By Ron B. Davis Jr., Georgetown University
Gold can be dissolved in the presence of a very powerful oxidizing solvent, aqua regia (a mixture of nitric acid and hydrochloric acid). This remarkable reaction has many applications in the preparation of gold-containing compounds. But its most unusual impact on history might have taken place during World War II, involving a German, the Nobel Prize and Adolf Hitler. How?
Being so far below hydrogen on the activity series, gold even resists dissolution in most acids, which is different from many other elements commonly alloyed with gold to provide hardness.
And yet, for centuries, it has been known that gold metal can indeed be dissolved by a very special acidic solution called aqua regia (Latin for ‘king’s water’). This very special solvent is actually a mixture of two strong acids, nitric acid and hydrochloric acid. When mixed, these two materials provide a one-two punch that can dissolve even gold.
The nitric acid acts first as an oxidizer, tearing electrons away from the gold atoms in a solid sample, to form gold(III) ions. Then, the chloride ions from the hydrochloric acid swoop in, in groups of four, to complex with the gold(III) ions. Together, they form complex ions of tetrachloro-aurate, which remain dissolved in solution.
Hitler and the Nobel Prize
After the 1935 Peace Prize was awarded to Carl von Ossietzky, a vocal critic of the Third Reich, a furious Hitler banned any German from accepting the prize. As his armies pushed into Denmark, in 1939, it was a safe assumption that any Nobel Prize medals that were confiscated would face certain destruction.
Hence, in an effort to conceal the two medals in his care from the invading Nazi forces, chemist George de Hevesy, of the Niels Bohr Institute, dissolved the medals using aqua regia, placing the resulting solutions in unlabeled bottles on his laboratory shelf.
De Hevesy’s plan worked, and the two dissolved Nobel medals sat unrecognizable in their jars on his lab shelf until the end of the war. Afterwards, the gold was recovered and the medals were re-forged by the Swedish academy.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
When it comes to its appearance, interestingly, because of its softness, gold is often alloyed with copper, silver, nickel and other metals. This is done in order to increase its durability without altering its appealing appearance. Of course, this means that any object that appears to be gold might be an alloy of other metals with gold, or not gold at all.
The noble nature of gold makes it fairly easy to test any object suspected to contain less noble materials. This is, in fact, the origin of the term ‘acid test’. If the composition of a gold object is in question, a jeweler might scrape a small amount of the metal off, using what is called a ‘touchstone’.
The resulting metal dust is then exposed to increasingly strong acidic solutions, observing whether the metal dissolves or discolors. If it does so, it must contain more active metals, much closer to—or higher than—hydrogen on the activity series.
Thus, although gold is valued for its inertness and longevity, gold still possesses a rich chemistry.
Gold and Extra-terrestrial Bodies
Studying gold’s chemistry, a question that often comes up is that given that gold is so dense, why are we able to find any appreciable amount on Earth’s surface? Shouldn’t most of it have sunk to the core? Geologists believe that most of the Earth’s initial gold did, indeed, sink to the core during differentiation. However, just as the dense and ‘iron-loving’ element, iridium (found on Earth’s surface, offering clues to the demise of the dinosaurs) can arrive from space, why not gold?
It is believed at least some of the gold we find on Earth’s surface may have been delivered by extra-terrestrial bodies that impacted the cooling Earth after differentiation took place.
The Asteroid Belt
A surprising hint of this possibility is a huge object orbiting the Sun, about 300 million miles from Earth, in the asteroid belt. It is a 120-mile-wide metallic object that’s bigger than the ‘Death Star’ is said to be, in the Star Wars movies.
NASA scientists have confirmed, this object is actually a very large asteroid that appears to be made primarily of metals. And they believe that this object, named ’16 Psyche’, may be a large piece from the core of a protoplanet, exposed after one or more massive collisions with other large bodies tore away the rocky outer layers and created pieces of the asteroid belt.
If this is really the core from a shattered planet, the composition of 16 Psyche will be of great interest, since it may offer clues to the composition of Earth’s core.
Mining 16 Psyche
Popular media seize on this notion whenever NASA announces information about its scheduled mission to send a probe to the large asteroid, planned to arrive is 2026. If 16 Psyche is indeed a piece of the metallic core of a former protoplanet, we’d expect to find oodles of ‘iron-loving’ elements in it.
It is likely to include enough precious metals like gold and platinum to make every person on Earth a multi-billionaire at today’s prices!
Mining 16-Psyche, or even latching the asteroid to a rocket and towing it back to Earth, is a fun fantasy to entertain. If we could pull that off, we might be able to literally pave the streets with gold. Pretty, but not a good engineering choice, considering how soft this metal is.
Common Questions about the Nobel Prize, Extra-terrestrial Bodies, and Gold’s Chemistry
After the 1935 Peace Prize was awarded to Carl von Ossietzky, a vocal critic of the Third Reich, a furious Adolf Hitler banned any German from accepting the Nobel prize.
This is because, it is believed, that some of the gold we find on the Earth’s surface may have been delivered by extra-terrestrial bodies that impacted the cooling planet after differentiation took place.
16 Psyche is likely to include precious metals like gold and platinum.