Sciencemadness Discussion Board

Flame heating (larger) RBFs?

Funkerman23 - 18-9-2018 at 20:52

Hello!

As dumb as this might be.. I need to ask you learned folks. As was all the rage here some time back, I was mulling over the idea of distilling some scrap sulfuric acid and how to best heat a 1 liter flask. Most heating mantles can't reach the 350C that is needed for the task and hotplates at that range are at risk for burning out. At this point this is all theoretical but its enough to make me ask.

In short: how where round bottomed flasks safely flame heated way back? Where there special ceramic dishes for heating flasks? Was wire gauze molded to fit the round flasks? How was this done before electric heating was standard? I've had some older texts but I can't seem to recall the proper pre-mantle/hotplate/oilbath way to get tp 350C. Thank you all for your time and patience.

Sulaiman - 19-9-2018 at 00:57

I would not like to boil sulphuric acid with a bare flame,
although I'm sure it can be done, with care, I consider the risk too high,
especially with my cheap Chinese glass.
Overall even heating is ideal, such as a heating mantle.
(the nichrome/glass heating element should be good to over 450oC, the internal wiring and insulation may not ! )

You could try a sand or salt bath, just be a little patient with the heating.

If possible use a pot for the sand bath that is large enough for the boiling flask and sand
and able to contain (even if only temporarily) the entire contents of the boiling flask
- in case the worst happens.

Thermally insulate everything that has upward-flowing vapour,
even the pot with the sand needs insulation.
(loosely wrapped with Al foil is good enough)

Use an air condenser (to do the bulk of the condensing)
followed by a water-cooled condenser (to reduce sulphuric acid lost to the exhaust / fumes )

Although you may get away with it if you are lucky,
I would not try to condense conc. sulphuric acid directly with a water cooled condenser,
due to the temperature difference/gradient across the glass.

Boiling or distilling sulphuric acid is one of the scariest things I've done in chemistry.
Think twice when planning this, then twice again before you start.

Sulphuric acid expands quite a lot when heated from ambient to b.p.
and is infamous for its bumping,
so I'd probably start with 500ml or less in a 1000ml flask, others may advise even less.

Just to get an idea of what you will be dealing with,
carefully boil a little conc. sulphuric acid in a test tube.

Forgive me if you already know all of this,
(from your question I guessed a lack of experience, not that I have much myself)
but this procedure genuinely scares me.

[Edited on 19-9-2018 by Sulaiman]

Ubya - 19-9-2018 at 01:53

in the past i read about molten metal baths, like a sand or oil bath, but for higher temperatures using molten tin or lead

DrP - 19-9-2018 at 01:59

Quote: Originally posted by Ubya  
in the past i read about molten metal baths, like a sand or oil bath, but for higher temperatures using molten tin or lead


Wouldn't these baths require a hotplate though? He wants to use an open flame. I've not tried it but the sand path seems plausible and safe with an open flame.

I've only really ever considered an oil bath on a hot plate or a heating mantle.

JJay - 19-9-2018 at 03:02

In high school we used a metal gauze below glass objects that we were heating. I don't remember ever having any problems, but I'd be a bit reluctant to heat a liter of sulfuric acid that way. I've had bad luck with sand baths not heating evenly and breaking the flask, but there are worse places to drop 500 mL of boiling sulfuric acid than into a pan of sand.

I use a quartz flask for distilling sulfuric acid. It's fine over a flame.

Dr.Bob - 19-9-2018 at 06:18

And remember, the thicker the flask, the less evenly it will heat and expand, so thin glass is better than thick for flame heat. I would be extremely careful with such a task. But I will also say that larger heating mantles are capable of going above 350C, if well insulated and carefully regulated. I have seen someone melt a large flask when they heated it to dryness in a mantle that was NOT plugged into a variac (or temp controller) by mistake.

morganbw - 19-9-2018 at 06:48

Doug's Lab on youtube has a video where he distilled sulfuric acid with flame as the heat source.

I am not advocating this method, but he pulled it off successfully.

Whatever method is used, it needs to be very well thought out.

SWIM - 19-9-2018 at 07:33

There are high temp mantles out there which should be able to do the trick.

Most mantles are rated at 450C(which means much less than that in the flask, of course), but the STS serial numbered mantles made by Glas-Col are rated for over 600C. These can be found on Ebay, but figure on $50 to $150 (US) for one unless you get lucky.

I bet one of these would have little trouble with distilling H2SO4 with a reasonable still design.







macckone - 19-9-2018 at 11:46

a 450C mantle with insulation on the flask should be sufficient to boil sulfuric acid.

obviously a quatz flask with a high temp mantle works better and a quartz flask with direct flame is ideal since quartz is almost impervious to thermal shock. But not all of us can afford a $200 flask.

Funkerman23 - 19-9-2018 at 15:28

Quote: Originally posted by Sulaiman  
I would not like to boil sulphuric acid with a bare flame,
although I'm sure it can be done, with care, I consider the risk too high,
especially with my cheap Chinese glass.
Overall even heating is ideal, such as a heating mantle.
(the nichrome/glass heating element should be good to over 450oC, the internal wiring and insulation may not ! )

You could try a sand or salt bath, just be a little patient with the heating.

If possible use a pot for the sand bath that is large enough for the boiling flask and sand
and able to contain (even if only temporarily) the entire contents of the boiling flask
- in case the worst happens.

Thermally insulate everything that has upward-flowing vapour,
even the pot with the sand needs insulation.
(loosely wrapped with Al foil is good enough)

Use an air condenser (to do the bulk of the condensing)
followed by a water-cooled condenser (to reduce sulphuric acid lost to the exhaust / fumes )

Although you may get away with it if you are lucky,
I would not try to condense conc. sulphuric acid directly with a water cooled condenser,
due to the temperature difference/gradient across the glass.

Boiling or distilling sulphuric acid is one of the scariest things I've done in chemistry.
Think twice when planning this, then twice again before you start.

Sulphuric acid expands quite a lot when heated from ambient to b.p.
and is infamous for its bumping,
so I'd probably start with 500ml or less in a 1000ml flask, others may advise even less.

Just to get an idea of what you will be dealing with,
carefully boil a little conc. sulphuric acid in a test tube.

Forgive me if you already know all of this,
(from your question I guessed a lack of experience, not that I have much myself)
but this procedure genuinely scares me.

[Edited on 19-9-2018 by Sulaiman]
I wouldn't say I am inexperienced, but I will say I am rusty. I started my Undergrad Chem degree year before last and that has been worth every moment...but it changes things. I spend more time on calculations than I ever thought I'd need. From Hobby to the real deal. Still, I am not sorry in the slightest.

Sulaiman - 19-9-2018 at 17:18

sorry, I'm just a paranoid hobby chemist,
I find it difficult to judge the abilities of posters
so for 'dangerous' advice, I'm over-cautious - I hope.

Funkerman23 - 19-9-2018 at 18:06

Quote: Originally posted by Sulaiman  
sorry, I'm just a paranoid hobby chemist,
I find it difficult to judge the abilities of posters
so for 'dangerous' advice, I'm over-cautious - I hope.

No No! Please, pardon me if I was rude! I used to be very active here but life changed. I don't nearly do as much lab stuff as theory now. It just feels like its been a very long time and this was a thought experiment more than anything else. Its safer to ask and look dumb then the alternative.