Sciencemadness Discussion Board - High Temperature Magnetic Stir Bars?

High Temperature Magnetic Stir Bars?

CaTaLiZr - 26-4-2016 at 00:45

I have a magnetic stirrer and a teflon-coated stir bar. Unfortunately, I don't think the teflon stir bar can survive experiments involving high temperatures (e.g. boiling sulfuric acid for distillation). Is there any kind of stir bar I can use for temperatures above the melting point of teflon?

Fulmen - 26-4-2016 at 02:29

The simplest DIY solution would be a magnet sealed in a glass tube. Neodymium would probably be a poor choice as it's curie point is in the 300-400°C range. A steel rod could work of your stirrer has a strong magnet.

[Edited on 26-4-16 by Fulmen]

j_sum1 - 26-4-2016 at 03:00

Yeah except sealing the glass tube is likely to zap the magnet too.

Sm magnets are better since they have a higher curie point. You could always seal one in high temp epoxy.

I am not sure that distillation of sulfuric scid really needs stirring. A lot of people manage without.

BromicAcid - 26-4-2016 at 03:26

I have used the glass tube method before with some success. I bought a magnet to fit inside one of my lengths of borosilicate, sealed one end, dropped in magnet, sealed the other. Issue ended up being that the magnet was too small to stir that much glass well, also spinning glass on glass was a bad idea I only realized in retrospect.

For high temperature reactions that I have done in the professional setting we have used overhead stirring and replaced the standard PTFE stir paddle with a stainless steel one. There are also other options like performing your distillation under vacuum to lower the temperature necessary, or use a capillary tube. Unfortunately, at least in my experience, boiling chips did not work, they floated.

hissingnoise - 26-4-2016 at 03:32

Quote:

I don't think the teflon stir bar can survive experiments involving high temperatures (e.g. boiling sulfuric acid for distillation).

I can't think why you'd need to distill sulphuric acid ─ or why you'd think stirring is needed . . . ?

And anyway boiling H₂SO₄ at NTP might severely weaken your glass!

I've often boiled acid in heavy pyrex cookware to reconcentrate it but not ina RBF!

If you really have to distill, a good vacuum is needed to drop the BP to something more managable than 339°!

Oh, and welcome to scimad . . .

XeonTheMGPony - 26-4-2016 at 03:53

I'll borrow this thread any one have a P-T chart for Sulfuric? (P-T = Pressure temperature)

CaTaLiZr - 26-4-2016 at 23:35

Quote: Originally posted by hissingnoise

Quote:

I don't think the teflon stir bar can survive experiments involving high temperatures (e.g. boiling sulfuric acid for distillation).

I need some 98% H2SO4 with some WFNA to synthesize trinitrobenzene from benzene. I couldn't find 98% H2SO4 anywhere so I decided to try to distill it from drain cleaner.

j_sum1 - 27-4-2016 at 00:07

Our resident expert on concentration of sulfuric acid is ave369. She calls it boiling the bat. (Bat being short for battery acid.)
It is a routine lab procedure for her and she has posted on it numerous times. Look up her posts and follow her procedure. Both concentrating via boiling and also distillation are covered.
I don't think she uses a stir bar at all.

Another option is to boil H2SO4 until fuming occurs which is about 70%.
Then produce some SO3 through thermal decomposition of sodium bisulfate and dissolve that in the sulfuric acid to bring it up to a higher concentration. The procedure is on the board here. Do a search. SO3 is nothing to be trifled with however. There are significant safety issues with this procedure and things can go wrong and badly wrong. Make sure you do your research thoroughly and proceed with caution.

hissingnoise - 27-4-2016 at 01:09

Quote:

I couldn't find 98% H2SO4 anywhere so I decided to try to distill it from drain cleaner.

When SA gets hot enough to produce dense white fumes of SO₃ its concentration will be 98%!

Draincleaner I get is >95%, and almost colourless so only a short boil (easier on the lungs!) is neccesary . . .

But TNB seems a strange choice since it's only marginally more effective than TNT ─ its preparation by direct nitration of benzene is waaay too wasteful of acid and the only practical synthesis is that of oxidising TNT with chromic acid in acid soln..

