OCR exam board Tips for recrystallization of Aspirin for chemistry PAG?

I have done this a few times now, and at the stage I cool the mixture I get no crystals to filer.

What I did:

Crushed an aspirin tablet and place din conical flask - slowly added hot ethanol until the solid dissolved, leaving me with a cloudy liquid (the PAG states to add enough to 'just' dissolve the 'impure' aspirin).

I then filtered this under pressure, gently washing with more hot ethanol.

This was then transferred to an ice cold water bath, and I stirred with a glass rod to apparently encourage crystal formation....

Found this method which is different form the PAG but seems to get good results:

 
We always get it to work by using a hot solvent mixture of ethanol: water in a ratio of 1:3. Students add minimal amounts until all the crude aspirin is dissolved. We then cover the beaker with parafilm and slowly let it cool to room temp.
 
We don't use an aspirin tablet, they recrystallise their product from part 2.
Dissolve in minimum amount of hot solvent - keep both the solvent container and dissolving sample on the hot plate while you do this.
Remove from heat and allow crystals to form. I was taught to do this at room temp for larger, easier to handle crystals. Only use ice if nothing is happening.
Filter under reduced pressure and wash with ice cold ethanol.

The extra filtration step (not really needed if you don't use a tablet, as there shouldn't be any insoluble impurities) is probably where you're going wrong - rinsing the filter paper with much more hot ethanol means you have now added too much solvent and you won't get any crystals out. It's better to lose a tiny amount of product in the filter paper than ruin the whole recrystallisation by adding too much solvent.
 
Thank you both.

The extra filtration stage was on the OCR instructions - I used a tablet in this instance as this was requested by a teacher.
 
We always get it to work by using a hot solvent mixture of ethanol: water in a ratio of 1:3. Students add minimal amounts until all the crude aspirin is dissolved. We then cover the beaker with parafilm and slowly let it cool to room temp.
So you don't do the filtering stage with hot ethanol? At some point someone should review these PAG's methodologies
 
So you don't do the filtering stage with hot ethanol? At some point someone should review these PAG's methodologies
A proper recrystallisation would include the hot filtration to remove insoluble impurities, and it's necessary for some syntheses where you have a load of crud floating around; but taking into account the students' skills and the particular method we skip it in favour of them actually getting a product :laughing:
 
So you don't do the filtering stage with hot ethanol? At some point someone should review these PAG's methodologies

Sorry I was just commenting on the recrystallization step for the crude product. I do Edexcel, the procedure does not involve a hot filtration.
 
Sorry I was just commenting on the recrystallization step for the crude product. I do Edexcel, the procedure does not involve a hot filtration.
If it works I don't care what any procedure says! I will give this a try and report back :)
 

karen b

COMMITTEE
Thank you both.

The extra filtration stage was on the OCR instructions - I used a tablet in this instance as this was requested by a teacher.

But was the intention to filter a crude product made by the student? Then you need this step, but if you are using a pure aspirin tablet you don’t need to filter it.
 
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