Author Topic: Improved recrystallization  (Read 33318 times)

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  • Guest
Improved recrystallization
« on: November 03, 2004, 07:46:00 AM »
SWIP wouldn't presume to teach anything to the real chemists in this forum, and maybe not to some of the more experienced bees either. But he ran into an improvement on the procedure he believes most bees use, and while the changes are very small, they made a huge difference. SWIP bets that you'll be pleasantly surprised at the results if you try it, but only if you carefully follow the procedure.

You will need:
  1-2 ehrlenmeyer flasks or ghetto equivalent
  2 small beakers, ~50 ml depending on the size of your sample.
  Denatured alcohol (the kind with methanol as denaturant works well)
  Eye dropper

Gently heat a small beaker of denatured alcohol. (Not on a flame, of course, and don't forget to aim a fan to blow the fumes away from any source of ignition.) Put the substance you want to recrystallize in a small ehrlenmeyer flask or ghetto equivalent, and add hot denat drop-by-drop until the sample disolves and cloudiness in the solution just clears.

If the solution has any suspended particles, add additional hot solvent to prevent crystals from forming, then hot-filter the solution through a moderately compacted Charmin plug. Don't use vacuum or crystals may form and get stuck in the filter. Rinse the original beaker 2-3 times with a little hot denat and pour it through the filter. Finally, pour some denat directly on the filter to minimize losses. Examine the solution, and if there is still any trace of suspended particles, repeat filtering but use a more tightly compacted Charmin plug.

Put the clear solution on heat and evap until solids start to appear. Blow air from a hair dryer across the surface if you want to speed things along. Then add hot denat drop-by-drop until all has redisolved and the cloudiness in the solution has just disappeared.

Poor a small amount of acetone into another small beaker. With an eyedropper, add the acetone drop-by-drop to the denat solution until it just turns cloudy. You'll have to look close because it's not really easy to see. Now add just enough hot denat--maybe a drop or two--to make the cloudiness disappear.

Let the sample cool, cover, then place in the coldest spot you can find in the freezer. Be prepared for a surprise when you return in about a half hour.

decant the mother liquor off of the crystals, then rinse the crystals with ice cold acetone. Don't throw away the remaining mother liquor, as it still contains a little of your sample. When you're ready you can evaporate/recrystallize this to recover a little more product.

Like SWIP said, this isn't a departure from the normal procedure, but there are some indicators that help to get repeateable results. SWIP has been dreaming about recrystallizing for several years, and thought he had become somewhat proficient. The first time he dreamt about trying these changes, though, he had to do it over three times before he got it right.

When he finally finished the process and thought the flask was cool enough to put in the freezer, he was disappointed to see no crystals beginning to form. Not even anything on the surface. He stuck it in the freezer anyway, thinking he might get something to fall out anyway. He checked a half hour later to see if it looked promising, and was astonished to see that the flask was crammed with large crystals! They filled the flask up to the line of the solvent, and many were at least 1/4 inch long and 1/8+ inch wide. After less than 1/2 hour! Imagine what it will look like after sitting overnight, or after a second recrystallization. One big diamond maybe?



  • Guest
« Reply #1 on: November 03, 2004, 09:42:00 AM »
All the freezer is doing is making the crystals form way to fast to purify. Thats why the majority will purify there product by slow recrystallization. And whats new about duel solvent recrystallizing that you are doing? Other than rushing the formation of the crystals..


  • Guest
Yeah i cant see how this is any different to...
« Reply #2 on: November 03, 2004, 01:03:00 PM »
Yeah i cant see how this is any different to the dual re-x procedures stated here million times over.
Can you point it out prepuce and how it makes it better than the procedures already stated, because all i can see is rushing of the crystal formations which gives a worse product.


  • Guest
Activated Charcoal
« Reply #3 on: November 04, 2004, 03:56:00 AM »
Some recrystallisation methods use Activated Charcoal for extra purification.


  • Guest
What's different
« Reply #4 on: November 04, 2004, 06:25:00 AM »
Jacked, what's new about this is definitely subtle, and I'm sure there are some to who there is nothing new about it. SWIP is assuming that the way he learned to do recrystallization, strictly from reading posts to the Hive, is similar to what others are doing. Namely, disolve the sample in a little solvent, heat until it gets to that "just right" point, then flood it with acetone. If any crystals fall right out, add just a bit more solvent. Nothing wrong with that procedure at all. SWIP has been using it for several years.

The only trouble he's had has been getting the solution to just the right stage every time. Even with all the practice he's had, he still misses from time to time, and some results are better than others.

What he likes about this procedure is that there is a signal  to say when it's had enough of the solvent, and another to say when it's had enough acetone. SWIP has been able to produce consistent results for the first time by following these steps. But in addition, you end up with less total liquid and SWIP thinks that's significant in the formation fo the crystals. While it's surely obvious to many, but the filtering technique as it's applied is very efficient and all but lossless.

SWIP has never before suceeded in growing such large crystals on the first recrystallization, even when left overnight. He couldn't believe it when they grew in about 1/2 hour, but he agrees with you--that's rushing things. If crystals like this form in a half hour, what will they look like after growing all night?

To summarize, the technique offered SWIP consistent and better results than he's ever previously acheived. Realistically, he never expected to teach you anything about growing crystals!



  • Guest
Been meaning to look into charcoal
« Reply #5 on: November 04, 2004, 06:29:00 AM »
WX, SWIP has been meaning to investigate the use of charcoal to clarify a solution before recrystallization. It is his understanding that charcoal is especially effective in removing an off-color. He's been worndering if carbon, such as that sold for acquarium filters, could be finely ground and used in place of charcoal. For the ghetto chemist it looks like it's easier to pick up good quality carbon than charcoal.



  • Guest
good, but take care
« Reply #6 on: November 04, 2004, 08:51:00 AM »
Some recrystallisation methods use Activated Charcoal for extra purification

Actived Charcoal (carbon) is readily available (aquarium shops, home brew barns..).  Get the best quality.

Word of caution.  Activated carbon can absorb some of the end product.  The main factors are time, ammount of ActC used (active sites), and end product/impurity ratios. 

Also keep in mind that the ActC will need to be filtered/removed.  The mechanical loss (esp is conc. solutions) can be a yield killer. 

Swib has used it to clarify fb-ephedrine steam distillate from fine organic matter.  Worked ok.  Haven't had a need to do it on the end product; so can't vouch for that.