Sciencemadness Discussion Board

Heating a round bottom flask with a hotplate?

Upsilon - 1-11-2015 at 13:54

I have an issue - I need to heat a 1000mL round-bottom flask with a flat hotplate. I don't have a heating mantle, and I don't want to spend the money to buy or even make one from premade heating tape. I also do not have a reliable flame source to use. Submersion in a bath of some liquid isn't really possible for me since I need to be able to achieve fairly high temperatures.

Does anyone have a good idea here? I was considering buying some wire like nichrome and making my own ghetto heating mantle, but I don't know what kind of insulation I could use, as this wire will get incredibly hot and won't play well with direct contact with the glass. Not to mention the electrocution hazard of uninsulated wire. I see some cheap glass fiber wire sleeving on eBay but they are rated to 600C, and being glass fiber I don't think this would be any better than direct contact with the flask.

JJay - 1-11-2015 at 14:02

I am very curious about this as well. I'd like to distill sulfuric acid and glycerine. I've considered buying a heating tape but so far have not taken the plunge.

[Edited on 2-11-2015 by JJay]

Crowfjord - 1-11-2015 at 14:09

I use aluminum foil to make what is sometimes called an "air bath." I crunch lengths of foil into rings that fit around the bottom of the flask being used. I usually use one or two of those to fill the space between the bottom of the flask and the hot plate. For higher temperature applications, I'll add more layers of foil around all of that, along with some fiberglass or ceramic insulation, and a thermometer to measure the "bath" temperature. Works quite well.

I should also add that I only use good borosilicate glass.

[Edited on 1-11-2015 by Crowfjord]

ziqquratu - 1-11-2015 at 14:24

Air bath works, but sand might be better. It's certainly much slower to heat up and cool down, but the transfer should be better and the losses less than a simple air bath.

As an added bonus, it'll support the weight of whatever is in the flask very well, reducing your reliance on the clamp at the top.

ave369 - 1-11-2015 at 16:46

Sand bath, man, sand bath. Take any old tin can, fill it with sand and put your RBF in the sand.

blogfast25 - 1-11-2015 at 17:10

Or you can just... heat your RBF with your hot plate and nothing else! It's not ideal but people do it, I do.

All these 'that's better' or 'no, this is better' only make sense if you specify precisely what it is you're trying to achieve. Generic statements really don't enlighten much and the devil is in the detail.

[Edited on 2-11-2015 by blogfast25]

gdflp - 1-11-2015 at 17:28

Personally I use air baths when heating round bottom flasks on a hot plate. I have cracked ceramic hot plates when using sand baths, so I tend to avoid them if possible. I do agree with blogfast though, it depends on the application.

Upsilon - 1-11-2015 at 18:13

I suppose I'm making it more complicated than it needs to be, then. However, I am really starting to like the idea of the homemade heating mantle - a thin length of heating element wire insulated with a glass-fiber sleeve. I could then fill in the extra space in the sleeve with sand. I think I was too worried about the effect of the hot wire on this sleeving - if it is relatively pure SiO2 like glass fiber should be, then it should have no problem with this, especially with a sand barrier between it and the wire.

I think this project could be done for under $20. The glass fiber sleeving is quite cheap, and there's this Kanthal heating wire stuff that's dirt cheap. Besides that I would need a cheap light dimmer switch for temperature control and some material to make a housing out of.

blogfast25 - 1-11-2015 at 18:18

Quote: Originally posted by Upsilon  


I think this project could be done for under $20. The glass fiber sleeving is quite cheap, and there's this Kanthal heating wire stuff that's dirt cheap. Besides that I would need a cheap light dimmer switch for temperature control and some material to make a housing out of.


Probably but a badly designed home-made mantle heater is almost certainly less safe than 'naked' heating of an RBF with a decent hot plate.

Upsilon - 1-11-2015 at 19:30

Quote: Originally posted by blogfast25  

Probably but a badly designed home-made mantle heater is almost certainly less safe than 'naked' heating of an RBF with a decent hot plate.


True; in that case I just won't design it badly :D

In all seriousness though it probably is just a better idea to use the hotplate with an air bath, but my inner DIY is calling. I'll make sure to take care with this one and thoroughly test it before trying to use it for actual lab work. Depending on how well this goes, I may also use this same design for my homemade lab hotplate/stirrer. I've got some math to do to figure out exactly how I'm going to do this; I'll post here with progress (that is, if I ever make any with the little free time I have).

TheAlchemistPirate - 1-11-2015 at 19:38

I once had the idea to use a bath of some kind to heat my round bottom flasks,. But, instead of using sand or oil to convey the heat, I would instead use aluminum spheres. They would be small enough to "form" around the shape of the vessel, but large enough to be easily collected. I originally decided upon aluminum spheres due to its chemical inertness but have settled on zinc-coated BBs instead, due to the cost of aluminum ones. These would be contained in a thin-walled aluminum pan just large enough to house the flasks. I will keep you all updated when I actually attempt this

JJay - 1-11-2015 at 19:50

Slightly offtopic but... I just picked up a gas regulator for my Bunsen burner. I think it will work for high temperature distillations if I use a gauze. Now where to get some gauzes....

Sulaiman - 2-11-2015 at 04:52

I have been looking at hot air guns on eBay,
1500 to 2000 W, up to 600C and cheap.
e.g. http://www.ebay.co.uk/itm/2000W-Hot-Air-Heat-Gun-Dual-Temper...
Also for a few GBP the heating elements are available for a diy air bath.
e.g. http://www.ebay.co.uk/itm/Hot-Air-Gun-Heating-Element-Core-M...
may be viable ?

