of the
Nobel Prizes

26 June 2018

Back in Nazi Germany, Hitler had made it a state crime to send gold out of the country and the party was collecting all the precious metal they could lay their hands on to fund the war.

Knowing this, two Nobel laureates, James Franck and Max von Laue, the first Jewish, the second who strongly opposed Nazism, smuggled their Nobel prize medals – made entirely of 23 carat gold (96% purity) – to Niels Bohr, whose institute in Denmark had become a refuge for scientists fleeing Nazism in the 1930s.

But in April 1940, the Nazi’s invaded and occupied Denmark. Bohr knew that his institute would be searched imminently and if the gold medals, which were struck with the names of their owners, were found, it could mean dire consequences for von Laue and Frank.

George de Hevesy was a chemist working at Bohr’s institute and the two sort to hide the precious medals from their invaders. To quote Adventures in Radioisotope Research (Vol. 1, p. 27, Pergamon, New York, 1962), de Hevesy wrote:

I suggested that we should bury the medal, but Bohr did not like this idea as the medal might be unearthed. I decided to dissolve it. While the invading forces marched in the streets of Copenhagen, I was busy dissolving Laue’s and also James Franck’s medals.

— George de Hevesy

Dissolving gold is not an easy job. Gold is cherished for its stability: it doesn’t tarnish, it isn’t very reactive and it doesn’t really dissolve in anything. I say really, because you can dissolve gold, but only under really harsh conditions.

Meet aqua regia, meaning “royal water”. This beautiful solution is a mixture of two acids: hydrochloric and nitric. It is pretty dangerous stuff, and is absolutely amazing at dissolving (and in most cases destroying) pretty much anything. Neither one acid is capable of dissolving gold, but together they can get the job done.

Nitric acid is a powerful oxidiser and rips a tiny amount of the gold into solution, generating the gold cation Au3+, and then the chloride anion Cl of hydrochloric acid sweeps in to create chloroaurate AuCl4. This process is in equilibrium, but favours the formation of the chloroaurate, and so the metal slowly dissolves into solution.

This is exactly what de Hevesy did. He submerged the two medals in aqua regia, and very slowly they dissolved. There’s no record of how long it took, but it was a success, and Bohr and de Hevesy simply placed the bottles of the orange solutions on a shelf in the institute with the other chemicals.

When the institute was occupied, it was searched from top to bottom, but they found no medals, and Bohr and de Hevesy had succeeded in protecting Frank and von Laue.

After the war, the gold was reclaimed from the acid and returned to the Royal Swedish Academy of Sciences in Stockholm, where the two medals were recoined and delivered back to the scientists.

It’s one of my favourite stories in science.

And fortune would have it, that just three years later, de Hevesy received his own Nobel Prize in Chemistry (1943) for the development of radioactive tracers to study chemical processes.

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