Sciencemadness Discussion Board

RFNA and Vacuum Distillation of H2SO4

UranylPeroxysulfate - 29-4-2021 at 01:06

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.

Antigua - 29-4-2021 at 01:17

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 - 29-4-2021 at 01:30

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 - 29-4-2021 at 02:16

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 - 29-4-2021 at 02:21

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 - 29-4-2021 at 03:54

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.


Quote: Originally posted by draculic acid69  

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?


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.

monolithic - 29-4-2021 at 04:10

Quote: Originally posted by UranylPeroxysulfate  
Hi,
My vacuum pump pulls a vacuum of about 6.5 mmHg.


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 - 29-4-2021 at 04:34

Quote: Originally posted by monolithic  
Quote: Originally posted by UranylPeroxysulfate  
Hi,
My vacuum pump pulls a vacuum of about 6.5 mmHg.


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.

Sulaiman - 29-4-2021 at 04:45

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]

Oxy - 29-4-2021 at 05:07

Quote: Originally posted by UranylPeroxysulfate  


Bubbling air is not an option. Any tube used for the bubbling will be instantly destroyed.


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 - 29-4-2021 at 05:17

Quote: Originally posted by UranylPeroxysulfate  
Quote: Originally posted by monolithic  
Quote: Originally posted by UranylPeroxysulfate  
Hi,
My vacuum pump pulls a vacuum of about 6.5 mmHg.


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 - 29-4-2021 at 07:21

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 - 29-4-2021 at 17:36

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.

UranylPeroxysulfate - 29-4-2021 at 17:38

Quote: Originally posted by monolithic  
Quote: Originally posted by UranylPeroxysulfate  
Quote: Originally posted by monolithic  
Quote: Originally posted by UranylPeroxysulfate  
Hi,
My vacuum pump pulls a vacuum of about 6.5 mmHg.


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.