Sciencemadness Discussion Board - Concentration of HNO3 with H2SO4

Concentration of HNO3 with H2SO4

Schleimsäure - 4-7-2015 at 15:17

Hallo,

so I wanted to make 99+% HNO3 by concentrating the 65% solution with conc. H2SO4.

I read that using a vacuum destillation prevents the production of NOx thus just resulting in a clear product.

So I mixed the acids in a ratio of 5:4 HNO3/H2SO4.
The joints of the destillation setup all have been greased well.
The already hot acid mixture immediately started to boil after vaccum was put on. The heating mantle I put on just thereafter, but it needs some minutes to heat anyway.

The HNO3 came over just seconds after vaccum was laid on. Destillation in the 400mm Liebig was very fast - and clear. No NO2 at all.

But after 1 or 2 min or so a cloud of NO2 formed in a heartbeat and went through the Liebig into the receiver flask and finally contaminated the clear HNO3 deyeing it reddish.

I immediately interrupted the vaccum to stop the destillation. Fortunately I put a washing bottle between vaccum adapter and oil pump, which had actually collected some NO2.

So my question is what went wrong? I have no manomter yet, so I don't know the strength of the vacuum. I read that the particuar pump I use creates a good vacuum of 0,05 mbar, although it is a relatively cheap one with no extras like manometer.

Maybe the vacuum setup had a leak? I forgot to grease the joint of the securing washing bottle.

[Edited on 4-7-2015 by Schleimsäure]

aga - 4-7-2015 at 15:57

Vacuum should have taken care of major leaks in a ground-glass rig.

Hazarding a guess, the mix got heated too much for that reduced pressure, decomposing the HNO₃.

Hopefully an illumni will come along and illumni-ate.

Edit :

A quick wiki and i found
https://en.wikipedia.org/wiki/Nitric_acid

If it does decompose to NO₂ and water, the conc Sulphuric acid should sequestrate the water, giving you the brown gas rather than the nitric acid.

Interesting.

Possibly not the best way to get RFNA, yet interesting.

[Edited on 5-7-2015 by aga]

Schleimsäure - 4-7-2015 at 16:30

Actually on versuchschemie a longtime reputable member uses this method with great success, meaning without any noticable NO2 formation. Thus he actually gets WFNA.
He measured density of his clear acid and got 1,51g/cm3, which is not too bad. He used 53% HNO3 and found a ratio of 5:4 or 4:3 HNO3(53%):H2SO4 (96%) is the best after some runs.
To further concentrate it you have to repeat the whole procedure with the 1,51g/cm3 acid product of your first run.

Also he made a test and nitrated a gram of Hexamine with success.

On my attempt the first 30-50 ml actually came over clear as it should be for up to 2 min.
Maybe the vacuum was too strong. I don't know.

[Edited on 5-7-2015 by Schleimsäure]

hissingnoise - 5-7-2015 at 02:20

Quote:

But after 1 or 2 min or so a cloud of NO2 formed in a heartbeat and went through the Liebig into the receiver flask and finally contaminated the clear HNO3 deyeing it reddish.

Did you use stopcock grease to seal your joints?

Fulmen - 5-7-2015 at 03:13

Since it boiled around room temp with the reduced pressures a heating mantle could cause unnecessary high temps. A water bath should give more control over the process and reduce the risk of overheating. When I researched this (pre-web) I ended up with a 50/50 mix (by volume) of 67% HNO3 and 98% H2SO4, this seems to be the standard in industrial setups. Yield was appr 1/2 the initial volume of the HNO3.

Schleimsäure - 5-7-2015 at 06:03

Quote: Originally posted by hissingnoise

Quote:

But after 1 or 2 min or so a cloud of NO2 formed in a heartbeat and went through the Liebig into the receiver flask and finally contaminated the clear HNO3 deyeing it reddish.

Did you use stopcock grease to seal your joints?

I used Silicone grease from LIQUIMOLY. It's recommended on versuchschemie and had no problems with it so far.

http://www.amazon.de/Liqui-Moly-3312-Silicon-Fett-transparen...

Schleimsäure - 5-7-2015 at 06:15

Quote: Originally posted by Fulmen

Since it boiled around room temp with the reduced pressures a heating mantle could cause unnecessary high temps. A water bath should give more control over the process and reduce the risk of overheating. When I researched this (pre-web) I ended up with a 50/50 mix (by volume) of 67% HNO3 and 98% H2SO4, this seems to be the standard in industrial setups. Yield was appr 1/2 the initial volume of the HNO3.

Yes, but the thing is the NO2 formed already when the heating mantle wasn't even warm yet after 2 min.

One issue could be that we have the warmest days in decades here in Germany. Room temp is arround 30 degrees C.
The nitrating acid in the reaction flask already had a temperature of up to 100 degrees C on its own. This plus high vacuum plus high room temperature could have led to decomposition of some HNO3. For two minutes temperature was ok and must have risen without external heat to an undesirable point.
I just wonder, because the guy on versuchschemie used vacuum plus heating mantle without decomposition. Unfortunately he didn't say what kind of vacuum source, hence strength, he used. It's from a very old thread from 2005 or so. Maybe he used just water-jet vacuum back then...

[Edited on 5-7-2015 by Schleimsäure]

papaya - 5-7-2015 at 07:08

Couldn't you use conc. sulfuric acid instead of grease ?

Schleimsäure - 5-7-2015 at 07:29

Yes, that's a possibility. But in the end I don't really know if this is the problem at all.

Here's a picture.

Don't know whether to go on from here or use a new setup.