Quote:

Another option is to boil H2SO4 until fuming occurs which is about 70%.

I don't know where you got that bit of misinformation . . . ?

The dense white fumes appear because at its boiling point H₂SO₄ partially decomposes ─ this is the reason that SA cannot be concentrated beyond 98% by heating!

j_sum1 - 27-4-2016 at 01:26

Well my memory might be incorrect but I thought I read it here.
I have boiled down many batches of electrolytically produced sulfuric acid and stopped boiling when the fumes presented a rust hazard to my shed. I don't think I was getting it to 98%. I haven't titrated or done a density measurement but my product does not have all the properties of 98%. In particular it is less effective at absorbing moisture.

Tdep - 27-4-2016 at 01:36

Yeah it does start to fume at around 70%, depending on humidity. The sulfuric acid starts to boil off along with the water because azeotropes are a thing that exist. This sulfuric acid then forms an aerosol, and this is what's seen as the fumes. That's how I understand it anyway. I've boiled down 50% acid and it fumes.

hissingnoise - 27-4-2016 at 02:02

The fumes produced at low concentration are steam with a little SO₂ and can appear bluish . . .

But the heavy, dense white fumes which appear @ 339° are umistakable!

When the acid cools it will be found to char paper instantly and this is one indication of its high concentration!

And BTW, the fuming hasn't caused a rust problem in my shed!

XeonTheMGPony - 27-4-2016 at 05:03

http://www.sciencemadness.org/talk/viewthread.php?tid=5495 < Synthesis of Sulfur Trioxide and Oleum: The persulfate method

http://www.sciencemadness.org/talk/viewthread.php?tid=10217 < Sulfur trioxide from sodium hydrogen sulfate

there is a method that was very successful at moderate low temps using metaphosphoric acid I'll add it when I find it

With out those you simply can not get to any where dry, even vacuum distilling at 200c at .01 bar (7.5 Torr/750mmHg) there you will get 97/98% and you'll lose a better part of your acid.

After lots of research the only viable method is adding SO3 to get anhydrous Sulfuric acid.

Quote: Originally posted by garage chemist

(HPO3)n is a real kickass dehydrating agent, stronger than oleum for example. It dehydrates H2SO4 to SO3, look at the procedure I posted in the "oleum and SO3" thread.

Now, let's talk about its preparation.

Its ability to attack glass in the hot stage is a problem.
As I found out today, it attacks also iron very badly, a lot of hydrogen is evolved and the crucible started to dissolve!
Then I used a quartz crucible, and incredibly, it was also attacked!

However, the attack was much less severe than with glass. My beaker in which I did the first preparation got attacked considerably, and the walls are now noticeably thinner.
On the contrary, the walls of my quartz crucible now feel as if they were a bit THICKER. Maybe the quartz forms some sort of protective layer after some time which prevents further attack.
For the next batches of (HPO3)n, I'll use my quartz crucible.

My design was to make some of that, mix with a sample of sacrificial purified sulfuric, use that SO3 to dehydrate my cleaned Sulfuric batch, and vial the remaining SO3 for when needed.

Saying SO3 is nasty is like calling a cloud of HF an irritant it is a massive understatement! use extreme safety precautions!!!

[Edited on 27-4-2016 by XeonTheMGPony]

careysub - 27-4-2016 at 05:20

The effectiveness of using oleum to produce smoke screens is well known to depend on the humidity (it usually worked very well for naval smoke screens for example). So disagreements about when boiling sulfuric acid visibly fumes, or how much it fumes, may be due entirely to the local humidity.

People opining on this should describe their local humidity (or location, as a proxy). Florida and the Southwest likely have different experiences.

hissingnoise - 27-4-2016 at 07:37

Quote:

So disagreements about when boiling sulfuric acid visibly fumes, or how much it fumes, may be due entirely to the local humidity.

SO₃ fumes from boiling SA are much denser than steam, an opaque white almost, and when seen, the heat should be removed and the acid almost covered by a lid to minimise reuptake of moisture!

When the container has cooled enough to be handled, it can be stored!