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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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[*] posted on 21-7-2018 at 22:20


I have a question: Why are you using 225g KNO3 and 100mL of H2SO4? I have tried to calculate the necessary reagents needed and got different values:

(125mL HNO_3)/1 x 68/100 x (1..51g HNO_3)/(1 mL HNO_3 ) x (1 mol HNO_3)/(63.01g HNO_3 ) x (1 mol H_2 SO_4)/(1 mol HNO_3 ) x (98.08g H_2 SO_4 )/(1 mol H_2 SO_4 ) x ( ml H_2 SO_4 )/(1.84g H_2 SO_4 )=108 mL H_2 SO_4


(125mL HNO_3)/1 x 68/100 x (1..51g HNO_3)/(1 mL HNO_3 ) x (1 mol HNO_3)/(63.01g HNO_3 ) x (1 mol KNO_3)/(1 mol HNO_3 ) x (101.1 g KNO_3)/(1 mol KNO_3 )=205.94 g KNO_3

The amount of sulfuric acid is close, but 8 mL difference is still a bit off. Also, I am not sure if my stoichiometry equation is right or its because you are using pottery grade KNO3 so you needed more of it to compensate for its impurity?




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[*] posted on 26-9-2018 at 05:33


I'm following MagPie's preparation, as written, with the addition of 1.18g (0.5% w/w% of the KNO3) ammonium persulfate. However, upon heating, the temperature rose to ~80°C, distilling over a liquid from 67°C to 80°C. Then, the temperature started to swing wildly, down to 70°C, back up to 80°C, etc..

Any ideas what's going on? I've only collected maybe 20mL so far, but its never reached past ~80°C in 7 hours on a sand bath with insulation. The sand itself reads 197°C, so that seems more than sufficiently hot even with heating losses. It was initially held at 135°C, but I was getting nowhere, so I pushed it up to around 197°C, which is the rated maximum setting on my hotplate for a sand bath (#3 on the dial, the 197°C is just where the temperature was measured). Eventually, I had to shut it down and go to work after pulling an all-nighter, so I'll heat it up again tonight.

Also, anyone have a vapor-liquid equilibrium chart with the temperatures and percentages marked on the axes for 760 Torr?

[Edited on 26-9-2018 by weilawei]
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[*] posted on 26-9-2018 at 17:25


You shouldn't have to heat it so much. The azeotrope comes over at 120°C. Did you add the required amount of water?



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[*] posted on 26-9-2018 at 19:15


I added 100mL of H2SO4 to 48mL of water in an ice bath and let it cool.

I reheated it today after insulating more thoroughly. Same heat settings, though I have some question as to the veracity of the sand bath temperature; the thermocouple probe looked pretty bad after I pulled it out. The last time a thermocouple got melted, the readings were unrealistically high.

I ran the distillation until it reached 114°C (density of 1.34g/mL at 28.4°C, 57.6% for the first fraction), then swapped receivers as it seemed to not be climbing. The temperature dropped to around 110°C and never rose again. I ended heating when I saw heavy red clouds of NO2 puffing into the condenser. Haven't measured density for the second fraction yet.

I'll definitely run this again. I'm thinking I'll do several runs, combine all the fractions, and do one final distillation to get 68% HNO3. I also think I'll switch to an oil bath. As to insulation, that seemed critical, so maybe I'll get out the glass wool. I'd used Al foil this time--ultimately in 3 layers.

Edit: Took the density of the second fraction, got 1.36 g/mL at 22.7°C, which is 61.0%. I'm satisfied with this as a first attempt. Certainly, my technique could stand improvement, but the reaction worked with relatively little fuss. I did not suffer major issues with foaming or clogging--I found it to be rather well behaved for such a solids-heavy slurry. I employed a dumbbell-shaped stirbar just in case. I found the microwave trick to be extremely useful in cleaning my reaction flask. As for the thermocouple: it did not survive. A couple pictures, the thermocouple and the combined fractions before the acid is transferred to an amber bottle.

20180929_072719.jpg - 261kB 20180929_072807.jpg - 1.8MB

[Edited on 29-9-2018 by weilawei]
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