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Author: Subject: Recrystallization with dual solvent system?
Fyndium
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[*] posted on 27-9-2020 at 01:37
Recrystallization with dual solvent system?


Some substances have very high solubility in water and meanwhile too low solubility into some other solvents like ethanol, acetone, etc. I recently recrystallized urea with ethanol, and carefully added water until it cleared up and recrystallized it with very good results.

So, could the solubility of various substances be adjusted by:

1) using direct mix of two solvents,
2) adding the lower solubility solvent and then the higher solubility until all dissolves,
3) adding higher solubility solvent to dissolve all and then add the lower solubility solvent just until it starts to precipitate?

Fourth method, not being an actual recrystallization is to add large amount of low solubility solvent to induce fast crashout.

EDIT For the record, this would apply to any compound and solvent mixture of polar and nonpolar type suitable, water+ethanol just being a practical example.

[Edited on 27-9-2020 by Fyndium]
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medchemist
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[*] posted on 27-9-2020 at 01:47


This is an established method, I believe it's even mentioned on the wikipedia page for recrystallization.

Of course using a single solvent is preferable as it's more replicable and less of a pain in the ass but there are substances that require mixed solvents if you wish to recrystallize them which is why chromatography is more preferable in 99% of cases in the lab and you can get far better yields with chromatography (even if running a column is more technically demanding)

On large scale pharmaceutical synthesis, recrystallization is preferred however, this is because scaling up chromatography will mean using tons (literal tons) of silica to purify kilograms of product, so recrysts are preferred here.
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monolithic
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[*] posted on 27-9-2020 at 05:12


MIT OCW has a short guide. https://ocw.mit.edu/courses/chemistry/5-301-chemistry-labora...

I've done this a few times on compounds where recrystallizing from a single solvent system was difficult because solubility was too great. Using a dual solvent system allowed for a greater combined volume of solvent and a more usable solubility curve, so post-recrystallization purity was comparatively better.
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cyriac1999
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[*] posted on 5-10-2020 at 23:23


Well Medchemist, I'm absolutely disagree with your affirmation that the single solvent method is preferable. Recristallization is almost best done with a binary solvent in an appropriate ratio (totally replicable if you fix the ratio which is found in litterature or by the experiment). Some solvents combination work for many products and sometime of course you have to find a good one.
Two major advantages : often far better purity after few rinsing with better yield and (this is related to the better purity) but : with two solvents, you increadibly increase the chance to keep in solution all impurities.

Sure it's less cheap than using a single solvent so when a single method works very well --> no reason to change but when no really satisfactory single solvent method exist, before to consider chromatography (which is a method that I use really often in my work), try with binary solvent.
Chromatography is often (in term of purity) the best methode (if you choose in this case too appropriate single, binary or even ternary or quaternary solvent and suitable stationary phase) but it's far more expensive, far more difficult to implement AND it's not applicable in amateur lab for more than grams scale samples.
So for amateur, recrist is very often the best choice (a good ressource to find purification protocols is "Purification of laboratory chemicals" by Wilfred L.F Armarego).
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