http://courses.chem.psu.edu/chem36/HTML/Experiments/recrstal.pdf

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.

A table of solvents and their dielectric constants can be found at

http://wulfenite.fandm.edu/Data/Table_26.html

The technique of recrystallization involves 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. 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.. Benzoic acid is also detected in car exhaust gases, presumably as an oxidation product of toluene.

A. 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.).  Weigh out about 1.0 g of impure benzoic acid.  Use a few milligrams to determine the melting point.  Transfer the remaining solid (after weighing accurately on the milligram balance) to an Erlenmeyer flask containing a few milliliter of water.  Heat the mixture with a hot plate.  Add a boiling stick (a wooden applicator) to the test tube 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).  Stir until the solution dissolves.  Record the total volume of water used.  Cool the flask in an ice bath.  As the solution cools, the solubility of the solid decreases and the solid crystallizes.  Once the flask is cool and crystals have precipitated out, collect by vacuum filtration.

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

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.
 
 

B. 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.
Obtain a great flow chart taken from http://www.wsu.edu/~pkuzmic/chem240/lab/crystal.html

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.
 

Recrystallize another accurately weighed sample (about 1 gr)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 to form a solution.  Then add the second solvent, dropwise again, to induce crystallization.  Collect, wash, and dry the crystals as above.   Weigh the dried,  purified crystals and obtain a melting point
 

Compare the melting points of the impure benzoic acid to the melting points of the pure samples.  Comment on the two methods of recrystallizations.  Keep in mind that there are many situations where a solvent pair is the only available option.  Explain your choice of solvents in the solvent pair recrystallization.  Make suggestions for improvements to the experimental methods for recrystallization in our chemistry lab.

Hand in your combined purified samples of benzoic acid in a properly labeled vial.  (Name of the sample, combined mass, and your name.)

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.

For more information about recrystallization see:

Everything you want to know about recrystallization:

http://www.chem.ufl.edu/~barbaro/2211L/experiments.html
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
http://courses.chem.psu.edu/chem36/HTML/Experiments/recrstal.pdf

This website explain the importance of slow crystallization.

http://orgchem.colorado.edu/hndbksupport/cryst/crystdiag.html