Sciencemadness Discussion Board

Problems with vacuum distilling HNO3

gnitseretni - 11-11-2007 at 08:29

ok, so i got a new distillation kit from UGT and i couldn't wait to try and distill some HNO3 from KNO3/H2SO4. I used to do it the brainfever way but i always got yellow HNO3 and now i thought i was gonna get me some clear HNO3 but i ended up with next to nothing. I used a water aspirator as the vacuum source. As soon as i turned on the pump i noticed that all the HNO3 vapour got sucked out of the system.. it doesn't get a chance to condense. So uhm.. how do i fix this?

I use a little submersible pump to cool the condenser. It's in a bucket with more ice than water so that oughta be cold enough. The temp i distilled was about 65C. I placed the thermometer in the water bath instead of in the distilling adapter because i didn't think the nylon bushing would withstand the HNO3 vapour, so i replaced the thermometer adapter wit ha glass stopper.

I let the pump for the water aspirator run continuously so i have a constant and even vacuum, but i was thinking am i suppose to have a valve in my vacuum line that i close after the water aspirator reached max vacuum? Say after one or two minutes or so? That would keep the HNO3 vapour from being sucked out of the system and being discarded in the water that circulates through my pump which i imagine isn't very healthy for my pump.

Any suggestions would be appreciated.

[Edited on 11-11-2007 by gnitseretni]

garage chemist - 11-11-2007 at 09:06

Dont use vacuum, simple.
The distillation of crude HNO3 from KNO3 and H2SO4 is usually done at atmospheric pressure.
And it doesnt really matter if your crude HNO3 is yellow or white since it is far from being anhydrous anyway, so useless for RDX even if it was white.

Whats best done under vacuum is the redistillation of already nearly anhydrous (distilled from crude HNO3 and conc H2SO4 at atmospheric pressure, fraction at 86°C used) yellow HNO3 to remove most of the nitrogen oxides, followed by the urea treatment.

I see people use vacuum where it is not required or even does more harm than good so often. Before you do a vacuum distillation, ask yourself if the usage of vacuum is really justified.

vulture - 11-11-2007 at 12:47

Quote:

As soon as i turned on the pump i noticed that all the HNO3 vapour got sucked out of the system


Something doesn't check out here. Either way you have a leak, but then you shouldn't have HNO3 vapour at that temperature (did you check if it's really HNO3?) or your condensor isn't efficient enough/not properly installed.

gnitseretni - 11-11-2007 at 13:09

i use my HNO3 to make PETN with. So far I've always done it with yellow HNO3 but i read that you're suppose to use clear HNO3 because the yellow HNO3 contains impurities(like dissolved nitrogen??) that may cause unwanted sidereactions or byproducts. That's what i read anyway, and to me that translates into: using clear HNO3 is safer. And I thought clear HNO3 is obtained by distilling it under reduced pressure which is why i got the glassware.

But so you're telling me that using yellow HNO3 is fine or do i still have to treat it with urea to make it clear before making PETN?
Quote:
Originally posted by garage chemist
Dont use vacuum, simple.
The distillation of crude HNO3 from KNO3 and H2SO4 is usually done at atmospheric pressure.
And it doesnt really matter if your crude HNO3 is yellow or white since it is far from being anhydrous anyway, so useless for RDX even if it was white.

Whats best done under vacuum is the redistillation of already nearly anhydrous (distilled from crude HNO3 and conc H2SO4 at atmospheric pressure, fraction at 86°C used) yellow HNO3 to remove most of the nitrogen oxides, followed by the urea treatment.

I see people use vacuum where it is not required or even does more harm than good so often. Before you do a vacuum distillation, ask yourself if the usage of vacuum is really justified.

garage chemist - 11-11-2007 at 13:20

I wanted to tell you that it is nonsense to use vacuum for the distillation of HNO3 from KNO3/H2SO4, as the acid wont be anhydrous- you will have to redistill it from H2SO3 to make it anhydrous, and then finally in vacuum to make it nearly colorless- followed by urea treatment to remove last NOx traces.

