Aqua Regia Definition
Aqua regia is a mixture of hydrochloric acid (HCl) and nitric acid (HNO3) at a ratio of either 3:1 or 4:1. It is a reddish-orange or yellowish-orange fuming liquid. The term is a Latin phrase, meaning “king’s water”. The name reflects the ability of aqua regia to dissolve the noble metals gold, platinum, and palladium. Note aqua regia will not dissolve all noble metals. For example, iridium and tantalum are not dissolved.
Also Known As: Aqua regia is also known as royal water, or nitro-muriatic acid (1789 name by Antoine Lavoisier)
Aqua Regia History
Some records indicate a Muslim alchemist discovered aqua regia around 800 AD by mixing a salt with vitriol (sulfuric acid). Alchemists in the Middle Ages tried to use aqua regia to find the philospher’s stone. The process to make the acid was not described in chemistry literature until 1890.
The most interesting story about aqua regia is about 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 into aqua regia. He did this to prevent the Nazis from taking the medals, which were made of gold. He put the solution of aqua regia and gold on the shelf in his lab at the Niels Bohr Institute, where it looked like just another jar of chemicals. de Hevesy returned to his laboratory when the war was over and reclaimed the jar. The recovered the gold and gave it to the Royal Swedish Academy of Sciences so the Nobel Foundation to re-make the Nobel prize medals to give to Laue and Franck.
Aqua Regia Uses
Aqua regia is useful to dissolve gold and platinum and finds application in the extraction and purification of these metals. Chloroauric acid may be made by using aqua regia to produce electrolytes for the Wohlwill process. This process refines gold to extremely high purity (99.999%). A similar process is used to produce high-purity platinum.
Aqua regia is used to etch metals and for analytic chemical analysis. The acid is used to clean metals and organics from machines and laboratory glassware. In particular, it’s preferable to use aqua regia rather than chromic acid to clean NMR tubes because chromic acid is toxic and because it deposits traces of chromium, which ruin NMR spectra.
Aqua Regia Hazards
Aqua regia should be prepared immediately before use. Once the acids are mixed, they continue to react. Although the solution remains a strong acid following decomposition, it loses effectiveness.
Aqua regia is extremely corrosive and reactive. Lab accidents have occurred when the acid exploded.
Depending on local regulations and the specific use of aqua regia, the acid may be neutralized using a base and poured down the drain or the solution should be stored for disposal. Generally, aqua regia shouldn’t be poured down the drain when the solution contains potentially toxic dissolved metals.