UranylPeroxysulfate
Harmless
Posts: 23
Registered: 11-6-2018
Member Is Offline
Mood: Sulfonated
|
|
RFNA and Vacuum Distillation of H2SO4
Hi,
I have a bottle of concentrated (98%) sulfuric acid made from boiling dilute sulfuric acid. Unfortunately, I used a dirty stir bar when boiling the
acid and I got the acid a bit cloudy and discolored. I want to clean up the acid by distillation, but at 337C (the boiling point of H2SO4), my stir
bar will demagnetize significantly and not be able to stir the acid properly, which means the H2SO4 will likely bump violently and destroy my
apparatus (this had happened to me multiple times). Is it a good idea to distill my H2SO4 under a vacuum so my stir bar won't demagnetize? My vacuum
pump pulls a vacuum of about 6.5 mmHg.
I'm also interested in making some red fuming nitric acid (RFNA). I want it to have high NO2 content but little to no water content. I plan to make it
by distilling a mixture of KNO3, concentrated H2SO4, and phosphorus pentoxide. I assume this will form HNO3 and N2O5; the N2O5 then decomposes to give
NO2 which dissolves in the nitric acid forming RFNA. What are your thoughts on this?
Thanks.
If something does work, sulfonate it.
|
|
|
Antigua
Hazard to Others
Posts: 155
Registered: 27-9-2020
Member Is Offline
|
|
Yes, it's a good idea as long as your pump can resist sulfuric acid or you have a cold trap or other tinkered device that can protect your pump from
awful sulfuric acid fumes. With your vacuum you could distill sulfuric acid in below 200C.
|
|
|
Oxy
Hazard to Others
Posts: 140
Registered: 1-12-2020
Member Is Offline
|
|
Instead magnetic stirring you could try to use an air pump and bubble air into boiling sulphuric acid.
I would not use vaccuum for H2SO4 personally. In case of violent bump it can destroy the apparatus and cause an implosion which may be catastrophic.
Regarding HNO3, what concentration do you want to achieve? You're able to get ~95% or even more by simply distilling HNO3 from mixture of 98% H2SO4
and KNO3.
Be aware that phosphorus pentoxide is so powerful that it can dehydrate H2SO4 to SO3. So maybe it's not the best idea to dump everything into 1 flask
and distill.
|
|
|
draculic acid69
International Hazard
Posts: 1371
Registered: 2-8-2018
Member Is Offline
|
|
Quote: Originally posted by Oxy  | Instead magnetic stirring you could try to use an air pump and bubble air into boiling sulphuric acid.
I would not use vaccuum for H2SO4 personally. In case of violent bump it can destroy the apparatus and cause an implosion which may be catastrophic.
Regarding HNO3, what concentration do you want to achieve? You're able to get ~95% or even more by simply distilling HNO3 from mixture of 98% H2SO4
and KNO3.
Be aware that phosphorus pentoxide is so powerful that it can dehydrate H2SO4 to SO3. So maybe it's not the best idea to dump everything into 1 flask
and distill. |
U raise a really good point there.p2o5 + h2so4= so3so it's a definite no-no. Is there a reason U need to distill your h2so4?
|
|
|
Garry63566
Harmless
Posts: 13
Registered: 22-2-2021
Location: Shenzhen, China
Member Is Offline
|
|
for distilling H2SO4 some ceramic pieces or things like that would probably do for boiling stones (so H2SO4 probably would not bump much and you might
not need the stir bar and consequently the vacuum)
|
|
|
UranylPeroxysulfate
Harmless
Posts: 23
Registered: 11-6-2018
Member Is Offline
Mood: Sulfonated
|
|
| Quote: Originally posted by Oxy | Instead magnetic stirring you could try to use an air pump and bubble air into boiling sulphuric acid.
I would not use vaccuum for H2SO4 personally. In case of violent bump it can destroy the apparatus and cause an implosion which may be catastrophic.
Regarding HNO3, what concentration do you want to achieve? You're able to get ~95% or even more by simply distilling HNO3 from mixture of 98% H2SO4
and KNO3.
Be aware that phosphorus pentoxide is so powerful that it can dehydrate H2SO4 to SO3. So maybe it's not the best idea to dump everything into 1 flask
and distill. |
Bubbling air is not an option. Any tube used for the bubbling will be instantly destroyed.
I want to achieve around 90-95% nitric acid with the rest being nitrogen dioxide (no water); I want the NO2 to be there so it has better combustion
properties (more likely to cause autoignition).
