Molecule of the Day

The noble metals seem to have the most interesting chemistry. Silver (I) nitrate, or AgNO₃, is a soluble compound. Its crystals are boring, whitish, maybe violet or gray if they’re a bit old, but only just. Its solution in water is clear. If you spill the dilute solution on yourself, it doesn’t sting at all (in fact, many hospitals still use silver nitrate eyedrops on babies, in case mom is harboring an STD that might make its way in through the newborn’s vulnerable eyes). Nothing too interesting happens until you go out into the sun. Then, photoinduced electron transfer from silver to, well, whatever’s lying around happens. It stains skin and clothes pretty readily. The silver (I) ion is reduced to silver (0), or silver metal. Interestingly, it looks boring and black, not silvery. Nitric acid will remove it, albeit at the risk of losing the use of your hands and/or pants. Better to try sodium thiosulfate.

Lots of metals look funny at this scale. Gold nanoparticles aren’t gold, they’re red! (See, for example, figure 4 here). True nanoparticles of silver will look yellow. The bizzare alchemy practiced by those who made stained glass was actually some of the earliest chemistry performed at the nanoscale; making red stained glass actually involves a touch of gold metal. No doubt this added to the magic. Gold being fabulously unreactive, it is one of the most durable red pigments around (you would still be pressed to beat it, actually; remember how old some of these still-red cathedral windows are).

Also interesting about silver nitrate is that it’s one of the only soluble silver compounds. Very few counterions will make silver soluble, and those that do (such as nitrate, tetrafluoroborate, and perchlorate) are those that are the kind that tend to be soluble with any counterion as their mates. This makes silver nitrate useful in the detection of halide ions (this is what I’m up to with it, actually - the test is ancient, but fast and easy). Here is an image of the process. Note that the movie doesn’t work. One of the cruelest things J. Chem. Ed. ever did was take their movie collection offline.

Another, more spectactular, thing you can do with silver nitrate is react it with magnesium. See, silver (I) is such a prodigous stain because of its tendency to get reduced to silver (0). Not coincidentally, magnesium metal is quite a good reducing agent. If you intimately mix silver nitrate and aluminum powders, make a small pile, and carefully add a drop of water, quickly moving away, you’ll see an incredibly intense flash of light. If you are not lucky, you will be hit by molten Ag(0) - silver metal. This is one of those incredible reactions kids used to see growing up - maybe you did, this is one of the classic flash-zoom-wow reactions, so if it sounds at all familiar, there’s a good chance it was this one. See, for example, here and here. It’s heartening to see that people still even consider doing it in a classroom!

Sadly, chemistry sets (I can’t find a decent link. They’re universally miserable. If your kid is interested in science, learn a little with her and buy your stuff elsewhere, like photo and hobby shops. Borax-elmer’s glue slime is neat, but it’s not going to cause any nitric-acid-acts-on-my-trousers moments that make people love science in a visceral way. Teach your kid to wear goggles and be careful. This is less dangerous than football if you use your head - my clumsiness aside, I have hurt myself many more times playing outside than playing in the lab, where I spend much more time.) aren’t what they used to be, and it’s not looking good for amateur chemists anywhere; the government is working on regulating all chemical sales to amateurs. While you still can, check out United Nuclear, an absolutely amazing company. Even though I work in a lab and can order a bit of tetrodotoxin on any old afternoon I please, I shake my head in amazement at this website every time I go there. I really wish I’d had half this stuff to play with in high school. If you like the idea of what they’re doing and worry about losing amateur access to reagents, you may wish to support them.

Undoubtedly I will stain my hands with silver tomorrow, like I always do when I work with it. Silver is not the lifesaving miracle the colloidal silver people make it out to be, but the odd milligram here and there on your skin isn’t going to kill you. I’d like to think my technique is pretty good; when I’m working with radioactive tracers there’s no hiding it: the geiger counter chirps if you spill something. My workspace rarely comes up hot, and if it is, it’s the odd droplet that can easily be cleaned up. (If you’re not careful and get the tiniest bit on your gloves, you can rapidly contaminate the better part of a room. It’s amazing how many things you touch without thinking.) No matter what, though, I seem to get some silver on my fingertips. My carelessness is your gain, though, you’ll have a better picture of a silver stain!
There is way too much to say about silver. It is used by crackpots, at the cost of gray skin. Including Stan Jones, a Montana gubernatorial candidate (!). It is used in photography. It can be used to make explosives. I believe the explosives are too unstable to be used as articles of commerce and are mostly fall under the auspices of “laboratory curiosity”/”toy of the terminally curious”.

See you tomorrow!