The Crystallization Procedure

 

Background

Recrystallization is a useful purification method for most organic compounds that are solids at room temperature.  The selection of a proper solvent is the most critical part of the recrystallization procedure.  Organic solids are usually more soluble in hot solvent than in a comparable volume of cold solvent.  The process of recrystallization involves dissolution of the solid in an appropriate solvent at elevated temperature and the subsequent reformation of the crystals upon cooling.  This way, many impurities will stay in solution and your target compound is purified.  However, it is not possible to recover your entire compound following a recrystallization.  To maximize your yield, it is very important to only use a minimum amount of hot solvent. 

 

Choosing the appropriate solvent is the most challenging part of the recrystallization.  In choosing a solvent, remember that “like dissolves like”, which means that a nonpolar compound will dissolve well in a nonpolar solvent, and a polar compound will dissolve well in a polar solvent.  Also note whether a solvent is flammable or not and find its boiling point.  This will enable you to select an appropriate heating device.  Generally speaking, solvents with boiling points below 100°C should be heated in a water bath (NO OPEN FLAME), above 100°C can be heated directly on a hot plate or you may use a sand bath.

 

Selecting a solvent is crucial for recrystallization of an organic solvent.  An ideal recrystallization solvent should:

 

A table of solvents and their dielectric constants can be found at http://wulfenite.fandm.edu/Data%20/Table_26.html

The best way to be certain that a solvent will be a good recrystallization solvent it to try and see it.  Test the solvent on a small amount of the compound.  If a solvent doesn’t dissolve the crystals when hot, or it readily dissolves the crystals at room temperature, try another solvent.  Remember to always use only a minimum amount of hot solvent!

 

Once you have selected your solvent you are ready to recrystallize your sample.  The process of recrystallization can be broken into the following steps:

  1. Selection of an appropriate solvent or mixture of solvents (consult table)
  2. Dissolution of the solute using a minimum amount of hot solvent
  3. Decoloration of the solution if necessary (with an activated form of carbon)
  4. Filter: Removal of suspended solids (through filtration of the hot solution)
  5. Crystallization of the solid from the solution as it cools
  6. Collecting the purified solid by filtration
  7. Washing the crystals with an appropriate solvent to remove impurities
  8. Drying the crystals

 

 

Your task will be to recrystallize benzoic acid from water and from a solvent pair of your choice. 

Benzoic acid is widely distributed in plants, such as anise seed, cranberries, prunes, cherry bark, cinnamon, and cloves.  In most plants, it acts as a natural preservative that inhibits the growth of bacteria, yeasts, and molds.  Its salt, sodium benzoate, is produced by the neutralization of benzoic acid with sodium bicarbonate, sodium carbonate, or sodium hydroxide. The salt is not found to occur naturally.  It is used a preservative in a variety of food products, such as jams, soft drinks, and cereals.  Benzoic acid is also an effective food preservative, but its sodium salt is more popular, partly because it is more soluble in water than benzoic acid. 

On a nastier note, benzoic acid is also detected in car exhaust gases, presumably as an oxidation product of toluene.  

 

 

Procedure

Single solvent recrystallization

(The solubility of benzoic acid in water is approximately 68.0 g per liter at 95 °C; and 1.7 g per liter at 0 °C.  Calculate the amount of hot water needed to dissolve your sample of benzoic acid.)

Always use a minimum amount of hot solvent to dissolve the crystals. 

 

Weigh out about 50 to 100 mg of benzoic acid.  Transfer the crude crystals into an Erlenmeyer flask containing a small amount of water.  Heat the mixture with a hot plate.  Add a boiling stick (a wooden applicator) to the flask to prevent bumping. 

Heat the mixture to reflux.  More solvent is added to the mixture in small portions using a Pasteur pipet until the solid just dissolves (saturated solution).  Don’t heat the solution too strongly or the solvent will boil away.  Record the total volume of water used. 

Remove the flask from the heat source and allow it to cool to room temperature undisturbed.  After about 10 minutes, further cool the flask in an ice bath.  As the solution cools, the solubility of the solid decreases and the solid crystallizes. 

Learn more about filtration at http://orgchem.colorado.edu/hndbksupport/filt/filtration.html

Once the flask is cool and crystals have precipitated out, collect by vacuum filtration.  If crystallization does not occur even after the solid is cold, scratch the inside of the flask with a glass rod or add a seed crystal.  If crystallization still does not occur, you probably added more solvent than was necessary.  Reduce the volume of the solution by heating it again just until the solution turns cloudy or crystals appear.  Add enough of your solvent to dissolve the crystals and let cool as you did before. 

Rinse the crystals with a small amount of a COLD 50/50 water/ethanol mix.  Allow the crystals to air dry on the filter, then weigh them, and take a melting point. 

