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Author: Subject: Crystalization & purification by evaporation - strange results
RogueRose
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[*] posted on 14-4-2016 at 14:52
Crystalization & purification by evaporation - strange results


Having let a saturated solution of ammonium sulfate sit in a large metal pot, I found some strange results after the water fully evaporated from the pot. A mixture of different size crystals (some larger than 1"^2) formed on the bottom of the pot as well as some strange "crust" which started to form and migrate away from the liquid level. The white crust has formed some very strange structures. The "crust" seems much less dense and almost powdery when pinched between fingers and some parts looks somewhat like what a patch of mold or fungus looks like when it grows. The crust also dissolves much more readily/quickly in water than do the crystals.


It seems that there are 2 substances here, one that forms the crystals and another that creeps up the side and forms the "crust" type substance. It is possible that some of the ammonium sulfate has decomposed to ammonium bisulfate but I don't know how to determine this exactly.

I would like to separate the two substances if it is indeed 2 different compounds. It seems that I could scrape the crust off the sides and separate from the crystals - re-dissolve the crystals and repeat until purity of the crystals is high. IDK if there is a way to convert the bisulfate to sulfate - maybe by dissolving in ammonia solution instead of just pure water?






crystal3.jpg - 281kB crystal2.jpg - 244kB crystal1.jpg - 387kB crystal4.jpg - 337kB crystal5.jpg - 281kB
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Detonationology
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[*] posted on 14-4-2016 at 18:12


I have experienced this phenomenon while evaporating ammonium chloride. I'm not entirely certain why this occurs, however I have noticed ammonium salts tend to sublime very easily at lower temperatures than listed on Wikipedia.



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OneEyedPyro
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[*] posted on 14-4-2016 at 19:09


I've noticed the same thing with other salts although not as dramatic.

I don't think this has anything to do with sublimation but rather a wicking effect.
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RogueRose
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[*] posted on 14-4-2016 at 19:54


I think it has to do with wicking as well. I just found some of those structures that were made really amazing!

Do you think the formation of the two types of structures, crystals vs "crust" are two different compounds?
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OneEyedPyro
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[*] posted on 14-4-2016 at 20:17


It could be sulfate and bisulfate but I think it's more likely just two different crystal structures/sizes of the same compound.
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hissingnoise
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[*] posted on 15-4-2016 at 01:38


Quote:
I'm not entirely certain why this occurs, however I have noticed ammonium salts tend to sublime very easily at lower temperatures than listed on Wikipedia.

Unlike the chloride and carbonate, the sulphate doesn't sublime, it decomposes as does the nitrate.
Quote:
I think it's more likely just two different crystal structures/sizes of the same compound.

Indeed, the clumping is similar to the crystal-phase recycling with temperature seen in ammonium nitrate!

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macckone
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[*] posted on 15-4-2016 at 12:19


Generally when recrystallizing material to improve purity, the first crystals and
last liquid are separated. If there are impurities the concentrate either in the first
Crystals or last liquid. The impurities can alter the crystals formed.
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aga
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[*] posted on 15-4-2016 at 12:39


Aha !

That explains why i sometimes get some powdery crap at the bottom of the beaker before the 'proper' crystals form.

Thanks macckone. I just learned a bit more.




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