Only the last of those three distillations requires vacuum. Why use vacuum thrice if you only need it once?
With vacuum your losses are higher due to the much lower boiling point of the acid, so to keep losses down, use the vacuum only when you NEED it.

gnitseretni - 11-11-2007 at 13:30

i don't think i have a leak. I checked for leaks before actually doing it. I turned on the pump and held my thumb to cover the hole where the thermometer adapter goes and there was vacuum allright.

As for whether it was really HNO3 vapour..
i first mix the H2SO4 and KNO3 in a big jar and then i pour it into my distilling flask(2000ml RBF) because you can't (or I can't anyway) stir anything in the RBF because of the small neck. Isn't the vapour that's released from the mixture (pretty much immediately after you add the KNO3) HNO3 vapour? I'm talking about the clear vapour not the nasty brown vapour which you get when you add too much KNO3 at once.

And not only the vapour I'm talking about above. I did have vacuum so the boiling point should've lowered a bit so at 65C i should at least have some HNO3 vapour coming over, right? What else could it be that i saw getting sucked out of the system?

The condenser is a 200mm liebig condenser, in case knowing that helps any.
Quote:
Originally posted by vulture
Quote:

As soon as i turned on the pump i noticed that all the HNO3 vapour got sucked out of the system


Something doesn't check out here. Either way you have a leak, but then you shouldn't have HNO3 vapour at that temperature (did you check if it's really HNO3?) or your condensor isn't efficient enough/not properly installed.

gnitseretni - 11-11-2007 at 13:40

using vacuum for the distillation of HNO3 is nonsense.. gotcha!

Is using clear HNO3 is safer then yellow HNO3 when making PETN nonsense too?
Quote:
Originally posted by garage chemist
I wanted to tell you that it is nonsense to use vacuum for the distillation of HNO3 from KNO3/H2SO4, as the acid wont be anhydrous- you will have to redistill it from H2SO3 to make it anhydrous, and then finally in vacuum to make it nearly colorless- followed by urea treatment to remove last NOx traces.

Only the last of those three distillations requires vacuum. Why use vacuum thrice if you only need it once?
With vacuum your losses are higher due to the much lower boiling point of the acid, so to keep losses down, use the vacuum only when you NEED it.

vulture - 11-11-2007 at 14:29

Man, man. Where to start. You're making PETN but you don't know why you need clear HNO3? Dissolved nitrogen? Make that dissolved NOx. And that's BAD. Figure out why yourself.

You shouldn't be messing with this stuff if you don't have basic theory and lab techniques covered. Start learning fast or end up on intensive care, or worse, the medical examiners table.

You do need vacuum distilling to get clear HNO3.

And use the edit function instead of double posting!

gnitseretni - 11-11-2007 at 16:12

so is using yellow HNO3 to make PETN fine or not?

Why didn't you answer that question vulture.. at least then your post would've been useful!

Sauron - 11-11-2007 at 16:21

Listen, interesting spelled backwards:

You are getting good advice from experienced chemists, which you clearly are not.

My advice, is: take it, and don't try to be a smart-ass.

In particular, do not be a smart ass with vulture, or your tenure on this forum may be very brief indeed.

garage chemist - 11-11-2007 at 17:41

You still dont get what I'm trying to say!
If your acid is not already anhydrous, using vacuum will not give you ANY advantage over atmospheric pressure distillation.
The final vacuum distillation is for REMOVAL OF THE YELLOW COLOR, and this is done AFTER you have synthesized your HNO3 and made it anhydrous by distillation from H2SO4, and those two procedures dont need vacuum.

