cnidocyte
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Hotplates + RB flasks
I invested in a hotplate thinking that it will be the only heat source I'll ever need but all my flasks are round bottomed and it seems a heating
mantle would be way more practical for heating RB flasks than setting up water baths and oil baths. I like water baths because quick and easy to set
up and clean up but I've noticed its impractical to try and distill anything with a BP of over 70C with a water bath. Oil baths drive mass amounts of
heat into flasks rapidly but they're messy and dangerous as hell. I don't know of any other ways to transfer heat from a hot plate to an RB flask.
Anyone know any other simple ways to do it?
Also are flat bottomed boiling flasks made to be placed directly onto hot plates?
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Picric-A
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What about air baths? Simply clamp the RBF hovering just above the hotplate and make a cone of aluminium foil around the flask and hotplate.
This heats the flask fairly evenly an you cant get much more easy to set up and clean!
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hissingnoise
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Flat-bottomed flasks can be placed on a hotplate as long as the plate isn't too hot to start with . . .
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cnidocyte
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An air bath, thats exactly the kinda thing I was looking for. Thanks.
[Edited on 25-1-2011 by cnidocyte]
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Arthur Dent
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And the other alternative, although it takes an eternity to warm up and to cool down, is a sand bath. An old 1L metal pot filled with sand will warm
up very evenly and can give temperatures well above 200C.
The downside is that it takes a long time to warm up, and your only alternative to interrupt the heat once a reaction is done is to take the vessel
out of the sand (not an easy task if you have a complex ground glass joint apparatus sitting on top of your vessel.)
So a sand bath would be recommended if your RBF is connected to whatever apparatus with flexible tubing and you can just "slide up" the clamp that's
holding it in the sand bath.
And one thing to do if you'll use this is to "cook" your sand a bit before using it, to get rid of the "smoking" organics and impurities. My throat is
still sore from choking on the fumes!  "cough"
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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cnidocyte
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Quote: Originally posted by Arthur Dent  | And the other alternative, although it takes an eternity to warm up and to cool down, is a sand bath. An old 1L metal pot filled with sand will warm
up very evenly and can give temperatures well above 200C.
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Forgot about sand baths. I've used them plenty of times in college. In my experience they take a ridiculously long time to heat up but I suppose you
could give it a head start by preheating it in the oven.
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cyanureeves
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hey! i like the air bath method.what about sand baths?i'm not suggesting rather asking because i've only used flames,but someone here used a crock pot
full of sand for a sand bath. would a crock pot heating plate deliver enough heat through sand to heat a RBF to distill nitric acid from sulfuric acid
and nitrate salts?also would an alcohol burner generate a hot enough flame to a glass retort if i want to distill nitric acid?i'm kind of intimidated
of hot oil.
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peach
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Foil skirts are the way to go. Get a foil beanie on at the same time to block the mind control rays.
Oil baths are absolutely horrible. The mess, the danger, not worth it at all. I've heated flasks right up to 200C and over using a foil skirt alone,
so it's not like you're loosing out much with one. If you're using everyday solvents in a reflux, the temperature's going to be tens of degrees anyway
- not 3,4 hundred.
There are other advantages to foil skirts, their low thermal mass and gentle heating makes it less likely you're going to shatter things and, my
favourite of all, if something gets carried away, you can very quickly, easily and safely slow it back down by simply removing the foil; whereas
moving a bath full of scalding hot, slippery oil in brittle glass is where people end up with 3rd degree burns.
If you find it's taking a while to warm up, or you're loosing too much heat, adding an extra layer, loosely over the first, makes for a useful boost
in insulation. The skirt shouldn't touch most the flask and, ideally, you want the base of the skirt to cover as much of the plate area as possible.
Leaving a small gap around the bottom and one at the top can help convection carry the heat through the tent.
Mantles are good, but you're also limited in the size of flasks they will take to a greater extent than you are with a hotplate.
Hotplates can usually go a bit higher in terms of temperature, and you can use them for heating flat bottomed objects - for instance, an evaporating
dish. I've also used mine to dry salts by simply sticking a piece of foil over the top and emptying them out onto that. If the surface ends up filthy
with muck, you can turn it all the way up and leave it on for a few hours, which will burn the surface clean again.
Hot air guns can reach 400 to 450C, and are also much more useful than I realised in terms of heating chemistry experiments, especially test tubes and
micro glass. Borosillicate glass shouldn't be run at more than 500C, and only for short periods and with gentle cooling. QuickFit use 500C open ended
conveyor ovens to anneal the glass. Around 500C, you can fix stress in the glass if it's not cooled properly; meaning it may spontaneously fail at a
later date. Once the blow torches come out, molten glass may be not far off. Making hot air guns a good choice. They're great because you can get
adjustable ones for about £20 and they come with lots of handy attachments.
I suppose if you could find a suitable metal bowl (I have a big stainless one in the kitchen for making cakes), and then wrap it insulation, you could
make an air bath mantle. Or you could use those stainless jars that are used to store coffee, sugar, herbs and tea in. Those have the added bonus of
coming with a metal lid, allowing you to cut a hole in it to sit the flask in.
Of coarse, you'll also need some dewalt ear plugs to counter the noise of the dewalt hot air gun, and once you've got them you may as well get
the dewalt ear defenders, shades, boots, googles, gloves and socks too*. 
*Yes, they do make all of these things, socks included. 
[Edited on 26-1-2011 by peach]
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cnidocyte
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Another exceptionally informative post from peach. Nice one. I'm gonna pick up a heat gun next time I'm in the hardware shop. I have a butane
blowtorch but those things go up to about 2000C which is severe overkill for the majority of the things I need to heat.
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cnidocyte
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I'm testing an air bath out now, I have the hot plate up at 150C (the BP of what I'm distilling is around 105C) and the still head has vapour in it
but the thermometer isn't giving a reading yet. I have the apparatus outside on a cold and windy day though so taking that into consideration I'd say
its working pretty well.
EDIT: I retract that statement. The hotplates been at around 200C for 30 minutes and the thermometer still hasn't got a reading. I covered the
stillhead in tin foil and pushed the thermometer down further but still no luck.
[Edited on 27-1-2011 by cnidocyte]
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peach
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Don't use the temperature settings on the hotplate as an assumed distillation temperature. They don't take into account for losses. This is why
chemists like using reflux points as temperature references - because it doesn't matter how hot the equipment is, the fact the solvent is boiling
tells you the reaction it's self is at that specific temperature. It's better to think of the hotplate not as a temperature control, but a source of
heat energy. The temperature the still reaches is based on the amount of heat energy going in versus the amount being lost. The numbers on the plate
may as well be 1,2,3,4 / A,B,C,D / &,%,£,@ or just Cold ----------> Hot
I can assure you, this tin foiling does work.
Can you take a photo of it next time you do it?
What are you heating and how big is the flask as well? It can take a good while to heat the bigger, higher specific / latent heat distillations. More
so if going from cold and if it's being done outside in the wind. If you're using a vigreux, it can take half an hour for the vapour to climb and warm
the column. Ideally, columns should be insulated, not cooled.
Once they're going, it can take three or more hours to finish a litre sized distillation.
[Edited on 28-1-2011 by peach]
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