chemkid
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Concentrating DIlute Ammonia
I need some concentrated (30%) ammonia. I can by it of course online, but i have a few liters of perfectly good dilute ammonia right here. I have read
some "distillation" methods but procedures have been vague. I have also searched the board and found no instructions for concentrating ammonia. Could
anyone explain how to concentrate dilute (5-10%) ammonia into at least somewhat more concentrated ammonia?
Chemkid
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Klute
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You could setup up a reflux apparatus, with a gas outlet attached to the top of the condenser, itself attached to some tubing fixed to an inverted
funnel. The funnel is immersed under the surface of some of your 15% ammonia in a beaker with magnetic stirring, covered with some celophane or wet
toilet paper to avoid excessive smell (it won't smell nice in any case  ). You
can use a distillation setup, and attach the tubing to the vacuum inlet.
As you heat up some of your dilute ammonia, the solubilty of the dissolved NH3 diminishes, and it is chased out as a gas, that is then absorbed in
your cooled solution contained in the beaker. You should be able to get a saturated solution that way. Using the distillation setup should get more
gas out i think.
Or you can neutralise some of your dilute solution with HCl, evaporate to obtain NH4Cl cristals, place them in a flask fitted with a addition
funnel containing conc. NaOH. Attach the tubing/funnel to the top of your pressure-equalised funnel, or above a condenser, and absorb the NH3 gas into
some of your dilute solution. A bit more work but you can calculate exactly how much you will need to add. Use excess NaOH.
NH3 gas is very soluble in water, so you won't need to keep your enriched solution that cold. The dissolution is exothermic IIRC, so applying a ice
bath now and then to keep the temp down will help.
Depending on what you need your conc ammonia for, you could just aswell introduce it as a gas during your reaction (like for amide synthesis, etc)
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MagicJigPipe
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I would think using a distillation (as opposed to reflux) setup would cause the water being condensed to dissolve some of the ammonia. It might not
take as long but I don't think you would get all of the ammonia out this way. I have never tried it so this is just speculation, however.
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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DJF90
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Yeah the reflux method sounds good. Saves having to neutralise and crystallise, and I would expect it achieves a similar result.
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MagicJigPipe
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Neutralize and crystallize what?
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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Klute
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I think is refering to the method using NH4Cl and reacting it with NaOH
Yes, it won't be possible to get all of the ammonia out even with reflux, i don't have any data concerning the solubilities though. Most of the time,
when i need to distill ammonia out of a reaction, i recycle the gas into commercial 20% or into some ester for amides, or simply neutralise with H2SO4
when i really don't want to smell any ammonia...
But if one can directly obtain a ammonium salt, reacting that with conc NaOH would be more precise.
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Pulverulescent
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I once tried leading NH3 from NH4N03/Na0H into water, but not having a stirrer made it very frustrating.
The funnel was approx. the diameter of the vessel and the ammonia avidly dissolved in the top few millilitres, and thereafter just blew out of the
funnel (cough!), hop-a-long fashion.
And all for want of a stirrer!
P
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chemkid
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I have no condenser, but perhaps i could use a flask to heat dilute ammonia, and then run a tube into another flask of cold ammonia and bubble it
through? Would this work?
Chemkid
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Klute
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Yes, to some extent. Some kind of large tubin could avoid excessive evaporation of the dilute ammed funnel or similar to help acheive godd dissolution
of the gas though.
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Pulverulescent
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Chemkid, if you can get ammonium sulphate fertiliser, it'll work with Na0H.
It should be quicker than refluxing, too!
If you're going to lead the gas directly into the dilute solution without using a funnel, watch out for suck-back caused by ammonia's high solubility.
If you don't have a stirrer, the outlet tube will need to go to the bottom of the receiving flask (NH40H is lighter than water), so the tubing should
be large dia. to accommodate possible suck-back without it reaching the first flask.
The funnel-method is great for a heavy gas like HCl; not so good for light NH3 without stirring!
P
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Pulverulescent
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The NaHS04 you're left with can be used for that S03 synth. everyone's on about!
P
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