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The term aqua regia is a Latin name meaning “king’s water”. Aqua regia is a mixture of hydrochloric acid (HCl) and nitric acid (HNO 3 ) in a ratio of 3 parts HCl to 1 part HNO 3 . It can also be prepared in a 4:1 ratio if a higher concentration is required. It is a fuming, reddish-orange or yellowish-orange liquid. The name of aqua regia reflects its ability to dissolve noble metals such as gold, platinum and palladium. It should be noted that aqua regia is not capable of diluting all noble metals; for example, iridium and tantalum do not dissolve with this mixture.
It is also known as royal water, king’s water or nitro-muriatic acid (name given in 1789 by Antoine Lavoisier).
History of aqua regia
In 1597, the German alchemist Andreas Libavius published a book, Alchemy , describing the process of making aqua regia. Some records indicate that alchemists in the Middle Ages tried to use aqua regia to find the philosopher’s stone. The process for producing this acid was not described in the chemical literature until 1890.
An interesting story about aqua regia describes an event that occurred during World War II. When Germany invaded Denmark, the chemist George de Hevesy dissolved the Nobel Prize medals belonging to Max von Laue and James Franck in aqua regia. He did this to prevent the Nazis from taking the medals, which were made of high-purity gold. He left the solution of aqua regia and gold on the shelf of his laboratory at the Niels Bohr Institute, where it looked like another jar of products .chemicals. The Hevesy scientist returned to his laboratory when the war ended and recovered the vial; consequently, he recovered the gold and gave it to the Royal Swedish Academy of Sciences for the Nobel foundation to redo the Nobel prize medals and present them again to Laue and Franck, the prize winners.
Uses of aqua regia
Aqua regia is useful for dissolving gold and platinum in the laboratory and on a small scale, but its most important application is in the extraction and purification of these metals on an industrial level. Chloroauric acid (HAuCl 4 ) can be obtained by using aqua regia to produce electrolytes during the Wohlwill process, which refines gold on an industrial scale and to extremely high purity (99.999%). Similarly, an analogous process is used to produce high purity platinum.
Aqua regia is also used to etch metals and to wash glassware in analytical chemistry laboratories. This mixture of acids is also used to clean metals and organic substances from machines and glassware in industrial laboratories; in particular, it is preferable to use aqua regia rather than chromic acid to clean nuclear magnetic resonator (NMR) tubes, because chromic acid is toxic and because it deposits trace amounts of chromium, which affect NMR spectra.
dangers of aqua regia
The aqua regia should be prepared immediately before use. Once the acids are mixed, they continue to react. Although the solution remains a strong acid after decomposition, it loses cleaning efficacy.
Aqua regia is extremely corrosive and reactive and produces vapors that can cause harm to those handling the mixture. For this reason, the precautionary rules in the laboratory must be followed to the letter and thus avoid accidents.
waste disposal
Aqua regia is a mixture of acids, which can be neutralized using a base. The solution must be stored for later disposal, following all safety protocols, both from the point of view of corrosion and the possible presence of traces of heavy metals.
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Regal water . Princeton University Environmental Health and Safety. Available at https://ehs.princeton.edu/laboratory-research/chemical-safety/chemical-specific-protocols/aqua-regia
aqua regia Chemistry.es: https://www.quimica.es/enciclopedia/Agua_regia.html
George deHevesy . Atomic Heritage Foundation. Available at https://www.atomicheritage.org/profile/george-de-hevesy
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