Sciencemadness Discussion Board - Weird things during nitric acid synthesis

Weird things during nitric acid synthesis

Nikotin - 30-10-2023 at 14:28

Hi, this is my first time posting here, not sure if this is the correct location.

Anyways, today I synthesized nitric acid from potassium nitrate and sulfuric acid.
This is my third time doing this, but the first time with a setup this "elaborate".

But weird things happened.

So the first pic shows my setup, so you have a clue what was going on.

The second picture shows the product, but for some reason two layers formed and both seem to be nitric acid. At the beginning there was just one layer but at some time the liquid stopped coming, so I thought it was done. But later, it started coming over again, bubbling into the "gas catcher" far more than the first time. This time whatever came over seem to be immiscible. I am not sure what this is or why it didn't want to mix really. I thought maybe it's just nitric acid with a lower concentration and therefor different density, but maybe someone here knows better. Maybe something bubbled over while I wasn't paying attention, seems to be unlikely tho. Also unlikely that the sulfuric acid got to boiling as my thermometer showed never more than 120°C, but the round bottom flask itself got up to 150°C at least, according to my laser thermometer.
After stirring it seems to have mixed, but I am unsure if it will separate later again maybe.

But this is not the one weird thing that happened, as my "gas catcher" water beaker absorbed the NO2, it turned into nitric acid of course. I tested the water even with a PH-Strip and it's at 1 or less. But weirdly my very dilute nitric acid it this catcher turned out blue? Does anyone have an idea why this could be? Only thing I can think of is the test strip somehow turning it a very light blue, even though it turned red with the acid.

As I am fairly new I just thought I may ask you guys, thank you already!

[Edited on 30-10-2023 by Nikotin]

[Edited on 30-10-2023 by Nikotin]

B(a)P - 30-10-2023 at 14:58

Hi Nikotin and welcome to the forum!
What was the concentration of your sulfuric acid when you started?
Is there any chance your reaction mix foamed over and entered the receiver?
I think you are right about the two different acid densities. It could be that you have near pure nitric acid on the bottom and red fuming on the top. You may have pushed the reaction too far and had decomposition of the nitric acid, which would give you a lower density and darker red coloured acid coming over. This would also account for the additional gas noted toward the end. If this is the case you will not get separate layers again now they have mixed. Have they remained separate?
For the beaker, did it turn blue when you put in the test strip? A better practice is to extract a little liquid then drip it on the test paper. If not any chance you had copper or some other metal contamination in the beaker?

Nikotin - 30-10-2023 at 15:14

First and foremost, thanks already for the fast answer.

According to the manufacturer it's 96%, but it's drain cleaner, so I am not 100% sure.
Yeah, it at one point foamed but i stopped it at the last second, maybe a few drops got over but all the Potassium salt stopped in the condenser before it could make it over. (this was fairly early)
As sulfuric acid is more dense it would be very weird if it would float on top and I saw that the drops that came over later penetrated thru the top layer into the bottom layer, when they came down they slowly floated to the top and mixed with the top layer.
If it is separate now, i do not know, as I put it into a brown glass bottle, but I will take a look tomorrow. I also thought that maybe some water got sucked in, I hear that other people had that happen with a similar setup, at most it should have been a few drops again.
I measured the density of it after i mixed it and it was almost exactly 15g at 10ml. which would mean a concentration of ~94%.

About the blue color, I only noticed it after the test strip, so it's very possible that it came from that. Thanks for the advice, next time i will just drop some on it and not put the strip into the liquid.
Copper contamination seems unlikely, as I haven't worked with copper yet, but some of the glassware is used, so it isn't impossible. But as I clean my glassware pretty well and have used it several times for different things since I purchased it, it seems fairly unlikely.

But I realized that the color might also be from the hose/tube I used, as the gases really took their toll on it and made it brittle.

[Edited on 30-10-2023 by Nikotin]

[Edited on 30-10-2023 by Nikotin]

Sir_Gawain - 30-10-2023 at 15:34

You might have heated it enough to drive some water out of the sulfuric acid. Also, 10 mL is not nearly large enough of a sample to get good density measurements. Try dripping some onto a nitrile glove. If it quickly bursts into flames, it's probably over 90%.

