General Recrystallization Procedure

General Recrystallization Procedure

Recrystallization of an Impure Unknown

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 andcold recrystallization solvent, or

Insoluble in both hotandcold recrystallization solvent.

Safety Note: If you need to add additional flammable solvent to a flask or beaker on the hot plate, remove the flask or beaker from the hot plate first, add the solvent, then put the flask or beaker back onto the hot plate. This will minimize fire danger.

Choosing a Recrystallization Solvent: Try water, 95% ethanol, toluene and acetone.

  1. Place about 50 mL of water in a 150 mL beaker, and heat it to boiling on the hot plate.
  2. In a medium test-tube, add a small amount (size of a pencil eraser) of solid, and 1-2 mL of solvent.
  3. If it dissolves in cold solvent, it won’t work, so try another solvent.
  4. 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.
  5. 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. Weigh out between 1-2 grams of your unknown. Record exactly what you use.
  2. Place your solid in a 125 mL Erlenmeyer flask. Add 10 mL of your solvent. Heat the solvent to boiling on the hot plate.
  3. 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.Allow the flask 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, then weigh them and take a melting point.

Insoluble Impurities Only

A.Add about 50% additional solvent, and heat to boiling.

B.Perform a hot gravity filtration on the hot plate.

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.

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 five mLs of new recrystallization solvent to the beaker, place the stemless funnel on the beaker, and heat the solvent to boiling.
  4. 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.
  5. 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.
  6. Carefully remove the hot filtration assembly from the hot plate, and allow it to cool to room temperature. Do A-D under “Best Case”.
  7. Save the filter paper and the material in it. You will TLC it, along with your crops of crystals. Allow the rest of it to dry, then weigh it.

Beaker / Stemless Funnel / Fan-Folded Filter Paper / Complete Assembly

Thin-Layer Chromatography (TLC) of

Unknown Recrystallization Products

In this experiment, you will be assessing the purity of your crops of recrystallized unknown. Your unknown is biphenyl, dibenzalacetone, or benzoic acid. Your unknown has various impurities added to it. Since the unknowns all show up under UV light, we can easily detect them with TLC.

Biphenyl Benzoic acidDibenzalacetone


Obtain two spotting capillaries and a TLC plate, and draw a light pencil line across the width of the plate, about 1 cm from the edge. Place 5 light “tic marks” on the line, approximately equally spaced.

In separate test-tubes, dissolve tiny amounts of the impurities from the hot filtration (note: all of the impurities may not dissolve), and your recrystallized product in about 1 mL of acetone. Using one of the spotting capillaries you made, spot a tiny drop of the acetone solution of your impurities on the second from the left “tic-mark” - you may want to practice spotting on a paper towel or tissue, to make small spots. Allow it to dry, and then check the plate under a short-wave UV light, to see if you have a visible spot. If not, spot your solution again on the same “tic-mark”. Do not spot any more times than needed - too much material may give results that are hard to interpret! Using a different capillary, spot the solution of your unknown on the leftmost “tic-mark”. After you have your samples spotted, take your plate to the standards in the hood, and spot benzoic acid on the middle “tic-mark”, dibenzalacetone on the second to rightmost “tic-mark”, and biphenyl on the rightmost “tic-mark”. Do not get the spotting capillaries for the standards contaminated! Check the spots under the UV light, to see if they are all visible.

To prepare a developing chamber, obtain a piece of 11 cm filter paper, and fold it about 1 inch from the edge, to obtain a flat edge. Place the filter paper, flat edge down, in your 400 mL beaker. Obtain 10 mL of the TLC solvent, and pour it into the beaker, swirling the beaker to wet the filter paper with the solvent.

Carefully place the TLC plate in the developing chamber, spotted side down, trying not to splash the solvent on the plate. The level of solvent must be below the pencil line. Cover the beaker with a watch glass. The solvent will rise up through the stationary phase on the plate. When the solvent has risen to 1-2 cm from the top of the plate, remove the plate, and draw a light pencil line across the plate, at the level to which the solvent rose. Allow the solvent to evaporate (waving the plate in the air will speed this up), and then look at the plate under the UV light. Circle all of the spots.

You will identify the components of your unknown tablet by comparing the amounts the components traveled up the plate with the amounts the standards traveled. These amounts are reported as Rf (retention factor) values.

XDistance the spot traveled

Rf = --- = ------

YDistance the solvent traveled

Measure the distance a spot moves from the center of the spot.

Cleaning Up

When you are finished with the experiment, pour the TLC solvent in the “Recovered TLC Solvent” container. The filter paper may be thrown away in the trash can. Used spotting capillaries should be placed in the “Clean Broken Glass” container. Your acetone solutions you used to spot with may be flushed down the sink with lots of water.

Lab Report Format Purification of an Unknown Compound by Recrystallization (Minilab 3), Identification of the Unknown Compound by Thin Layer Chromatography and Melting Point

  1. Title Page
  1. A descriptive title with between 15-25 words.
  2. Dates the experiment was performed.
  3. Course and section numbers.
  4. Your name.
  5. Tape your TLC plate to the bottom of the page. Label each column of spots so I know what is what.

2. Body of the report for Recrystallization of an Unknown (start a new page)

  1. Unknown compound number.
  2. Results of solubility tests in the different solvents. Summarize the results in a neat table.
  3. Best recrystalization solvent found - explain.
  4. Weight of compound used for recrystallization.
  5. Melting point range of unknown before recrystallization.
  6. Weight of recrystallized unknown.
  7. Melting point range of recrystallized unknown compound.
  8. Calculate the percent recovery of the unknown. Show your work.
  9. Calculate the Rf values of each of the spots on your TLC plate. Show your work.

3. Questions

  1. What is the identity of your unknown solid? Explicitly explain your reasoning, using the data you acquired.
  2. After you recrystallized your unknown, was it pure? Explain. What impurities were likely present? What evidence do you have for them?
  3. How well did you perform your hot gravity filtration? Explain your reasoning.