Sciencemadness Discussion Board - Using flask on electric stove top - should pan or sand/metal bath be used?

Using flask on electric stove top - should pan or sand/metal bath be used?

RogueRose - 14-4-2018 at 08:58

I'm trying to heat an ammonia/CaSO4 solution to drive off the gas but I've never used a stove top before and am wondering if the coil on the electric stove is going to damage the glass. I'm using a Kimax erlenmeyer flask and I know they make glass skillets/pans so I would think these are pretty similar and I'm not looking at putting this on high as I will only need fairly low heat.

I figure I can use an aluminium pan in between the element and the flask if needed and would be a good idea if a higher heat is needed. There is also the option of using sand or even oil or water bath. While maybe not necessary for this setup, I'm wondering if it is most always necessary to put something between the element and flask.

I also have super fine copper shavings (about 1-3mm long by 34/36 gauge or smaller - its the strands for speaker wire) that I'm thinking would work well for heat transfer and am wondering if anyone has used something like this?

Bert - 14-4-2018 at 09:30

You don't mention using a hot air bath. Running a variable temp heat gun pointed upward and at a distance from bottom of flasks or beakers, sometimes even configuring scraps of soft, thin flashing Aluminum sheets to make a hot air chamber around the glass has worked for me in a pinch.

Wouldn't advise a sand bath for any glass you value

I used an improvised sand bath for things like driving water out of (cheap! $30.00 for 5 gallons!) battery acid to get 80% or so Sulfuric acid, I used a (cheap! $3.50!) garage sale purchase deep fryer for heat.

Natural sand is cheap and easy to find. And harder than glass, with random shrap bits. Glassware is not so cheap. The glass I used was badly scratched after a few such uses, although it had not yet failed, I did not trust it and took it out of use. Who wants 1000ml or so of hot 80% acid getting loose, probably right when you are picking it up...

Copper is reactive. Fine Cu even more so, and Cu is PARTICULARLY reactive towards ammonia as vapor, liquid, or water soluble ammonia compounds. Someone I know made the mistake of screening an ammonium perchlorate oxidized star mixture through 20 and 40 mesh brass screens, one use and these screens literally fell apart. Your Cu wire sounds like a poor choice.

I still like my (soft) Aluminum punch press scrap better than an oil bath after a couple of uses.

Plus, I found it works nicely for a new use... Cleaning the (hard Aluminum or stainless steel or Titanium alloy) baffles from a suppressor. These are a nightmare to clean with a brush and the usual gun cleaning solvents, I now put a few baffles of identical composition at a time into a rubber rock polisher barrel with some water and "simple green" cleaner. An hour later, every nook and cranny is clean.

[Edited on 4-14-2018 by Bert]

[Edited on 4-14-2018 by Bert]

Sulaiman - 14-4-2018 at 09:30

I'd use a cooking oil bath.

Maybe most of the time you can use direct heating with no observable damage,
but I would try to not subject my glassware to too much thermal stress.

Bert - 14-4-2018 at 09:41

Oil baths work, I have used them. Liquids convect, which is a good thing for heat transfer too. But cleaning your glass after is more difficult than when using conductive metal pellets, and cooking oil is flammable- Overheating causes breakdown in edible oils, over time ANY heating will break down oil, which lowers the flash point. In a commercial kitchen, the deep fryer oil must be changed regularly, or the place will experience a fire. When I cooked, it was changed at least weekly, 2X per week if the kitchen was open for more than one 8 hour shift per day.

Oil fires in a kitchen are a common source of house fires, having your chemistry set in the middle of one would make such a fire harder to extinguish, plus any ramifications from the contents (and the fire departments of jumping to conclusions , they HATE meth cooks and may make the wrong assumptions)

[Edited on 4-14-2018 by Bert]

RogueRose - 14-4-2018 at 10:23

Thanks for all the suggestions. I think the sand is out for this experiment and water might be enough if I don't need to get above 212 and I don't think ammonia stays in solution above that point but I need to find out. I did try using water just a little bit ago and it worked great for the first 15 minutes or so and then the production dropped off a lot, so I think the concentration had fallen a lot.

I have 2L of solution and about 11 moles of NH3 (each mole being 17g). I used 1L of the solution and it got to the point where the water was boiling as well as the liquid inside were at a rolling boil but there didn't seem to be any gas coming over only a VERY tiny amount of bubbles, maybe 1ml every 5-8 seconds. I guess there could be a leak in the line, though I couldn't see a place where it could leak or smell any along the line. I don't see how it could look like it is boiling in the flask but have no gas coming over. I'm not using vacuum to boil this.