Admagistr - 30-10-2023 at 15:38

The colour is indeed indicative of the decomposition of HNO3 and the formation of NO2.I suspect that concentrated H2SO4 got into the HNO3 and decomposed it.You used technical utility H2SO4 and there is probably a problem here, unless the cause is an inconspicuous splash of H2SO4 into the flask with the product.It would be useful to find out what impurities and components your H2SO4 contains and whether it is usable for this particular reaction because of its content.Does the manufacturer give the complete chemical composition of the cleaner?

Nikotin - 30-10-2023 at 15:41

Thanks for the message, I will test the density tomorrow at 50ml, results may differ due to me having to use a less accurate scale for that and as I mixed it with a little nitric acid that was already in the bottle. I will also drip some on a nitrile glove.

Nikotin - 30-10-2023 at 15:48

Quote: Originally posted by Admagistr

It would be useful to find out what impurities and components your H2SO4 contains and whether it is usable for this particular reaction because of its content.Does the manufacturer give the complete chemical composition of the cleaner?

The source of the acid is "Normatek Hydraulik" drain cleaner, there is some polish datasheet but I can't find it right now, as I am not polish.

I am sure it contains sulfuric acid and the datasheet says 96%. The color is clear, that is all I know from the top of my head.

Edit: found it: https://te-ch.pl/pl/p/file/4c609b5f34b064dc62f371dfe369e0cb/...

Also I used it before and had no problems, I will do more research tomorrow, as it's pretty late.

Thank you guys!

[Edited on 30-10-2023 by Nikotin]

Sir_Gawain - 30-10-2023 at 16:09

150°C is far too hot for fuming nitric acid. Nitric acid decomposes into NO₂ and water upon excessive heating. Judging by the extremely dark color in the flask, I think that's very likely. The blue color of the gas trap water could also be from dissolved NO₂ or nitrous acid.

Keras - 31-10-2023 at 01:05

IMHO, using impure sulphuric acid to make nitric acid is not a good idea. The nitric acid will probably react with whatever additives are present in the drain cleaner and form a slew of compounds, some of which will pass with the nitric acid and stain your final product. Given than nitrous products have a knack for explosions, especially when heated, I would be very careful here and use only pure stuff. Distill your drain cleaner before using it.

That being said, the brown colour is pretty normal. It’s damn hard to get colourless nitric acid right from the distillation apparatus. The best way to do it is to bubble nitrogen or argon into your fuming nitric acid until all the NO₂ has been carried away.

woelen - 31-10-2023 at 01:21

The blue color in the water trap most lkely is due to formation of N2O3. This stuff can exist in dilute acidic aqueous solution for quite some time.

I have observed this blue color many times, when working with NOx fumes. Easiest way to observe it is dissolving some nitrite in water and then adding excess dilute acid, but you can also see it, when you dissolve NOx-fumes in water. The water turns blue, due to formation of HNO2, which in turn is in equilibrium with N2O3.

If you allow this to stand for a day, then the blue color should have disappeared though. N2O3 finally ends up as NO and HNO3 in water and if air is present, then it all will end up as HNO3.

If the blue color indeed persists after a day or so, then it must be some contamination, probably from the test strip.

Sulaiman - 31-10-2023 at 10:11

When using cheap (drain unblocker) sulphuric acid with nitrate salts to make nitric acid,
I use excess sulphuric acid (especially with calcium nitrate) to reduce hot-spots that decompose nitric acid.
If stoichiometric ratios of acid and nitrate are used then the sulphate produced makes even heating difficult.
I've read that (but not tried) using hydrogen peroxide in the gas trap will produce nitric acid directly.

Nikotin - 31-10-2023 at 10:53

Quote: Originally posted by Sulaiman

I use excess sulphuric acid (especially with calcium nitrate) to reduce hot-spots that decompose nitric acid.

Same, I used 400g of Potassium Nitrate and 250g of Sulfuric acid.

Also:

I tested the density again today and it came out at 94% again, 75g at 50ml, see picture.