I'm aware that phosphorus pentoxide dehydrates H2SO4 to SO3, but I only need a small amount of it (concentrated sulfuric acid has only 2% water I need
to get rid of, and generating 5-10% of NO2 and N2O5 wouldn't take very much P4O10). When sulfuric acid is in excess, the SO3 dissolves in it to form
oleum, which is far less dangerous.
Yes. I'm using my H2SO4 for organic synthesis. While dirty H2SO4 might not cause side reactions, it will make the purification steps a lot more
annoying.
If something does work, sulfonate it.
|
|
|
monolithic
Hazard to Others
Posts: 435
Registered: 5-3-2018
Member Is Offline
Mood: No Mood
|
|
Nomograph says 337 C at 1 atm = 177 C at 6.5 mm Hg. I would cover the apparatus in a towel or cover it with a DIY blast shield. Glassware implosion
under vacuum plus sulfuric acid at 177 C would probably kill you.
|
|
|
UranylPeroxysulfate
Harmless
Posts: 23
Registered: 11-6-2018
Member Is Offline
Mood: Sulfonated
|
|
| Quote: Originally posted by monolithic |
Nomograph says 337 C at 1 atm = 177 C at 6.5 mm Hg. I would cover the apparatus in a towel or cover it with a DIY blast shield. Glassware implosion
under vacuum plus sulfuric acid at 177 C would probably kill you. |
How likely is glassware implosion when doing vacuum distillations? I vacuum distilled methyl salicylate (b.p. 222C @ 1 atm) and benzyl chloride (b.p.
179C @ 1 atm) before, and there doesn't seem to be any problems.
If something does work, sulfonate it.
|
|
|
Sulaiman
International Hazard
Posts: 3555
Registered: 8-2-2015
Location: 3rd rock from the sun
Member Is Offline
|
|
Bumping during sulphuric acid distillation is one of the scariest things I've had happen in this hobby!
based on my limited experience :
- use dry boiling stones/chips
(distilling with stones still wet from a first run that did not bump I had two very violent bumps on a second run)
- expect and prepare for explosive ejections of extremely hot liquid and vapour.
If you do try vacuum distillation then I think that an air leak bubbler would be good - but I've not tried yet.
(see bottom of this page http://www.sciencemadness.org/talk/viewthread.php?tid=28461)
PS just for emphasis ;
BOILING SULPHURIC ACID COULD EASILY RUIN YOUR LIFE
- BE VERY CAREFUL
[Edited on 29-4-2021 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
|
|
|
Oxy
Hazard to Others
Posts: 140
Registered: 1-12-2020
Member Is Offline
|
|
Not really. You can use PTFE tubing and glass tube to provide the air.
Glassware can whitstand high vaccum. But the problem can occur during bumping as the whole apparatus can be shaken wildly.
Be careful with that, do it preferably outside and behind the cover
|
|
|
monolithic
Hazard to Others
Posts: 435
Registered: 5-3-2018
Member Is Offline
Mood: No Mood
|
|
| Quote: Originally posted by UranylPeroxysulfate | | Quote: Originally posted by monolithic |
Nomograph says 337 C at 1 atm = 177 C at 6.5 mm Hg. I would cover the apparatus in a towel or cover it with a DIY blast shield. Glassware implosion
under vacuum plus sulfuric acid at 177 C would probably kill you. |
How likely is glassware implosion when doing vacuum distillations? I vacuum distilled methyl salicylate (b.p. 222C @ 1 atm) and benzyl chloride (b.p.
179C @ 1 atm) before, and there doesn't seem to be any problems. |
Unlikely if you take care of your glassware and watch for vacuum bumping, but it's best not to risk it when the results of an implosion could be
absolutely devastating. I've vacuum distilled high boiling point organic compounds (200-300 C at 1 atm) and I still felt sketched out -- and they
weren't dangerously corrosive like sulfuric acid.
[Edited on 4-29-2021 by monolithic]
|
|
|
Deathunter88
National Hazard
Posts: 507
Registered: 20-2-2015
Location: Beijing, China
Member Is Offline
Mood: No Mood
|
|
Just have one stir bar you use for sulfuric acid. It will still work for stirring since once it becomes demagnetized it becomes like any other piece
of iron and will still spin on the magnetic hotplate.
|
|
|
UranylPeroxysulfate
Harmless
Posts: 23
Registered: 11-6-2018
Member Is Offline
Mood: Sulfonated
|
|
| Quote: Originally posted by Sulaiman | Bumping during sulphuric acid distillation is one of the scariest things I've had happen in this hobby!
based on my limited experience :
- use dry boiling stones/chips
(distilling with stones still wet from a first run that did not bump I had two very violent bumps on a second run)
- expect and prepare for explosive ejections of extremely hot liquid and vapour.
