I wanted to make some liquid NO2/N2O4 just as an interesting thing to do and because it looks pretty cool in an ampoule... I used copper and nitric
acid to generate it.
But it just won't condense! I don't get it. It's supposed to do so at 10°C as far as I know, but leading into a test tube submerged in
salt+snow+water at -15°C barely did anything. I got a few tiny droplets which I suspect was mostly N2O3 based on the greenish blue color, but the
majority of the gas just hopped right out of the test tube despite having a very slow flow.
What else do I need to do to condense it? What am I missing?charley1957 - 24-2-2024 at 08:15
The only other thing I can think of is oxygen in the air. So unless you’re trying to do it under vacuum, I can’t imagine what the problem could
be. Is your nitric acid good? Is it real copper? Junk_Enginerd - 24-2-2024 at 08:29
What about the oxygen in the air?
Yes, 60% technical grade nitric acid, pure copper. I mean, clearly plenty of NOx is being generated. That cloud of orange evil is hard to mistake for
something else.
I even tried capturing it in a plastic bag that I threw in the -20°C freezer. "Let's see you try escaping now!" But only a small fraction of it
actually condensed, and the rest just hung out as a brown cloud in the bag taunting me. I'm clearly missing something.Rainwater - 24-2-2024 at 08:53
Its likely to be something to do with heat capacity, heat of vaporization, thermal transfer amd partial pressure. Same as fog. I had the same problem
till switched to dry ice bathschornedsnorkack - 24-2-2024 at 12:04
How bright brown is NO2 at low concentrations in mixture?
An obvious alternative, especially given that the liquid was blue (N2O3) would be NO. If your NOx was mostly NO, with only some NO2, would it have had
the brown colour?clearly_not_atara - 24-2-2024 at 12:51
I think NO2 tends to be produced at pretty low pressure, and in the vapor phase, I'm pretty sure a mixture of NO2 and NO (colorless) looks very
similar to pure NO2.
The vapor pressure of NO2:
1 bar at 293 K
0.5 bar at 280 K
0.25 bar at 268 K
0.12 bar at 258 K (-15 C, you are here)
0.09 bar at 255 K (-18 C, common freezer)
0.06 bar at 250 K
0.03 bar at 243 K
16 mbar at 237 K
8 mbar at 231 K
(practically zero in dry ice = 195 K)DraconicAcid - 24-2-2024 at 13:35
If you cool it, most of it should dimerize to colourless N2O4, even if it doesn't condense.BromicAcid - 24-2-2024 at 15:34
Once upon a time I was trying to distill nitric acid from sodium nitrate / sulfuric acid. I opted to add a ton of sulfuric acid in the hopes of
getting anhydrous nitric instead of the azeotrope. Instead I distilled mostly NO2 - it was interesting because there was some water in there too so I
had a biphasic mixture. I don't remember having too much of an issue with it condensing just with an ice water condenser. woelen - 25-2-2024 at 12:50
The most likely problem of the OP is that the path between the place where NO2 is produced and where it is condensed is too short. The reaction
between Cu and HNO3 is quite exothermic, so the NO2 you get is hot. You first need to cool down that gas in a long tube (best is cooling it with cold
water) and then slowly lead it into a receiving flask, which is kept in a cold mix (snow or ice, mixed with salt should do).chornedsnorkack - 27-2-2024 at 08:27
I think NO2 tends to be produced at pretty low pressure, and in the vapor phase, I'm pretty sure a mixture of NO2 and NO (colorless) looks very
similar to pure NO2.
The vapor pressure of NO2:
1 bar at 293 K
0.5 bar at 280 K
N2O3 boiling point is reported as +3 Celsius, so 276 K.
0.25 bar at 268 K
0.12 bar at 258 K (-15 C, you are here)
0.09 bar at 255 K (-18 C, common freezer)
0.06 bar at 250 K
0.03 bar at 243 K
16 mbar at 237 K
8 mbar at 231 K
(practically zero in dry ice = 195 K)
And pure NO is 121 K.
There are two suggested reasons why NOx did not condense: it was too warm, or it was NO not NO2.
Since the condensate was blue(/green), this suggests that NO was part of the reason: pure NO2 should have been brown, maybe pale (N2O4) but without
blue.
But how bright are N2O3 blue and NO2 brown relative to each other? Is "greenish blue" mostly N2O3 (because mostly blue), or mostly NO2 (because the
blue of small N2O3 impurity is much brighter than NO2 brown)?
