Sciencemadness Discussion Board

Help preparing a HIGH oxide content RFNA

Delta-R - 4-1-2022 at 14:02

OK so this has been a real challenge. I can’t seem to find any literature on the topic whatsoever and experimentation has been pretty fruitless so far. I’m trying to prepare a Red Fuming Nitric acid with the highest dissolved oxide content possible, and without any copper contamination. Normally people are looking to reduce the oxide content and there’s plenty of literature on that, As well as literature on oxide content from the Oswaldt process but nothing about general nitrate salt/H2so4 distillation. What are the variables that effect NO2/N2O2 production during distillation of RFNA from a nitrate salt and H2SO4, and what variables affect the final content of the product? The beginning of the distillation always seems to produce lots of dark NOx, but it always seems to dissipate rapidly as the distillation progresses, eventually almost disappearing entirely and leaving a very light yellow Fuming Nitric acid. The balanced equations for the reaction are
2NaNO3 + H2SO4 → Na2SO4 + 2HNO3
And
4NaNO3 + 2H2SO4 → 2Na2SO4 + 2H2O + 4NO2 + O2
And
NaNO3 + H2SO4 → NaHSO4 + HNO3
Temperature is obviously a big one, but driving the reaction at higher temps and reducing the condenser and receiver temperatures hasn’t seemed to help. Using an excess of NaNO3, but again, hasn’t seemed to make a huge difference, experimenting with different nitrate salts including potassium, ammonium, lead, calcium, etc., as well as starting with H2so4 + Concentrated 70% nitric acid and H2so4 + RFNA haven’t seemed to make a huge difference in the end product. I’m quickly running out of ideas. Could a fractionating column packed with tin granules work to decompose some of the HNO3 into NO2/N2O2 + H2O? I just posted this in general chemistry hoping maybe the broader category would help, but I thought I would toss it in here too since you guys have lots of RFNA experience, but any thoughts, ideas or literature would be greatly appreciated.

caterpillar - 4-1-2022 at 21:46

H2SO4 + NaNO3 will be good if sulfuric acid is concentrated. Then add some P2O5.

RustyShackleford - 5-1-2022 at 07:54

why dont you bubble nitrogen oxides into cold pre-prepared nitric acid? seems a lot easier than trying to figure out a way to do it in 1 step, and as a bonus youd get saturated solution, which inevitably would not be the case with 1 step, since the acid condenses and escapes the condenser while still warm and is thus not able to dissolve as much as it could when cold.

Delta-R - 5-1-2022 at 12:58

So the strength of the acid isn’t a problem- the H2so4 is 98%+ and the final RFNA is 90-95%. I already tried bubbling NO2 through the final RFNA, and it does help somewhat - after bubbling the NOx from about 10g of Sn and 40g hno3 plus some H2O, absorbance at 500nm went from 1.035% to 1.055% - but The difficulty is first from the fact that there can’t be any copper contamination in the final product and copper + HNO3 is a much better way of producing NO2 than tin, so if I could find a tweak to the distillation itself that yeilds the same or better results, that would be much better than having to go through an entire separate process.

RustyShackleford - 5-1-2022 at 15:04

Quote: Originally posted by Delta-R  
So the strength of the acid isn’t a problem- the H2so4 is 98%+ and the final RFNA is 90-95%. I already tried bubbling NO2 through the final RFNA, and it does help somewhat - after bubbling the NOx from about 10g of Sn and 40g hno3 plus some H2O, absorbance at 500nm went from 1.035% to 1.055% - but The difficulty is first from the fact that there can’t be any copper contamination in the final product and copper + HNO3 is a much better way of producing NO2 than tin, so if I could find a tweak to the distillation itself that yeilds the same or better results, that would be much better than having to go through an entire separate process.


why not just filter the gas flow from the NO2? just through a simple bit of glass wool or something would surely remove 99% of the copper in the droplets that come, if you were really anal you could bubble the gas through sulfuric acid or something to remove the droplets and dry it.

If you really insist on distillation, NaHSO4 generates a ton of NOx when heated with nitrate, so that would be better than sulfuric acid for that purpose. Another tip would be to use a long condenser so the nitric has good time to cool down and absorb the gas in the setup.

Microtek - 10-1-2022 at 06:27

If you lead the gas from a Cu + HNO3 reaction through a vertical packed coloumn before absorption in the HNO3, no Cu-ions would be transferred to the product. You can't get metal ions into the gas phase at normal temperatures, so only droplets might be a problem. These would be eliminated with a coloumn (or a glass wool filter). I don't think it would be a good idea to bubble the gas through H2SO4 since that might form nitrosylsulfuric acid.