Aqua Regia 8/14/22

Science by Richard Bleil

Aqua regia sounds like water, doesn’t it?  But it’s anything but.  It’s a mixture of concentrated nitric acid with concentrated hydrochloric acid, forming a super acid.  Both nitric and hydrochloric acid are clear colorless liquids, but if you make aqua regia correctly, for a short time it will turn red.  It is the only acid strong enough to dissolve gold.

Gold is actually highly inert (non-reactive).  It is the only metal that is always mined in its elemental form, that is, as gold, not as an ore.  The only other metal that has a chance to do so is silver, where sometimes they will find a vein of pure elemental silver, but usually it’s also an ore.  To be an ore means the metal is in an oxidized, or salt, form.  Iron is usually mined as iron oxide.  Through the smelting process, the iron is separated from the oxygen and other inorganic elements to form the elemental metal.  Gold is mined as just gold.  Nothing outside of aqua regia is strong enough to dissolve it.

In the early 1930’s, there is the story of two Nobel Laureates, Max von Laue and James Franck, who needed to flee the Nazi’s.  In a story heroic in its own right, they managed to escape in the cargo bay of the last plane leaving the country before the Nazis shut down air traffic, but they knew they would never be able to escape with their Nobel prizes, medallions made of pure gold.  Their colleague George de Hevesy, dissolved the medals in aqua regia, marked it plainly, and left it on the shelf of his lab.

As expected, the Nazis searched the labs of von Laue and Franck, and of their friend de Hevesy, searching for the metals.  They tore through the labs quite thoroughly, but they were afraid to disturb the chemicals too much, so while they moved the bottles around, they left them intact.  In their ignorance, they didn’t realize what aqua regia was.  After the war, all three men returned to their labs, de Hevesy neutralized the aqua regia and reduced the dissolved metal back to its elemental form, and the Nobel commission kindly recast the gold back into the original metals. 

Just at the start of my graduate studies, or, more properly, the summer prior to the start, my institution agreed to pay me to teach the general chemistry lab for the summer courses.  This afforded me the opportunity to get to the city, find an apartment and settle in before classes began.  One of my earlier tasks was to mix up a batch of aqua regia.  Of course, I was a budding graduate student, so I knew more than I actually knew, like almost all graduate students. 

Instead of mixing the solution in beakers (capable of handling great changes in temperature), I decided, hey, why not just mix it right in the bottle in which it will be stored?  So I grabbed a four liter glass bottle, and poured in the proper ratio of the acids, sure enough generating that beautiful red color indicating that I had done it correctly.  Unfortunately, the reaction is also highly exothermic, and along with the red color, it heated the bottle to a very high temperature.  I knew I was in trouble, because the bottle was not properly tempered to handle excessive heat. 

Fortunately, I was intelligent enough to mix the acid in a fume hood, and in this fume hood was a sink (most do have sinks, although few of them were full-size like this one was).  I realized that there was a risk of the bottle breaking, so slowly, carefully, I picked up the bottle just high enough to move it, and as soon as I did, I heard a distinct “crick” sound. 

Yes, I realized the bottle was breaking, but it was still intact.  Slowly, carefully, I moved it towards the sink, being very cautious not to jar it.  Pretty much the instant it was over the sink, the entire bottom fell out of the bottle, dumping all four liters (about a gallon) WHOOSH into the sink, and straight into the city sewage system.  Immediately, I turned on the water faucet to try to dilute out the acid as much as I could, but, of course, I knew I was too late.

Now, having worked as a water treatment chemist, I now know that part of the treatment is biological.  A kind of algae is grown on a rotating cylinder which is half submerged at any given time to chew up any organics it can get a hold of to help purify the water coming through the treatment plant.  Even this much aqua regia, dumped into a major city wastewater, would surely have dropped the pH of the sewage so dramatically that it undoubtedly killed off that algae, probably making it necessary for the water treatment plant to replace it (at great cost), and to add neutralizing chemicals to bring the pH back up to a reasonable level.  To this day I feel guilty about this, but how many people can honestly say that they destroyed the wastewater treatment facilities for a major metropolitan area?

God help me, I sure do love being a chemist.

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