Sciencemadness Discussion Board

No NO2 contamination at high temps?!

Oscilllator - 9-10-2012 at 02:25


So recently I made nitric acid from distillation of sulfuric acid and potassium nitrate.
It all went more or less according to plan, however I made som very weird observations.
The first was that if the temperature inside the flask was pretty low (barely boiling), lots of NO2 evelved to the extent that the inside of the flask got very brown. I am pretty sure that this NO2 is formed from the decomposition of the HNO3.
However when I heated up the flask a little further, until there was steady and fast boiling, almost all of the NO2 disappeared, and only clear vapor evolved off.
If I heated the mixture even further, the boiling was intense enough that vapor came out the end of my condensing apparatus. The vapor was very white, and I assume that it was sulfuric acid vapors.
Now the quandary is: Surely as the temperature increases the decomposition of the nitric acid should also increase,not decrease as was observed.
A less confusing quandary is that the nitric acid when distilled was a yellow clolour and slightly cloudy, which I understand is normal. However I left it sitting on my bench for 5 days (having gone on holiday) and when I arrived back I found that the acid was completely clear, like water. I assume this is because the dissolved NO2 reacted with trace amounts of water to produce nitric acid.
I dont have any numbers I'm afraid as my thermometer is made of metal and I figured it wold be a bad idea to stick it into some nitric acid.

Can anyone offer any explanation as to why the boiling was clear at higher temperatures ? And also how pure would the

nitric acid I distilled off be?

hissingnoise - 9-10-2012 at 03:23

If your H<sub>2</sub>SO<sub>4</sub> was concentrated (98%) and your nitrate freshly dried, NO<sub>2</sub> should be very noticeable throughout the distillation!
Temperatures in the flask shouldn't exceed 120°C, to prevent possible evolution of oxides of sulphur!
A glass thermometer (bulb located at the height of the adapter side-arm) is pretty essential for any distillation at normal pressure.
H<sub>2</sub>SO<sub>4</sub> boils at ~337C (which will strain even borosilicate glass), but <i>some</i> SO<sub>2</sub> can be expected below the B.P..
The acid produced may possibly be contaminated by SO<sub>2</sub> and diluted by water coming over towards the end of the process . . .



hissingnoise - 9-10-2012 at 03:32

Also, SO<sub>2</sub> is readily oxidised by NO<sub>2</sub> (lead-chamber process); the resulting mix of SO<sub>3</sub> and NO is colourless!

Oscilllator - 9-10-2012 at 18:52

Ok, it seems like the NO2 was oxidised by the SO2. I'll go buy a glass thermometer to measure the temp of my distillation, been meaning to buy one for a while anyway.

hissingnoise - 10-10-2012 at 00:43

NO<sub>2</sub> was <i><b>reduced</i></b> by SO<sub>2</sub>!

Oscilllator - 10-10-2012 at 01:06

shoulda picked up on that myself :mad:
On the other hand, I think distillation may be better at high temperatures when preparing nitric acid that will be used in conjunction with sulfuric acid e.g. in nitrocellulose because it is much faster, and it might have a small amount of sulfur trioxide dissolved in it, which should help get rid of any water that may have weaseled its way into the acid.

hissingnoise - 10-10-2012 at 05:02

Sodium nitrate is the preferred salt for this distillation ─ its fixed nitrogen content is higher that of the potassium salt, its bisulphate, being more soluble is easily washed from flasks and it's cheaper!
Its hygroscopicity, though, must be allowed for.

Overdriving a distillation increases vapour production without affecting soln. temp..
Too high vapour production can overwhelm the condenser cooling leading to loss of product, releasing toxic, corrosive vapours to the environment.
A moderate boiling rate is called for and when the temp. rises above 120°C the distillation is essentially finished!

Oscilllator - 10-10-2012 at 21:45

Quote: Originally posted by hissingnoise  
Sodium nitrate is the preferred salt for this distillation ─ its fixed nitrogen content is higher that of the potassium salt, its bisulphate, being more soluble is easily washed from flasks and it's cheaper!
Its hygroscopicity, though, must be allowed for.

