Clan-destined
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Effecting a separation of two, nearly identical compounds
Ok, this is a question that I've had a most difficult time trying to figure out. It involves a mixture containing two very similar compounds, and how
one may go about separating them.
For example, suppose I have two crystalline solids in powder form: ammonium nitrate and urea. Both are highly nitrogenus. Both have similar molecular
structure. Both have similar melting points (one differs from the other by around 30*C).
Assume I have a mixture of these two compounds. How would I go about effecting a separation of them?
I can't solvent-extract, as both are miscible in this and insoluble in that.
I cant hope to use a mesh filter-like system, whereby the porosity is such that one compound will fall through the sieve, whilst the other remain in
the filter media.
One thing that I considered doing is heating to melting point, and tell me if this will work or not: urea melts at 150*, and ammonium nitrate melts at
180* or so. Why not just get to 170*, then just vacuum filter? Is it tenable that the urea, now in a liquid phase, would pass the filter, leaving the
still-crystalline ammonium nitrate behind; in effect, separating the two? Or, will the presence of one compound alter the mp of the other, as is the
case with something like essential oils (steam-distill eucalyptus with water. Eugenol, which usually boils at 230* at standard pressures, will, by the
presence of water steam, see a lower boiling point; effectively the boiling point of the water itself, and shall come over the adapter along with the
water steam)?
How would this be accomplished?
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DraconicAcid
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If you dissolve the mixture in water and boil it, the urea will hydrolyze, leaving the ammonium nitrate.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Praxichys
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I do not think heating to decompose the urea will work. At best you will be left with a mess of cyanurates and biuret.
As for melting, urea is well known for coprecipitating with a wide variety of compounds. With heating, the ammonium nitrate will dissolve into the
urea, and on crystallization the two will form singular crystalline structures with mixed composition.
Attached is a pdf with good info on the behavior of various salts coprecipitated with urea.
Attachment: u1370538.pdf (1MB)
This file has been downloaded 309 times
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Endo
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Could you run a soxhlet extractor with anhydrous ethanol?
Solubility table for urea in ethanol
Solvent extraction with acetonitrile
Abstract on Urea Acetonitrile mixtures
[Edited on 22-4-2021 by Endo]
[Edited on 22-4-2021 by Endo]
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Praxichys
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I was not able to find solubility data on urea oxalate but did find mentions that it was fairly insoluble in water. Ammonium oxalate, however, is
quite soluble. The urea could be precipitated from the solution as the oxalate salt through the addition of oxalic acid.
I initially thought solvent extraction as well but their solubility seems too close in most inexpensive solvents I looked into.
For instance, ethanol:
Urea, 5 g/100 ml
Ammonium nitrate 3.8 g/100 ml
Generally there needs to be about an order of magnitude for efficient separation, and fractional crystallization is likely not possible due to
coprecipitation.
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Swinfi2
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Another way would be flash chromatography. But not that useful for such cheap stuff as urea. You'd spend less time and effort buying more than
recovering all the copious amounts of solvents.
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Fyndium
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I would likely feed then to sodium hydroxide, recover the ammonia formed from urea and ammonium nitrate and take the sodium nitrate formed.
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Belowzero
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Another idea:
If you have a cheap source of sulfuric acid you might be able to distill nitric acid from the mixture, if you lead this into an ammonia solution you
can obtain ammonium nitrate directly.
Right after typing that I realized you might just end up making urea nitrate, which is an interesting direct route too, not what you intended..
Fyndium's idea is probably better 
[Edited on 23-4-2021 by Belowzero]
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njl
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Depending on what you actually want from the two, Fyndium has the right idea. I would use Ca(OH)2 rather than NaOH since the formed calcium nitrate
can directly be turned back into ammonium nitrate with ammonium sulfate.
Reflux condenser?? I barely know her!
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clearly_not_atara
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All of the commenters point to the same problem: ammonium nitrate isn't very stable. Normally, you would recover the ammonium and the nitrate
separately.
One possible protocol is as follows:
1. add sodium (not potassium) bitartrate to a solution of NH4NO3 / urea in water
2. collect NH4 bitartrate precipitate
3. add ethanol, reduce volume, collect NaNO3 ppt [1]
4. evaporate ethanol to retrieve urea
5. stir NH4 bitartrate with Ca(OH)2 in cold water to freebase ammonia
6. add NaNO3 to ammoniacal filtrate
7. gas with excess CO2 to ppt Na+ as NaHCO3
8. evaporate to obtain mostly pure NH4NO3
This is an incredibly long and frustrating process to separate a mixture of two highly unremarkable chemicals. If you were willing to share what
you're trying to accomplish, we might be able to offer a more practical solution.
Sodium bitartrate in particular is already more expensive than either of the chemicals you'd be using it to separate!
