dissolves to chloroauric acid, HAuCl4; platinum to chloroplatinic acid, H2PtCl6
•It does not dissolve iridium, ruthenium, tantalum, osmium, rhodium or titanium, and silver is only superficially attacked because silver chloride passivates the surface
•In April 1940 George de Hevesy dissolved Max von Laue's and James Franck's Nobel medals in it at the Niels Bohr Institute to stop the German occupiers seizing them; taking gold out of Germany was then a serious offence
•The flask stood untouched on a shelf through the occupation; after the war the gold was precipitated out and the Nobel Foundation recast the medals, which were re-presented in 1952
•De Hevesy himself was awarded the 1943 Nobel Prize in Chemistry, for his use of isotopes as tracers
•It is used industrially to refine gold and platinum, to etch semiconductors, and in the laboratory to clean stubborn residues from glassware
•It must be made fresh and never stored in a sealed container, since the gases it gives off will burst the vessel

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Sources

American Institute of Physics — The Invisible Prize The de Hevesy Nobel medals story
The Nobel Foundation — The Nobel Medals
Compound Interest — The Nobel Prize Medals (and How to Make Them Disappear)