Sciencemadness Discussion Board

Direct heating of flat-bottom flasks

melvinthedestroyer - 18-7-2011 at 19:06

Hello sciencemadness!

So I have been reading several posts on this website (as well as some other internet articles) describing the direct heating of flat-bottom flasks using hotplates.

In my university lab (this was back when dinosaurs roamed the earth), any time a distillation was carried out, we used round-bottom flasks and either a water / oil / sand bath, or a heating mantle. Flat bottom flasks were only used for... well actually, now that I think of it, we pretty much never used them.

Anyway, is the direct heating of flat-bottom flasks on a hotplate considered safe laboratory technique? What is the proper procedure for carrying this out? I would presume you would heat the vessel from room temperature, slowly, on the hotplate.

Is there a maximum allowable temperature, at which you should not perform this? Is the temperature distributed to the flask contents evenly? Will any type of glass do, or just borosilicate? Is there a preferred ratio of hotplate surface area to flask volume, such that there would be a maximum flask size for a given hotplate? Are there any other considerations?

If this is not proper technique, I assume these vessels may be heated using a bath, right? FYI, I AM aware that you should never pull a vacuum on a flat-bottom flask.

I ask because I am inheriting a 2L, 3-neck flat-bottom and a 6L flat-bottom and I want to know if I can use them in a distillation setup. I've never used them before!

Thank you for the information!

White Yeti - 29-7-2011 at 06:06

Usually, flat bottom flasks are used when you need to ramp up the temperature really high to perform a dry distillation. The sad truth is that if you perform a dry distillation in a flat bottom flask, you run the risk of destroying said flask. The deposit is also rock hard and almost impossible to remove.

I think that a flat bottom flask should be fine for performing reactions that require low temperatures, haloform reactions for example. Other than that, I wouldn't use such a flask for distilling liquids that boil at temperatures higher than 100C. So just to air on the side of caution, I would only use this flask for temperatures not exceeding 100C. This is by no means a bad thing, there are plenty of reactions you can run within that temperature range. 100C is also an arbitrary number, you can go higher, but don't be surprised if things start getting dangerous.

S.C. Wack - 29-7-2011 at 11:14

It depends on what the piece of glass is spec'd for. Anything with thick glass was definitely not designed for heating. Off-brand, used, etc. glass weirdness is always something you just have to sort out for yourself, preferably in a way that doesn't burn your house down and all.

smuv - 29-7-2011 at 21:16

I've heated flat bottom flasks with a hotplate directly. It is really not the best thing to do to the flask, problems. Even with a thick walled 1L.

The thing is, just putting a flask on your hotplate kind of sucks if you want to get to a high temp or run a distillation of anything above say 100c. There just isn't enough contact area with the hot plate. I hate oil baths...but if you have no mantle, bite the bullet and use one.

For low temps though, I do this all the time. At work I heat my schlenk tubes to about 40c just by putting the butt of them on the hot plate. I have done this hundreds of times. Never a problem.

GreenD - 1-8-2011 at 10:25

Actually, the erlenmeyer flask was made just for recrystallizations - i.e. heating up hexanes or solvent pair systems...

The more you know :D

dann2 - 2-8-2011 at 14:26

Not a bad method of heating flasks in a heat gun. It's not very professional I guess but it gives good even heat. You can heat very slowly at first if you so wish as you have good control of temperature. Maximum temperature can be quite high.
You could go the ostentatious route and make a fancy holder for the heat gun!
A hair dryer would do for those jobs requiring the very gentle touch.


jon - 3-8-2011 at 19:43

another trick is to wrap that flask in aluminum foil so your heat is spread out evenly and you don't have areas of localised high temperature differentials that's what cause flasks to break.
i remember using a snapple bottle heating it to 200 C this way in a salt bath and nothing bad happened just be sure to allow it to cool naturally don't let it cool to fast or it will shatter.
patience is a virtue.
and guys don't use oil baths use sand or salt even iron filings for heat retention.
that way you don't have to worry about a disaster happening if a flammable is boiling in an oil bath and the flask breaks in a flammable oil.
or something toxic that could poison and incinerate you at the same time.
if something bad happens dump a shitload of sand on top of it.
buckets of sand, class b fire extingushers, nomex, and wet towels, are a chemist's best friend.

[Edited on 4-8-2011 by jon]

smuv - 3-8-2011 at 22:28

Quote: Originally posted by dann2  

It's not very professional


You would be surprised what heat-guns are used for in 'professional labs' :P

Mildronate - 4-8-2011 at 04:21

It is professional its not educational. In university always we use roundbottomet with glycerine bath, but at home i heat direct on hot plate. Just need good glass.

fledarmus - 4-8-2011 at 04:29

This is very useful for heating solvents to their boiling point for recrystallizations. It isn't as easy to control the temperature as an oil or sandbath and a round bottom flask with a magnetic stirrer would be, but it's quick and clean to set up and if all you're doing is boiling a solvent, not too bad. At least, unlike a heat gun, your hands are free to do something else at the same time.

The biggest problem is actually the hardness of the hotplate. I've broken a couple of erlenmeyers tapping them against the side of the hotplate when I was moving too quickly.

ksj_6808 - 13-8-2012 at 08:24

So i bought a 4,000mL flat bottom flask the other day, it's brand new, pyrex glass. I definately want to use it to build a big distillation apparatus. The main uses would be distilling nitric acid and solvents such as chloroform, diethyl ether, ect... The hottest i would ever run it would be 150C, and use it completely sealed up from the distill flask to the receiving flask and use the vacuum adapter to bubble the unused vapors into a beaker of water. So are there any problems with this, it's not like i would be heating it up fast at all, anytime i do a distillation i heat it up very slow. Just tell me what you think about it. Oh and i love the idea of a sandbath, i used to use an oil bath and the oil gets cooked on the glassware and you can't get it off!! Thanks for that tip!!

ksj_6808 - 13-8-2012 at 08:25

Pretty much just got a good deal on it, found it for 50 bucks brand new

Hexavalent - 13-8-2012 at 08:42

There is a comprehensive thread on here somewhere about different heating baths - oil, water, sand etc.

Personally, my favourite method is to use copper BBs in a metal container as a heating 'bath'.

Poppy - 14-8-2012 at 07:18

I always heat my 1L beakers directly upon flame, starting with a short flame until reaching boiling point, then changing to maximum flame in the stove. It's said such beakers can hold a 150°C temperature gradient without cracking. 2L or bigger should never be heated like this becausethey will likely crack. I have a thick walled Buchner flask which can withstand higher temperatures, but that requires a bath and directly flame would almost surely crack it aswell.
I have a beaker up to 8 years old and it never cracked this manner.