EDIT: 1: https://espace.curtin.edu.au/handle/20.500.11937/505
[Edited on 23-4-2021 by clearly_not_atara]
[Edited on 04-20-1969 by clearly_not_atara]
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unionised
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https://en.wikipedia.org/wiki/Urease
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Clan-destined
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Quote: Originally posted by Belowzero  | Another idea:
If you have a cheap source of sulfuric acid you might be able to distill nitric acid from the mixture, if you lead this into an ammonia solution you
can obtain ammonium nitrate directly.
Right after typing that I realized you might just end up making urea nitrate, which is an interesting direct route too, not what you intended..
Fyndium's idea is probably better
[Edited on 23-4-2021 by Belowzero] |
Yeah, I was looking more for non-destructive testing.
Btw, I knew the separation problem between urea and ammonium nitrate would be technically challenging and I was right! Not that I want to be right; in
this instance, it would be nice if I were wrong for a change.
Anyone else have any ideas how I can effect this separation without destroying either compound in the process?
[Edited on 27-4-2021 by Clan-destined]
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Clan-destined
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| Quote: Originally posted by Praxichys | I do not think heating to decompose the urea will work. At best you will be left with a mess of cyanurates and biuret.
As for melting, urea is well known for coprecipitating with a wide variety of compounds. With heating, the ammonium nitrate will dissolve into the
urea, and on crystallization the two will form singular crystalline structures with mixed composition.
Attached is a pdf with good info on the behavior of various salts coprecipitated with urea.
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Now you say that once the urea has melted and thus a liquid you say that the ammonium nitrate would "dissolve into" the melted urea. But are you
sure? By virtue of both compounds being molecularly (or at least chemically) similar, shouldn't we consider the urea to be a 'super-saturated'
condition of the ammonium nitrate? In other words, the AN cannot dissolve in the urea because the urea super-saturates the AN, in much the same way
that adding additional salt to a saturated solution of salt water will not yield any more dissolved salt. Correct? Not about the salt water, but about
the AN/urea brin6g super saturated?
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Clan-destined
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Yes, I wasnt going to let this post escape me without first acknowledging the contribution...
I was considering something along these lines, and in a larger context I always thought that these enzymes that animals produce would work wonders
from a synthesis and chemo standpoint. Just look at the impressive array of chems and enzymes the liver makes just to metabolize drugs! And perform
it's work with zeal and diligence! It's like the liver can do anything! Certainly far more than even the best equipped labs.
But in this instance; in the clandestine instance, I was hoping to keep things simple with regards to separating urea and AN; I see the introduction
of enzymatic action as the gift of the liver, with tons of side rxns or metabolized side products emerging from urase introduction.
Nevertheless, such a solution may be my only choice at this moment, no?
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Endo
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| Quote: Originally posted by Praxichys |
I initially thought solvent extraction as well but their solubility seems too close in most inexpensive solvents I looked into.
For instance, ethanol:
Urea, 5 g/100 ml
Ammonium nitrate 3.8 g/100 ml
Generally there needs to be about an order of magnitude for efficient separation, and fractional crystallization is likely not possible due to
coprecipitation.
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This degree of solubility is more than enough to be done using a soxhlet extractor. If you fill the cup with around 15 grams of the mixture, and each
solvent cycle takes off ~3 grams of urea you would probably have most of the urea off in an hour or so of active reflux. Allowing a couple hours for
each refill one could get a very clean separation. The only difficulty is that the system needs to be anhydrous.
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njl
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PubChem claims that AN is somewhat soluble in acetone, while urea is not.
Reflux condenser?? I barely know her!
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Fyndium
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That would enable efficient extraction with soxhlet. I thought earlier of purifying NaCN from sodium carbonate with methanol, as carb is insoluble.
The reagents would have to be anhydrous, otherwise they will dissolve mostly everything. Making AN anhydrous needs temperature and patience.
[Edited on 27-4-2021 by Fyndium]
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BaFuxa
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1) How about adding H202, it forms urea peroxide. Then precipitate from solution at about 60°C.
NH4NO3 solubility in water at 20°c is 150g/100ml
Urea peroxide solubility in water at 20°c is 500G/L or 50 g / 100ml, it should precipitate first.
That is of course assuming no reaction between H202 and ammonium nitrate.
2)Other than that maybe electrochemical separation ? NH4NO3 are cations and anions in solution, make them flock to anode and cathode, the urea should
be left in the center  of the cell ?
3) Or an ion exchange resin, available in some water filters, should trap your ammonium nitrate and let the urea go through.
Potential counts for nothing until realized.
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Fyndium
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Could you use sodium percarbonate to deliver H2O2 into the solution, and having urea hydrogen peroxide adduct, sodium carbonate and ammonium nitrate?
These could be more separable than U+AN alone.
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