I also looked at the acid and it didn't seem to separate, but that was in the brown bottle, so I put it in a beaker and waited for half an hour, as I didn't have more time to wait, but it didn't separate.

I also put about 2ml on a nitrile glove and it started fuming instantly and caught fire within like 5 sec.

[Edited on 1-11-2023 by Nikotin]

B(a)P - 31-10-2023 at 12:32

Nice work, looks like you have a good product, so long as the dissolved nitrogen oxide is ok for your purposes. How did the beaker of water turn out, has it lost its colour? I had no idea you could have a dilute solution of N2O3 in water such that you would actually see the blue colour. Very interesting.

Nikotin - 31-10-2023 at 13:08

Sadly I did not keep the water, sorry.
If it happens again I will post it here.

Thank you all very much btw!

[Edited on 31-10-2023 by Nikotin]

Sir_Gawain - 31-10-2023 at 14:37

To up the purity and remove most of the NO₂, re-distill it with some sulfuric acid. Just make sure to heat very carefully and make sure your cooling water is ice cold.

Nikotin - 31-10-2023 at 23:23

Thanks for the advice Gawain, I have not heard of this method so far, but I am willing to try it.
But are you sure that the 4% Water in the sulfuric acid will not make the product worse, or do you think the acid is hygroscopic enough to pull more water out actually?

Rainwater - 1-11-2023 at 01:17

You can use a dehydrating agent to remove more water from your nitric acid.
H2SO4 is a great choise as it loves water
Use of an excess will trap water while allowing you to distill a more pure nitric acid.
Useing the H2SO4 boiling point chart (found here) you can see that 96% sulfuric acid has a much higher boiling point than nitric acid(78c)

By knowing the amount of water in your nitric and the different boiling points, you can crudely determine the amount of sulfuric acid needed to completely dry your product.

Calculate the weight of the water you wish to remove, and add enough sulfuric acid to ensire that its boiling point remains above 120c
According to the chart Wiki provided, thats about a 50% (125c) consentration of H2SO4

Havent had my coffee yet, this math isblikly to be wrong
Nitric at 94%
Sulfuric at 96%
Amount to dilute sulfuric acid to 96-50=46%
Amount of water per gram of nitric 100-94=6
weight ratio 6/46=0.13

So add at least 0.13g of sulfuric acid to every gram of your nitric and distill.
The fraction that comes over at 78-79c will be anhydrous nitric acid.
Anything after that will have water in it.
Stop collection after 120c.
See here for a professional amateur
https://m.youtube.com/watch?v=88gbfCnrV8o

Edit. The 78c is wrong. I was thinging ethonol. Mitric acid will be different

[Edited on 1-11-2023 by Rainwater]

Sir_Gawain - 1-11-2023 at 05:56

Pure nitric acid boils at 83°C, 68% azeotropic at 120°C.
If you want to purify your sulfuric acid to 98% and get rid of any organic contaminants, add a few mL of hydrogen peroxide and heat it to 330°C. I also use drain cleaner, and after concentrating it I achieved 98-99% nitric acid with one distillation.

Nikotin - 1-11-2023 at 06:36

Can you really purify sulfuric acid by adding hydrogen peroxide? If so what concentration of hydrogen peroxide is needed to do so?

About what Rainwater said, couldn't I use a big excess of Sulfuric acid with 50%? I have way to much of 50% sulfuric acid anyways.

[Edited on 1-11-2023 by Nikotin]

Sir_Gawain - 1-11-2023 at 06:57

1. Yes, hot sulfuric acid plus hydrogen peroxide will destroy any organic contaminants like the dyes and inhibitors in drain cleaner. Heating sulfuric acid to about 330°C will drive off water until it's about 98% pure; this will also work for 50% sulfuric acid. Do it outside as it releases a large amount of sulfuric acid vapor. Here's a video about it.

2. Rainwater mentioned 50% because that's the minimum concentration of sulfuric acid that won't distill over with the nitric acid. You should use the most concentrated sulfuric acid you have. Adding about half the amount of sulfuric acid to nitric should work. Make sure not to heat far past the boiling point of pure nitric acid or it will decompose.

