Sciencemadness Discussion Board

Dring HNO3 with MgSO4, possible?

Per - 23-2-2007 at 10:31

I wanted to know if it´s possible to dry a 86% solution of HNO3 with dry MgSO4, normally Mg(NO3)2 is used here but preparing dry MgSO4 is much easier than preparing dry Mg(NO3)2.

Or is it possible to dry the acid by azeotrod distillation with CHCl3?
Or would the CHCl3 be nitrificated by the nitric acid?

Biginelli - 23-2-2007 at 12:10

There's a method: HNO3 is mixed with H2SO4 (BOTH LIQUIDS MUST BE COOLED) and distilled in vacuum. bp 37-40 centigrade at 22 mm Hg. A receiver flask must be cooled with dry CO2 in the process.
Distillation is repeated once more, thius time bp equals 36-38 C.

Biginelli - 23-2-2007 at 12:11

Another method is vacuum distillation over P4O10.

Biginelli - 23-2-2007 at 12:15

First method is used in nitric acid industrial production, though distillation is not vacuum

Furch - 23-2-2007 at 16:09

My spontaneous thought is that there is no water present in 68% nitric acid, so I think it's going to be a bitch to dry it (there is nearly 2 mol equivalents of HNO3 per molecule of H2O in 68% nitric acid, which means there is only H3O+, NO3- and HNO3 present in the solution - no water).

The reason that H2SO4 is capable of "drying" HNO3 (aq) is that it's a strong enough acid to protonate NO3- to form HNO3, also occupying all the water/oxonium ions in the solution, allowing HNO3 to be distilled, and yes, at atmospheric pressure. Lots of nasty NOx in that distillation though. I bet they are minimized when vacuum distilling.

[Edited on 24-2-2007 by Furch]

Fleaker - 23-2-2007 at 19:23

Vacuum distillation over P4O10 should give N2O5 depending on temperature. It is unstable though so I'm not certain. I do not think P4O10 is a good reagent to use when sulfuric works much better and is much cheaper.

I distill my nitric-sulfuric mixtures at about 80*C.

Per - 24-2-2007 at 08:03

The problem is that I haven´t a cacuum pump so I have the problem with NOx.
Wouldn´t a azeotrop distillation work with CHCl3 as a tug(not sure if it´s the right word for "Schlepper").
My spontaneous thought is that there is no water present in 68% nitric acid,

Mine has 86%, not 68;)

Sauron - 24-2-2007 at 09:22

Nitric acid is an oxidant, when you oxidize CHCl3 what do you?


Do not mix HNO3 and chloroform. We'd hate to lose a member to pulmonary edema.

Let us know if you find a miscible liquid that will azeotrope the water out, yet be inert to HNO3.

HINT: Benzene, toluene and xylene are not inert to HNO3.

ANOTHER HINT: conc HNO3 is an azeotrope itself. Constant boiling. If you distill HNO3 alone, you get HNO3 same as you started.

FINAL HINT: do protect your vacuum system from NOx and all that.

Cold traps, emptry wash bottle, maybe a water wash with a drying tube and a one way valve don't want moisture in, you don't want suckback, and you don't want acid vapors getting to your pump or out into what you breathe.

[Edited on 24-2-2007 by Sauron]

Furch - 24-2-2007 at 12:02

I feel pretty confident in saying that no drying agent will solve your problem, Per. You need something that reacts with water, and more so than nitric acid.

There is no water present in concentrated nitric acid, so drying agents won't work.

Concerning reacting with the water/oxonium ions, I recommend either the sulphuric acid method, which definitely seems to be the most feasible way to go about the problem... Someone mentioned nitric acid anhydride (N2O5), which would react with water and form more nitric acid. This route however is not feasible, as N2O5 spontaneously decomposes somewhere around 0-5 °C.

I'm sure this topic has been thoroughly covered elsewhere, and I recommend you ask in the energetic materials forum instead, as nitric acid in all concentrations is frequently occuring in that kind of chemistry :)

Good luck anyway!

unionised - 25-2-2007 at 05:12

I'm perfectly confident that the 68% stuff is an azeotrope and that if you alter the pressure you will alter the concentration of the azeotrope.
I don't know which way the change will go but, imagine doing the distillation at 2 atmospheres. Either the azeotrope composition will drop below 86% and so you will be able to remove a weaker acid by distillation and so leave a stronger (than 86%) solution behind in the still pot; or the azeotrope will rise above 68% so you will be able to distill off a higher than 68% solution.
Either way you can get a solution that is higher than 68%.
OK, I realise that someone is going to tell me that the acid will decompose when it gets that hot. Fair enough- reduce the pressure and distill it. The same thing will happen; the azeotrope will change so, one way or another, you can get a solution over 68%
All this without a drying agents, in this sense , Furch's assertion that "I feel pretty confident in saying that no drying agent will solve your problem " is correct.
Another way to influence the azeotropic distillation would be to add a soluble material like KNO3 and I strongly suspect that MgSO4 would work too.
Would asking this question again in another forum be cross posting?

Sauron - 25-2-2007 at 06:33

The behavior of an azeotrope at varying pressures is predictable.

But I suspect it would take a hard vacuum to get the desired result with HNO3.

As to P2O5 you only get N2O5 if you use enough P2O5 to satisfy the stoichiometry:

2 HNO3 + P2O5 -> N2O5 + 2 HPO3

And that is only after all water is removed by still more P2O5 forming H3PO4 then additional P2O5 dehydrates that to HPO3.

