Aqua regia

Aqua regia is a highly corrosive liquid mixture used in chemistry. Its name comes from Latin, meaning "regal water" or "royal water."

This name was given by alchemists because the liquid can dissolve noble metals. Noble metals, such as gold and platinum, are very stable and do not react with most other substances. Aqua regia is unique because it can break these metals down into a liquid form. This ability makes it a vital tool for refining precious metals and cleaning laboratory equipment.

To create aqua regia, scientists mix concentrated nitric acid with concentrated hydrochloric acid. The most effective mixture uses a molar ratio of 1:3. This means there are three parts hydrochloric acid for every one part nitric acid.

When these two acids combine, a rapid chemical reaction occurs. This reaction produces volatile gases known as nitrosyl chloride and chlorine gas. Because these gases are released, the liquid is described as "fuming." The mixture also undergoes a rapid color change. It starts as a colorless liquid but quickly turns yellow, orange, or red. As the gases escape the solution, the aqua regia loses its potency. For this reason, scientists usually prepare it immediately before they use it.

The way aqua regia dissolves gold is a multi-step chemical process. Neither acid can dissolve gold on its own. First, the nitric acid acts as a powerful oxidizer. It dissolves a very small amount of gold to create gold(III) ions. Next, the hydrochloric acid provides a steady supply of chloride ions. These chloride ions react with the gold ions to produce tetrachloroaurate(III) anions. This reaction is an equilibrium reaction that favors the formation of these anions. By pulling the gold ions into the solution, the acid allows even more gold to be oxidized. Eventually, the solid gold turns into a liquid called chloroauric acid.

Dissolving platinum with aqua regia is a much more complex process than dissolving gold. The initial reaction produces a mixture of chloroplatinous acid and nitrosoplatinic chloride. The nitrosoplatinic chloride is a solid product that does not stay in the liquid. To achieve full dissolution, scientists must perform repeated extractions using concentrated hydrochloric acid. They can also oxidize the chloroplatinous acid into chloroplatinic acid. This is done by saturating the solution with molecular chlorine while applying heat. This complex chemistry was useful for historical discoveries. When scientists used aqua regia on platinum ores, they discovered iridium and osmium. These two metals are extremely dense and did not dissolve. Instead, they collected as an insoluble metallic powder at the bottom of the vessel.

History shows that humans have used these types of mixtures for centuries. Records from 660 AD suggest that nitrates were used to create similar substances. In Europe, aqua regia appeared in writings by pseudo-Geber. Early alchemists used symbolic art to explain its power. One famous illustration from 1678 shows a fox eating a rooster.

In this symbol, the rooster represents gold and the fox represents aqua regia. This imagery captured the intense chemical struggle between the metal and the acid. Later, in 1789, the chemist Antoine Lavoisier referred to the substance as nitro-muriatic acid.

Today, aqua regia serves several critical roles in modern science and industry. One primary use is the Wohlwill process, which is used to refine gold. This process uses chloroauric acid as an electrolyte to produce gold with a purity of 99.999%. It is also used for etching and in various analytic procedures. In laboratories, it is a preferred method for cleaning glassware, such as NMR tubes. While other methods like chromic acid baths exist, they are highly toxic and can cause explosions. Aqua regia is also safer for certain types of spectroscopy because it leaves no paramagnetic chromium traces behind. However, it remains very corrosive and can cause explosions if mishandled.

Aqua regia has even played a role in protecting scientific history during wartime. During World War II, chemist George de Hevesy used the liquid to hide gold Nobel Prize medals. He dissolved the medals belonging to Max von Laue and James Franck into the acid. He placed the solution on a laboratory shelf in Denmark to hide it from the Nazis. The soldiers did not realize the gold was hidden in a common chemical jar. After the war, de Hevesy successfully precipitated the gold out of the solution. The gold was used to recast the medals, which were then returned to the original winners.