CaTaLiZr
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High Temperature Magnetic Stir Bars?
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?
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Fulmen
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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]
We're not banging rocks together here. We know how to put a man back together.
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j_sum1
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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.
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BromicAcid
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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.
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hissingnoise
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| Quote: | | I don't think the teflon stir bar can survive experiments involving high temperatures (e.g. boiling sulfuric acid for distillation).
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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 . . .
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XeonTheMGPony
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I'll borrow this thread any one have a P-T chart for Sulfuric? (P-T = Pressure temperature)
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CaTaLiZr
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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).
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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 . . .
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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.
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j_sum1
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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.
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hissingnoise
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| 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!
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j_sum1
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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.
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Tdep
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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.
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hissingnoise
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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! 
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XeonTheMGPony
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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]
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careysub
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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.
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hissingnoise
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| Quote: | | So disagreements about when boiling sulfuric acid visibly fumes, or how much it fumes, may be due entirely to the local humidity.
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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!
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