sternman318
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Crystallizing Solutions
This might be a dumb question, and I dont know why I have such apprehensiveness towards it, and searching the forums for any term related to
crystallization yields a plethora of results.
For crystallizing a salt solution ( let us use a dilute solution of CuSO4 as an example), would the procedure be
You boil it down, removing as much water/solvent as you can. Is it possible to boil off ALL of the water? I feel like many lab procedures have one
boil the solution down then either cool it or have it evaporate till crystal formation.
If no crystals for, a seed crystal or scratch of the glass is probably required then?
Can some solutions never form a crystal under normal conditions? For example, I have a solution of [Cu(Br)4] 2- that simply will not form crystals,
even if left on the hot plate for a long time and then cooled.
Can salts decompose in solution? For salts that decompose when heated in their anhydrous state.
Are there any online texts that deal with laboratory technique/practices that are accessible? I feel like reading one of those would answer most of my
questions
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trb456
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Ideally you are not suppose to boil away the solvent. The goal of recrystallization is to purify the sample, and if you boil away the solvent you
simply deposit impurities on the product.
What you are suppose to do is find a solvent where:
1) The product is very soluble when the solvent is hot;
2) The product is very insoluble when the solvent is cold; and
3a) Any known contaminants are still soluble when the solvent is cold; or
3b) The impurities are insoluble when the solvent is hot.
It can be tough to find a solvent that does all this!
If your case includes 3a, you decant off the supernatant, and wash the crystals in cold solvent.
If 3b, you filter the hot solution, wash it down the funnel with more hot solvent, then chill the filtrate to get the crystals.
I was recently trying to recrystallize iodoform using acetone. Problem is that iodoform is still quite soluble in cold acetone. But fortunately,
acetone is miscible with water, and iodoform is insoluble in water. So this is a mixed solvent recrystallization: dissolve in hot acetone, cool, then
add water and filter/wash.
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