Hiding treasure from Nazis

12 May 2020

Interview with 

Ljiljana Fruk, University of Cambridge

NOBEL-PRIZE

A Nobel Prize on a black background

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After the war ended life began to return to some kind of new normal. And for two scientists, they found something special in their labs in the University of Copenhagen. Max von Laué and James Franck were Nobel Prize winning scientists. In 1940 the Nazi troops invaded Denmark. And the Nazis had a rule that no one could own a Nobel Prize after pacifist Carl von Osseitsky won the Peace Prize. Understandably, they did not want to lose their gold medals, but their colleague George de Hevesy had a plan. To hide them by dissolving them in a chemical called aqua regia. University of Cambridge chemist Ljiljana Fruk told Adam Murphy more about aqua regia...

Ljiljana - So Aqua regia is the mix of nitric acid, concentrated nitric acid, and concentrated hydrochloric acid in a particular ratio. So you would usually have one part of nitric acid and three parts of hydrochloric acid, and then you would get this really powerful acidic mix. And the name stemmed from... It means like kings acid, noble acid, because it was used by all chemists as well to dissolve certain materials that would not dissolve in any other acid or solvent. And one of these materials is gold.

Adam - But is dissolving gold an easy thing to do?

Ljiljana - This is one of the mixes that is dissolving gold in its form. And the reason for this is that gold is so precious because it's very unreactive. So it can survive treatment with acids, it doesn't oxidize, that means it doesn't react with oxygen. So you would never have a gold ring that will change color with time. So not many things will dissolve gold at all. And Aqua regia is almost the only one that will interact with gold. The other material that interacts with gold is mercury and you wouldn't like using mercury.

Adam - Right then the Nobel prize metals have been put in this stuff. How do they dissolve? Chemically what's going on in the jar?

Ljiljana - Well, there are certain acids that can kind of react in a slow way with noble metals like silver and copper and gold, so nitric acid would actually start interacting with gold. But reaction will stop very quickly, so it's not going to go further to dissolving the entire amount of the gold. But if we add hydrochloric acid, this reacts with some of the species that are produced in the first process of interaction between the nitric acid and the gold, and these intermediate pieces are then strong enough to push the equilibrium of the reaction in the direction of the gold dissolution. So what you basically have, you have then the reaction which is pushed from elemental gold, which we know as a gold column, to the ions of the gold, which give out this very nice yellowish orange solution when they are formed.

Adam - Seems like pretty potent stuff. Why would a chemist just be mixing this stuff up in a lab other than thwarting Nazis of course.

Ljiljana - So one of the reasons where you actually can use this as an acid as well is to get the gold ions made. And the gold ions are very useful for nanotechnology. So what we do today is we use some of these gold ions, which are produced, and we use them as precursors, as starting materials to make gold nanoparticles. So what we are basically doing is we dissolve the gold into the ions and then we transform these ions again into the gold, but now using different procedures so that we can get very tiny structures of gold, which have really wonderful properties which have been used for electronics and also in medicine. So it's very useful for kind of producing these gold ions that can be transformed in nanotechnology into the new material.

Adam - Eventually the scientists had to leave Denmark and the jar behind, but it sat there, an unremarkable looking beaker of liquid, until after the war. But when they got back, they didn't want a jar of potent acid did they? How can you get the gold back?

Ljiljana - So you can have, for example, you can simply - one of the ways how you can have the gold back from the solution is simply evaporate what you have left. During the reaction, and this is the beauty of chemistry, during the reaction, if you do the reaction between Aqua regia and gold, you will see the bubbles forming. Because the products of these reactions are gold ions but also gases. And so the gases will get removed. And you are also left with a little bit of water and with these kind of gold ions. And so you can just simply evaporate the water. And then you are left with a really nice powdery compound, which is now the salt of gold ions.

Adam - And then it's a pretty simple reaction to turn that gold salt back into pure gold.

Ljiljana - You know, this is the power of knowledge: how you can hide something also in plain sight. Who would have said that this orange solution is basically gold!

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