Overdriving a distillation increases vapour production without affecting soln. temp..
Too high vapour production can overwhelm the condenser cooling leading to loss of product, releasing toxic, corrosive vapours to the environment.
A moderate boiling rate is called for and when the temp. rises above 120°C the distillation is essentially finished!


so the speed of the distillation won't affect the purity of the end product because the temperature isn't changed?

Also potassium nitrate literally costs me $2AUD/kilo, so cost is not a problem for the nitrate.

UnintentionalChaos - 10-10-2012 at 22:30

Ammonium nitrate is even better. It's quite soluble in the sulfuric acid and the resulting ammonium bisulfate in the stillpot is molten at atmospheric distillation temps.

Oscilllator - 11-10-2012 at 01:56

I dont have any ammonium nitrate, and I probably cant produce it in the quantities required to make any meaningful amount of nitric acid

hissingnoise - 11-10-2012 at 03:31

Why this stubborn preoccupation with increasing the speed of distillation?
Quote:
. . . so the speed of the distillation won't affect the purity of the end product because the temperature isn't changed?

The benefits of careful work are plainly self-evident, surely?
I mean, if drastically reduced yields coupled with the real risk of pulmonary oedema are what you <i>want</i> then bang away!
And as for the ammonium salt, it's more hygroscopic even than NaNO<sub>3</sub> making it difficult to dry fully and NH<sub>4</sub><sup>+</sup> becomes prone to oxidation by hot H<sub>2</sub>SO<sub>4</sub> as distillation progresses!

Oscilllator - 11-10-2012 at 13:17

Is is probably safer to increase the speed of distillation if it means you convert your poisonous NO2 into NO, which isn't toxic at all (they use it as an anesthetic in dentistry) Also the sulfur trioxide produced would be condensed aswell. If it didn't get condensed than surely you would see white fumes coming out the end of the distiller. I did not see any such fumes.

So I think that the reduced toxicity of the by-products, combined with the increased speed of the reaction mean that it would be beneficial for me to speed up the distillation.
If I have missed out on anything that might kill me, please let me know.

plante1999 - 11-10-2012 at 13:46

Quote: Originally posted by Oscilllator  
Is is probably safer to increase the speed of distillation if it means you convert your poisonous NO2 into NO, which isn't toxic at all (they use it as an anesthetic in dentistry) Also the sulfur trioxide produced would be condensed aswell. If it didn't get condensed than surely you would see white fumes coming out the end of the distiller. I did not see any such fumes.

So I think that the reduced toxicity of the by-products, combined with the increased speed of the reaction mean that it would be beneficial for me to speed up the distillation.
If I have missed out on anything that might kill me, please let me know.


I laughed hard! Nitrogen monoxide turn into nitrogen dioxide in the air, dentist use nitrous oxide or dinitrogen monoxide. It is not beneficial at all to use high temperature!

tetrahedron - 11-10-2012 at 13:50

1. NO is immediately converted to NO2 upon contact with air. the non-toxic anesthesia gas is N2O!

2. SO3 gets partially decomposed to SO2 which will largely escape to the atmosphere; moreover, sulfuric acid unnecessarily contaminates your product. this cannot be removed as easily as NO2 and can turn out detrimental to some reactions.

Bot0nist - 11-10-2012 at 14:13

Plus, a concentrated, or even an azeotropic HNO<sub>3</sub> distillation is not a procedure to be rushed. To do it right you ideally would keep constant track of both the heating bath, and still head temperature. Always check the joints first too. It is easy to underestimate how bad a seal popping can be when aggressivly heating. The cloud of NOxious gas spewing out ain't laughing gas.

Oscilllator - 11-10-2012 at 20:42

ok, low temp it is.

hissingnoise - 12-10-2012 at 02:48

Good for you ─ recipients of the 'Darwin Award' rarely voice appreciation of the honour they've received . . .
And if you're intent on nitrating something organic, try not to go at it with a head full of shite!