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February 1, 2012
Potassium Hydride Is Not Your Friend
Noted chem-blogger Milkshake seems to have had a close call with a fire started by a tiny potassium hydride residue. It looks like he made it through without serious injury, but that sort of thing will definitely shake a person up.
I hate potassium hydride. Its relative sodium hydride is a common reagent, but it's much tamer (and even so, can cause interesting fires - I knew someone who ignited a heap of it on the pan of a balance while he was weighing it out, which slowed things down a bit). Sodium hydride is usually sold as a 60% dispersion, a dark grey powder soaked with mineral oil to keep it from deteriorating too quickly (and to keep it from setting everything on fire). You can buy 95% sodium hydride, the dry stuff, and there are people who swear by it, but I tend to sweat at it. You never know if it's been stored properly; you may be adding a slug of sodium hydroxide to your reaction without knowing it. And there's the fire part. You'll want to move briskly if you're using the 95%, and I'd pick a day when the humidity is low.
But potassium hydride, that's another beast entirely. It makes the sodium compound look like corn meal, in terms of how forgiving it is. You can't get away with the clumpy oily powder form at all - traditionally, KH is sold as a gooey dispersion of grey powder sitting under a few inches of mineral oil. If it's well dispersed, it's supposed to be 35%. You shake the stuff up until you think it's even mixed, then pipet out the amount of gunk that corresponded to the KH contained therein. Sure you do. What actually happens is that you pipet out the stuff, noticing while you do that it's already settling out inside the pipet, thereby to clog it up when you try to transfer it. No fun.
It's becoming available now dispersed in a block of wax, which is not such a bad idea at all. Wax isn't any harder to get out of your reaction than oil is, and you can carve off chunks and weigh them without so many what-am-I-doing moments. But Milkshake worries that this ease of use will lead to more fires during workups (which is where his reaction ran into trouble), and he may well be right. If you're going to use KH, don't let your guard down.
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