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Procedure for purifying HNO3 by vacuum distillation?

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Dr. Shark

Tue Jan 23 2007, 04:28PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Out of general curiosity and because I hear that it makes a superb rocket fuel, I've been pondering the possibility of creating 100% HNO3 by distilling a mixture of HNO3 and H2SO4 under reduced pressure.
Back in the days when I was interested in thing that go Boom! (I' still thankful to my guardian angel I didn't loose any limbs back then), I think I had a rather detailed account of safety procedures, temperatures, pressures etc., but I'm not able to find that anymore. Does anyone happen to have the procedure on hand?

Oh, and would one of the more experienced members (Bored Chemist comes to mind) like to comment on the general sanity of attempting this? My equipment is basically limited to an all-glass distillation aparatus, no fancy fume hoods, blast shields, &c. If it's unsafe to attempt this in any quantity, I'll forget about it, but it's so tempting when you have all the apparatus for it.

Bored Chemist

Tue Jan 23 2007, 06:37PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

"Oh, and would one of the more experienced members (Bored Chemist comes to mind) like to comment on the general sanity of attempting this? "
well, I think it's insane, but I'm not qualified as a psychiatrist.
It would be safe enough to do this on a sub- gram scale but that's no use to you.
Even on a small scale any vacuum distillation should be done behind a screen of some sort. The next question is how badly do you want to inhale the nitric acid vapour when (not if) something breaks?
" it's so tempting when you have all the apparatus for it."
Good point, but since you lack a fume cupboard and blast shield you don't have all the apparatus.
I realise that someone will reply to this pointing out that they have distilled buckets of acid in their kitchen while feeding the dog and changing the baby. I'm told there's a guardian angel who looks after fools, drunks and children, but I don't want to bank on it and I don't want anyone's accidents on my conscience.
If you persist in this idea, at least try to track down as much information as possible.
This is probably a good start

Hazmatt_(The Underdog)

Tue Jan 23 2007, 08:11PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

A long time ago when I was still into dangerous stuff too, I was trying to distill Nitric as well. I doubt that I ever succeeded, but what I can tell you is that it's not really that easy. If you're attemting to condense the gas like older methods, then you will loose quite a bit of it during the process and its very noxious! Not to mention toxic.

If you really want to do it, the glassware is avaialable if you look in the right places. There is some on ebay all the time, and swapmeets, and universities. Someone is always shutting down a lab. However, you don't know what's been in the glass so its a big risk factor to your safety. It should be properly cleaned with Chromic Sulphuric Acid before use, and getting the cleaning solution is almost impossible for someone without chemical buddies.

If you want to do it at reduced pressure you will need a reaction flask rated for it, and safety too. You could use a round bottom flask, but the trouble is there is hot Nitric and Sulphuric acid in that flask. If it cracks, then you have a world of problems on your hands.
I have some old low pressure glassware and what they did was to tape the outside up, not all of it, but they made a web like covering over the outside. To do this to a boiling flask you wouldn't be able to use PVC tape because it would melt. BUT you could use a silicone Fiberglass tape because of its heat resistance and mechanical strength. These tapes are available by 3M but are rather expensive. If you do tape the flask, don't bother trying to get the tape off, it will bond to the glass.

Reducing pressure, well, that is easy with a vacuum pump, but its not easy to do it properly. Vacuum pumps are not intended to have corrosive or organic material find its way into its pumping medium. So as a system design you need traps, neutralizers, bubblers, dessicators, and a bunch of other expensive stuff.

I think you'd be better off trying to buy it. I can't even buy Nitrate here unless its through one of my chemical connections, and they would want a justification for a 50 lb. bag of Nitrate.

Here's some stuff you would need to get started:

Here's the home for my Calcium metal

Dr. Shark

Wed Jan 24 2007, 03:13PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Bored Chemist wrote ...

It would be safe enough to do this on a sub- gram scale but that's no use to you.
Even on a small scale any vacuum distillation should be done behind a screen of some sort.

Well I wasn't going to pruduce liters of the stuff, 5ml or so would be about the right quantity to have some fun with. I'm not so worried about the danger of imploding glass though: Even back at university we did lots of distillations under reduced pressure, and never had any safety screens. I've got all the proper glassware, and I think the danger of that failing just from air-pressure (no explosion involved) is close to nil.
But in case of a failure the HNO3 would possibly decompose explosively (correct?) and then one deep breath could kill me - not something I'm keen on right now.

Hazzmat, you were also on the Powerlabs board, right? I think I had a thread there once about the same topic, and I got flamed by Sam, but in retrospect I think he was right (but only that one time

) As I said, I have roundbottom flasks etc, and I am using one of these tap-water powered pumps, so all the nasty stuff goes straight down the drain.
You've got quite a nice collection of glassware too, btw, but at least mine was more expensive since I bought the stuff before eBay came along.

Steve Conner

Wed Jan 24 2007, 03:16PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think Chris (not Russell) has distilled nitric acid in a similar way before. He recently posted some pics of about a half bottle of red fuming nitric acid, that I assume he made himself. Hopefully he'll comment on this.

