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Author Topic: Getting a salt out of an aqueous solution  (Read 6865 times)

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horsebox

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Getting a salt out of an aqueous solution
« on: October 22, 2010, 05:08:37 PM »

I've been doing a bit of home chemistry lately for the fun + gain extra experience and lately I've realized how difficult it is to boil off the water in a salt solution to recover the salt. The colligative properties of the salt raise the BP a fair bit and it takes ages to boil it off. Is there a better way to recover a salt from the solution (not involving metathesis reactions)? If I was to cool the solution would the water start to crystallize while the salt remains in solution or will the salt just get trapped in the ice?
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Borek

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Re: Getting a salt out of an aqueous solution
« Reply #1 on: October 22, 2010, 10:02:16 PM »

It doesn't have to boil to evaporate.

Whether solid will crystallize after cooling the solution depends on how the solubility changes with temperature. Usually it goes up but sometimes it either almost doesn't change or even goes down, in which case crystals that can form in hot solution will dissolve on cooling.
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horsebox

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Re: Getting a salt out of an aqueous solution
« Reply #2 on: October 23, 2010, 02:50:07 PM »

Can you gimme an example of a salt thats solubility goes down with temperature?
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Grundalizer

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Re: Getting a salt out of an aqueous solution
« Reply #3 on: October 23, 2010, 03:11:37 PM »

What reactions are you doing?  Boiling the water for 20-30 mins is shorter than letting it evaporate over days and days. 

Just do your experiments in smaller solvent (water) volumes.

There is no other easy way to get salts to crystallize out of solution
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Borek

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Re: Getting a salt out of an aqueous solution
« Reply #4 on: October 23, 2010, 09:56:49 PM »

Can you gimme an example of a salt thats solubility goes down with temperature?

http://www.newton.dep.anl.gov/askasci/chem03/chem03726.htm

Boiling the water for 20-30 mins is shorter than letting it evaporate over days and days.

I was aiming at heating without boiling.
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nj_bartel

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Re: Getting a salt out of an aqueous solution
« Reply #5 on: October 24, 2010, 07:32:59 AM »

Heat the solution in something like a cookie sheet - much bigger surface area.
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horsebox

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Re: Getting a salt out of an aqueous solution
« Reply #6 on: October 25, 2010, 09:02:12 AM »

What reactions are you doing?  Boiling the water for 20-30 mins is shorter than letting it evaporate over days and days.  

Just do your experiments in smaller solvent (water) volumes.

There is no other easy way to get salts to crystallize out of solution
I bought an instant cold pack containing a mixture of ammonium nitrate and urea (I don't know what ratio) and want to isolate the NH2NO3. I don't know the ratio of AN to urea so I didn't bother doing any calculations, I just dissolved the substance in about 400ml of water and boiled the solution in order to decompose the urea.

Heat the solution in something like a cookie sheet - much bigger surface area.
I used a pyrex baking dish but a cookie would work way better. I'll keep that in mind for the future.

EDIT: I decided to finish boiling the water off a few minutes ago and went to my lab and found that a solid white slate had formed at the bottom of the pyrex dish. This slate is about 5mm thick and so solid I had to snap bits off it to get it into a storage container. If there were still urea in there would it have crystallized into its own layer in this slate? I boiled for a very long time so its unlikely that theres any urea in there but I have a fairly large amount of solid and thats just what crystallized out at about 10C.
« Last Edit: October 25, 2010, 09:29:09 AM by horsebox »
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vmelkon

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Re: Getting a salt out of an aqueous solution
« Reply #7 on: October 31, 2010, 09:19:08 AM »

You can try this to make NH4NO3
http://www.youtube.com/watch?v=w2IG49Nw1Fo

and I think there was another video that was about getting it from cold packs made by the same guy.
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horsebox

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Re: Getting a salt out of an aqueous solution
« Reply #8 on: November 13, 2010, 09:31:00 AM »

I boiled off most of the water and left it in the shed. When I left it not a single crystal had formed but when I came back, there was a slate (about 1cm thick) of white solid on the  bottom of the pyrex cook dish. Guessing thats my NH2NO3  :D
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macman104

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Re: Getting a salt out of an aqueous solution
« Reply #9 on: November 14, 2010, 11:21:59 AM »

Note, ammonium nitrate = NH4NO3, NH2NO3
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Zerm

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Re: Getting a salt out of an aqueous solution
« Reply #10 on: November 23, 2010, 01:12:33 PM »

I once tried to crystallize a solution of ammonium nitrate by boiling.  I boiled it down really far and the temperature kept rising and rising.  I thought I would see it crystallize any second but it never did.  Finally, the solution turned a pale yellow color and smelled of ammonia.  I then realized I was looking at boiling ammonium nitrate!  This is quite dangerous as, though rare, pure ammonium nitrate can detonate under extreme conditions.  Not wanting to find out exactly how extreme, I removed the heat immediately and left the room until it had cooled down. 

A much safer and smarter method would have been to predict stoichiometrically how much product would just dissolve in a volume of solution near boiling point, boil the solution to slightly above that volume, and then remove the vessel to an ice bath or refrigerator to crystallize quickly.  In general, attempting to crystallize any compound from a boiling solution can lead to unwanted decomposition and other undesirable results.  For evaporation crystallizations, I find keeping continuous air flow over the solution with a fan greatly increases rate of drying without the need for heat. 

Hope this helps!
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nj_bartel

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Re: Getting a salt out of an aqueous solution
« Reply #11 on: November 23, 2010, 03:23:03 PM »

Ammonium nitrate decomposes before boiling. That's why you were smelling ammonia.
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Zerm

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Re: Getting a salt out of an aqueous solution
« Reply #12 on: November 23, 2010, 03:49:59 PM »

Sorry, that's what I meant.
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