Nox Purging Reaction

[mongo blongo]

Does anyone know what the reaction is for purging Nox from HNO3 with urea? I'm sure I have seen it here before but this certain post is proving very elusive.
I'm assuming that urea nitrate is formed first but I can't see what happens when O2 is introduced. Is the reaction catalytic?
If urea nitrate is the first reaction product then I presume that higher concentrations of HNO3 could be achieved by using urea nitrate instead of urea.

[simply RED]

Little urea is added to red nitric acid to lower the Nox content in it.
The Nox react with urea to produce N2, CO2 and H2O.
Urea nitrate in such low quantity is not formed due to urea nitrate solubility in the acid.

[mongo blongo]

Wouldn't that then make it less concentrated with more H2O in it? I remember reading posts stating that after titrations, it was more concentrated after purging.

[cyclonite4]

Wouldn't that then make it less concentrated with more H2O in it? I remember reading posts stating that after titrations, it was more concentrated after purging.

The H2O probably escapes as water vapour

[simply RED]

Water reacts with NOX to form HNO3 again. It exists in balance.
Even white nitric acid will turn red if it stands in warm place.
Urea can not make it more concentrated...

Anyway my reaction is from chemical manual. Correct or not...

"The main purpose of adding urea to red fuming HNO3 is to purge it of HNO2 (formed by NOx) which could influence out-of-control reactions:

H2N(-C=O-)NH2 + 2 HNO2 --> CO2 + 3 H2O + 2 N2 "

The usanet article helped to translate the manual.

[FUTI]

Bubbling air through acid will make it concentrated, if it is dry and warm, and remove NOx in process too. Bubbles that comes out of solution contain a small amount of water depending of water vapour pressure...if it works for sea-salt producing facilities that use wind to speed the evaporation I can't see reason not to work here also.

[grendel23]

Dry air will remove NO₂, it will not concentrate acid that is already over 68%. At concentrations above 68% more HNO₃ will evaporate than water, below 68% more water will evaporate, but it would take forever and you would lose acid in the process.

[DF90]

Would it be possible to place the acid in a vacuum flask (a buchner flask with a solid bung) for a short period of time, to "suck out" the NOx? It's just an idea but I think it could work :D I'm sure I have seen it on the forum before but I have spent the early hours of the morning searching for it with no results :(

[mike16]

I once put 10ml of yellow 70% HNO3 in a desiccator with 98% H2SO4 as the desiccant, in the hope that the H2SO4 would suck the water out of the HNO3 to concentrate it.

The next day as expected I had the 10ml back, so it was still only 70% HNO3, but it was crystal clear!!

The H2SO4 had somehow taken the NOx out of the nitric acid.

[alancj]

Would it be possible to place the acid in a vacuum flask (a buchner flask with a solid bung) for a short period of time, to "suck out" the NOx? It's just an idea but I think it could work :D I'm sure I have seen it on the forum before but I have spent the early hours of the morning searching for it with no results :(

Yes, I have tried that before. It takes a while; from my one time experience :). I used a side arm flask, with a rubber stopper in the neck, a vacuum pump (for food canning) and pulled a vacuum while gently warming the acid.

From a couple hundred mL of yellow acid at start I got a water clear acid after about an hour. My acid wasn't very concentrated, less than azeotropic, but it still ate up the stopper. Agitation would help a lot.