Recently a friend convinced me to watch Breaking Bad, a show on AMC. (Mild Spoilers Ahead!)
The protagonist of the show is a high school chemistry teacher named Walter White who, upon discovering he has lung cancer, decides to start a life of crime in order to earn money for his family before he dies. Specifically, he decides to apply his chemistry expertise to become a crystal meth supplier, and with his CalTech technique, his meth quickly surpasses the competition in quality, as well as obviating the need to scrounge around for pseudoephedrine. I’ve never purchased Sudafed, but my more cold-prone (drug-addicted?) friends tell me that it’s impossible to buy in quantity from the same drug store. When Walter gets into trouble, he uses chemistry to get out of it, sort of like a MacGyver or even Michael Westen, but more rooted in pure science. A great example of this, that actually made me regret not studying more chemistry, was when Walter uses the highly explosive mercury fulminate to dominate a local mob boss. Suffice to say, we never made that in chem lab (although I believe we learned the formulas for making meth, or was it synthetic cocaine, in orgo).
Besides being a great show, it would be cool if Breaking Bad inspires previously bored high school students to pay attention in their chemistry classes by revealing its practical, sometimes everyday applications. Of course, a chemical engineering degree is a great way to make money, even without resorting to drug dealing.
Watching Breaking Bad and thinking about the practical applications of chemistry reminded me of a thrilling story I once heard about hiding gold. As in the Purloined Letter, the best place to hide anything is in plain sight.
During World War II, Jewish scientists from Germany fled to (among other places) Copenhagen’s Niels Bohr Institute. Two Nobel Prize-winning scientists, Max von Laue and James Franck, brought with them their Nobel Prize medals, which at that time were still minted in 23 carat gold (since 1980 they’re just 18 carat with 24 carat plating; hardly worth winning anymore, I’d say). When the Nazis invaded Denmark, another (future) Nobel laureate and chemist George de Hevesy decided to hide their gold medals. Instead of burying them, or secreting them in some hidden drawer, he thought like a scientist and dissolved the gold in aqua regia; stored the mundane, reddish-brown solution in his lab among all the other bottles; then fled to Sweden. The Nazis, upon searching his lab, saw only bottles of common chemicals. After the war, de Hevesy returned to Copenhagen and found his bottles undisturbed. He precipitated the gold out of solution and the Nobel Foundation recast the medals for von Laue and Franck.
So instead of hiding gold Krugerrands or kilobars in expensive safes, easy pickings for whatever Neal Caffrey that seeks to crack it, you should be hiding gold dissolved, in bottles of “red wine vinegar” or maple syrup”!
Just for fun (and since I know amateur gold scavengers Google this stuff), here’s How to Dissolve and Precipitate Gold Using Aqua Regia:
Aqua regia is a 3:1 molar ratio of concentrated nitric acid and concentrated hydrochloric acid. Its pH is near zero, so it’s HIGHLY acidic. It is so named (“royal water”) because it has the unique ability to dissolve noble medals like gold and platinum (see what I did there?).
- Au (s) + 3 NO3– (aq) + 6 H+ (aq) → Au3+ (aq) + 3 NO2 (g) + 3 H2O (l) and
- Au3+ (aq) + 4 Cl– (aq) → AuCl4– (aq).
Solid gold reacts first with the nitrate from the nitric acid which oxidizes the gold into the gold ion. However, this equilibrium leans toward the left, meaning very little gold is actually dissolved… yet. This is why nitric acid alone can be used to dissolve other metals/compounds in the purification of gold. The second reaction removes Au 3+ from the first reaction by converting it into aurochloride, and drives the first reaction to the right (LeChatelier’s principle).
Now to precipitate the gold and get it back into a solid state, I found this helpful patent online. You add a strong base like NaOH until the pH of the solution rises to 2.8-3.0. At this higher pH, the nitric oxide remaining in the solution will no longer oxidize gold and just redissolve anything we precipitate. After filtration, add butyl stearate (an emulsifying agent to prevent coalescence of gold particles during precipiation) and sodium sulfite (or I’ve seen sodium metabasulfite elsewhere). The sodium sulfite (or sodium metabasulfite) forms sulfur dioxide, a strong reducing agent:
Na2SO3 + 2 H+ → 2 Na+ + H2O + SO2
Which leads us to (I think):
2 AuCl4– + 3 SO2 + 6 H2O → 2 Au (s) + 3 SO42- + 8 Cl– + 12 H+
Filter, wash the gold powder with acetone and water, bake, melt, and you’re back to your solid gold at ~98% recovery!