Nikotin - 1-11-2023 at 08:36

I have distilled sulfuric acid before, albeit with mediocre success, from about 50 to 85%, as I ran out of time. I have a lot of 50% acid, so I will most definitely try it again.

I would think that it will be better now, as I have a better setup and am a bit more experienced.

Only hydrogen peroxide is one thing I don't have, as regulation is a small hell. Max is 12% as you probably already know.

The concentrated acid I already have is already at 96% and is clear and has no smell, so I don't really think that I have to clean it up for anything I use it for. It seems to be only acid with water and nothing else, I can't be 100% sure tho of course. If I ever come across worse drain cleaner I will use your mentioned method.

Let's see how it will work out next time and thank you very much <3

Rainwater - 1-11-2023 at 13:35

The beautiful thing about sulfuric acid, other than how when heated will instantly destroy your flesh on contact, it that is 99% recyclable.
Within reason, the consentration of the sulfuric acid doesnt matter, the more consentrated it is, the less you need.
You can always boil the water out if it to bring the consentration back up.
Just remember safty first.
These are powerful things to work with.
Treat them like they can kill you.

Adding H2O2 to the sulfuric acid will accelerate a lot of reactions, turning carbon contamination into co2 that bubbles out, but so will nitrate salts.
I have used some perty nasty looking sulfuric acid for the production of HNO3, it was never an issue.

Fyi, the byproduct, potassium bisulfate is very useful.
1kg properly neturalized is enough to fertilize about 1 acre of farm land for a year.
The pyrolysis of this reagent results in a compound that will being iridium into solution.

Nikotin - 1-11-2023 at 14:51

Hey, thanks for the advice but I have 2 questions.

Quote: Originally posted by Rainwater

Fyi, the byproduct, potassium bisulfate is very useful.
1kg properly neturalized is enough to fertilize about 1 acre of farm land for a year.

Isn't potassium sulfate the main product, and isn't it also a good fertilizer? Also how do I separate the potassium sulfate from the bisulfate? The last few times it was a solid chunk at the bottom of my flask and the only way to get it out without destroying my glassware was washing it out with a lot of water, so I would have to boil the water away or leave it outside in hot weather for a few days maybe to get rid of the water, if I removed it from the flask like that.

Quote: Originally posted by Rainwater

The pyrolysis of this reagent results in a compound that will being iridium into solution.

And I did not at all understand what you tried to tell me with that sentence, but maybe I am just stupid, so can you explain maybe, because it sounds interesting?

Rainwater - 2-11-2023 at 01:16

Pyrolysis- heating while protecting from air.
Causes the compound to decomose into other things.
https://www.sciencemadness.org/smwiki/index.php/Potassium_bi...

Iridium is one of the most corrosion-resistant metals known. Immune to aqua rega and hydrofluoric acid, and temperatures over 2000c, this metal is difficult to bring into a chemical reaction.
Its basicly the badass of the chemical resistance literature

Rainwater - 2-11-2023 at 16:59

Quote:

Also how do I separate the potassium sulfate from the bisulfate?

I havent tried that before.
it should be possible through recrystallization or extraction, lets search the wiki
With water
The sulfate salt has a solubility between 20 and 100c of 111g-240g
The bisulfate salt has a solubility between 20 and 100c of 49g-121g.
So thats no good. Requires a lot of math and precision

But looky here, sulfate is insoluble in liquor and the bisulfate is..
It could be used as an extraction solvent.
But thats a waste of good drink.

Sir_Gawain - 2-11-2023 at 17:02

You could just add an excess of sulfuric acid to convert it all into bisulfate.

Nikotin - 3-11-2023 at 01:12

Because there are not enough K+ Ions, so only one per sulfuric acid molecule(=potassium bisulfate), instead of two (potassium sulfate), right?

Keras - 3-11-2023 at 03:39

Quote: Originally posted by Nikotin

Because there are not enough K+ Ions, so only one per sulfuric acid molecule(=potassium bisulfate), instead of two (potassium sulfate), right?

