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How Not to Do It

How Not to Do It: Sodium

We have a lot of pyrophoric substances in an organic chemistry lab – things that burst into flame when they encounter normal air. Liquids are handled by syringe and needle, using bottles fitted with gas-tight septae. That works pretty well, once you get the hang of it. The main things you have to learn are to provide some inert gas as a replacement if you’re removing a large volume, and to not twitch your arm and hand muscles while you’re holding a syringe full of stuff. (That can provide a spectacular flamethrower effect, which is fine if that’s what you’re after, but we rarely are.)
Pyrophoric solids are a bit trickier. Some of them (like sodium hydride) are often sold as a fine powder mixed with mineral oil, which coats everything and keeps it from igniting. Of course, you have to get rid of it at some point, because it’s sure not going to go anywhere. You can either wash off the mineral oil before you begin, while the solid is inside your reaction flask, or clean it away from your compound at the end of the reaction. I usually opt for the latter, on practical grounds: by the time, I’ll know if the reaction worked, and I won’t have wasted the initial effort on a loser. Besides, most of those reactions need purification anyway.
You can buy dry sodium hydride, but I’m not a fan of the stuff. It goes bad too quickly, and an NaH fire is a beast to put out if it really gets going. A carbon dioxide extinguisher usually isn’t up to the job, and it’ll blow the powder all over your lab, which isn’t recommended. And you most surely don’t want to throw water on the stuff, although you’d have to be the village idiot to try that one. You’d certainly get a village idiot’s reward for your efforts. The only reliable way to put one of these things out is to bury it in sand or some other inert powder. Then you have to let it cool down for a while – if you don’t, it’ll just whoomph up on you again when you try to clean the place up. If that doesn’t make you feel like you’re wasting your day, I don’t know what will.
The related potassium hydride is invariably sold in a hard-to-handle suspension that’s mostly oil. I’ve never seen it packaged dry, and I don’t want to. It ignites much more easily than the sodium compound, to the point that people even manage to start fires with the oil-soaked item. At least the flames are prettier.
And the metals themselves are usually sold and stored under oil. Sodium metal, as my fellow chemists know, is interesting stuff. It’s soft enough to cut with a metal spatula (the texture is rather like cold butter), and is very shiny indeed until the air hits it. You can work with it out like that, if you move with reasonable speed and get it under some inert solvent. Potassium metal is much less forgiving, and I have no desire to work with the heavier metals in the series (rubidium, cesium) as their elemental metals, because they just get worse as they go up.
You can mess up the area with just plain sodium, though, oh yeah. Some collagues of mine had a summer undergraduate (here’s where experienced chemists start to grin and pull their chairs closer), and they were teaching him how to handle sodium metal: take it out of the oil, have your beaker of hexane ready, cut it like so, pick it up (the point of the spatula works well), drop it in the solvent, and so on. Everything went fine. So the next day, one of the guys tells him to go down and weigh out, say, five grams of sodium for a reaction. The summer student scampers off, and a few minutes later, the grad student wanders down to have a look, just to make sure things are going OK.
And there the guy is, with his beaker of hexane, sawing away with a spatula at a big cylinder of sodium metal which he is gripping in his bare left hand. Well, he hadn’t been told not to do that, true, but neither had he been warned not to fetch it in his teeth. You just sort of take these things for granted. The summer student had no doubt taken the skin of his left hand for granted, too. (It took a few weeks, but he recovered.)

4 comments on “How Not to Do It: Sodium”

  1. It certainly sounds like a lot more fun than some of the things people can catch if they make mistakes with level 3 biohazard material. But then we do try to keep students out of those areas ;-). Your story does remind me of another type of blooper which I have had to observe (and perform first aid on afterwards).
    When performing certain sterile techniques a glass/metal implement is often kept in 70% ethanol, which is removed by holding it briefly into a flame and letting the ethanol burn off.
    Needless to say that before you start work, you have to sterilise your hands and forearms with…. ethanol (we don’t use that anymore unless we can help it, but that’s what we had at the time). Most people will see where this is going now.
    My bright-spark co-worker student rinsed his hands, immediately picked up one of the implements and held it in the flame. The fire went up to his elbows and caused second-degree burns before I could get a blanket on him. of course most of the ethanol had burnt off by that time, possible rendering making my quick reaction unnecessary. The skiver had his arms in bandages for two weeks which meant I had to do all the experimental work as well as the typing of course.

  2. Linkmeister says:

    Makes teletype practical jokes (go get me a box of carriage returns and line feeds) pale in comparison.

  3. The Un-Candidate says:

    I might have been like that summer undergraduate, but dumber:
    http://30days.typepad.com/30_days_til_my_exam/2004/06/real_stories_of.html

  4. Mike says:

    Ahh, sodium fires!!! It was the winter of 1979 and I was partnered with another undergraduate, who is now the senior researcher at a major US photoresist company, who shall therefore remain nameless, beyond TOM. TOM and I were doing our Na-catalyzed organic reaction, and I was the one sent to get the 5g of sodium. No problem, I knew not to fetch it with my teeth. Well, he decided we needed 7g to get a really good yield, and he went back to get 2g more. The reaction went just fine, and I was clearing up the reagents. Sodium chunk safely back in the big bottle, and 20ml of hexane in beaker (where the 5g and 2g of Na had been held prior to reaction) went into the sink (1979! ah, what a decade). Instant sink-shaped pyre of incandescent flame! TOM had neglected to tell me that in his desire to add an additional 2g Na to our reaction, he had shaved a bit off his second piece, to get exactly 2.0 g, and had left a few nearly invisible slivers in the hexane beaker. By the time I had gotten back to the sink with the 3foot tall CO2 extinguisher, the hexane was nearly burned out, but what the hell, how often would I get a chance to empty one of those giant extinguishers? No harm done, fun had by all.

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