Listen, I have made nearly anhydrous, fuming nitric acid very often, as it is enormously expensive when bought, while 65% HNO3 is cheap and easy to get.
I turn the 65% HNO3 into 99% yellow (I dont make high explosives, but rather do syntheses like 2,4-dinitrochlorobenzene, which require fuming HNO3 but not the colorless quality) HNO3 by distilling it twice from its own volume of concentrated sulfuric acid, while only collecting the fraction coming over at 86°C (the boiling point of anhydrous HNO3) during the second distillation. This gives 99% HNO3 with light yellow color, perfectly suitable for most applications.
This procedure is taken from the orgsyn preparation of tetranitromethane, which requires fuming HNO3.

If I did this in vacuum, it would not work as good as at atmospheric pressure. Why? Because the ability of sulfuric acid to bind water is lower at reduced pressure and elevated temperature. Only at atmospheric pressure does the sulfuric acid bind to the water strongly enough that nearly anhydrous HNO3 can be obtained from 65% HNO3 in just two distillations.
With vacuum, you are simply pulling more water out of your H2SO4/KNO3 mix, and also out of the H2SO4 at the second (dehydration) step.

Eclectic - 12-11-2007 at 05:53

Are there any dessicants that can bind up the last bit of water? 3A sieves or B2O3?

vulture - 12-11-2007 at 07:57

Mg(NO3)2 is good for drying HNO3 if you don't want to mess with sulfuric acid.

gnitseretni should also consider using the DCM extraction method (UTFSE).

kilowatt - 16-11-2007 at 18:27

I had always been under the impression that a significant (at least 10%, even more) portion of HNO3 decomposed at 1atm distillation to NOx yielding RFNA during the dehydration step, which would later be drawn off during your third distillation, or just drawn off fractionally with one distillation.

Garage chemist, what sort of column (height or theoretical plates) do you use? You should be able to distill out first NO2 and then later clear HNO3 from a single distillation (or with a fractionating still) of the previously made crude distillate, and in fact get a relatively dry product even from the crude, should you not? Since the high boiling azeotrope is 32% water it seems to me you should be able to distill out some anhydrous HNO3 (followed by azeotrope) in one pass with sufficient reflux, so long as the initial water is below that. I used a lot of excess sulfuric when I did my prior crude RFNA runs to retain water, but I believe the temperature was too high and I got very high NOx, based on the properties of my product. The second distillation would further dehydrate it as well as remove NOx.

I'm assuming gnitseretni is only using a simple still though, as most of the "distillation kits" I've seen are little or nothing more than that. Is this true or are you getting a reflux rig gnitseretni? I also have not yet taken the distillations this far (I've only made crude RFNA, but in a decent reflux column with dry precursors) and am most interested in what issues there are to face, especially when I go to distill Ostwald derived HNO3 which is quite dilute to start.

Quote:

Are there any dessicants that can bind up the last bit of water? 3A sieves or B2O3?

Nitric acid will attack zeolite, as far as I know. Zeolites are based on silicates, you'll end up with silica gel and nitrates. B2O3 isn't much of a dessicant. Anhydrous Ca(NO3)2 should work (or any nitrate that forms hydrates), as well as the Mg(NO3)2 vulture mentioned, not that it's any cheaper or more convenient than H2SO4 though. It's even more difficult to dry out afterward than H2SO4.

alphacheese - 7-12-2007 at 13:11

Quote:
Originally posted by garage chemist
The ability of sulfuric acid to bind water is lower at reduced pressure and elevated temperature. Only at atmospheric pressure does the sulfuric acid bind to the water strongly enough that nearly anhydrous HNO3 can be obtained from 65% HNO3 in just two distillations.
With vacuum, you are simply pulling more water out of your H2SO4/KNO3 mix, and also out of the H2SO4 at the second (dehydration) step.


So would distilling HNO3 at increased pressure raise H2SO4's ability to bind to water, or is that ONLY at atmospheric pressures?

MagicJigPipe - 12-12-2007 at 15:10

These people know what they're talking about. Follow their instructions. Doing it increased pressure would be absurd and difficult.

Just use the most concentrated H2SO4 and the driest KNO3 you can get for your first distillation to reduce, as much as possible, the amount of water in your first distillate.

And, from experience, I always get a little NO2 even at near room temperature.