If you do try vacuum distillation then I think that an air leak bubbler would be good - but I've not tried yet.
(see bottom of this page http://www.sciencemadness.org/talk/viewthread.php?tid=28461)
PS just for emphasis ;
BOILING SULPHURIC ACID COULD EASILY RUIN YOUR LIFE
- BE VERY CAREFUL
[Edited on 29-4-2021 by Sulaiman] |
I actually had multiple explosive ejections of boiling sulfuric acid before (and boiling nitric acid). Fortunately, in none of them, I was standing
close to the distillation apparatus, though the acid still destroyed my heating mantle. Another part of the reason I wasn't killed is that I attached
a fractionating column and above the boiling flask, which really limits the amount of sulfuric acid that is ejected out. I also wore an acid-resistant
face shield.
The boiling goes fine when the concentration of H2SO4 is below 70% but starts bumping crazy once it reaches above 70%, even with boiling stones. It's
only when I start stirring the sulfuric acid while boiling that I was able to reach above 80% sulfuric acid. However, the bumping still goes out of
control when the concentration of H2SO4 reaches 98%; I assume this is because my stir bar demagnetized. The good thing is that 98% sulfuric acid is
already more than concentrated enough for organic synthesis.
Another scary moment I had when boiling H2SO4: I boiled a batch of H2SO4 to 98%. The acid is at 337C and I was tired of waiting for it to cool down,
so when it is still around 230-240C I tried to cool the acid by putting the flask in some water (looking back, this is just idiotic). I heard a crack.
I immediately took the flask out of the water; it started to slowly leak sulfuric acid. As fast as I can, I grabbed another flask and poured the
sulfuric acid into it. I wasn't injured, but this moment is VERY SCARY and I was very lucky to not be killed. If I realized a crack a few seconds
later, the flask might have exploded and I would be dead. If I wasn't using a flask made of high-quality borosilicate glass, the flask would have
likely exploded and I would be dead. The lesson: leave hot sulfuric acid alone; never mess with it.
Conclusion: Never boil sulfuric acid above 70% concentration if you can't stir the sulfuric acid while it boils. Wear a proper lab coat and face
shield. Use a fractionating column. Don't try boiling sulfuric acid above 98% concentration. Never handle hot sulfuric acid; let it cool down to at
least below 80C before handling it.
If something does work, sulfonate it.
|
|
|
UranylPeroxysulfate
Harmless
Posts: 23
Registered: 11-6-2018
Member Is Offline
Mood: Sulfonated
|
|
| Quote: Originally posted by monolithic | | Quote: Originally posted by UranylPeroxysulfate | | Quote: Originally posted by monolithic |
Nomograph says 337 C at 1 atm = 177 C at 6.5 mm Hg. I would cover the apparatus in a towel or cover it with a DIY blast shield. Glassware implosion
under vacuum plus sulfuric acid at 177 C would probably kill you. |
How likely is glassware implosion when doing vacuum distillations? I vacuum distilled methyl salicylate (b.p. 222C @ 1 atm) and benzyl chloride (b.p.
179C @ 1 atm) before, and there doesn't seem to be any problems. |
Unlikely if you take care of your glassware and watch for vacuum bumping, but it's best not to risk it when the results of an implosion could be
absolutely devastating. I've vacuum distilled high boiling point organic compounds (200-300 C at 1 atm) and I still felt sketched out -- and they
weren't dangerously corrosive like sulfuric acid.
[Edited on 4-29-2021 by monolithic] |
I'll keep that in mind. Another question: is it necessary to grease all the joints when doing vacuum distillation? I never greased them and the
distillation appears to go fine.
| Quote: Originally posted by Deathunter88 | | Just have one stir bar you use for sulfuric acid. It will still work for stirring since once it becomes demagnetized it becomes like any other piece
of iron and will still spin on the magnetic hotplate. |
It does spin, but it doesn't spin fast enough to stop the sulfuric acid from bumping.
If something does work, sulfonate it.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
