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Author: Subject: Nitric acid synthesis with bisulfate weirdness.
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[*] posted on 23-11-2020 at 02:45
Nitric acid synthesis with bisulfate weirdness.


I tried making nitric acid with the sodium bisulfate and nitrate salt method twice over the last day both with really bad yields. I was basically following Nurdrage's video on the topic.

My first attempt I put 200g of KNO3 and 400g of NaHSO4 in a round bottom flask, added a 400mm vigreux and a straight condenser and then heated the whole thing with an oil bath. The mixture appeared to have melted before I got any liquid over so I assumed it was monohydrate NaHSO4 (58.5C melting point for monhydrate).

But as the distillation progressed I got very little acid and ridiculous amounts of brown NO2. My home made gas wash bottle bubbled ridiculous amounts of brown bubbles and fumes even with loads of NaOH in solution and when I added extra water hoping that would keep the NO2 down I had NO2 leak through the PTFE tape joints and wreck several keck clips.

I figured that maybe I didn't have pure monohydrate but instead a mixture of some sort so I added about 100ml of water.

I was eventually left with 178g of nitric acid with a density of 1.21 g/cm^3 or about 35% concentration. This is a yield of about 50% by my math. Going by the fumes I suspect I lost a lot to the atmosphere and wash bottle. Interestingly when I inspected the hose to the wash bottle it was clearly crusty on the inside and when burned it burned like a low order explosive (think nitrate soaked paper)

I made a second attempt with 300g NO3 and 600g NaHSO4. I added 100ml of water at the start. This time there was still more fumes than I expected but I watched the temperature much more closely and stopped once I hit ~125C at the top of the column. The oil bath was also much hotter than I expected at around 220C.

Yield was garbage though: almost exactly the same concentration and amount. except I used more starting ingredients so I got 33% yield.

With 3 mol of nitrate 5 mol water should have been plenty if it had been completely anhydrous but I ended up with a very pathetic yield.

Can anyone shield some light on the issue?

My only thought is the sodium bisulfate converting to monohydrate and taking the water. I've seen different sources say different things but either the monohydrate decomposes at 58C and the released water makes it "melt" or it melts but retains the water, decomposing to anhydrous at a higher temperature. I figure in the latter case its possible I'm not getting a good yield because I've not hit the decomposition temperature but that seems unlikely given I can't find a temperature other than 58C.

I would rather not push it higher than I already have with an oil bath (and I probably would have stopped if I realised it was that high earlier) but I thought this reaction happened around the azeotrope of nitric acid. I don't really have anything good I could heat my round bottom flaks with other than oil or direct gas flame.

Another possibility is KNO3 instead of NaNO3. KNO3 is easier to get (A$63 for a 20kg bag!) so that's what I used. KSO4 does not form hydrates though.

[Edited on 23-11-2020 by Download]
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[*] posted on 23-11-2020 at 05:52


Distilling under vacuum can reduce NOx formation
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[*] posted on 23-11-2020 at 11:40


Thought about trying this, though now, I think I'll just leave it.



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[*] posted on 23-11-2020 at 13:35


Having a temperature of well over 200 C at the boiling flask is way too high. You meed that to get a sufficient high temperature at the top, where the condenser starts. Nitric acid decomposes at that temperature. Why do you have a column of 40 cm above the boiling flask? I have done distillation of HNO3, with a round-bottom flask and a condenser connected to that through a simple adapter in which I had my thermometer. This gives at most 7 cm length or so above the boiling flask before you are at the condenser. This allows you to boil the acid at much lower temperature.

[Edited on 23-11-20 by woelen]




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[*] posted on 23-11-2020 at 17:43


Quote: Originally posted by woelen  
Having a temperature of well over 200 C at the boiling flask is way too high. You meed that to get a sufficient high temperature at the top, where the condenser starts. Nitric acid decomposes at that temperature. Why do you have a column of 40 cm above the boiling flask? I have done distillation of HNO3, with a round-bottom flask and a condenser connected to that through a simple adapter in which I had my thermometer. This gives at most 7 cm length or so above the boiling flask before you are at the condenser. This allows you to boil the acid at much lower temperature.

