Molecule of the Day

As we’ve discussed, gold is among the so-called “noble metals,” named as such for their lack of reactivity. Gold won’t dissolve in concentrated solutions of nitric acid or hydrochloric acid. Both are strong acids, and nitric acid is a potent oxidizer, which tends to help quite a bit in dissolving metals. It turns out chloride and an oxidizer are the necessary and sufficient conditions to dissolve gold.

Enter aqua regia, which is just a mixture of the two acids (providing both the chloride plus the oxidizer). Typically you use ~25% concentrated nitric acid and ~75% concentrated hydrochloric acid, but other proportions are known (the other one I see some people use is 75/25, which I think is quite a bit nastier.

Like so many colorfully named classics, aqua regia derives its name from alchemy. As you have no doubt figured out, it is from the Latin for “royal water,” from its gold-eating superpowers. It was discovered in 800 AD by the alchemist Abu Musa Jabir ibn Hayyan.

One unique thing about aqua regia is that it decays after being mixed up, so you always have to make it fresh. The nitric acid slowly works on the chloride ion, generating chlorine gas, leading to a pleasant swimming-pool aroma if you just catch a whiff, or choking fumes if you get more than that (Chlorine is really a violent poison in much higher concentrations than you run into at the pool, and it has been used as a war gas).

Also generated is the toxic nitrosyl chloride (NOCl), which is a beautiful reddish-orange. So you mix concentrated nitric acid (colorless, or maybe tinted just barely yellow, but mostly clear), concentrated hydrochloric acid (colorless), and you get a red-orange, bubbling, smelly solution that can dissolve gold. Can you imagine what the alchemists must have thought of this?

My very favorite story about aqua regia is this: during World War II, a Hungarian chemist living in Denmark, George de Hevesy, dissolved two fellow scientists’ Nobel Prizes in aqua regia literally as the Nazis stormed into Copenhagen so they wouldn’t be stolen (he assumed, correctly, that the Nazis would just leave the chemicals alone). After the war, he recovered the gold, and the Nobel committee recoined the prizes. You can read more about it here.

See you tomorrow.