A recrystallization involves purification of a solid mixture of compounds by dissolving the solid sample in an appropriate solvent and then allowing crystallization of the sample. The solvent must be hot (boiling) for the sample to initially dissolve. The sample must be cold (immersed in ice-water bath) to get the best yield of pure crystals.
Once you have dissolved your desired material, check to see if there are any undissolved impurities. If you, you will need to perform a hot filtration. If everything dissolves (with sufficient solvent when boiling), then the hot filtration may be omitted and the recrystallization is greatly simplified. The steps outlined below include the hot filtration.
1. Place sample and a small stirring bar to a 50 mL Erlenmeyer flask. video (10 sec.)
2. Add a small amount of solvent. The exact amount is not critical since you will adjust the volume later as needed. Avoid using a large amount. Too little will be okay since you will be using more later. video (10 sec)
3. Heat the mixture to boiling on a hot plate. A clamp will keep it from spilling. video (10 sec)
4. Prepare fluted filter paper in a glass funnel. Secure the funnel in a ring over a 125 mL Erlenmeyer flask. video (15 sec)
5. In a beaker, bring some additional solvent to a boil to be used later to rinse the 50 mL Erlenmeyer and to wash the sample through the filter paper. video (5 sec)
6. Once your sample is boiling, pour it through the filter paper. Make sure it does not cool down very much. You might use a paper towel (see video) or other device to hold the flask to avoid burning yourself. video (15 sec)
7. Rinse the original flask with boiling solvent to filter all of your sample. You will also need to pour boiling solvent through the filter paper as much as necessary (in combination with the trick in step 8) to rinse all of the crystals through the filter paper. video (15 sec)
8. One trick to rinse all of your crystals through without using too much solvent is to place the 125 mL Erlenmeyer on the hot plate once you have collected solvent through the filter. The vapors that boil off will re-condense and help rinse your crystals through. Take the funnel off the ring and place in on the top of the Erlenmeyer. Take care that you do not boil to dryness. video (15 sec)
9. Once the hot filtration is complete and all crystals have been rinsed through the filter paper, you need to adjust to the correct volume. The solution must be saturated while boiling. You will recognize this by looking for either cloudiness or solid particles in the sample while it is boiling. Do not evaporate to dryness. video (15 sec)
10. Once you have achieved the proper volume, allow the solution to cool to room temperature (the slower the better). video (10 sec)
11. Make a bath of 50% ice and 50% water. video (5 sec)
12. Once the solution has reached room temperature, cool in the ice-water bath to 0 degrees C. video (10 sec)
13. Start cooling some additional solvent in the ice-water bath to be used later for rinsing. video (5 sec)
14. Prepare for vacuum filtration. Clamp a side-arm flask and attach the hose to the vacuum line. Place one neoprene adapter on the Buchner funnel and put it on the flask. Wet the paper with a small amount of solvent. video (15 sec)
15. Turn on vacuum. video (5 sec)
16. Collect crystals by pouring the ice-cold mixture through the Buchner funnel. video (10 sec)
17. Using the cold solvent, and a spatula if necessary, rinse out remaining crystals into the funnel. Keep the vacuum on for a while to dry the crystals. video (10 sec)