From: ILPI Support <info**At_Symbol_Here**ILPI.COM>
Subject: Re: [DCHAS-L] Benzene flammability teachable moment
Date: Tue, 23 Jan 2018 08:59:24 -0500
Reply-To: ACS Division of Chemical Health and Safety <DCHAS-L**At_Symbol_Here**PRINCETON.EDU>
I used to do that all the time - in an inert atmosphere glove box. Benzene was the solvent of choice for my starting material preparation because the byproduct salts were insoluble in it, and I never found a substitution that would work (for example, the salts were soluble in toluene). I would evaporate the benzene on a rotavap to recover my product and, of course, evaporation causes cooling which not only slows down the evaporation rate, benzene will also freeze if you have any significant volume. The ordinary way of dealing with that in bench chemistry is to use a water bath, but that's not an option when you're operating in a no-oxygen, no-water environment. We generally used compact Clairol 1200 watt blow dryers as heat guns given their cheap price and small size. Went through one of those a year. But there's no real ignition threat in a glove box unless you really screw up.
But still, evaporating benzene is a real pain in the neck. Thermal transfer to glass to a frozen solid is slow, particularly with benzene which has a relatively high bp (80 =B0C). I could easily see this happening if they weren't pulling a vacuum on the flask at the time and were trying to thaw the stuff in open air.
A simple example of sublimation causing issues is what happens if you try to stick your head into a dry ice storage chest to get a block at the bottom. It's an IDLH situation - it's nothing but CO2 down there and if you inhale, you risk asphyxiation. Similar situations have caused death. Here are a couple more; not all include sublimation, however:
And, of course, dry ice bottle bombs are an all-to-real example. As I've discussed here previously, I consulted on a case where a K12 student lost an eye because his teacher decided building these (a felony in most states) was a good "demonstration" of sublimation.
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I am very interested in identifying opportunities to use safety examples to illustrate scientific principles as a teaching strategy for both safety and science.
Along these lines, a colleague shared this anecdote with me during a discussion on a different topic:
"I had a very cautious student take a heat gun to a flask of frozen benzene to warm it up. They were truly surprised by the fire that resulted."
It was not intuitively clear to me what was going on here until I looked up the melting point of benzene (42 degrees F) and the flashpoint (12 degrees F) and saw that solid benzene is still flammable. Am I correct in interpreting this to mean that frozen benzene is sublimating enough gas to support a flame if the vapor doesn't disperse too quickly? This seems to me like an opportunity to connect the scientific concept of sublimation to a memorable lesson by using safety data and connect science and safety in an educational way.
I wonder if other people have similar examples of Science taught through considering Safety Scenarios.
Thanks for any thoughts on this.
Ralph Stuart, CIH, CCHO
Environmental Safety Manager
Keene State College
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