I've seen the BP of ammonia solution listed from 76F to 99F but I don't think mine started producing until about 140-180 or so. The worst part is when I turned everything off, I forgot about suckback, but I caught it just as it stated and only lost about 1/2 of my new solution. I'll just have to boil this off again I guess. This is turning out to be much more trouble than I had thought it would be.

aga - 14-4-2018 at 10:49

A sand bath is a PITA.

Tried it once and it behaved badly. The sand heats up slowly and loses heat slowly.

If your reaction is exothermic, a sand bath is a big no-no.

A lab jack would kinda help, but not in a controllable way (more of an emergency exit).

Sulaiman - 14-4-2018 at 10:54

I use cooking oil in an old cooking pot on a used portable electric hob/hotplate for heating my flasks,
mostly 2l florentine and 2l rbf (thank you Santa)
(Other than for 500ml as I have a suitable mantle, or very high temperatures such as H₂SO₄)

Once you have a favourite heating pot it is quick and simple to set up,
a little oil is lost each time I clean up, but it is very cheap.
Washing cooking oil off an rbf is simple using domestic 'washing up liquid'.
Even when a water bath could be used, I prefer oil, due to steam/condensation.

EDIT: As I have no Al beads or similar,
I would use a sand bath for anything with b.p. above about 150^oC
(The cheap 'vegetable oil' that I use has a smoke point around 220^oC and it is surprisingly easy to exceed 50^oC difference in temperature between inside and outside the flask)

[Edited on 14-4-2018 by Sulaiman]

happyfooddance - 14-4-2018 at 12:16

As far as oils go, corn or hydrogenated cooking oil have the lowest smoke points for the cost. Safflower is good too, but is a little pricey. I run my baths at upwards of 200°C with these oils, with no smoking.

Cheap vegetable oil you can't run as high.

aga - 14-4-2018 at 12:42

To be honest, while the hotplate/stirrer is great for many things, a simple spirit burner works really well for most of the simple stuff i have done so far.

arkoma - 14-4-2018 at 13:33

use a branded "Pyrex" 2L Erlenmeyer on my stovetop regularly and have no problems. I think about thermal shock--don't put cold glass on a hot element

aga - 14-4-2018 at 13:38

Yay !

arky! arky! arky!

It's a Good day when arkoma posts.

Today has been a shit day work-wise.

Now i can go to bed much happier !

JJay - 14-4-2018 at 13:41

I've had horrible luck with sand baths. I've used table salt heating baths at 300-400 C a couple of times and did not have any problems, but I ramped the heat up very slowly.

Bert - 14-4-2018 at 14:21

Quote: Originally posted by happyfooddance

As far as oils go, corn or hydrogenated cooking oil have the lowest smoke points for the cost.

HIGHEST smoke point is what I would look for, from a fire risk standpoint if using cooking oils for a heat transfer medium?

https://en.m.wikipedia.org/wiki/Smoke_point

Bert - 14-4-2018 at 14:22

Quote: Originally posted by JJay

I've had horrible luck with sand baths. I've used table salt heating baths at 300-400 C a couple of times and did not have any problems, but I ramped the heat up very slowly.

Thanks, I just learned something...

happyfooddance - 14-4-2018 at 14:39

Quote: Originally posted by Bert

Quote: Originally posted by happyfooddance

As far as oils go, corn or hydrogenated cooking oil have the lowest smoke points for the cost.

HIGHEST smoke point is what I would look for, from a fire risk standpoint if using cooking oils for a heat transfer medium?

https://en.m.wikipedia.org/wiki/Smoke_point

Yeah, it was clear from context what I meant, though.

JJay - 14-4-2018 at 14:46

It's hard to go wrong with soybean oil as a heating bath. It is typically the cheapest vegetable oil available, sold in every grocery store as "vegetable oil." It has one of the highest smoke points. IIRC, avocado oil has a slightly higher smoke point but is much more expensive, and I don't think the difference in smoke points would matter for many applications.

ninhydric1 - 14-4-2018 at 18:36

Which type of cooking oil as the lowest heat capacity? I currently use vegetable oil ($7 for 4 L), but it doesn't heat up very quickly. Any suggestions?

happyfooddance - 14-4-2018 at 18:50

Quote: Originally posted by ninhydric1

Which type of cooking oil as the lowest heat capacity? I currently use vegetable oil ($7 for 4 L), but it doesn't heat up very quickly. Any suggestions?

Since they are all triglycerides with pretty much similar fatty acid profiles, I am guessing the difference will be negligible...

RogueRose - 14-4-2018 at 19:47

Quote: Originally posted by ninhydric1

Which type of cooking oil as the lowest heat capacity? I currently use vegetable oil ($7 for 4 L), but it doesn't heat up very quickly. Any suggestions?

There is very little difference between any of them, just pick the least expensive and you can even use old/re-used oil from an old fryer if you have that.