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Maceriam
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[*] posted on 11-1-2020 at 13:22
Crystallization Techniques


Hey There!

Currently I am trying to get some Monopotassium phosphate crystals that are monocrystalline, but I am having a very hard time. Everytime I seem to get a big layer of very small crystals which are not good enough for seed crystals.

Now I know there are some different techniques out there like Vapour Diffusion and Liquid-Liquid Diffusion (https://web.mit.edu/x-ray/cystallize.html)

But I am not sure if they work very well at bigger scales. Since the scale I use for chemistry is just a poor kitchen scale and those techniques are usually done at milligram level.

I was wondering if you guys knew more.

Thanks in advance for the help!
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karlos³
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[*] posted on 11-1-2020 at 13:35


Small crystals are usually purer than big crystals, as they trap less impurities in the crystal.
Do you want pure crystals or big crystals?
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unionised
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[*] posted on 11-1-2020 at 14:03


Quote: Originally posted by karlos³  
Small crystals are usually purer than big crystals, as they trap less impurities in the crystal.
Do you want pure crystals or big crystals?

You have that the wrong way round.
Bigger crystals are usually purer.

Crystals that are grown slowly are usually bigger and cleaner.

Filtering solutions before crystallizing often helps.

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Maceriam
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[*] posted on 11-1-2020 at 14:04


I don't mind if the crystal isnt too pure but I would like it to be mono-crystalline and big.

I do know that if going slow enough the crystal will get more pure/clear
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Swinfi2
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[*] posted on 11-1-2020 at 22:10


While it's not always true that salts are more soluble in hot solutions than cold ones, it is 99% of the time.

My advice is to filter the solution, boil it down until it starts crusting up (saturated boiling) then add 5-10% boiling water and insulate the container really well so it cools down over a few hours. Should give bigger crystals the slower it cools.

Unless you get unlucky and it supersaturates (crystalises all at once after cooling/shaking/ect).
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Sigmatropic
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[*] posted on 12-1-2020 at 00:26


I use slow evaporation to obtain large single crystals. The seeds sometimes by heating&cooling of near saturated solution but I get there by evaporation.
Can take several weeks, so I hope your not in a rush.
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markx
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[*] posted on 12-1-2020 at 03:35


Working with clean glassware, thoroughly filtered solutions (lack of cristallisation centers) and giving the process ample time (weeks, sometimes months) to proceed shall usually yield bigger cristals.
Oversaturation and temperature fluctuations should also be avoided as this will create spontaneous seeding and formation of small cristals will be the result.




Exact science is a figment of imagination.......
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Sulaiman
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[*] posted on 12-1-2020 at 06:13


I have only a little recrysallisation experience but based on reading and some experimenting ...
(e.g. http://www.sciencemadness.org/talk/viewthread.php?tid=27851&... half-way down the page)
... you can't get purer than a single perfect crystal.

I am convinced that when recrystallising for purification ... the slower the better.
(I suppose because it gives time for molecules to
- diffuse from the very slightly supersaturated bulk of the solution, into/through the thin layer of very very slightly supersaturated liquid surrounding the forming crystal
- neatly extend the crystal strucure of the slowly growing crystal, excluding 'contaminating' (less well fitting) molecules from easily joining the crystal lattice,
and not forming inclusions that could contain anything)

silver nitrate turned out to be more difficult to recrystallise than I expected,
it has very high solubility (122g/100ml@0oC, 733g/100ml@100oC)
and tends to easily form supersaturated solutions.
(99.9% silver plus nitric acid formed silver nitrate with a green tinge,
recrystallising by unintentional crashing-out (twice) made negligible difference,
the crystals from the above post have no discernable colour)




CAUTION : Hobby Chemist, not Professional or even Amateur
(suffering from separation of me and my chemistry stuff)
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Maceriam
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[*] posted on 12-1-2020 at 07:03


I was thinking of maybe putting the beaker that is slowly evaporating in a large container which contains something like isopropanol. So the solubility decreases slowly as the isopropanol vapours will mix with the water. Not sure how much isopropanol I would need for that tho
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monolithic
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[*] posted on 12-1-2020 at 08:22


When you've added enough boiling solvent to reach saturation (plus a bit more), turn your hot plate off and leave the beaker on the hot plate. This will allow it to cool more slowly than taking the beaker off the hot plate and placing it on a counter top, as the ceramic/metal tops have some thermal mass to them. You could also insulate the beaker with aluminum foil or glass wool, and cover it with plastic wrap (with a few holes poked in it.)
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