[fordfiestamt]

Hi to all,

          Really interesting website full of usefull information. I am doing an experiment at school recrystallization of ASA (acetylsalicyclic acid, Aspirin), and would like to know of any possible sites giving any SOP's (standard operating procedures) about this experment.

Thank you in advance and hello to all.
Greetings from Malta

[pantone159]

Try this:

Dissolve the ASA in as little hot ethanol as possible.  (If you are starting from consumer aspirin tablets, there will probably be insoluble binders/etc that will not dissolve.  If so, filter them out.)  Add a little hot water until the solution becomes a little cloudy, then add a bit more hot alcohol until it is clear again.  Let cool to room temp, then cool more in the freezer.  You should then have plenty of nice crystals.

[Jared]

Hi from Canada, I bet the weather is better in Malta

Part 1: Synthesis of Crude ASA

Prepare a water bath by half-filling a 250mL beaker with water and heating it on a hotplate to near boiling (it will be steaming visibly).  Add a few boiling chips to prevent "bumping" if it begins to boil.

Weigh accurately about 1.5g of salicylic acid, using the analytical balance, and record the weight on your data sheet.  Place the salicylic acid in a 50 mL Erlenmyer flask.  Add to this, 5mL of acetic acid anhydride using a graduated cylinder - keep the container of acetic anhydride tightly closed when not in use or the moisture in the air will hydrolyze and decompose it.  Record the exact volume of acetic anhydride used.  Add about 5 drops of concentrated sulphuric acid to the flask, and mix.

Using the test tube holder as a convenient handle, carefully place the small Erlenmeyer flask in the hot water bath for 10 minutes.  Remove the flask from the bath and slowly add about 2 Ml of distilled water using an eyedropper.  Wait about 2 minutes, and during this time replace the hot water in the beaker with cold water-and-ice slurry.

After the 2 minutes, add another 10 mL of distilled water to the flask, and place it in the ice bath.  As the solution cools, a precipitate - crude aspirin - should begin to form.

Part 2: Recrystallization of ASA

Fold the filter paper and prepare to filter the precipitate from the solution.  After the liquid has drained through the filter paper, wash the precipitate with two 10mL portions of cold, distilled water.  After the rinse water has drained, carefully scrape the solid from the filter paper into a clean, dry flask or beaker using the scoopula.

Prepare another hot water bath in the 250mL beaker.  Using the graduated cylinder, add 5mL of ethanol to the solid ASA in the flask, and swirl to dissolve.  If it doesn't all dissolve, place the flask in a hot water bath until it is all dissolved.  However, do not let the ethanol boil: remove the flask immediately if it does.

After all the solid has dissolved, add about 15 mL of distilled water, mix, and place the flask in an ice bath.  Immediately, set the hotplate to low heat for later use in drying the product.  After 10 minutes, most of the ASA should have precipitated. [If no precipitate has started to form after 10 minutes, gently scratch with a glass rod the inside surface of the flask, below the surface of the solution.  This should promote precipitation by providing a rough surface on which the crystals can start forming]

Prepare another filtration set-up (with a new sheet of filter paper) and filter the recrystallized ASA.  After all the solution has drained through the filter, inspect the filtrate to see if any addition precipitate has formed. [ If so, filter the solution through the same filter paper.] Wash the precipitate with two 10mL portions of cold, distilled water.  When the liquid has finished draining, remove the filter paper, and place directly on top of the hot plate set at low heat.  Care must be taken to avoid overheating, which can lead to charring of the aspirin, or redissolve it into the filter paper, from the fibres of which it then cannot be recovered.

When the purified product is dry, weigh the filter paper and the ASA, and record the weight.  Empty all the aspirin from the paper and re-weigh the filter paper without ASA.  Record this weight.  Calculate the weight of aspirin obtained in this synthesis.


And a question for anybody out there, I'm not quite sure what they want but...In general terms, describe the properties required of the solvent chosen for a recrystallization?

[mike]

Generally recrystalisation solvents are chosen so that the desired product is soluble at raised temperatures and insoluble at low temperatures. This allows you to do hot filtration at raised temperatures and then recover the solid at lower temperatures.

[Jared]

Thanks, just what I needed


C00kie for youuuuu

[pantone159]

In the case of a mixed-solvent recrystallization (e.g. the EtOH/H2O mix described here), you use two miscible solvents, where one dissolves the solute easily, and the other hardly at all.  (ASA is soluble in EtOH, only very slightly in H2O).  For some reason, which I don't really understand, this mixture acts like the good solvent when hot (e.g. as EtOH, so the solute dissolves very well), and the poor solvent when cold (e.g. H2O, so the insoluble solute precipitates).

The solvents have to be miscible.  It is possible that the solute will cause the previously miscible solvents to separate, which will mess this up.  E.g. if you tried a mixed H2O / EtOH solvent for NaCl, you'd likely find the NaCl making the solvents separate into layers, and this scheme wouldn't work.

[fordfiestamt]

Well thanks to all of you.
The weather at the moment is fair not so much of rain.

I managed to the experiment quite well thanks to all of the info all of you gave me. To tell you the truth we were also filmed on national tv (to see how we scientists are) and most important was the fact that me and my colleague really showed an impressive experiment.

Thanks to all