cnidocyte
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Recrystallization
I'm unsure of how recrystallization really works. Lets say I have a mixture consisting of 40% Salt A and 60% Salt B and need to separate the 2. First
scenario lets say they are equally soluble in the solvent I choose and I dissolve the mixture in the hot solvent then cool to allow the salts to
crystallize out. Will 1 salt crystallize out before the other? Would it be Salt B because theres more of it? Would a mount of only Salt B crystals
keep forming until the solution is no longer supersaturated or would Salt A simultaneously start to crystallize out. If its the latter then would Salt
A crystals form in a different area to Salt B?
2nd scenario lets say the 2 salts have different solubilities in the solvent I choose. Would it be only the less soluble salt that crystallizes out
until the solution is no longer supersaturated?
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12AX7
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What's the solubility curve of each salt?
Shall we assume they do not form a compound salt (like KCl + MgCl2 <==> KMgCl3), and will not result in a double displacement reaction (like
2KCl + Na2SO4 <==> 2NaCl + K2SO4)?
Using KCl + NaCl as an example, NaCl's solubility curve is level, so you will precipitate KCl during cooling (with constant solvent), and NaCl during
evaporation (at constant temperature). This breaks down at the edges, where NaCl's temperature coefficient is noticable (<5%), or where KCl is too
weak to precipitate, etc.
Subsequent recrystallizations of the products will increase purity, by precipitating the majority product, partitioning the impurity into the solvent
(at the expense of some of the major product, which can be fed back into the process to seperate it again).
This process overall can be very tedious, where both salts have low tempcos or similar solubilities, or where compounds are formed, etc. I've seen at
least a few occasions where a range of salts are precipitated simultaneously, e.g., NaCl, CuCl2 and Na2CuCl4. Simultaneous precipitation is analogous
to freezing a eutectic mixture; an unpleasant fine-grained mixture results.
Tim
[Edited on 8-23-2010 by 12AX7]
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crazyboy
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Generally recrystallization is not used for the type of scenario you described unless they have very different solubility. Generally I would use
recrystallization when I have a solid crystalline product contaminated with an oil or other liquid or if the contaminant is highly soluble in the
solvent at a range of temperatures but the product has a steep solubility curve.
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psychokinetic
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Yer, in most cases I'd think you'd get both crystals.
“If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found
the object of his search.
I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him ninety per cent of his labor.”
-Tesla
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cnidocyte
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Quote: Originally posted by crazyboy  | | Generally recrystallization is not used for the type of scenario you described unless they have very different solubility. Generally I would use
recrystallization when I have a solid crystalline product contaminated with an oil or other liquid or if the contaminant is highly soluble in the
solvent at a range of temperatures but the product has a steep solubility curve. |
So how do you separate salts without the addition of another salt? Lets say you had a 1:1 mixture of NaOH and NaNO3 and needed to separate the 2 how
would you do it? Adding enough nitric acid you could get pure NaNO3 but you'd lose the NaOH. You could try melting one salt but in this case theres
only a 10 degree difference in melting point. You can probably come up with a long, complicated array of metathesis reactions that will result in the
separation of the 2 but this kinda thing is a pain in the ass, is there not a simpler way to do this kinda thing?
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ScienceSquirrel
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It all depends on the different solubilities, for example Lo Salt which is a mixture of potassium and sodium chloride is fairly easily fractionally
crystallised to obtain quite pure potassium chloride.
A hot saturated solution will preferentially deposit potassium chloride leaving the sodium chloride in solution.
Potassium chlorate has a very steep solubility curve so potassium chlorate will crystallise almost completely from any solution containing potassium
and chlorate ions leaving sodium chloride, potassium chloride, etc in solution.
See the solubility table here
http://en.wikipedia.org/wiki/Solubility_table
[Edited on 24-8-2010 by ScienceSquirrel]
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cnidocyte
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Ah right I get it. In my example the solubility curve of NaOH has a slope of 1.5 whereas NaNO3 curve has a slope of 0.8 so I'm guessing NaOH could be
crystallized out in this case.
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