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Topic: Preparation of pure potassium nitrate from ammonium nitrate  (Read 660 times)

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Offline sophiearmbruster

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Preparation of pure potassium nitrate from ammonium nitrate
« on: October 19, 2022, 10:46:07 AM »
Hi all!

I used to do only theoretical chemistry and am rather new to applied chemistry as a hobby.

During my experiments, I tried (and failed) to produce pure potassium nitrate from ammonium nitrate and potassium hydroxide.
What happened is, that the ammonia gas (which was released from the reaction of ammonium ions and hydroxide ions) went into solution again, leaving me with a solution of ammonium nitrate and potassium hydroxide.

So I figured that the key of getting pure potassium nitrate is to have as little water as possible.

My thoughts on this are that I should have used saturated NH4NO3 and KOH solutions?

What are your thoughts? Since chemistry is such an expensive hobby (at least on my budget), I'd like to avoid more mistakes ;)

Thanks for your input!

Offline Hunter2

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Re: Preparation of pure potassium nitrate from ammonium nitrate
« Reply #1 on: October 20, 2022, 02:59:04 PM »
What about boiling the solution, to push out the ammonia.

Offline sophiearmbruster

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Re: Preparation of pure potassium nitrate from ammonium nitrate
« Reply #2 on: October 21, 2022, 07:56:43 AM »
I don't think that will work because the ammonia is already in solution (I also tried that without success).
In this video https://www.youtube.com/watch?v=0-jJ5QF-EVE&t=309s boiling out the ammonia from a solution of sodium hydroxide and ammonium nitrate, only a very small amount of ammonia could be released.

I noticed that at a later point in the video https://youtu.be/0-jJ5QF-EVE?t=436 they suggest to use ammonia nitrate and sodium hydroxide (potassium hydroxide would probably work just as well) without or with just "catalytic" amounts of water.
But the video is about making ammonia gas, not about making potassium nitrate from ammonium nitrate. Can a complete reaction be guaranteed if there is almost no water?

Offline Borek

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Re: Preparation of pure potassium nitrate from ammonium nitrate
« Reply #3 on: October 21, 2022, 08:16:36 AM »
You need to wait for the ammonia to run out, this is not a quick process. In general KOH/NH4NO3 mixture that is not kept in a closed container will loose all the ammonia. Using less water to keep the solution more concentrated, elevating the temperature to speed the process will definitely help, but you are all the time limited by kinetics.
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Offline sophiearmbruster

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Re: Preparation of pure potassium nitrate from ammonium nitrate
« Reply #4 on: October 21, 2022, 10:25:33 AM »
Here is what my experiment looked like:
I solved NH4NO3 and KOH (masses according stoichiometry) in water and mixed the solutions, I boiled the mixed solution for two hours. During this time, almost no ammonia gas was produced (at least by smell test). Since nothing happened after 2 hours, I stopped boiling it.
Then, I've let the solution sit outside (shielded from rain) for 2 weeks until it was dry.

Then I tested the resulting salt:
- adding slightly wet KOH to it produced ammonia gas
- solving the salt in water increased the pH

So I think that both ammonia ions and hydroxide ions are still present.
In the original mixture, I used the stoichiometric numbers and dried both the NH4NO3 and KOH before weighing.
Now that I've seen NurdRage's video I linked above, it seems I'm not the first one who couldn't get the ammonia to leave the solution.

Do you think it will still help to put the salt into solution and boil it down again?
Another thought (with bad yield): maybe I could purify it by recrystallization. KNO3 should be the least soluble salt in my mixture.

Offline Enthalpy

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Re: Preparation of pure potassium nitrate from ammonium nitrate
« Reply #5 on: October 24, 2022, 07:55:08 AM »
Boiling ammonium nitrate as a chemistry hobbyist...

Is that a path towards suicide?

Warm ammonium nitrate detonates, even without a fuel.

Offline sophiearmbruster

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Re: Preparation of pure potassium nitrate from ammonium nitrate
« Reply #6 on: October 26, 2022, 08:15:46 AM »
I didn't boil a concentrated ammonium nitrate solution, even as a theoretical chemist I know that can be dangerous ;) I boiled the mixture of ammonium nitrate and potassium hydroxide solution in order to "boil out" the ammonia gas.
As a precaution, I did that outside and I did not stay in close proximity (~5 meters) of the beaker.

I got the idea of boiling out the ammonia from these articles: https://www.wikihow.com/Make-Potassium-Nitrate and https://www.instructables.com/How-to-Make-Potassium-Nitrate/

Edit: I just verified that solid ammonium nitrate and ammonium nitrate in aqueous solution is non-combustible. Only if you melt it and continue to apply heat to it, it becomes dangerous since it slowly begins to decompose exothermically which contributes to a heat buildup. Since I had a dilute solution and some of the ammonium ions already escaped as ammonia gas, I don't think my process was unreasonably dangerous.
« Last Edit: October 26, 2022, 10:17:15 AM by sophiearmbruster »

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