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Germans Max von Laue and James Frank received the Nobel Prize in Physics in 1914 and 1925, respectively. The first was a clear opponent of the Nazi regime and the second was Jew. When World War II began, these two physicists sent their commemorative medals awards to Niels Bohr’s Laboratory (Nobel Prize in Physics in 1922), in Copenhagen, so that he had a safe place until the end of the war. But medals are gold and gold export from Germany was a very serious offense at that time. Regrettably, Denmark was invaded and as the Bohr’s lab had become a refuge for Jewish physicists and would received a Nazi visit, so he had to hide medals but the name of the winner was recorded in them.
To avoid the capture and the likely punishment to these researchers, the Hungarian George de Hevesy (Nobel Prize in Chemistry in 1943) chose the chemistry way “to hide" the medals. But gold, being a noble metal, does react with almost nothing, adding more difficulty to the problem. Finally, de Hevesy chose to dissolve the medals in aqua regia, a solution discovered in the ninth century AD, composed of nitric acid (HNO3) and hydrochloric acid (HCl) in a rate 1:3.
The power to dissolve of aqua regia comes from the combined properties of each acid. In the first step, HNO3 acts as oxidizer and leads gold from the oxidation state 0 to +3.
The chloride ions in the HCl react with Au3+ ions, generating complexed auric tetrachloride ions, stable and soluble compounds.
The resulting orange solution was poured into a dark bottle and gold of medals was unnoticed for years in Bohr's laboratory.
When finished World War II, de Hevesy decided to recover the dissolved gold by precipitating, ie, returning back to its solid state. For that, he used sulfide dioxide from sodium sulphite as a powerful reducing (Na2SO3 + 2H+ → 2 Na+ + SO2 + H2O).
Finally, gold was sent to the Swedish Academy, which produced new medals that were returned to their rightful owners in 1950, on the 50th anniversary of the Nobel Prizes.