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Texium
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Posts: 4580
Registered: 11-1-2014
Location: Salt Lake City
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Mood: PhD candidate!
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The organic chemistry stockroom at my university has been continually promoting the filling of heating mantles with sand. And they wonder why they
have such a high failure rate! That doesn't fly in the sections that I teach. I will continue to wage my private war against them for the rest of this
semester. Then I'll be done teaching and couldn't care less about how many perfectly good mantles they burn out...
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Fyndium
International Hazard
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Registered: 12-7-2020
Location: Not in USA
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Sand, dehydrated gypsum or even ceramic tiles are such good insulators they will burn even thicker gauge kanthal wire, I've experienced this so many
times with my electric oven that I moved to propane heated after trying to mess with it.
I would absolutely never pour anything into heating mantle. Not sure about hotplates how they could be protected, possibly by using a thick steel
plate as temp buffer/conductor between it and the sand bath? I'd got 20mm dumbbell plates 5kg each for this purpose. Air gap might not be any better,
since air is the best insulator of all easily available materials after vacuum, in practice.
There are plethora of instances where open flames pose great fire hazard, so a closed circuit heating element for sand baths would be necessary. In my
instance, I just make it case sensitive and use sand bath for +250C operations where I make the environment suitable for gas flame, and for anything
below 200, oil bath with induction heater works best, and then there is also the heating mantle without any additives. My heating mantle is rated up
to 400C, but I'm gonna go easy with it, don't want to burn the heating elements and likely never gonna hit above 200 with it.
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Sulaiman
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Open flames provide an obvious ignition source,
but the surface of a hotplate heating a sand bath will quite likely be above the autoignition temperature of common solvents - a not so obvious
hazard.
PLUS, if a flask does fail it will dump the boiling liquid into the sand - which will be at a MUCH higher temperature than the already boiling
liquid...
flash boiling!
What fool had the idea to put sand in a heating mantle?
Even a hobby chemist (like me) is not that dumb.
To distill concentrated sulphuric acid I recently used cheap Chinese glassware ABOVE (not in) a gas flame
(portable butane gas stove)
No matter how you heat glassware you should EXPECT it to fail and prepare for it,
if the glassware survives consider it a bonus.
(I've not had glassware break due to heating yet, but I do try to prepare for it)
CAUTION : Hobby Chemist, not Professional or even Amateur
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Texium
Administrator
Posts: 4580
Registered: 11-1-2014
Location: Salt Lake City
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Mood: PhD candidate!
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I don't know who had the idea, but it has infected
the entire chemistry department at a major research university... I'm definitely going to have a chat with the stockroom manager. It's always
refreshing to have what I take for common sense validated here.
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Fyndium
International Hazard
Posts: 1192
Registered: 12-7-2020
Location: Not in USA
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Quote: Originally posted by Sulaiman  |
To distill concentrated sulphuric acid I recently used cheap Chinese glassware ABOVE (not in) a gas flame
(portable butane gas stove)
No matter how you heat glassware you should EXPECT it to fail and prepare for it,
if the glassware survives consider it a bonus.
(I've not had glassware break due to heating yet, but I do try to prepare for it) |
I used to heat all glass with direct open propane flame or on metal hotplates when I was young and very reckless. I still say some good words that I
never cracked a single glass; I smashed them only when cleaning or handling them otherwise, including dropping and entire operating distillation
apparatus on the floor once. The most amazing part was that only the distillation head broke. Gosh, I get creeps thinking those times.
One should not trifle with safety, though. All it takes is one cracked flask half-full of low boiling solvent, and you have instant out-of-control
fire that will burn down your entire house unless you have a large scale distinguishing capacity within hand's reach.
Sulfuric acid is one of these exceptions where I'd just use sacrificial glass and open flame, and put it in some sort of container than can limit the
volley of boiling sulfuric acid if a catastrophic failure is to occur. When I vacuum distilled a little sulfuric acid from battery acid, I built a
quick enclosure from plexiglas and plywood around the boiling flask in case of failure and kept a pressurized water hose at hand. I did this in a
space with all-concrete surfaces where such a splash wouldn't cause too much issues.
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