I'm trying to prepare a nitration mixture to nitrate a quinazolin-4-one compound, I did so by adding H2SO4 dropwise to 69% HNO3 through a dropping funnel in ice/salt bath in 1:1 ratio, the first time I did this I saw lots of reddish brown fumes turning to white fumes inside the flask, however I didn't see that on repeating the experiment and the color of the mixture is colorless, on standing I noticed the mixture turning to slightly reddish yellow color.
Is it a must to see reddish brown fumes turning to white fumes as an indication of formation of fuming HNO3 acid in-situ?
For conc. H2SO4―HNO3 mixed aq. solutions, nitronium cation generation is expected to occur between both conc. acids: (i) H2SO4 + HNO3 ⇌ HSO4− + NO2+ + H2O; (ii) 2H2SO4 + HNO3 ⇌ NO2+ + H3O+ + 2HSO4−.
Nitric acid can undergo autoprotolysis in very conc. (fuming) sol.: (iii) 2HNO3 ⇌ NO2+ + NO3− + H2O.
Conc. nitric acid often contains dinitrogen tetroxide (N2O4) in equilibrium with nitrogen dioxide: (iv) N2O4 ⇌ 2NO2, where NO2 imparts yellowish colour to the acid. It may contain also nitrogen monoxide (NO). Very conc. (fuming) nitric acid typically contains a significant amount of N2O4 / NO2.
The nitronium cation can combine with bisulfate anion to to yield nitrosulfuric acid (nitronium hydrogen sulfate); a strong oxidizing agent: (v) NO2+ + HSO4- ⇌ NO2HSO4. Nitronium can also combine with nitrate anion, to form dinitrogen pentoxide: (vi) NO2+ + NO3- ⇌ N2O5.
Nitronium hydrolysis, and that of N2O5, also occurs: (vii) NO2++ 3H2O ⇌ NO3- + 2H3O+; (viii) N2O5 + H2O ⇌ 2HNO3.
Sulfur trioxide (SO3) evaporates along with H2SO4 from conc. H2SO4 sol. Nitric oxides (NO, NO2, N2O4, N2O5) ― 'rutilant vapours' ― tend to evaporate along with HNO3, H2SO4 and H2O, from conc. H2SO4―HNO3 mixed aq. solutions.
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