RogueRose
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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?
    
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Detonationology
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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.
“There are no differences but differences of degree between different degrees of difference and no difference.” ― William James
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OneEyedPyro
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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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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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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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| 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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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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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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