RogueRose
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Crystalization by lowering temp of saturated solution - Question
So I was thinking of different ways to crystalize various compounds and I was wondering if there are benefits or drawbacks from saturating a solution
at a high temp and then allowing to cool (close to freezing) which, I would guess, would precipitate some of that which is dissolved - then filter
precipitate.
The only other way I have been able to solidify some compounds is to boil off liquid until I see a color change while monitoring temp to verify it
isn't rising above 212.
I would think that there is some water left in the precipitate and wonder if washing precipitate (with H2O or alcohol) would be beneficial and then
adding wash liquid back into evap/precip liquid container.
Does one method yield a better or higher quality product?
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nezza
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For compounds with normal solubility curves that are not too soluble or hygroscopic cooling a hot saturated solution works. There are notable
exceptions though. Sodium chloride - very little difference in solubility between hot and cold and most lanthanide sulphates are more soluble in cold
water than hot. It is difficult with exceptionally soluble materials such as silver nitrate and some solutions tend to supercool such as sodium
thiosulphate. Adding alcohol to reduce the solubility sometimes works. Washing with water or water/alcohol is advisable for most crystallisations.
There are online solubility tables that should give you an idea how well crystallisation will work.
If you're not part of the solution, you're part of the precipitate.
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Artemus Gordon
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Amplifying on what nezza wrote, first I assume you are doing this crystallization to purify your substance. I have done this with Copper Sulfate.
CuSO4 is highly water soluble at 100 degC, much less so at 0 degC, so it is ideal for this procedure. As nezza said, NaCl would not be. You need to
find a table of solubilities at various temps for your substance of interest to see if it is a good idea or not.
As for washing, that is done to sluice away impurities that are still sitting on the surfaces of your new crystals. Cold H2O can be used, but it will
redissolve some of your product and wash it away as well. I used acetone on my CuSO4 because it is a much poorer solvent than water in this case.
Thorough drying is then needed to remove the residue of your washing liquid of choice.
Another technique that can sometimes work is to simply let your solution dry out completely very slowly. I put the liquid I had collected after
filtering my cold crystals into a beaker covered with a watchglass and let it sit for several days. Eventually it dried completely and I was left with
large blue CuSO4 crystals and some other crystals that looked like needles made of brown glass. These were easily separated with tweezers. Of course,
if your desired product and its impurity are both colorless crystals, this would be much harder to do.
[Edited on 8-11-2015 by Artemus Gordon]
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