RogueRose
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Purifying chemicals - filtering & crystalization - repeat
I have read a number of posts and articles about purifying chemicals and they usually say to dissolve, filter, evaporate, repeat. I've read some
people say that this can be done 1-5 times, the more, the more pure the product.
What I find confusing is what the additional dissolving, filtering and evaporation steps achieve - unless a different filter or filtering process is
used. I can also see how a final crystallization by very slow evaporation will give higher quality crystals vs boiling off the liquid giving more of
a sludge/paste consistency of final product.
So my question is, if the initial filtration is done with a high quality filter and evaporated slowly, how can repeated processes produce higher
quality product?
Can anyone explain how this effects the results and how to get the best results?
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j_sum1
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Very simply, a forming crystal will have higher purity than the solution it comes from. Therefore if you allow something to crystallise and drain off
the last of the liquid, most of the impurities remain in the discarded liquid.
Each time you repeat this process you eliminate more impurities. Several repetitions can give very high purity.
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brubei
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also, during the crystallisation process, impurities are commonly trapped in small quantity into the crystal structure. That's why it had to be
repeated.
The nature of the solvent, crystal, impurities, temperature and evaporation ratio are critical to achieve a perfect crystallisation.
Dissolving solid to give crystals is called recrystallisation
[Edited on 24-3-2016 by brubei]
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PHILOU Zrealone
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During the crystallisation process molecules gathers with molecules of the same type depending on saturation of the given molecule into the solvent.
As a general rule, they tend to exclude impurities or different molecules from the crystal lattice.
There exists few exceptions like in cocrystallisations but then the amount of impurity is very high to such an extend that you can't say wich of the
two compound is the impurity (imagine a 30/70 or a 50/50 mix).
So if you have a pure solvent (thus not containing minor ingredients that could polluate your crystals while evapo-concentrating phase to
saturation--> (mono- bi- or more- distillated solvent) and if your solvent is unreactive with the substances you wish to crystallisate; then
crystallisation will be a convenient way to purify.
In the event of a low reactivity between solvant and crystallisate, then each time you use the solvent for a crystallisation step, you would produce a
tiny quantity of a new compound polluting the initial one.
The exclusion of impurities from crystals comes also from colligative properties of matter (like the lowering of the melting point (solidification
point)) then solidification of the pure compound happens at a higher temperature than the solidification of the mix containing impurities...This
effect is used in a different crystallisation purification technique called "by migration of zone". This is used to get ultrapure silicon crystals by
annealling and moving the melting zone along the crystal from the bottom to upside (if the solidus is denser than the liquidus- or the other way
arround if the solidus floats on the liquidus).
The melting zone passes for example (ds>dl) from the lower layer to a higher one very slowly; then the lower part start to crystallise and exclude
its impurities in the melting zone during the process the melting zone enrich itself in impurities; once at the top of the reactor, you start again
from the bottom that is now purer (major part of the impurities are frozen in the solidified top layer)...you can do the process as much as wanted and
at the end the unpure top layer is cut off.
[Edited on 25-3-2016 by PHILOU Zrealone]
PH Z (PHILOU Zrealone)
"Physic is all what never works; Chemistry is all what stinks and explodes!"-"Life that deadly disease, sexually transmitted."(W.Allen)
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Magpie
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Quote: Originally posted by RogueRose  | I have read a number of posts and articles about purifying chemicals and they usually say to dissolve, filter, evaporate, repeat. I've read some
people say that this can be done 1-5 times, the more, the more pure the product.
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This might make more sense if stated this way:
1. crystallize (by evaporation or cooling)
2. filter (discard filtrate)
3. dissolve crystals
4. repeat
The single most important condition for a successful synthesis is good mixing - Nicodem
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