That'a way to see things. The (mostly accepted) view is one of pH equilibrium. KHSO₄ is a weak acid, therefore the proportion of KHSO₄ vs K₂SO₄ is pH dependent. But both are equivalent. That is, if you start from one mole of H₂SO₄ and add one mole of KOH, considering that the former is a strong acid and the latter a strong base, you should end up with one mole of KHSO₄. But since that KHSO₄ is in solution in water, there will be a non-zero number of HSO₄⁻ anions that will give away a proton to a water molecule and transform into SO₄²⁻, which you can consider as diluted potassium sulphate.

Beside, if you heat it over 600 °C, potassium bisulphate will lose SO₃ (that you can make sulphuric acid back with) and transform into potassium sulphate:

2 KHSO₄ → K₂SO₄ + SO₃ + H₂O

Rainwater - 3-11-2023 at 23:53

Quote:

Beside, if you heat it over 600 °C, potassium bisulphate will lose SO₃ (that you can make sulphuric acid back with)

I tried that once, not willing to do it again.
That stuff is nasty. Like really nasty, total containment is a must.
It eats gloves, mask, wood. Watch my new tongs rust before my eyes.
Leave that reaction to the professionals please.

Keras - 4-11-2023 at 02:23

Quote: Originally posted by Rainwater

Quote:

Beside, if you heat it over 600 °C, potassium bisulphate will lose SO₃ (that you can make sulphuric acid back with)

Leave that reaction to the professionals please.

I’m not sure why you say that. The setup is very simple and there is really little danger if you do things correctly. I use a quartz tube, a borosilicate bent tube and a condenser. Of course, do that outside. My starting point is sodium bisulphate, that you can buy cheaply as pH minus for swimming pool. I got 5 kg for 2 € at the local LIDL.

akmetal - 21-11-2023 at 16:31

I am happy I found this site. I have built a lab to make 99.X% nitric and going through the struggles

Stopping NO2 leaks and corrosion is the first thing - I found these along with the PTFE coated metal clips (https://www.labunlimited.com/cms.jsp?menu_id=29544&descr...)

I installed a separate birkeland eyde reactor that will have a cooled liquid jacket and stiring and I am going to heat the NO2 post arc reactor.

The first stage distillation to 68% nitric seems to be straight forward with a glass rashing ring distillation stage I just need to seal it up, got up to about 110 F in the bulb then just started running out of liquid which suggests poor yield in the creation of dilute nitric.

I have a separate still for distilling sulpheric and still need to break up a bunch of tiny glass for that still.

It sounds like from above the best mixture for the final distillation is 50% 90%+ sulpheric and 50% 68% nitric

I hope these little seals work because the thin teflon sleves didnt and its safety first, I will abandon this entire buisness venture if I cant do it safe.

Rainwater - 21-11-2023 at 18:28

I use a water asporator to place the appratus under reduced pressure.
Any leaks, leak in, keeping total containment.
The asporator with a baking soda solution, is an almost perfect gas scrubber
You get less decomposition of your product.
And of your slick, you can include a bubble capillary to prevent bumping.

akmetal - 22-11-2023 at 22:49

Quote: Originally posted by Rainwater

I use a water asporator to place the appratus under reduced pressure.
Any leaks, leak in, keeping total containment.
The asporator with a baking soda solution, is an almost perfect gas scrubber
You get less decomposition of your product.
And of your slick, you can include a bubble capillary to prevent bumping.

Thats a great idea, I just ordered this - https://www.amazon.com/dp/B071X63B2R?psc=1&ref=ppx_yo2ov...

With 24/40 connections - one connection will be the bubbling frit introducing the NO2 another will be a suction tube (I have a 500 micro vacuum pump connected to the still to create enough vacuum to force the liquid over to the boiler.

The 3rd is the vent that I could put the aspirator on (or eductor maybe?) - can you recommend one that I could use like a fish tank bubbler instead of water. I dont have water running to this building but i have a large water tank I could circulate with a pump but using air as the motive fluid would be easier.

Could the still be operated in a similar manner with an asperator on the outlet of the top side condenser it would be mixed phase flow (liquid water and water vapor mixed with nitric carry over)