RECRYSTALLIZATION of BENZOIC ACID
Prelab

DISCUSSION

    Recrystallization is a method of purifying a solid.  An impure solid contains, for this discussion, two types of impurities:  those more soluble in a given solvent than the main component, and those less soluble.  (If there are any impurities that have the same solubility as the main component, then a different solvent needs to be chosen.)
As a simple example, say you accidentally put salt into your sugar bowl.  The solid contains sugar and salt, both of which are very soluble in water.  However, salt is less soluble in alcohol than is sugar.  Mix your solid with alcohol - the sugar will dissolve but not the salt.  Filter the resulting suspension and the solid salt gets left behind on the filter paper.  The filtered solution (the filtrate) contains sugar.  In this case the easiest way to get back solid sugar would be to let the alcohol evaporate (using either heat or patience).

    Here are two other recrystallization scenarios:

1.  An impure polar solid is put into a polar solvent.  Some impurities do not dissolve; the solution is filtered to remove those impurities.  A less polar solvent is then slowly added 1 to the original solvent.  If all goes well, crystals of the desired compound will form, and impurities remain in solution.  [Slow addition of solvent ensures that the crystals are not too small - small crystals usually clog the filter, and small crystals also adsorb 2 impurities.]  The resulting crystals are then cooled and filtered, usually by the vacuum method, although gravity is used on very small samples sometimes..  The crystals are then usually washed with a small amount of cold non-dissolving liquid.  Then the crystals are allowed to dry 3.  [You can of course substitute the term nonpolar for polar in the preceding discussion.]

2.  An impure solid is placed in a solvent with a large solubility temperature gradient.  The solid does not dissolve in the liquid until the liquid is heated.  The solid dissolves in the hot solvent, and the mixture is gravity filtered.  Insoluble impurities stay behind on the "fluted" filter paper.  The solution is then allowed to cool slowly - for example, by placing it back onto the hotplate that has now been turned off.  As the temperature slowly drops, the solubility of the solid decreases, and crystals will form.  When room temperature is reached, cool the flask further in an ice bath. Then vacuum filter and wash with a few milliliters of cold solvent.  Allow the purified crystals to dry.

    Regardless of crystallization method, the purity of the solid can be verified by taking the melting point.  Be sure that the solid is dry before attempting this -  in 245 we usually wait until the following lab period.

    The actual laboratory we will do is the recrystallization of benzoic acid from water using method #2 above, using the temperature gradient method. (See below).  Benzoic acid is not very soluble in cold water, but it is soluble in hot water.

Footnotes:  1.  Slow can mean drop by drop with stirring.  The slowest mixing is accomplished by "vapor diffusion".  The solution and the other solvent are both placed in a closed container.  Over a day or two the solvents mix by vapor exchange.
2.  Spelled correctly!  Adsorption means that the impurities will stick to the surface of the crystals.  Paint is absorbed by a sponge, but adsorbed by a wall.
3.  In a research lab drying would be done using heat, vacuum, or a combination of both.  We just wait until the next lab meeting.



 
PROCEDURE
Precautions:  Hot benzoic acid has a high vapor pressure and the vapors are extremely irritating.  Hot solutions must be handled under the hood.  Heat-resistant gloves are available in the wooden cabinet where the heating mantles are.

Take about 0.8  - 1.0 gramsof impure benzoic acid.  This is a dirty gray-black color; not white crystals.  Dissolve this in 10 mL of hot (~95oC - not boiling) water.  Filter the hot solution by the gravity filtration method.  Use the fluted filter paper, stemless1 funnel, and small Erlenmeyer (seen on the right in the picture).  The receiving Erlenmeyer flask should be warmed on a hot plate (on low heat) to prevent rapid crystallization (on the right in this photo).  A few drops of water can be put in the receiver  to "steam up" the flask.  Pour the hot slurry2 onto the fluted filter.
There will be solid left on the fluted paper!  However, if you see a lot of white crystals on the paper, that would be undissolved benzoic acid.  You may add another 5 mL of very hot water to the paper.  After the gravity filtration is complete, turn off the hot plate, and let the flask cool for about five minutes.  Then put the flask on the benchtop.  Crystallization should begin as the flask cools.  Pretty, aren't they?  After the flask is cooled enough to handle comfortably, put the flask in an ice bath3.  Let the solution chill thoroughly before proceeding with the vacuum filtration.
Vacuum filtration uses a filter flask, a rubber adapter, a Buchner funnel, and flat fitted filter paper as shown on the left in this picture.  They're shown assembled on the left here.
Attach a vacuum hose to a convenient aspirator, and turn on the water.  Wet the filter paper with a little water (or other suitable solvent in other lab experiments).  Apply the vacuum, and then pour the chilled mixture into the Buchner funnel.  The water should filter quickly - if not, check for vacuum leaks.  Get all the crystals out of the flask using a spatula or stirring rod.  Rinsing with 1 or 2 mLs of cold water helps get the crystals out of the flask, and rinsing helps remove impurities.
Let the aspirator run for a few minutes to start air-drying the crystals.  Then use a spatula to lift the crystals and paper out of the Buchner funnel.
Carefully tranfer the crystals (and paper) to one of the envelopes provided.  TAPE, do no lick, the envelope to seal it.  Leave the envelope in your Chemkit cupboard to dry. The next time your laboratory meets, weigh your crystals, and obtain their melting point.
 
 

1 Stemless to prevent crystallization in the funnel.  We want the crystals in the flask, not in the stem.
2 Slurry: A heterogeneous solid/liquid mixture.
3 In this and subsequent labs, ice bath means both ice and water mixed together.