What is Aqua Regia?

Golden triangular patterns

Aqua regia is a highly corrosive mixture of concentrated nitric acid and hydrochloric acid. With the ability to dissolve noble metals, it was once used by alchemists on their quest to find the philosopher’s stone.

Properties

Aqua regia appears as a yellow-orange fuming liquid. The presence of nitric and hydrochloric acid makes this substance extremely corrosive, especially to metals and tissue.

  • IUPAC Name: Nitric Acid Hydrochloride
  • Other Names: Aqua Regis, Royal Water, Nitro-Muriatic Acid
  • Chemical Formula: HNO3+3HCl
  • Molar Ratio: 1:3
  • Melting Point: -42°C
  • Boiling Point: 108°C

Aqua regia undergoes quick decomposition because of how its components react. When nitric acid and hydrochloric acid are combined, they produce nitrosyl chloride (NOCl) and chlorine gas. These are volatile products that cause the solution to fume.

Nitrosyl chloride and chlorine gas escape from the solution in these fumes. The more volatile products escape, the less potent aqua regia becomes. This why a solution of aqua regia is usually mixed immediately before it is used.

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Aqua regia was revered in the Middle Ages for its ability to dissolve gold. Even today, it is still used to not only dissolve gold but to also refine it to an extremely high purity level.

How is it Made?

You can prepare aqua regia by mixing hydrochloric acid and nitric acid together in a molar ratio of HNO3+3HCl (1:3). If preparing the solution according to volume ratio, it is approximately 4 parts hydrochloric acid to 1 part nitric acid. This is because a concentrated solution of HNO3 (65%) is much stronger than a concentrated solution of HCl (35%).

When mixing a solution of aqua regia, it is important to ensure that you are adding nitric acid to hydrochloric acid, not the other way around. This is because, when HCl is added to nitric acid, a lot of heat is produced because of the water content in hydrochloric acid. This heat is able to convert drops of water into steam, which could cause the corrosive mixture to splash back.

It is always important to use a fume hood when preparing aqua regia because of the production of chlorine gas and nitrous oxide. You can dispose of the mixture by pouring aqua regia over a large amount of ice and neutralising it with a 10% solution sodium hydroxide. Once the mixture has been neutralised, it can be poured down the drain.

History of Aqua Regia

Ancient alchemists gave aqua regia its name because of its colour and ability to dissolve gold. Literally translating to ‘royal water’ in Latin, it was first discovered by an Arab alchemist in c. 800 AD.

In the middle ages, its ability to dissolve metals like gold and platinum led alchemist’s to use aqua regia in their quest to uncover the philosopher’s stone. Yes, you read that correctly. More than just a Harry Potter book, the philosopher’s stone is a legendary substance that ancient alchemists believed could cure illness, transform base metals into gold or silver, and even grant immortality by providing the elixir of life. For a long time, finding this mysterious stone was the most important goal in Western alchemy and aqua regia played a big part in this quest.

Aqua regia was also used by George de Hevesy in World War II to dissolve two golden Nobel Prize medals in an effort to prevent Nazi soldiers from taking them. He kept the dissolved gold in a jar in his lab where it blended in with his many other chemicals. These Nobel Prize medals belonged to Max von Laue and James Franck. After the war ended, de Hevesy retrieved the jar and the medals were able to be reformed and returned to their owners.

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Ancient alchemists used aqua regia as a way of finding the legendary philosopher’s stone, which they believed could cure illness, transform base metals into precious metals and grant immortality.

Applications

The main application for aqua regia is in the Wohlwill process, a chemical procedure that is used to refine gold. Aqua regia is used in this way to produce chloroauric acid, which plays an important role in this gold-refining technique. The result of the Wohlwill process is high purity gold (99.999%).

Aqua regia has a range of applications and uses, including:

  • In the etching of different metals
  • In the analytic chemical analysis
  • To remove trace metals and organic compounds in the cleaning of machinery and glassware

Most famously, it can dissolve noble metals like gold, platinum and palladium. Therefore, it often finds use in the extraction and purification of these substances.

Dissolving Gold

The fact that aqua regia can dissolve gold is especially interesting given that neither nitric acid or hydrochloric acid, by themselves, can dissolve this metal. In fact, gold doesn’t easily react with any chemical. It is only when these strong acids are combined that this sturdy metal can be dissolved. This is because when mixed together, nitric acid and hydrochloric acid perform two different functions: nitric acid is a powerful oxidising agent while hydrochloric acid is a good supplier of chloride ions.

In the case of gold, the oxidising powers of nitric acid can attack the gold and dissolve a small proportion of it. This results in gold ions (Au3+) breaking apart from the metal and entering the solution. But there is an equilibrium happening: as some gold ions go into the solution, they are balanced out by other gold ions reattaching to the surface of the metal. This is why nitric acid on its own would never be able to dissolve gold, and also why the presence of hydrochloric acid is so important.

The chloride ions from HCl are able to attack and react with the gold ions in the solution, converting them into a different compound (tetrachloroaurate III) so that they can’t go back to the gold’s surface. The more gold ions that come off the metal, the more gold ions get converted into this new compound.

This process continues until all gold ions are eventually removed from the solution and the gold is converted into another, more soluble acid: chloroauric acid (HAuCl4). This compound is very miscible with water and will allow all of the gold to successfully dissolve, causing the solution to change colour.

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ReAgent has nitric acid and hydrochloric acid available for sale in our online chemical store, where you can choose from a variety of concentrations and pack sizes. If you’re looking for aqua regia, we have all the chemicals and technical advice you’ll need. Just contact us today for more information.

Hydrochloric Acid (General Use)

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