Sciencemadness Discussion Board

Alternate Methods for clean HNO3 dist.

trezza - 4-3-2011 at 04:28

As we know distilling Nitric acid can be difficult if we're trying to achieve high concentrations with very little NOx contamination.
I'm a member of a small forum with a pyrotechnics and chemistry section. One of the members explained a distillation he carried out. Using A H2SO4 and HNO3 solution in a retort he managed to get about 15ml of yellowish 95%+ Nitric acid, He said after his distillation he immediately refilled his retort with a fresh crop and found that in his following distillation he was getting no NOx contamination. This led us to believe that all of the available oxygen inside the retort had been pushed out by the Nitric acid vapour leaving nothing for it to react with hence there was no nitrogen oxides produced. If before each distillation someone were to blow a gas (which wouldn't react with any of the acids) through a retort after it was filled, you would end up with a clean crop of HNO3 distillate from the first distillation? Are we correct in saying that in an oxygen free environment, nitrogen dioxide can not form or did we confuse using low air pressure in a distillation set up requiring less heating thus less decomposition of the hot acid?

[Edited on 4-3-2011 by trezza]

bbartlog - 4-3-2011 at 06:23

I have not done this; however, I do not think that excluding air will solve the problem. There is plenty of oxygen (and oxidizing power) in the H2SO4 and HNO3 already. Look what happens if you (in the abstract) dehydrate HNO3:
2HNO3 -> H2O + 2NO2 + O
...not to say that this reaction takes place per se but you can see that there is extra oxygen in the mix already, even with no air.

hissingnoise - 4-3-2011 at 08:12

The thermal instability of HNO<sub>3</sub> means that it will always decompose when heated - even in vacuum!
Your friend trezza, is mistaken if he thought NO<sub>2</sub> was not evolved.

Random - 5-3-2011 at 02:16

Maybe NO was evolved which is colorless, but in the presence of oxygen it will produce NO2.

hissingnoise - 5-3-2011 at 02:54

The thermal decomposition of HNO<sub>3</sub> does not produce the colourless NO . . .

White Yeti - 16-8-2011 at 11:38

Simply storing white fuming nitric acid at room temperature will cause it to decompose into nitrogen dioxide, water and oxygen...

What temperature was the receiving flask at? Nitrogen dioxide exists in equilibrium with dinitrogen tetroxide and this equilibrium is strongly affected by temperature. Dinitrogen tetroxide is a faint yellow, nitrogen dioxide is a reddish brown.

hissingnoise - 16-8-2011 at 12:16

The tetroxide itself is colourless - but stained yellow by residual NO<sub>2</sub> . . .

White Yeti - 18-8-2011 at 01:09

Sorry for making that silly mistake. I knew dinitrogen tetroxide was colourless -.-