Sciencemadness Discussion Board - Effecting a separation of two, nearly identical compounds

Effecting a separation of two, nearly identical compounds

Clan-destined - 20-4-2021 at 14:51

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?

DraconicAcid - 20-4-2021 at 15:10

If you dissolve the mixture in water and boil it, the urea will hydrolyze, leaving the ammonium nitrate.

Praxichys - 20-4-2021 at 16:11

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 453 times

Endo - 22-4-2021 at 09:55

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]

Praxichys - 22-4-2021 at 10:28

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.

Swinfi2 - 22-4-2021 at 11:16

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.

Fyndium - 22-4-2021 at 23:14

I would likely feed then to sodium hydroxide, recover the ammonia formed from urea and ammonium nitrate and take the sodium nitrate formed.

Belowzero - 22-4-2021 at 23:21

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]

njl - 23-4-2021 at 05:06

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.

clearly_not_atara - 23-4-2021 at 05:50

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]

unionised - 23-4-2021 at 07:22

https://en.wikipedia.org/wiki/Urease

Clan-destined - 26-4-2021 at 21:07

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]

Clan-destined - 26-4-2021 at 21:13

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.

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?

Clan-destined - 26-4-2021 at 21:24

Quote: Originally posted by unionised

https://en.wikipedia.org/wiki/Urease

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?

Endo - 27-4-2021 at 04:42

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.

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.

njl - 27-4-2021 at 04:59

PubChem claims that AN is somewhat soluble in acetone, while urea is not.

Fyndium - 27-4-2021 at 07:19

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]

BaFuxa - 30-4-2021 at 23:33

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.

Fyndium - 1-5-2021 at 03:27

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.