Note:  The products of a chemical reaction should always be dried to constant mass, meaning its mass after drying should not change significantly between two successive weighings. 

 

If your sample of benzoic acid contains an insoluble material, you will need to filter the insoluble impurity after you have added enough of your solvent to allow the benzoic acid to completely dissolve.  In this procedure, care must be taken to prevent the loss of the desired component due to premature precipitation.  After filtration. proceed by reducing the volume of the filtrate until the solution turns cloudy.  Add just enough solvent (water in your case) to bring it back in solution, heat, then let cool and allow for crystals to form. 

 

Tips:

·         Adding too much solvent to dissolve the solute will prevent later recrystallization causing the loss of the product.

·         After the solute is dissolved, do not use vacuum filtration to remove the solid impurities because the air flow will cause recrystallization of the solution before the impurities are removed.

·         Never use boiling chips in crystallization because they are hard to separate from the crystals, resulting in an impure product.

Once crystals are drying on the filter paper and watch glass, do not heat the watch glass in an attempt to speed up the drying process because the crystals may melt.

 

 

Mixed solvent recrystallization

Sometimes it is difficult to find a solvent that will dissolve the compound when it is hot, but will precipitate it out when it is cold.  If no solvent can be found with the appropriate solubility parameters, sometimes it is possible to create an appropriate solvent by mixing two inappropriate solvents.  By using two solvents, or a solvent pair, it is possible to successfully complete the recrystallization.  The solvent pair should be chosen such that one solvent dissolves the solute at elevated temperature and the other one does not.  Refer to a table of dielectric constants for solvents to choose your appropriate solvent pair but remember that the two solvents you select must be miscible.  This means they mix in all portions, such as ethanol and water.  To ensure that the two solvents selected are miscible, put 1-mL portions of each solvent in a test tube and shake.  If two phases appear, the solvents are immiscible. 

 

Following is a flow chart for the recrystallization procedure: 

 

 

 

Sometimes it is necessary to induce crystallization.  This becomes necessary, if a solution does not crystallize upon continuous cooling.  Several techniques may be used to induce crystal formation.  One technique involves scratching the inside surface of the flask with a glass rod. The rod should not be fire-polished and is moved vertically in and out of the solution.  Enough force should be used so that an audible scratching is heard. Seeding is the second technique.  A small amount of the original crystal is saved, and then placed in the cooled solution.  This seed can promote crystal formation. 

 

 

Procedure (mixed-solvent)

Recrystallize another accurately weighed sample (about 50 – 100mg) of benzoic acid using a solvent pair of your choice.  Consult the table of solvents and their dielectric constants and/or practice before hand with a small amount of benzoic acid.

Start with the solvent in which the substance is soluble.  Always only use a minimum amount of solvent.  Continue to add the hot solvent in which the compound is more soluble dropwise.  Add just enough hot solvent to completely dissolve the solid, nor more.  Then add the second solvent, dropwise again, until the solution turns cloudy.  This is called the “cloud point”.  The cloudiness is due to the crystallization of the solid.  Add 1 or 2 more drops of the first solvent to just redissolve the crystals and cause the cloudiness to disappear.  Allow the solution to cool to room temperature, and then continue cooling in an ice bath.  Collect, wash, and dry the crystals as above.   Weigh the dried, purified crystals and obtain a melting point

 

 

 

Analysis

Compare the melting point for both benzoic acid recrystallization procedures and compare the efficiency of each.  To do so, calculate the percent recovery. 

Percent recovery  = weight of pure crystals recovered x 100

                                            weight of original sample

 

Compare the melting points of the impure to the melting points of the pure samples.  Comment on the two methods of recrystallization and explain your choice of solvents in the solvent pair recrystallization. 

Questions

1.  Briefly describe how insoluble impurities are removed during a recrystallization?

2.  Briefly describe how soluble impurities are removed during a recrystallization?

3.  Both benzoic acid and 2-naphthol melt around 122 ºC.  If you have an unknown sample of either of these two solids, describe how you could determine if the solid was benzoic acid or 2-naphthol. 

4.  What are the characteristics of an ideal recrystallization solvent?

5.  How much solvent should be used in recrystallization procedure?

 

 

 


For more information about recrystallization see:

 

Everything you want to know about recrystallization:  http://www.chem.arizona.edu/courses/chem243/recrystallization.html

 

Great site with step-by-step information about recrystallization as well as calculations:  http://ice.chem.umbc.edu/Chem351l/mp_notes.html

 

A little bit of theory about recrystallization:  http://www.chem.ualberta.ca/~orglabs/recrystallization/rexmaster.html

 

This website explain the importance of slow crystallization.  http://orgchem.colorado.edu/hndbksupport/cryst/crystdiag.html