Recrystallization Practice
Solubility Properties Needed for a Recrystallization Solvent
The Material we are trying to purify must be:
Soluble in the hot recrystallization solvent, and
Insoluble in the cold recrystallization solvent.
The Impurities must be:
Soluble in both hot and cold recrystallization solvent, or
Insoluble in both hot and cold recrystallization solvent.
Choosing a Recrystallization Solvent
1. In a medium test-tube, add a small amount of solid, and 1-2 mL of solvent.
2. If it dissolves in cold solvent, it won’t work, so try another solvent.
3. If it doesn’t dissolve in the cold solvent, heat the test-tube up. If the solid dissolves, cool it, and see if crystals form. If they do, it’s a good solvent. If not, try another solvent.
4. If not all of the solid dissolves in the hot solvent, add more solvent a little at a time and heat it to boiling. Do not fill the test-tube more than half-full. If not all of the material dissolves (other than insoluble impurities) in the hot solvent, the solvent won’t work.
General Recrystallization Procedure
1. In an Erlenmeyer flask, dissolve your sample in the minimum amount of hot solvent. You can either add your solid to cold solvent and heat to boiling, or boil the solvent first, then add it to the sample. Use the hot plate to carefully boil your solvent.
2. If not all of the sample dissolves, add 5 mL more solvent, and heat to boiling. Continue to do this until all of the sample has dissolved. Watch out for insoluble impurities!
Best Case: Clear, colorless solution
A. Remove the flask from the hot plate and allow it to cool to close to room temperature on the bench top.
B. Cool your flask in an ice-bath. (How long? Until your flask is cold!)
C. Collect the solid by suction filtration, and wash it with a little cold solvent.
D. Transfer the crystals to a watch glass, allow them to dry, weigh them and take a melting point.
Suction Filtration
Assembled Apparatus Filter Flask Buechner Funnel Neoprene Adapter
1. Clamp your filter flask to a support rod.
2. Place a neoprene adapter in the neck of the flask.
3. Place the Buechner funnel on the neoprene adapter.
4. Place a piece of 5.25 cm filter paper in the Buechner funnel, and wet it with a few mLs of your solvent.
5. Connect the filter flask to the water aspirator with your thick tubing, and turn on the aspirator to start the vacuum.
6. Pour the solution into the funnel. You may need to press down on the funnel to initiate the vacuum. Scrape the crystals out of the flask with your spatula. You may rinse the flask with a few mLs of cold solvent to get the last little bit out.
Insoluble Impurities Only
A. Add about 50% additional solvent, and heat to boiling.
B. Perform a hot gravity filtration on the hot plate. See below.
C. Do A-D under “Best Case”.
Colored Solution, With or Without Insoluble Impurities
A. Add about 50% additional solvent to the flask, and heat to boiling.
B. Remove from heat, and allow solution to just stop boiling.
C. Obtain about one-half spatula-full of decolorizing carbon.
D. Add the decolorizing carbon.
E. Heat the suspension to boiling for about 5 minutes.
F. Perform a hot gravity filtration on the hot plate. See below.
G. Do A-D under “Best Case”.
Performing a Hot Gravity Filtration
1. Fan-fold a piece of 11 cm filter paper, as demonstrated in pre-lab.
2. Choose a beaker that is at least twice the volume as the amount of recrystallization solvent you are using.
3. Add a few mLs of new recrystallization solvent to the beaker, and place it on the heat source.
4. Place the stemless funnel on the beaker, and allow the boiling solvent to heat the funnel for a couple minutes.
5. Place your fan-folded filter paper in the funnel. Pour the hot recrystallization solution, a little at a time, into the filter paper, being careful to not miss the filter paper. Keep the recrystallization solution hot.
6. After all of the recrystallization solution has been poured into the filter, place a few mLs of recrystallization solvent in the flask, heat it to boiling, and pour it into the filter.
7. Carefully remove the hot filtration assembly from the heat source, and allow it to cool to room temperature. Save the filter paper and the material in it. Allow it to dry, then weigh it and TLC it, along with your crops of crystals.
Beaker Stemless Funnel Fan-Folded Filter Paper Complete Assembly
Possible Recrystallization solvents for this exercise: water, 95% ethanol, toluene
Report Format
Title: “Recrystallization of Unknown #”
Your name
Course and Section Number
Amount you initially attempted to crystallize
Recrystallization solvent used
Amount recovered
Percent recovery. Show your calculation.
Melting Point Range
Why did you lose the amount you lost? Be specific!
Melting Point Determination
Melting point (although more accurately it should be called melting range) is a fundamental physical property of a substance. A pure substance should have a melting range of one degree of less. The melting range should also be close to the reported value in the chemical literature. Impure substances usually have wide melting ranges (over several degrees), and generally melt lower than the reported value.
There are several techniques and pieces of apparatus that can be used to take a melting point. We will be using the Meltempa apparatus. Although rather simple-looking, this apparatus costs over $300 to replace now, so handle it carefully!
The melting point of a substance is determined in the following manner.
1. Introduce a small amount (about 1-2 mm) of dry, finely powdered crystals into a capillary tube. Be sure that the crystals pack tightly. This can be accomplished by tapping the closed end of the capillary tube on the benchtop.
2. Place the capillary tube (closed end down) in the slot of the Meltemp?. Be careful not to break the tube in the apparatus. Notice that the Meltemp? can hold three tubes at once.
3. Heat the sample and record the melting range. This range starts when the sample just begins to liquefy, and ends when the sample has totally liquefied. You will have to look in the eyepiece and watch the sample. When the sample just begins to liquefy, you will have to look up at the thermometer and record the temperature. You then look back down at the sample and watch it until it has totally liquefied, then look back up at the thermometer, and record this temperature.
The rate of heating is critical for obtaining a good melting point. If you heat the apparatus too fast, you may miss the end of the melting range, because the sample will have completely melted before you can look back down at it. Therefore, you should set a rate of heating of no more than about 5° per minute while your sample is melting. This is done by adjusting the voltage appropriately.
For example, let’s assume you have a sample that melts at 150°. Looking at the table of heating curves, you can see that some voltage settings would not be appropriate. Voltages of less than 40 would never reach 150°. Voltages of 80 or higher would have the temperature increasing so rapidly, you would miss the melting point. A voltage in the range of 50-60 would have a reasonable rate of temperature increase, and would probably work well. Notice that at 50 volts, it would take approximately 12 minutes to reach 150°.
So what do you do if you don’t know what the melting point is supposed to be? One approach is to start with a reasonably low voltage (such as 40), then gradually increase the voltage to maintain a level of heating of about 5° per minute. The disadvantage of this approach is that it will probably take a long time if the melting point is >150°. A second approach would be to take a melting point at a high voltage, such as 90, first. This melting point would be inaccurate, but would give you a rough idea of what range you are looking for. You would then go back and take a second melting point, using a more appropriate voltage setting. The disadvantage of this approach would be having to use two samples, and the need to let the Meltemp? cool back down to about 20° below the melting point before you could use it again. Of course, if another Meltemp? is available, you could just use it.
Precautions:
1. The heating block of the Meltemp? gets very hot (surprise!). Therefore, do not touch it with your fingers or nose (yes, your nose can get close to it) when taking a melting point.
2. Check the temperature on the thermometer before you insert your capillary. If the temperature is higher than the melting point of your sample, it will melt immediately, and you will have to prepare another sample.
3. Don’t break off the capillary tubes in the Meltemp?. If you do, or if you find broken tube pieces in a Meltemp?, please notify your instructor so it can be cleaned out. Broken tubes reduce the efficiency of the apparatus.
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