March 29, 2024, 02:40:06 AM
Forum Rules: Read This Before Posting


Topic: Questions about the safe heating of a flask on a hotplate  (Read 9650 times)

0 Members and 1 Guest are viewing this topic.

Offline PeterDav

  • New Member
  • **
  • Posts: 3
  • Mole Snacks: +0/-0
Questions about the safe heating of a flask on a hotplate
« on: January 15, 2017, 11:06:58 PM »
I'm having challenges with a simple starter task of boiling water.

I tried to record the details of my experience in order to help answer potential questions, as a good record of experiments is essential.  This was a ‘dry-run’ of future lab experiments, so my measurements are approximate.  A real experiment I would have recorded more information and significant figures. Safety goggles are worn by myself and my wife when doing an experiment.
My motivation is that while I'm an engineer in a very separate technology, my wife is majoring in chemistry, and I'd like to gain some information / skills in chemistry.  You know, I feel the need to be helpful.
I tend to over think things, but find that in technology design, this is not always a flaw.
I am starting to work my way through:
    Illustrated Guide to Home Chemistry Experiments
    https://www.amazon.com/Illustrated-Guide-Home-Chemistry-Experiments/dp/0596514921
Lab: 6.2 - Distillation: Purify ethanol.
The distillation apparatus is an overly simple pair of Erlenmeyer flasks 2 stoppers a little glass tubing and some rubber hoses.  (similar to this setup:  http://www.scienceforums.net/topic/33873-homemade-distilation-apparatus/).

Foolishly, I changed the main chemical from drugstore ethanol to hardware store metal can “Denatured alcohol.”  When I get back to this experiment, I’ll switch to the Walgreen’s rubbing ethanol alcohol.
I did not see any bubbles until 70 C.  This may have been from an allotrope of Acetone of Methanol in the solution.  When I boiled the alcohol, the experiment produced a lot of vapor which caused me to cancel the experiment and took some time to clear.  This may have been the acetone (bp 57 C) added to as part of the alcohol.
I discovered an error.  The apparatus really has to have a condenser (though the experiments book does not mention one in the book, and I am seriously disappointed in the author over the potential safety problem this exposes the reader to).  The Periodic Table of Videos has a very clear warning for those attempting distillation (https://youtu.be/iX5ZTieAUXI) You need a condenser to reduce fumes or you might get an explosion!  In the case of the book, adding a coil of tubing between the distillation flask and the receiving flask with that coil submerged in a little cool water might seem to be enough.
After an appropriate dressing down from my wife, I ordered a proper 24/40 distillation rig (https://www.southernlabware.com/laboratory-glassware/distillation/liebig-condenser-distillation-kit-complete-500ml.html), tubing, an aquarium pump, …
This is all being heated by a hotplate stirrer (https://www.amazon.com/gp/product/B0147TTNJS/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1).  I have a little fuel driven camping stove heater for bending glass, or flame tests.  I think for distillation, avoiding flame is a good idea.

New temporary goal:  Distill ordinary water

The previous vapor problem motivated me to read more about distillation problems and accidents.  Apparently, even borosilicate glass can break given enough thermal shock, so I wanted to heat the 500 mL flask slowly and evenly.
I was reading about different ways of heating the Distillation Flask, and decided that a water bath would not work.  I got a 2.5 qt. stainless steel pan.  I read an article from someone using baby oil.  I setup the new rig and put 300 mL tap water into the first flask + 2 drops green food coloring.
I poured about 600mL baby oil into the pan around the round flask (to the side of the round flask), until about 2 cm of the flask was submerged into the oil.  Added a stirring rod to the flask.  With help of a 3-fingered clamp, dipped a thermometer into the oil and turned on the meter.
Applied heat.  (long story short).  After about 60 C the baby oil smell is increasing.  At 80 C, the smell warranted opening more windows, adding more fans.  The water is starting to create vapors as well (little droplets on the top of the distillation flask all the way to the bottom of the 24 / 40 joint.  At 90 C, my wife is concerned about the vapors and so watches the setup as I reread the MSDS on baby oil (white mineral oil) to double check the BP of the oil). 
There are too many inconsistencies in the SDS of white mineral oil.  Frequently boiling point is simply listed as N/A.  And the flash point is between 25 C and 230 C. 
We decide the fumes are getting dangerous.  I turn off the hotplate, and also unplug it.
When the oil reaches below 40 C again, I recover it back into the bottles, disassemble the gear and clean all the glass and tubing in Alconox.   I like this soap, when used separately, it takes tea stains off tea and coffee cups easily!
Fumes again!  I need to get help. 
I eventually found this article on baths by U Iowa:
https://ehs.research.uiowa.edu/print/1365
But, it still does not answer all my questions.

Questions:
How do you recommend heating flasks for distillation (< 150 C) without fumes from the setup?
Should I place a Flat-bottomed beaker (Round or Erlenmeyer) straight on the hotplate?
Or, should I place a wire gauze square with a clay center between the heating surface and flask?
Would you recommend using a sand bath (pure SiO2 in a stainless-steel pan)?
Other arrangement?
Are there any key questions I am missing and should know about?
Should I run the output from the vacuum adapter out a window when working with volatiles like ethanol?

