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Author: Subject: heat loss b/w flask and plate
peace c
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heat loss b/w flask and plate

hello all,
am wanting to distill a liquid from a flat bottom flask. Can this be placed directly onto a hot plate/stirrer?

Also is there going to be any heat loss between the actual plate temp and the temp of the contents of the flask and if so how big is this difference likely to be? For eg if i am wanting to distill at atmospheric pressure of say 250 degC and the hplate/stirrer is only rated to 300 degC will it maxout in trying to heat the liquid to 250 because of heat loss?

What could be done to minimize such loss? Foil wrapping etc.

Sorry for these basic questions.

hope some 1 can help.
Magpie
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I don't know why you couldn't do as you propose assuming your flask is borosilicate glass. I've heated Pyrex beakers directly on a stirrer/hotplate with no problem many times. However, use of a heat distribution screen placed under the beaker is probably a better practice.

Yes, there will be a delta T between the hotplate and your liquid. The rule-of-thumb that IIRC someone stated on this forum was 30 deg C.

An aluminum foil wrap will help insulate your flask. A foil-fiberglass-foil sandwich would be even better.

The single most important condition for a successful synthesis is good mixing - Nicodem
aldol
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heat loss

yes magpie there is no chance of heating a flat bottom flask to that temp on a hotplat even with insulation
i moderfied my hotplate it is alluminium and i welded sides on it and its now a oil bath .
or tip out the oil and use dry it will work mutch better
regards aldol
neutrino
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It’s not that the temperature difference will automatically be 30*C, it’s that it shouldn’t exceed this.
peace c
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Thanks for the replies. Um so am i right in saying that the contents of a flat bottom flask cannot be heated up to 250C on a hplate/stirrer? That the diff intemp between the liquid in the flask and the plate will be approx 30C?

-p.c.
neutrino
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The difference shouldn't exceed 30*C, although it very well could. If you’re distilling under atmospheric pressure, this shouldn’t be much of a problem. For example, when distilling things with a bp of ~100*C, I often see the wires in my heating mantle glow red. I don’t know if thin wall vs. thick wall matters (I imagine that it should), but my flask is standard (thin) walled Pyrex.
Organikum
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A wrap wont be of much use, but a kind of tent around the flask works wonders.

/ORG

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peace c
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Ok thanks all. If one is doing standard distillation should i go with a heating mantle/rbf or hplate/stirrer and a flat bottom flask.

Is one just as efficient as the other. Want to get this straight before i shell out the bucks. Seems that hp/stir is more versatile in that it can use different size flasks and is cheaper.

Wraps- What otc material could one use?

thanks,

pc
neutrino
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Any kind of fiberglass matting/cloth should work well. Beware of fiberglass insulation, it usually has a layer of plastic on it which could burn off. As for heating efficiency- a mantle would certainly be more efficient, but a hotplate is more versatile. I'd recommend the plate for its versatility. Try to get a used one off ebay, I got a broken stirring hotplate for ~$40 and fixed it somehow. If you're lucky, you'll find a used mantle on ebay for ~$15. Just wait for it, the deal will come.
Organikum
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Hey! trust me on this: Building a nice "tent" of Al-foil around your flask beats every insulation whatever! Its just a variation of the "Hot Air Bath" and works perfect!
You can even leave a small slit open for watching.

/ORG

[Edited on 31-3-2005 by Organikum]

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runlabrun
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so org your saying build a cap of alu foil (few layers? to make it thick and ridgid?) from the flask neck to the hotplate surface? air bath the flask over the hotplate? or do you mean to build the tent over the top of the oil bath container?

-rlr
neutrino
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That’s an ingenious solution, Organikum. I will make sure to use it sometime.

runlabrun: I’m guessing that all of those would work, the one with the hotplate is the one that works with a simple plate.
Magpie
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I think that one reason the aluminum foil works so well is that radiated heat is reflected back into the flask. My lab text says it is best to keep the foil shinny side in (glass side).

The single most important condition for a successful synthesis is good mixing - Nicodem
runlabrun
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Maybe as per heat insulation info i have seen make a aluminium foil - fibre glass - aluminium foil sandwich and use that....
This might be more efficient and be easier to craft to the correct shape/size etc...

Many thoughts...
-rlr

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