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Author: Subject: Preparation of Nitric Acid
Monte Carlo

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[*] posted on 22-6-2017 at 18:25

Persulfate for cleaner WFNA from atmospheric distillation (and as NO2 scavenger): US3113836

Ammonium persulfate is an excellent NO2 scavenger, useful both during distillation from sulfuric acid (whether using a nitrate salt or azeotropic nitric acid) and for clarifying and stabilizing the distilled product.

The persulfate reacts with water and NO2 to make bisulfate and nitric acid, with a net reaction as follows:

(NH4)2S2O8 + 2 H2O + 2 NO2 ==> 2 NH4HSO4 + 2 HNO3

I have found that adding a small amount of ammonium persulfate (less than 1% of the weight of the nitrate source) to the pot seems to significantly reduce the NO2 that comes over during fractional distillation at atmospheric pressure. (It also helps to exclude light from the parts of the apparatus where HNO3 is hot -- the pot and the column -- by wrapping them in aluminum foil, which also acts as heat shielding for better fractionation.)

The distilled product still has a yellow tinge which can be clarified to water-white either with a small amount of persulfate or urea.

The advantages and disadvantages are,
* Urea slightly dilutes the acid when scavenging NO2 (water is a byproduct), but it does not introduce any contaminating ions
* Persulfate actually scavenges water, dehydrating the acid while removing NO2, and even a tiny excess of persulfate also stabilizes the white fuming nitric acid product during storage. However, persulfate does introduce a small concentration of contaminating ions. For most WFNA uses, this has no impact.

The stabilization is described in patent US3113836.
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