Praxichys - 2-11-2015 at 05:50

I don't know if the flask was just cheap but about 4 years ago I cracked a 1L 24/40 RBF by heating it on a bare ceramic hotplate. It has a 3/4 circumference crack around the point of contact with the plate. I think the plate was about 300C and I was distilling nitric acid at the time.

I still have the flask in the glassware graveyard if anyone is interested in a pic.

Upsilon - 2-11-2015 at 08:50

Quote: Originally posted by Sulaiman  
I have been looking at hot air guns on eBay,
1500 to 2000 W, up to 600C and cheap.
e.g. http://www.ebay.co.uk/itm/2000W-Hot-Air-Heat-Gun-Dual-Temper...
Also for a few GBP the heating elements are available for a diy air bath.
e.g. http://www.ebay.co.uk/itm/Hot-Air-Gun-Heating-Element-Core-M...
may be viable ?


That's actually an interesting idea - I do have a 1500W heat gun. It may be cumbersome to set up but it would likely be better than heating it on a flat hot plate.


In order to get the ball rolling on my project, I have ordered 15 feet of 6mm diameter glass fiber insulation sleeving, as well as 50 feet of 22 AWG (0.64mm) Kanthal heating wire. Cost me less than $10 USD altogether. I plan on using 10 feet for the heating element, which should theoretically give me 1000W of power to heat with. In reality it will be higher since the resistance of the wire increases with temperature.

For my design, I'm going to run the wire through the sleeve and fill in the empty space with sand. The wire melts before the sand, so if there is a problem I at least won't have exposed wiring. I'm going to have to melt the ends of the Kanthal wire to some copper wire with a torch (solder won't work because it melts far too easily) to connect it to the power source. I still need to get a dimmer switch but I need one rated to about 10 amps - all of the cheap ones I see are rated to 3 amps. I'll include a fuse in the design to prevent too much current from moving through the element as a safety measure. I have yet to come up with something to use for a housing, but that shouldn't be too difficult.

ParadoxChem126 - 2-11-2015 at 08:51

I advocate for the sand bath. It takes a while to heat up but works fairly well for high temperature applications.

Texium - 2-11-2015 at 09:51

I've used sand baths, water baths, and air baths before and they all have their different strengths and weaknesses.

-Water baths: Fairly good temperature control and very safe, but temperature is limited and condensation will collect on everything nearby

-Sand baths: Can reach high temperatures and is very safe, but has bad temperature control and can't be cooled quickly.

-Air baths: Very easy to control the temperature- insulation can be easily removed to allow rapid cooling, but is a bit unwieldy and may not heat evenly if improperly insulated.

starman - 2-11-2015 at 22:22

Personally I use molecular sieves in the bath. Works really well as a heat regulator and much faster heat up than sand. As a bonus they're dehydrated and ready for reuse.

subsecret - 3-11-2015 at 19:59

Be careful not to let the bottom of the flask actually touch the hotplate surface; this could cause a good bit of stress in the flask. I generally get mine within about 2 mm of the hotplate. "Tent" the flask with aluminum foil, and maybe wrap this with glass cloth.

Funkerman23 - 4-11-2015 at 20:10

If I wasn't afraid of killing my hotplate's magnetic stirrer, I'd use a BB bath myself. Granted I don't know how well it'd be for things over 3 liters but no one makes solid copper shot and even then...( round and round the topic goes). Aggh

Upsilon - 12-11-2015 at 17:30

I started making my heating tape today for the mantle. I cut 10 feet of the Kanthal wire and a little longer length of the glass fiber sleeve. I ran the wire through the sleeve, and soldered copper wire to the ends of the Kanthal wire. By soldered I mean melted; actual solder would most certainly melt from the heat of the Kanthal wire. It was difficult to do this, and I didn't really get it completely melted but I got it soft enough (very bright yellow heat) to press them together to make a solid connection. I used a regular propane torch to do this. I then used some Kaptan tape (a high temperature tape) to seal the glass fiber sleeve to the copper wire.

Next I need to fill the sleeve. I don't think I'll use sand because it is too thermally insulative. I'll probably use Al2O3, but I need to make some. I'm thinking that I can probably dissolve a bunch of aluminum cans in HCl, filter all the paint and crap out, and then add sodium hydroxide to precipitate Al(OH)3, then heat to decompose this to Al2O3.

j_sum1 - 12-11-2015 at 17:39

Did you check the resistance a sample of the kanthal? preferably at what you will believe is operating temperature?
Once you know the resistance per unit length, then you can calculate the total resistance and using specs of your power supply, calculate the power output. P=V2/R

You really want to tweak your design if possible so that it gives the outcome you want. Length and number of thicknesses of the wire are two easy parameters to alter.

Upsilon - 12-11-2015 at 17:42

Quote: Originally posted by j_sum1  
Did you check the resistance a sample of the kanthal? preferably at what you will believe is operating temperature?
Once you know the resistance per unit length, then you can calculate the total resistance and using specs of your power supply, calculate the power output. P=V2/R

You really want to tweak your design if possible so that it gives the outcome you want. Length and number of thicknesses of the wire are two easy parameters to alter.


Yeah I calculated it ahead of time based on information I found on Google, but I guess I haven't physically tested it yet. Still, what I calculated should give me about 1000W of heating power at maximum. Like I said I'll be adding some form of dimmer switch to be able to adjust this.

j_sum1 - 12-11-2015 at 17:44

Just checking. You don't want to either blow your circuits or burn out your wire.

And why isn't superscript working for me at the moment?

Upsilon - 12-11-2015 at 17:48

Quote: Originally posted by j_sum1  

And why isn't superscript working for me at the moment?


At the moment? I don't think I've ever gotten it to work.

Texium - 12-11-2015 at 18:29

You need to use the HTML tags rather than the bb code ones. (use <> instead of [ ]) <sup>like this!</sup>