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Aqua Regia: Overview, Chemical Inventory & More

By Matt Bedford, Chemist

Every now and then I come across a chemical that gets my attention. Normally the chemical is in a researcher's satellite accumulation area and the researcher is enquiring into the best way to get rid of the chemical waste. Recently this scenario brought me into contact with (not literally, I was wearing nitrile gloves) Aqua Regia, a mixture of concentrated nitric and hydrochloric acids. Like all good field chemists who want to know more about a chemical I utilized my number one resource...Wikipedia, and found out an astonishing array of facts about this acidic mixture.

Aqua Regia, clear when freshly mixed but quick to turn yellow, is highly acidic and is also a strong oxidizer. In the laboratory setting it can be used to clean glassware, such as NMR tubes, of organic compounds.

From a safety point of view the storage of Aqua Regia is not recommended. After mixing, the solution rapidly decomposes, liberating nitrosyl chloride, nitric oxide and chlorine. This causes a loss of potency, but if the liquid is stored in a closed container the resulting buildup of gas can cause the bottle to explode. If it is unavoidable to store Aqua Regia it should be kept in a fume hood, away from flammables and in a container that allows it to off-gas.

One of the more interesting facts about Aqua Regia is that it is capable of dissolving gold. I found this interesting, as by themselves neither nitric nor hydrochloric acid are able to do this, but together they each perform a different task to form the gold solution chloroauric acid. This property was put to good use during World War II. As Germany invaded Denmark, scientist George de Hevesy dissolved the gold Nobel Prize medals of Max von Laue and James Franck to prevent the Nazis from stealing them. The jar containing the gold / acid solution was placed on a shelf and consequently ignored by the Nazis thinking it was just another chemical. After the war the gold was reclaimed and sent to the Nobel Foundation to be re-cast and presented again to Laue and Frank.

So the next time you come across a new chemical, and have a couple of minutes to spare, dig around a little, there might just be an interesting back story to it.

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