Thanks!
« Last Edit: January 16, 2017, 12:14:37 AM by PeterDav »

Offline billnotgatez

  • Global Moderator
  • Sr. Member
  • ***
  • Posts: 4399
  • Mole Snacks: +223/-62
  • Gender: Male
Re: Questions about the safe heating of a flask on a hotplate
« Reply #1 on: January 17, 2017, 02:08:33 AM »

Offline PeterDav

  • New Member
  • **
  • Posts: 3
  • Mole Snacks: +0/-0
Re: Questions about the safe heating of a flask on a hotplate
« Reply #2 on: January 17, 2017, 02:31:13 AM »
Thanks billnotgatez,

That article answers a lot of my questions.

I'll give it a try ... Saturday.

Offline shiffdaddy

  • Regular Member
  • ***
  • Posts: 17
  • Mole Snacks: +0/-0
Re: Questions about the safe heating of a flask on a hotplate
« Reply #3 on: January 25, 2017, 10:27:03 PM »
In reply to heating flasks for distillation I've always just had a pot of sand where I put my flasks for distillation, seems to work well

Offline kamikaze762x39

  • Regular Member
  • ***
  • Posts: 9
  • Mole Snacks: +0/-0
Re: Questions about the safe heating of a flask on a hotplate
« Reply #4 on: February 01, 2017, 10:02:01 PM »
I had the same trouble, but I was using ceramic shielded wire mesh over an alcohol burner.  That was no where near enough heat to boil anything, so I crushed the shielding off of the wire mesh, used aluminum foil, and rigged my own propane tip.  Still not hot enough.  I encased an actual torch head and hose into a cement / sand mix for a DIY bunsen burner.  It produces a lot more concentrated heat than a bunsen burner, so shielding is essential.  I found that placing the bottom of an aluminum can, crushed flat, offers sufficient shielding for that tight flame.  I boil easily now and no exploding glass so far.

Offline RDX

  • Regular Member
  • ***
  • Posts: 15
  • Mole Snacks: +1/-5
Re: Questions about the safe heating of a flask on a hotplate
« Reply #5 on: February 08, 2017, 06:22:32 PM »
1. Buy Pyrex everything.
The borosilicate cheap Chinese crap just breaks too much.

2. Why aren't you just directly heating the flask on a hot plate?
It's not like you're heating it on an open flame, so it won't break as quickly. Yes, I said as quickly, which leads me to 3.

3. Expect the glass to break.
Just prepare for it to explode, because in time, it will. Even with Pyrex. Using water around the pan is what I used to do, because I was worried about directly applying the flask to the hot plate. But that's what a hot plate was designed for. Get one. Pyrex flasks take a good duration of use before they break.

Offline billnotgatez

  • Global Moderator
  • Sr. Member
  • ***
  • Posts: 4399
  • Mole Snacks: +223/-62
  • Gender: Male
Re: Questions about the safe heating of a flask on a hotplate
« Reply #6 on: February 08, 2017, 07:12:51 PM »
1. Buy Pyrex everything.
The borosilicate cheap Chinese crap just breaks too much.

2. Why aren't you just directly heating the flask on a hot plate?
It's not like you're heating it on an open flame, so it won't break as quickly. Yes, I said as quickly, which leads me to 3.

3. Expect the glass to break.
Just prepare for it to explode, because in time, it will. Even with Pyrex. Using water around the pan is what I used to do, because I was worried about directly applying the flask to the hot plate. But that's what a hot plate was designed for. Get one. Pyrex flasks take a good duration of use before they break.

@RDX
I am not sure I would totally agree or disagree with your opinions.
Did you read the link I provided earlier in this thread?

Offline P

  • Full Member
  • ****
  • Posts: 639
  • Mole Snacks: +64/-15
  • Gender: Male
  • I am what I am
Re: Questions about the safe heating of a flask on a hotplate
« Reply #7 on: February 09, 2017, 04:33:15 AM »
I think I used to put my glassware into an oil bath that was heated on a hot plate. You can control the temperature a lot more accurately and the glass won't break.

Say you want to run a reaction at 395 degrees...  you won't get that spot on by heating the flask directly....  you bring the oil bath to 395 degrees, let it stabilize and then clamp your flask from a retort into the oil bath.
Tonight I’m going to party like it’s on sale for $19.99!

- Apu Nahasapeemapetilon

Offline P

  • Full Member
  • ****
  • Posts: 639
  • Mole Snacks: +64/-15
  • Gender: Male
  • I am what I am
Re: Questions about the safe heating of a flask on a hotplate
« Reply #8 on: February 09, 2017, 04:36:04 AM »
PS - regarding safety  - obviously you need to make sure your oil is rated for the temperature you are heating to otherwise it can burn. :-)
Tonight I’m going to party like it’s on sale for $19.99!

- Apu Nahasapeemapetilon

Offline PeterDav

  • New Member
  • **
  • Posts: 3
  • Mole Snacks: +0/-0
Re: Questions about the safe heating of a flask on a hotplate
« Reply #9 on: February 09, 2017, 07:23:26 PM »
Thanks everyone for the input. 

Having a couple quality flat bottomed flasks and heating them directly on the hotplate worked fine.
The first time I raised the heat 5 C at a time until it boiled.  I can probably do better. 

I also got some SEOH crystallizing dishes for future baths.  But, mostly I'm buying Pyrex now.

Got a chain clamp to stabilize larger vessels.

Peter

Sponsored Links