Sciencemadness Discussion Board - Steel wool on hot plate for even heating??

Steel wool on hot plate for even heating??

Twospoons - 30-11-2020 at 01:06

Scrunched up aluminium foil might be better. Better heat conduction than steel wool.

nzlostpass - 30-11-2020 at 01:17

I use steel wool on hot plates with rbf's and it seems to work fine. Have used the same steel wool pad now for well over 12 months....I'm sure ive gone over your intended temperature also, plenty of times.

macckone - 30-11-2020 at 01:38

BBs are your friend.
You can easily get copper coated shot.
Solid copper shot is going to be harder to find in the US but is readily available in the EU.

Copper scrubbers are going to be more conductive than steel wool (thicker strands and better conductivity)

Aluminum foil is more accessible and can be layered to improve overall heat conduction.

Having said all of that cooking oil that has a smoke point above 210C is readily everywhere.

https://www.masterclass.com/articles/cooking-oils-and-smoke-...

Mineral oil is also going to have a very high smoke/boiling point.
Baby oil is the most commonly available mineral oil and is generally available anywhere on the planet.

The final point is that distillations at that temperature are usually done as a vacuum distillation to reduce the necessary temperature and reduce breakdown of the product.

Σldritch - 30-11-2020 at 09:44

I usually put one layer of aluminium foil around RBFs/beakers and most of the hotplate surface to keep the hot air in. Never had an issue doing it.

Arthur Dent - 30-11-2020 at 10:22

I never put glassware directly on my hotplate, unless it is at very low heat. When I need to use high heat on a vessel, i use a sand bath.

Years ago, I tried a sheet of aluminium paper. At high heat, it created tiny nucleation points where the glass fused with the foil, and shattered the vessel by creating spiderweb cracks at each nucleation point.

So I don't use foil anymore (except to isolate a vessel's path to a condenser). A sand bath conducts the heat perfectly and preserves your precious labware from thermal destruction. Plus bonus, a rbf will hold in place and not tilt to the side, for erlenmeyers, beakers and flat-bottom flasks, it's perfect.

Fyndium - 30-11-2020 at 10:49

How do you use sand baths properly? I have need for 170-200C temps sometimes, last time I used avocado oil which apparently was not very refined as it smoked plenty and while the smell was nice at first, it became unbearable at the end. Had to use respirator.

Should they be heated slowly, how deep the flask should be immersed, I suppose sand scratches glassware if rolled around too much? I use induction hotplate to heat the vessel so it should not burn out so easily as coil plate, but apparently the bottom temp can rise very quickly very high if heated too intensely.

Doesn't metal BB high thermal conductivity cause issues to glassware by creating hotspots? When people tell to avoid metal contact when heating glass, I see a possible issue here.

Vacuum distillation is mandatory for higher BP stuff yes, but it also requires either very high vacuum level. 15mbar makes 230C boil at 90C, but for 330C it already goes close to 200 and getting much lower vacuums seem a bit challenging, at least from my experience. Maybe with 2-stage pump with good traps. Thermal gradient also limits somewhat - CaCl2 bath has basically water's thermal conductivity and it needs a much lower gradient than silicone, mineral or vegetable oil. Last vacuum distillation I did at 140C needed the bath to be at 180-200C for reasonable takeoff rate.

Arthur Dent - 30-11-2020 at 14:01

I bought from a craft store some fine white "decorative" sand (slightly beige

) and I cleaned it thoroughly with water (3 or 4 washes, until the wash water is clear), then washed it with acid (muriatic pool/drain cleaner) and then one final wash in water to remove the acid. Then I cooked the sand in the over for 2h at 400F to dry it off completely.

I pour this fine sand in a steel bowl (2 for a buck at the dollar store) so that it contacts the first 1/2 to 3/4 inch of the vessel being heated. The heat is very uniform and heats up quickly (quicker than the vessel on the hotplate directly) because of the greater contact surface. Scratches are not an issue because this is fine sand and is very free-flowing, it's not like emery grit!

170 ºC to 200 ºC should be okay. Anything higher requires a high power heating mantle (mucho $$$) or a direct open flame from a bunsen burner directly to the bottom of a rbf, which always terrifies me...

.

I've used the same sand for the past two years. Whenever there's a bit of a boilover, just pick-up the sand crud with the spilled material and throw it away, and pour a bit more sand to compensate. Never had a single damaged piece of labware since and I feel it's easier to stop the reaction before you boil it to dryness. If your hotplate is thin enough, you can use an elevator platform to pull the sand bath away from the vessel.

aromaticfanatic - 30-11-2020 at 19:42

Has anyone ever tried dot 5 brake fluid? I saw someone using it in a separate thread.

Sand baths are painfully slow and insulate heat too well causing damage to the heating element according to another SM user.

I'm not sure if I want to try and use steel wool or the brake fluid.

I've tried BBs before but they were pretty crappy in my opinion.

What about this?
https://www.walmart.com/ip/Scotch-Brite-Copper-Coated-Scouri...

Seems pretty convenient in terms of shape and if you compress the wool a little you should be able to get good conduction, right?

