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Author: Subject: Separating Ammonia and Hydrogen Chloride from Decomposing Ammonium Chloride
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[*] posted on 24-11-2024 at 09:20
Separating Ammonia and Hydrogen Chloride from Decomposing Ammonium Chloride


When ammonium chloride is heated, it decomposes into ammonia and hydrogen chloride. Both are extremely useful gasses, but the problem is the fact that they immediately combine as soon as the temperature falls. How could the gasses be separated before the temperature falls?

The easiest way would be to have a container that has two gas outputs, each one having a membrane, one that allows ammonia to pass through and the other one hydrogen chloride. But do such membranes exist (they also have to be able to withstand >300°C)?

Could the gasses' density difference be utilized to separate them?

Is there any solid (e.g. silica gel) that could absorb one of the gasses (and liberate the other one upon heating, or some other way)?

And lastly, if oxygen was added to the mixture of gasses and it was passed over copper chloride at 450°C (the Deacon prosses), the HCl should convert into chlorine gas (even more useful than HCl), but would the ammonia decompose?

Are there any other ways to separate the gasses before they turn back into ammonium chloride?

Thanks.




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[*] posted on 24-11-2024 at 11:01


For the amateur chemist there is no easy means of separating these two gases, so the short answer is "no".

Of course, you can do interesting things with ammonium chloride. Releasing only ammonia is certainly possible. I actually did this myself to make gaseous ammonia for experimenting purposes.

Mix some NH4Cl with solid NaOH in appr. equimolar ratio. A good starting point is 50 grams of NH4Cl with 40 grams of NaOH (this is a little excess NaOH, but in practice, hardware-store NaOH contains some water). Next, add a few drops of water. The reaction sets off, and a nice flow of NH3 is produced. If you want it as dry gas without water vapor, you can lead the gas through a tube of solid NaOH.

You can also make HCl from NH4Cl, but there are more economic ways of making that. Simply mixing NaCl with NaHSO4 (swimming pool pH-minus) and then gently heating the mix produces gaseous HCl, which can be dried by leading it through a tube of CaCl2 (drying agent from the hardware store).

If you do these experiments, do them outside, or in a good fume hood. Both gases are nasty, and HCl is particularly so, because it leads to severe rusting and corrosion of all nearby metal items! Avoid breathing the gases!

[Edited on 25-11-24 by woelen]




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[*] posted on 24-11-2024 at 14:50


Agree with Woelen, there are membranes that would likely allow the passage of HCl or Ammonia but not the other but you'd have to embark on some massive R&D to find one to make on your own. The big issue with these sorts of membranes that I have encountered in production is that the throughput is garbage. Usually one side of the membrane is under vacuum and the flow side with both chemicals is under pressure, sometimes many atmospheres. Then the membrane manufacturers prepare a membrane surface with tons of folds and valleys to maximize surface area, not just a flat sheet or anything. Even then with the types of membranes we've used in the lab like that the throughput is in grams per hour if you're lucky, and I mean like 1-3 grams per hour. Obviously results may vary but I have thus far not been impressed with these semi-permeable membranes except for some of the liquid/liquid separators I've seen in use. But even those are prone to fouling.



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[*] posted on 26-11-2024 at 09:57


I would consider electrolisys of a water solution as a more easy way to get HCl and NH3 out of NH4Cl.

2NH4+ + 2e- -> 2NH3 + H2
2Cl- + 2e+ -> Cl2
NH4Cl + Cl2 -> NH2Cl + 2HCl

So, the primary products (when using graphite electrodes) on an anode are HCl (100 parts), NH2Cl (7-43 parts depending on the concentration and current) and Cl2 (3-10 parts) (Gmelin Ammonium 1936, p. 175). And a cathode should produce the ammonia/hydrogen mix.

Depending on the temperature/electrode type chloramine is partially decomposing to N2 and Cl2 (which reacts with NH4Cl to form HCl) increasing the HCl yield.

[Please be aware that when using platinum electrodes on some special conditions (mixing/temperature) NCl3 could form, but it could be the case only when the platinum electrodes are used]

Also separating NH3 and HCl by NH4Cl heating could be difficult only when you need both in a pure form, but you can use the stream directly for chemical reactions, for example, by heating of NH4Cl with some metal or metal oxide powders one can obtain valuable anhydrous chlorides. And there is some field for experiments because some oxides and chlorides (e.g. Cu2O or FeCl2) will react with ammonia to form nitrides. So, you will not get NH3 and HCl but probably could get different nitride/chloride mixes.


[Edited on 26-11-2024 by teodor]
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