How is aqua regia stored?

Question

How is aqua regia stored as it dissolves all metals?

Answer 1

How is aqua regia stored as it dissolves all metals?

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No one stores aqua regia for extended periods of time, because it decomposes rapidly, yielding volatile products that escape from the solution resulting in a loss of potency.

From the Wikipedia article on aqua regia,

Due to the reaction between its components resulting in its decomposition, aqua regia quickly loses its effectiveness (yet remains a strong acid), so its components are usually only mixed immediately before use.

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Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency.

$$\ce{HNO3 + 3HCl -> NOCl + Cl2 + 2H2O}$$

The nitrosyl chloride formed is, essentially, a decomposition product of $\ce{HCl}$ and $\ce{HNO3}$. This itself, is further decomposed as,

$$\ce{2NOCl -> 2NO + Cl2}$$ $$\ce{2NO + O2 -> 2NO2}$$

The mixture itself is created by combining $\ce{HCl}$ and $\ce{HNO3}$ in a 3:1 molar ratio in glass vessels immediately before use.

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Aqua regia: Why does it dissolve gold but not glass?

From what I gather from this PDF, nitric acid is a notoriously strong oxidizing agent and is quite capable of oxidizing elemental gold (a "noble metal"). $$\ce{Au(s) + 3NO3^{-}(aq) + 6H^{+}(aq) <=> Au^{3+}(aq) + 3NO2(g) + 3H2O(l)}$$

However, we wouldn't see that happening even if we plopped a gold coin in a jar of nitric acid and waited a lifetime. This is because the reaction has an extremely small equilibrium constant (I'm having trouble getting the actual value of the constant myself, feel free to give me a heads up if you know where to find it), meaning only a minuscule (almost negligible) quantity of gold actually reacts/dissolves.

So for all practical purposes, nitric acid doesn't react with gold (though strictly speaking, it does).

So you want to dissolve gold, and you know (for real) that nitric acid can do it (but not very "efficiently", if you will). What do you do?

The trick is to throw in some concentrated $\ce{HCl}$ as well.

If you remember your high school chemistry lessons on equilibrium and Le Chatelier's principle, you'll find this easy to follow: The $\ce{Cl^{-}}$ generated by the $\ce{HCl}$ complexes with the $\ce{Au^{3+}}$ and essentially "plucks" out the $\ce{Au^{3+}}$ from the earlier equation, resulting in the equilibrium shifting to the right, causing more gold to react.

$$\ce{Au^{3+}(aq) + 4Cl^{-}(aq) <=> AuCl4^{-}(aq)}$$

Now according to this Quora answer, the reason aqua regia doesn't attack glass, is because silicates (the stuff that constitutes glass) are fairly inert to the oxidizing power of nitric acid (on account of their strong $\ce{Si-O}$ bonds).

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Aqua regia doesn't readily react with glass. So while you could keep it in a glass retort for years, it would be nothing like the original mixture (you be hard pressed to even call it "aqua regia" after a day, much less, after a few years).

A rather famous incident regarding the "storage" of aqua regia, was when de Hevesy dissolved two gold Nobel medals in aqua regia to hide them from Nazi Occupiers, and kept the solution in a glass retort on a shelf. He returned to collect it in about 5 years (the bottle was still intact)

When Nazi Germany occupied Denmark from April 1940, during World War II, de Hevesy dissolved the gold Nobel Prizes of Max von Laue and James Franck with aqua regia; it was illegal at the time to send gold out of the country, and had it been discovered that Laue and Franck had done so to prevent them from being stolen, they could have faced prosecution in Germany. He placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute. After the war, he returned to find the solution undisturbed and precipitated the gold out of the acid.

There is, however, an interesting alternate interpretation of de Hevesy's heroic deed,

It is commonly suggested, even years after his death, that he only chose to dissolve the gold for story-telling purposes as opposed to practically. Perhaps a much simpler and efficient option was for him to bury the Nobel Prizes.

_{You can also read up on how aqua regia "dissolves" gold here. And for those of you who want to know a way to easily recover the said gold, have a look at this Nurdrage video}

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Also, your question assumes that all metals are dissolved in aqua regia, which is apparently, not the case, as discussed under this post.