It's copper coated (just saw that) but I think it could still get the job done, right?

[Edited on 1-12-2020 by aromaticfanatic]

Fyndium - 30-11-2020 at 21:55

What I've tested:

- Brake fluid starts to smoke quickly of thick smoke and smells terrible, never measured temp but based on other stuff it hits between 150 and 200C
- Vegetable oils start to smoke between 150-180C no matter what the smoke point seems to be
- Paraffin oil starts to smoke slightly past 150C but goes to 200 with not too bad smoking
- Silicone oil smokes at least initially from 150C, but the smoking lessens upon use as apparently more volatile stuff polymerizes
- CaCl2 works well up to 150C and it has basically the same TC as water, but eventually more water need to be added
- Salt bath is very slow and conductivity seems to be an issue
- Sand bath I haven't tested to extent I can say for sure, but I have read from multiple accounts and it is reasonable to imagine that the insulation burns out heater elements of hotplates

I personally prefer most CaCl2 for temps up to 130-150C as it's very versatile, easy to rinse off and allows for immersing the flask with jack. Silicone oil is right next to it. But for higher temps I'm still in look for a fluid that is thermally stable up to 250 or higher.

aromaticfanatic - 30-11-2020 at 22:56

Quote: Originally posted by Fyndium

What I've tested:

- Brake fluid starts to smoke quickly of thick smoke and smells terrible, never measured temp but based on other stuff it hits between 150 and 200C
- Vegetable oils start to smoke between 150-180C no matter what the smoke point seems to be
- Paraffin oil starts to smoke slightly past 150C but goes to 200 with not too bad smoking
- Silicone oil smokes at least initially from 150C, but the smoking lessens upon use as apparently more volatile stuff polymerizes
- CaCl2 works well up to 150C and it has basically the same TC as water, but eventually more water need to be added
- Salt bath is very slow and conductivity seems to be an issue
- Sand bath I haven't tested to extent I can say for sure, but I have read from multiple accounts and it is reasonable to imagine that the insulation burns out heater elements of hotplates

I personally prefer most CaCl2 for temps up to 130-150C as it's very versatile, easy to rinse off and allows for immersing the flask with jack. Silicone oil is right next to it. But for higher temps I'm still in look for a fluid that is thermally stable up to 250 or higher.

Thanks for the very informative reply! My distillation will get up to 200C. Sadly no vacuum to pull temp down

In my experience the sand baths just warm up pathetically slow and the heat transfering capability is too low to properly distill at a normal rate. Just my experience though. I wish I would use oil or water baths for everything but this will be the first 150C+ distillation and I don't have many options. I am really eyeing the copper coated wool. It shouldn't stick or melt onto the glass and cause shattering but I don't know how guaranteed that is. I want to try and push down on the metal a little to make sure there is a decent spread of heating.

I like my flasks so I'm trying to prevent that whole shattering thing from happening but so far not many options. I know KNO3 has been used before but I don't trust liquid oxidizer near something that is generally not flammable but it's got carbons so it can burn!

Molten metal is out of the question. I have Field's metal but the sample is far too small.

I was originally going to use fine steel wool but I want to try out that copper coated abrasive sponge. I hope it works well but frankly, I don't know.

I was hoping the brake fluid would work but I figured I wouldn't be so lucky.

Man, if only I had the budget for a heating mantle lol.

TriiodideFrog - 1-12-2020 at 04:16

Just use a sand bath or an oil bath. An oil bath might get messy and splash around though.

aromaticfanatic - 1-12-2020 at 11:46

Sand baths take too long and are too inefficient at heat transfer to be of use to be. They also damage the hot plate heating element. Oils don't go to 200C without fuming.

itsallgoodjames - 1-12-2020 at 11:54

I've never had any issues with a sand bath, besides poor heat transfer. But I've used one for distillations and refluxs for hundreds of hours total. It is yet to be damaged. For things that I need better heat transfer, I use a mineral oil bath.

aromaticfanatic - 1-12-2020 at 17:03

Quote: Originally posted by itsallgoodjames

I've never had any issues with a sand bath, besides poor heat transfer. But I've used one for distillations and refluxs for hundreds of hours total. It is yet to be damaged. For things that I need better heat transfer, I use a mineral oil bath.

Hmmm perhaps I will give it another try. How much of the flask do you cover with the sand?

itsallgoodjames - 1-12-2020 at 18:22

Usually around half way covering a round bottom flask, with the rest covered with foil. Sand doesn't transfer heat very well, so even with a 1600w hot plate, I can't really expect to get hotter than around 120-150° with a 500ml rfb, and a 1l flask won't go over around 110° , at least from my experience. Just a warning, the sand will scratch your glassware over time. That's not really an issue for me, but if you care about your glassware looking nice and perfect, that could be a concern.

aromaticfanatic - 1-12-2020 at 22:23

Thanks for the information. I'll try to use that copper coated thing first because my distillation needs to be at around 200C. If I had a vacuum pump I could really get some better distillations going. The lack of a vacuum pump also caused me to fail my last synthesis because I couldn't freeze and filter out the isomer separation efficiently enough. Sucks.