Fulmen - 5-7-2015 at 07:58

Quote: Originally posted by Schleimsäure

The nitrating acid in the reaction flask already had a temperature of up to 100 degrees C on its own.

That could be the problem. The point of vacuum distillation is to keep the temperature down, so why start with hot liquids?

macckone - 5-7-2015 at 08:03

Likely culprit for vacuum loss, the thermometer seal.
Unless you replaced it with a viton O-ring,
it will get eaten up in nothing flat.

Recommendation, get teflon seals for your joints.
These are available at United Glass Tech.
Next get a thermometer well from
Roccoware Glass Works.

This will leave everything as teflon or glass in contact
with your nitric acid.

Another recommendation is to replace what appears
to be a rubber hose with a pvc hose.
Exploding hoses are no fun. And I mean nitrated
and then exploding not just rupturing.
It looks like your second hose coming off of the trap
is already pvc. But it is hard to tell from the picture.

Good luck with your experiments.

Schleimsäure - 5-7-2015 at 08:33

Quote: Originally posted by Fulmen

Quote: Originally posted by Schleimsäure

The nitrating acid in the reaction flask already had a temperature of up to 100 degrees C on its own.

That could be the problem. The point of vacuum distillation is to keep the temperature down, so why start with hot liquids?

Good point. Next time I'll try to use a chilled down acid mixture.

[Edited on 5-7-2015 by Schleimsäure]

Schleimsäure - 5-7-2015 at 08:39

Quote: Originally posted by macckone

Likely culprit for vacuum loss, the thermometer seal.
Unless you replaced it with a viton O-ring,
it will get eaten up in nothing flat.

Recommendation, get teflon seals for your joints.
These are available at United Glass Tech.
Next get a thermometer well from
Roccoware Glass Works.

This will leave everything as teflon or glass in contact
with your nitric acid.

Another recommendation is to replace what appears
to be a rubber hose with a pvc hose.
Exploding hoses are no fun. And I mean nitrated
and then exploding not just rupturing.
It looks like your second hose coming off of the trap
is already pvc. But it is hard to tell from the picture.

Good luck with your experiments.

Also a good hint. thank you. I don't exactly need the thermometer, so I could just put in a NS14 stop cock. On the other hand the thermometer has a NS14 boro glass joint, no adapter. It's all high quality labware. Contacts are actually only glass besides the hoses.

Yes the second hose is PVC. Maybe I find a proper second PVC one for the vacuum adapter.

If vacuum loss has taken place I still suspect the ungreased washing bottle joint.

[Edited on 5-7-2015 by Schleimsäure]

Bot0nist - 5-7-2015 at 21:24

The following ma be veering slightly off of the OP, since I cant offer much advice for or improvements on existing procedures for distilling concentrated WFNA from strong sulfuric acid and some common alkali nitrate salts. Conc. acid, dry sodium nitrate, careful control of pot temperature, and reduced pressure with the proper ground glass joints, sealed.with a smear of sulfuric acid or inert vacuum grease are all good tips that have been dished out here over the years. The initial volume of raw nitric acid obtained can then be further concentrated and purified to an extent if mixed with more conc. sulfuric acid and distilled again, with a careful eye kept of the distillate temp coming over.

I Hope though that it may be of use to anyone making concentrated nitric acid, but that may be lacking a vacuum source, or other accessories needed to produce concentrated, nearly clear HNO3 with minimal nitrogen oxide contaminates IIRC, in the past several members, myself included, have had some success in clearing up smaller volumes of freshly made RFNA (red fuming nitric acid), with dissolved and fuming nitrogen oxides) by slowly bubbling dry air, CO2 or even dry ozone supplemented air, with the use of an aquarium pump or regulated cylinder. Employ a clay bubble stone to aid in proper gas diffusion to speed the reaction up, and, of course, lead the gas through inert to strong, oxidizing nitric acid, and through a drying tube and then allow.it to bubble through the concentrated, red fuming isolate for some time until.it clears.

Doing this will slowly clarify the isolate to a decent quality of WFNA (white fuming nitric acid) which will work just a good for most applications, and yet is suitable for use in procedures where the RFNA won't do, and clean WFNA is called for.

I think our very own hissingnoise had been, at least in the past in the past a proponent of this slightly time consuming, yet practical, cheap and simple solution to cleaning up home made RFNA into quality WFNA.

Good lick, and stay safe.

[Edited on 6-7-2015 by Bot0nist]

blogfast25 - 6-7-2015 at 09:07

Low pressure favours formation of gases (to 'restore' pressure again), so also the decomposition of nitric acid. Too high temperature is almost certainly to blame here, as an aggravating factor.

The whole method seems like a sad waste of conc. H2SO4, though.

Why not displace an ionic nitrate with conc. H2SO4? That should yield above the azeotropic composition.

[Edited on 6-7-2015 by blogfast25]

hissingnoise - 6-7-2015 at 11:39

Quote:

I think our very own hissingnoise had been, at least in the past in the past a proponent of this slightly time consuming, yet practical, cheap and simple solution to cleaning up home made RFNA into quality WFNA.

Indeed yes Bot0nist, though I'd be somewhat loath to leave a stirrer and compressor on overnight again.

Using the output of an airfed ozoniser will be quicker and more complete and can, in theory, increase acid density as nox products in the effluent are partly oxidised by ozone to form the anhydride, N₂O₅!

aga - 6-7-2015 at 11:45

In terms of applying the word 'concentrated' what w% would qualify for HNO₃ ?

Metacelsus - 6-7-2015 at 16:44

In my opinion, azeotropic and above.