Runningbear
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Size of flat bottomed flasks that can be used on hotplate.
I've read that larger flat bottom flasks can explode when used directly on hotplates. Is this only likely to happen when using heat and vacuum? Or
just heat only.
Also, what is the maximum size flat bottom flask you would apply directly to the hotplate? I have read that 150 ml is about as large as you want to
go. Is this correct?
thanks in advance,
Runningbear
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entropy51
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I have used one liter Erlenmeyers on a Corning hot plate for steam distillations, with the heat just high enough to boil water, without incident.
I definitely wouldn't crank the heat higher, and I would never pull a vacuum on one of these boys. I have refluxed small amounts of
flammable solvents in this way, but it's probably not a good idea. For low boiling solvents, better to immerse the flask in a pan of hot water on the
hotplate.
[Edited on 14-1-2010 by entropy51]
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bbartlog
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I've also used a 1 liter Erlenmeyer on a hot plate. Haven't turned it above 320C; frankly it hadn't occurred to me that this might be dangerous, since
I've previously heated it over a (low) natural gas flame on a stove without any incident. Though I suppose that with the hot plate you have extremely
local hot spots. Mind you, I make sure to put the flask on the plate and *then* turn on the heat; I wouldn't plop a cold flask on an already-hot hot
plate...
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protem
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actual practise
In fact this can be done with relative impunity - I have heated 24/40 ST ground glass jointed Florentine flasks under aspirator vacuum for years
without any failures. Flasks are mainly Borosil 250ml - 1L. Temps have been up to 160C under vacuum for hours. I lag the flasks with fibre cloth
and foil. Flasks always have good magnetic stirring and are heated and cooled gently. Hotplate is Cimarec 2 never set higher than 6/10. Flasks
should be inspected for flaws and retired after several uses, just in case. I have used individual flasks ?50 times in this way which may be too
many. Always use some form of shield when severely heating corrosive mixtures. Always have a fire extinguisher handy. Never leave unattended.
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sonogashira
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I was wondering, on a related concern, if a buchner flask can be heated? (Perhaps in oil/sand bath rather than directly on the hot plate).
I don't know much about the different types of glass used - but would it be suitable for heating?
My thought was to adapt one to do distillations with since there is the 'side-arm' already in place! - so it would need to be heated to high
temperatures. If anyone has any thoughts if this is a good/bad idea I would be grateful of the assistance! 
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