Brauer pages 489-492 contain procedure for preparing N2O5 from fuming HNO3 d.1.52 and P2O5. P2O5 is described as slowly decomposing at room temperature but fairly stable below 10 C.

More pertinently, the following monograph for HNO3 per se gives a procedure for preparing anhydrous HNO3 from fuming HNO3 of as high a density as possibke, and P2O5, then destroying any small amount of P2O5 if found by test, by adding just sufficient concentrated HNO3 (d.1.42) that the WATER PRESENT reacts with the N2O5 to form 2 HNO3.

It is true that conc HNO3 exists as nitronium and oxonium ions but that is an equilibrium with HNO3 and H2O, it's just an equilibrium that strongly favors the ionic species.

Now clearly it would take a lot of P2O5 to get to anhydrous HNO3 starting from the constant boiling concentrated acid rather than from 98-99% fuming acid (and yes you must remove the NOx before proceeding.) P2O5 is fluffy floculent and a pain to handle, on that basis alone I would say the H2SO4 procedure (also detailed in attachment) is better.

Reverting to the original question re MgSO4 I doubt this is practicable. Mostly because I have never seen any example of this, only the P2O5 and H2SO4 methods.

On the other hand, it could just be worth a try. There are three possible outcomes.

1. Nothing.
2. the mag sulfate absorbs water of hydration
3. the mag sulfate reacts with the nitric acid, forming mag nitrate and H2SO4, the H2SO4 dehydrates the nitric acid.

That's two out of three achieving the desired result. Worth a try even if it's a long shot.

[Edited on 25-2-2007 by Sauron]

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YT2095 - 25-2-2007 at 06:43

3. would never happen.

vulture - 25-2-2007 at 07:09

No, but in theory nitric acid is strong enough to protonate sulfate to hydrogen sulfate. Whether this actually happens is something I won't speculate about.

Sauron - 25-2-2007 at 08:02

All right.

Concentrated HNO3 d1.42, in the cold, and freed from NOx, mixed slowly with the calculated amount of 30% oleum, precooled and what do you have?

Do your arithmetic properly and you have anhydrous H2SO4, with a little left over SO3, and anhydrous HNO3. Readily seperable by fractional distillation. SO3 will come over first then HNO3 leaving H2SO4 in the pot.

This would be an expensive and wasteful method, but would work. It is really just a variation on fuming nitric plus conc sulfuric. SO3 + water gives H2SO4 + a lot of heat.

I'd hate to be the one to have to perform that distillation but there it is.

vulture - 25-2-2007 at 08:10

I've tried it with magnesium nitrate, but that stuff is a serious bitch to dry properly. Once you get it dry it usually forms a solid block which could cause nasty blunt force trauma.
Boiling down sulfuric acid is actually easier.

If you're willing to go through all that hassle, just optimize the DCM/HNO3 extraction method. Safe and pretty easy.

YT2095 - 25-2-2007 at 08:36

if it were a group 1 metal possibly, the Sulphuric is far stronger than Nitric, I can`t picture Any synth that would liberate Sulphuric acid from Nitric acid.

the only exception I know of is using Phosphoric to make HI, and That is much stronger acid, but it`s not Often things Scale up in that way.

Sauron - 25-2-2007 at 08:58

IIRC the usual advice about the sulfuric acid-nitric acid distillation method of preparing anhydrous HNO3 is:

If you bstart with fuming nitric acid you can do it in one distillation

If you start with conc nitric acid, it will take two steps.

Per - 26-2-2007 at 07:36

2CHCl3 + 2HNO3 -> 2CCl2O + CO2 + 2H2O + Cl2
:o,what a worse idea but maybe a good synthesis when you need phosgen.

I tried the method with the MgSO4, noething happened:(

My problem was that I just had 83% sulphuric acid and so I could only produce 86% nitric acid. But now I have 96 to 98% sulphuric acid and soon I´m going to produce HNO3 with that acid.
Firstly my mistake was that I heated my low concentrated sulphiric acid just to 200°C because it elvolved white fume at these temperature but now I know that it´s necassary to heat the acid to 300 to 350°C.

Thanks for your explanations for drying the nitric acid otherwiese.

apidej - 26-2-2007 at 08:49

Thanks for your explanations for drying the nitric acid otherwiese.

Sauron - 26-2-2007 at 09:03

Oxidizing CCl4 with hot fuming sulfuric acid, or oxidizing chloroform with nitric acid or chromic/sulfuric acid, are the standard ways to make phosgene and therefore, are best avoided.

I strongly suspect that DCM would undergo the same reaction.

If you make your own chloroform via haloform rxn or any other means, you must remember to stabilize it with a little methanol, and store it in dark bottles in a cool place and not in direct sunlight, or it will happily make phosgene all on its own.

Phosgene is an insidious poison, its irritant action is delayed and you can inhale a lethal concentration for long enough and never smell it. Very nasty.

Per - 2-3-2007 at 09:24

If you make your own chloroform via haloform rxn or any other means, you must remember to stabilize it with a little methanol, and store it in dark bottles in a cool place and not in direct sunlight, or it will happily make phosgene all on its own.

Mine is bought (acceptable 15€/litre) and technical and it´s stabilisized with amylin.