I think trying to distil nitric acid at home is pretty crazy, but I'm not a chemist, so I'm not qualified to comment.

Bjørn

Wed Jan 24 2007, 04:03PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I've got all the proper glassware, and I think the danger of that failing just from air-pressure (no explosion involved) is close to nil.

Quality glassware with a known history is quite safe, but never 100% safe. There is always a tiny chance that it will fail without notice. I can recall 5-6 incidents where glass have failed for no obvious reason, one of those involved a test tube filled with acid that caused a lot of problems.

If the glassware contains high pressure, low pressure or dangerous chemicals you should always assume that it will explode because it is only a matter of statistics before it fails. As Bored Chemist tried to explain there is only one way to err and that is on the safe side.

Bored Chemist

Wed Jan 24 2007, 06:39PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

The really important thing to know about "sub gram" is that it doesn't mean more than a gram.

" I'm not so worried about the danger of imploding glass though"
I am, possibly because, if it were not for the fact that I wear glasses, I might not be able to see to read this thread.

"I've got all the proper glassware, and I think the danger of that failing just from air-pressure (no explosion involved) is close to nil."
Imediately before any accident, if you asked the people involved they would practically always say " I think this is reasonably safe" and they were always wrong.
Just for the record, in this case the propper glassware includes a safety screen (and I'm quite happy to discuss whether or not polycarbonate is a glass but not in this thread).
IMHO there are 2 ways to err; on the safe side and on the dumb side. An interesting difference is that you can err on the safe side as often as you like.

Chris

Thu Jan 25 2007, 01:49AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

I have distilled about 400mL of red fuming nitric acid before, which is done at one atmosphere. It's easy if you have the proper equipment. If you wanted to make white fuming nitric acid, you would simply have to add a vacuum pump, probably a peristaltic pump, to reduce the boiling point and therefore the level of decomposition of HNO3 to NO2.

You will need to do this outside or under a fume hood or something if you do it at one atmosphere; there will be a lot of NO2 put off. If you use a vacuum pump, I imagine you would need to run the output to a condenser that returns to the reflux.

WATCH THE LEVEL in the boiling flask. Don't put too much in; I recommend at most enough dry reagent for 300mL of HNO3 in a 3L flask (you can do the math yourself). The anhydrous H2SO4/KNO3 mix is very thick and dough-like even if you use excess H2SO4. The other night I was distilling this stuff for the second time and had too much in there (enough for 750mL HNO3), and it started rising up into the column. It plugged up, pressure built, and I ended up in the hospital with 3rd degree chemical burns on my hand, stomach, and a little on my face. Very painful. Thank goodness I was wearing protective clothing and goggles, or it would be much, much worse *cringes*. Don't make this mistake! and don't stand near the aparattus when it's doing it's thing!

As long as you are careful with avoiding pressure buildup in the still, and the dangers of NO2 vapor (remember that stuff will desensitize your nose despite its strong odor, and even low concentrations can be dangerous or deadly, with no symptoms showing for several hours!) it should be a relatively safe procedure. If you don't have anywhere to vent the vapors, you can run it through a scrubber with baking soda, sodium or calcium hydroxide, or something. I use an all glass and teflon reflux rig to distill (quite expensive equipment, even if you get lucky on ebay, but anything less would not be sufficient), and a double boiler type setup to heat the boiling flask slowly, gently and evenly. Always keep plenty of baking soda around in case something breaks. Remember teflon has a very high thermal expansion compared to the very low expansion of borosilicate glass. Teflon plugs, adapters, and stopcocks will get tight when heated and cannot be removed until cool. There is always the risk, however unlikely, that the expanion could break glass joints. Never cram teflon stoppers in tight before they are warmed up.

Remember fuming nitric acid is a strong and potentially hypergolic oxidizer, and is not compatible with any materials besides glass, teflon, some nickel or stainless alloys, and special fluoroelastomers like Viton or fluorosilicone. Don't even think about keeping it in a plastic bottle.

Swany

Thu Feb 01 2007, 04:03AM

Registered Member #261 Joined: Mon Feb 27 2006, 12:34AM
Location:
Posts: 22

I would not be worried very much distilling nitric acid as long as you take proper precautions and take your time. If you have a vacuum pump, by all means use it. It is not totally critical however. I distill 2:1 molar mixtures of NH4NO3/H2SO4 frequently at standard pressure. I keep the temperture well below 120C, and avoid light. Adding a small amount of urea to the final product gives me roughly 96%+ white fuming nitric acid. If it is yellow, simply run some cold air through it with urea to clear it up.

However, I would not suggest distilling perchloric acid!

EDIT: And yes, when storing do NOT use anything but ground glass or teflon. Same goes for distilling, although plastic wrap is quite resistant to it. As is tinfoil.

If you only need small small amounts, and do not want to fiddle with a distillation setup.... this works, albiet slowly:

(go to syntheses' and then Nitric acid)

Shaun

Thu May 10 2007, 04:46AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

And If you dont have/cant get/wont buy a good quality vacuum pump, scitoys.com tells how to make a good PVC hand powered one.

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