Yttrium2
Perpetual Question Machine
Posts: 1104
Registered: 7-2-2015
Member Is Offline
|
|
Recrystallization technique
Just curious, but is the technique to recrystallize a compound to
1: dissolve compound in minimum amount of hot solvent
2: filter compound
3: slowly? Let the air evaporate the solution?
Or is it to dissolve the compound in a minimum amount of hot solution and then evaporate?
I've heard both methods being discussed when talking about recrystallizing.
|
|
|
Vosoryx
Hazard to Others
Posts: 282
Registered: 18-6-2017
Location: British Columbia, Canada
Member Is Offline
Mood: Serial Apple Enjoyer
|
|
Depends. The most common method that I use for recrystalization is to dissolve compound in hot solvent, filter, and then let it cool slowly.
Alternatively, i recrystallized dioxane by melting it and then placing in in a warm thermos.
The objective of recrystallization is to purify a compound. Crystals are brilliant at purity, and thus the best technique is to grow the largest
crystals possible, which is done by cooling slowly.
I've also used the method of dissolving, filtering, and then evaporating. There are many other methods too. Depends on what the objective, compound,
solvents, available equipment are.
"Open your mind son, before someone opens it for you." - Dr. Walter Bishop
|
|
|
Bert
Super Administrator
Posts: 2821
Registered: 12-3-2004
Member Is Offline
Mood: " I think we are all going to die. I think that love is an illusion. We are flawed, my darling".
|
|
Most of the ones I have done involve COOLING the hot solution of whatever is being re-crystallized, this may reduce the solubility sufficiently that
you can retrieve a large % without changing the volume of the solvent at all. Potassium nitrate has a steep solubility curve, this was a usual
purification step in the old days. It had the additional feature that a couple of the primary contaminants had high solubility even in cold water and
could be largely removed from the desired Potassium salt along with the filtrate.
Reducing the solvent through evaporation certainly works too, but takes longer or more equipment (vacuum chamber, pump, etc.
Also, you can tailor the crystal size somewhat by controlling how saturated the solution is, and how slowly or quickly it is cooled.
Similarly, a substance who's solubility in a particular solvent is drastically reduced by some other solvent freely miscible into the effective
solvent may be recrystalized by addition of the second solvent- As Potassium nitrate may be dropped from solution in water by the slow addition of an
alcohol, or PETN dissolved in acetone may be dropped by water.
Or a substance that requires a particular condition to be freely soluble, such as silver azide needing a fair concentration of ammonia to dissolve in
ammonia water may be dropped from solution by heating the water and driving the ammonia off.
Similarly, Mercury fulminate dissolves in strong ammonia water, and some have dropped it from solution by slowly neutralizing the ammonia with a drip
of glacial acetic acid.
Lots of ways to change solubilities and cause recrystalization from a solvent.
[Edited on 17-2-2018 by Bert]
|
|
|
WangleSpong5000
Hazard to Others
Posts: 129
Registered: 3-11-2017
Location: Oz
Member Is Offline
Mood: Curious
|
|
Cooling very slowly at a constant rate will ensure larger crystals... I do this procedure to as low a temp as is practical. For example I put the
solution in my bar freezer to cool as the main home freezer is far too cold.
Water should ideally be filtered or distilled if possible. Compound ahould also be as clean and pure as possible.
Look into dual solvent crystalisations too. I don't really know much about it tbh.
Hyperbole be thy name
|
|
|
aga
Forum Drunkard
Posts: 7030
Registered: 25-3-2014
Member Is Offline
|
|
The first step is to look up the solubility of the substance you're recrystallising.
If there is a write-up, easiest to follow that.
The filtering step is just to remove any solid junk, so if you do not have any, that can be skipped.
Things with very steep solubility curves will require that the filter is kept hot too, or your product will crystallise in the filter (e.g.
Na2SO4).
|
|
|
DavidJR
National Hazard
Posts: 908
Registered: 1-1-2018
Location: Scotland
Member Is Offline
Mood: Tired
|
|
Leaving it to evaporate is often very slow (of course depends on the solvent) so if you can it's best to just do it by temperature change. Also, if
you're going to put the solution in the freezer to precipitate as much out as possible, then it's also a good idea to remember to put your buchner
funnel in for a chill too.
For substances where the solubility doesn't change much between near-boiling solvent and near-freezing solvent then you might have to do it the slow
way (evaporation).
|
|
|
Reboot
Hazard to Others
Posts: 141
Registered: 8-8-2017
Member Is Offline
Mood: No Mood
|
|
Mixed solvent crystallizations can be an almost unbelievably handy technique.
Solubility table:
https://en.wikipedia.org/wiki/Solubility_table
For instance, let's say you want to recrystallize potassium nitrate. If you look up the solubility, you see that there's a huge difference in
solubility in water at 100 C (246 g/100 ml) vs. 0 C (13.3 g/100 ml.) So, you can just dissolve it in boiling water and then chill to crash out
crystals.
But, let's say you wanted to recrystallize potassium NITRITE instead of nitrate. Looking at the solubility in water, it's 410 g/100 ml at 100 C, but
still 279 g/100 ml at 0 C. We'll lose most of our product if we just try the classic 'dissolve and chill' method.
That's where mixed solvent crystallization can come to our rescue. Potassium nitrite is a very polar molecule, so what we're looking for is a
solvent that is miscible with water (any amount of it can mix freely with any amount of water) AND is relatively non-polar (has a significant amount
of hydrocarbon to its structure.)
An alcohol (methanol or ethanol) should work, but there are a ton of options. Potassium nitrite has a solubility of only around 0.3 g/100 ml in
ethanol, so let's try that.
As you add the ethanol to the saturated solution, the alcohol mixes into the water, making it on average less polar, and less able to dissolve the
highly polar nitrite salt. So, the solubility of the nitrite in the solution goes down and it starts to crystalize out.
Mixed solvent crystallizations can be used to crash out things that don't have a steep solubility curve (like nitrite in water) but if you know what
your contaminant is, they can also be tailored to give a cleaner crystallization by picking a second solvent that DOES dissolve your known
contaminant. For instance, you were purifying an organic salt, you might use a relatively non-polar second solvent to not only reduce the
solubility of the polar organic salt, but increase the solubility of relatively non-polar side products.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
