I know that this is the millionth nitric acid thread, but I can find very little information on the vacuum distillation technique.
I'm trying to produce some quality, >95% conc., white fuming nitric acid. I have a full 24/40 distillation setup with ground glass joints, a
ebulliator prevent bumping, and an aspirator to create a vacuum. I heat the nitric/sulfuric mix with a sand bath over a hotplate, and my vacuum gets
well below 50 torr.
I've tried this method a few times, with results ranging from moderately success with small yields to disaster.
The problem is that as my mixed acids are heating up, but while the vacuum is on, nitric vapors will be sucked out distillation setup, and end up
condensing in my vacuum hose and attacking my aspirator. I've tried putting the receiving flask in an ice bath, and cooling the water that runs
through the condenser, but neither of these help. I still get nitric vapors that come off and attack my aspirator. It seems like the nitric vapors are
being sucked away, and there's nothing I can do to stop them.
Can anyone who has distilled nitric under vacuum help me out? Anyone know why the nitric isn't condensing?chemrox - 13-3-2008 at 21:54
Try rigging a double trap setup between the receiver and the aspirator. I'd do it like this, one trap you could make with a small ehrlenmeyer
although a side-arm test tube would be better, anyway the first trap is for condensing vapor that got past the reciever, so place it in a bath as cold
as you can get ..dry-ice and acetone if you can .. the second trap is for catching acid vapors, it's a polycarb tube packed with KOH or NaOH flakes...
tube, one hole stoppers each end and tubing for the vacuum line.
What is your aspirator made of? I would think a chrome plated brass one would survive the vapors especially if you protect it as per above.Magpie - 14-3-2008 at 10:39
The brown gas being sucked out of your apparatus is NO2 and not nitric acid - but I suppose you know that. If you have an adequate condenser you
should be able to condense the acid. I'm wondering if you are not letting too much air into your system either purposely through the ebulliator or
through leaks.texaspete - 14-3-2008 at 12:17
Chemrox- I like the double trap idea. I have one vacuum trap, and I once used that submersed in ice-water and salt. I managed to catch some yellow
liquid, but some of the nitric gases still made it through. The double trap idea sounds good, if I can make the NaOH trap myself, because vacuum traps
are quite expensive. Still, I'm kind of afraid of loosing too much of my product this way.
Magpie- The "nitric vapors" I observed were clear, not brown. They are difficult to see, but they are visible. Once the distillation rig gets below
300 or 200 torr, all brown gases disappear.vulture - 14-3-2008 at 14:04
You have a leak. Once vacuum is attained there shouldn't be anything sucked out of your setup. Carefully examine all joints. Disassemble your setup
and use some soapy water around the joints to detect leaks.texaspete - 22-3-2008 at 16:51
Quote:
Originally posted by vulture
You have a leak. Once vacuum is attained there shouldn't be anything sucked out of your setup. Carefully examine all joints. Disassemble your setup
and use some soapy water around the joints to detect leaks.
Like I said, I'm using ground glass joints. I noticed when i have previously noticed that when I put water on the joints, the water was slowly drawn
into the chamber. I haven't had time to test my setup with soapy water, but I know that it will leak a little.
Should I be looking for major leaks, or is the small amount of air that comes through the joints enough? If the joints are the problem, what is the
solution? I know grease is supposed to be used with ground glass joints, but wouldn't the nitric attack it?organometallic - 22-3-2008 at 18:18
I don't believe Dow Corning grease is attacked by conc nitric acid. Although you might want to check thatgarage chemist - 23-3-2008 at 00:12
Perhaps your cooling water isn't cold enough, and/or your vacuum too strong, so that the boiling point of HNO3 lies below room temperature?
With strong aspirator vacuum, water boils at around like 26°C. Nitric acid has a boiling point of 86°C at atmospheric pressure, so at aspirator
vacuum it is going to have a boiling point way below 26°C!
Try running ice water through your condenser, and cooling your receiver with an ice/salt mix.
[Edited on 23-3-2008 by garage chemist]MagicJigPipe - 23-3-2008 at 00:22
What kind of condenser are you using? Are you sure it is adequate for condensing a semi-low boiling liquid under vacuum? In my experience, a simple
liebig (around 200mm jacket length) WILL NOT suffice.vulture - 23-3-2008 at 04:38
If you do vacuum distillation, you need grease or something else (teflon tape/jackets) to seal your joints. Otherwise it's an exercise in futility.mr.pyro - 26-4-2008 at 20:31
Is it possible for the vacuum to be to strong, and thus suck out the nitric acid fumes?
The last time I tried a distillation under vacuum, the fumes were also sucked out almost the same way as the OP. I have a roughing pump that pulls
10^-2 torr. I have checked all joints in my glass and everything is sealed. It seems as soon as the vapor is created it is sucked right out from
the low pressure of the pump.chemrox - 26-4-2008 at 22:13
Do you know the vapor pressure of the "fumes?" their composition? I also think the problem is leaks and you're dealing with fluid dynamics in your
system when you shouldn't be. But without knowing the expected condensation temp of the "fumes" I'm guessing. Is there NOx mixed with the nitric?Pulverulescent - 27-4-2008 at 06:35
With gas-tight connections and a good vacuum, loss of vapour sounds like the receiver isn't cold enough.
Dry ice/acetone might seem overkill, but acid freezing in the receiver shoudn't be a problem.
AFAIK, unlike water, HN03 contracts slightly on freezing!