[Edited on 23-11-20 by woelen]


I'm hesitant to boil anything that can foam without one.
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[*] posted on 23-11-2020 at 23:50


Don't put too much in the boiling flask. First try with a small amount (filled up to 25% or so). Add a little water to reduce foaming even more. The worst that can happen is that some of the foam makes it into the condenser and then your acid is spoiled. But if you work with small amounts, then the loss is limited.



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[*] posted on 24-11-2020 at 02:34


As Woelen said you don't really need that column, if you insist at least pack it very well.
The destillation path is way too long resulting in the destruction of your acid.
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[*] posted on 24-11-2020 at 03:25


I know that Tdep uses KNO3 and NaHSO4 for his nitric acid needs. He never claims high yield but I think he has it down to a pretty fine art. I don't recall the exact conditions and proportions that he uses however. Perhaps he will drop by and enlighten us.

Me, I got sick of battling foam. (NaNO3 seems to foam a lot less than KNO3 as far as I can tell.) I ended up buying a 2.5L bottle.
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[*] posted on 24-11-2020 at 06:15


Purity of the chemicals also has a strong influence on the amount of foam. I have fertilizer grade KNO3 (white spheres, covered by a very thin, but somewhat greasy layer), which when dissolved in water produces a layer of scum on the surface. If I crunch these spheres, mix that with H2SO4, and use that for making HNO3, then I have a lot of foaming. I also have food grade KNO3 (intended use is for treating meat), which is a fine crystalline powder, which dissolves perfectly clear in water. With that, I have no issues when making HNO3. I quit using fertilizer grade KNO3 altogether. I purchased 5 kilo's of the food grade KNO3 for EUR 30 or so, including shipping. This is a life time supply for me. I only use it for making HNO3 when I need some of the acid (I always make small amounts, 30 ml or so, good for a few experiments).

I never tried the mix of NaHSO4 and KNO3 though. Maybe I should try that as well, with the upcoming ban of H2SO4 in the EU.




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[*] posted on 27-11-2020 at 17:23


Alright, repeated the experiment. I rewatched Nurdrage and in his video he said "hotplate as hot as it will go". So I gave that a shot.

I scaled it back to 100g KNO3, 200g NaHSO4, 23g water (mostly arbitrary). I emptied out my only flat bottomed flask to use (some heads from a distillation I had let go on for too long and planned to redistil), ditched the column and the oil, and heated the flask on the hotplate directly.

I got about as much NO2 fumes as the second attempt. Got what was probably some water coming over at around 110-120C, then it slowed down, but what came over was definitely concentrated nitric acid. Under the aluminium foil the hotplate read 340C by the time it stopped while at the head the distillate was coming over at ~180C.

I measured the density as 1.429 g/cm^3 or about 70% HNO3. Yield was a bit under 85g, or about 90-95%. So this does work, it just has to be much hotter than nitric acid from nitrate and HSO4. I guess NaHSO4 -> Na2SO4 reaction happens at high temps.

I'm still pretty sure what I've got is partially anhydrous NaHSO4.

Edit: Looking it up, 2 mol NaHSO4 turns into 1mol sodium pyrosulfate and 1mol water. I assume pyrosulfate is what is needed for this reaction. The 2mol:1mol water ratio might also explain the significantly better yield when stoichiometrically you use a massive excess of NaHSO4. (I guess it's not stoichiometrically then is it? Nurdrage's math was off).

[Edited on 28-11-2020 by Download]
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[*] posted on 28-11-2020 at 07:04


I accidentally did a thing I think is very effective in this synthesis. I don't have a "normal" condenser, so I used the jacketed coil condenser I had and cooled it with ice cold water.

See, if you're not getting HNO3 from your flask, you are getting H2O, NO2 and O2 instead. These three combined makes HNO3 under the right circumstances.

I cooled my condenser pretty aggressively, because I wanted to also condense the NO2 mostly because I wanted to see it in action. The NO2 did condense, and then combinated with the H2O and O2 to nitric acid. So in the end barely any gaseous NO2 ever left the apparatus, both increasing the yield and getting rid of the nasty fumes.
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