Recrystallization of an Unknown Solid

Background


Crystalline compounds are generally purified via recrystallization. In a recrystallization, we dissolve the impure solid in a minimum amount of hot solvent and then allow crystals to form slowly. It is important to choose a solvent that will not dissolve the substance at room temperature so that the pure crystals may be recovered. If too much solvent is used, the recovery of sample will be decreased. If the solvent is not hot when the dissolution is carried out, too much solvent will be used, leading to diminished yield.

This purification method takes advantage of the differences in solubility between the compound and its impurities. Most of the impurities will remain dissolved in the cool solvent, allowing them to be removed when the sample is isolated by vacuum filtration. Some of the impurities may not dissolve even in hot solvent, requiring a hot filtration to remove them.

Occasionally the crystals may not form as the solution cools. A few steps can be taken to induce crystallization in these cases. These include: (a) using a seed crystal, (b) scratching the inside of the flask, (c) cooling the mixture in an ice bath, and (d) removing some of the excess solvent by boiling, then allowing the mixture to cool again. Seed crystals and flask scratching both induce crystallization by providing a surface on which the crystals can begin forming. It is important to always save some impure sample to use as seed crystals.

Once the crystals have formed, the sample should be cooled in an ice bath to maximize the recovery. The crystals are then isolated using vacuum filtration. In organic lab, these vacuum filtrations are typically carried out using a Hirsch funnel and a filter flask, as shown below. You should always use a clean filter flask (instead of using one left behind by a classmate), in case you need to re-filter the sample or take a second crop of crystals. Finally, you should wash your crystals with a small amount of cold solvent to wash away any impurities. Before you obtain a melting point or weight of your sample, be sure that the sample has dried adequately. Remaining solvent will give erroneously high weights, and it is an impurity that will influence the physical properties of your product.

Melting points

You should remember the discussion of colligative properties from CHE 132. (If not, reread that chapter in your general chemistry textbook.) Colligative properties, like boiling point elevation and melting point depression, depend on the number of solute particles in a solvent. Using salt to melt ice on the roads is an example of a colligative property. Just as salt lowers the melting point of the ice, impurities lower the melting points of organic compounds. Thus, the purity of a compound may be qualitatively assessed by taking its melting point. In addition to lowering the melting point, impurities also widen the melting point range. You should always report the entire melting range of a sample for that reason. Likewise, it is good practice to compare your experimentally determined melting point with the literature melting point of the pure compound.

Procedure

Heat a beaker of water to a gentle boil on a hot plate to use as a heating medium. You will first need to determine the solubility of your unknown in several common solvents (water, ethanol, petroleum ether, dichloromethane, and ethyl acetate). Place about 20 mg (a small spatula-tip full) of finely crushed unknown in each of several test tubes and add about 0.5 mL of each solvent to the different tubes containing the solid. Stir each mixture and determine whether the solid is soluble in each solvent at room temperature. If the unknown is not soluble in a given solvent, place the test tube in the hot water bath. Stir or swirl the tube, observing whether the solid is soluble in hot solvent. Allow the hot solutions to cool slowly to room temperature. If crystals form in the cooled mixtures, compare their quantity, size, color, and form with the original solid material. It may be helpful to construct a table containing the solubility data, from which you should be able to decide the solvent that appears best suited for recrystallization.

Once you have determined which solvent will be most effective for recrystallizing your unknown sample, dissolve your sample in a minimum amount of hot solvent. Save a spatulaful of your impure sample for seed crystals and to compare its melting point to your recrystallized sample. Using an Erlenmeyer flask, add the hot solvent to the solid. Do NOT add the solid to the hot solvent. Hot filter the sample if necessary. Allow the solution to cool slowly to allow crystals to form. Once the sample has cooled to room temperature, place it in an ice bath to complete the crystallization. If necessary, you may induce crystallization by scratching the inside of the flask with a stirring rod (rough end), cooling your sample in an ice bath, or adding a seed crystal. If your sample oils out, heat it up again and add more solvent before allowing it to cool again. Collect the crystals by vacuum filtration using a Hirsch funnel. Wash the crystals with a small portion of cold solvent. Allow the crystals to air-dry overnight before determining the weight and melting point of your purified sample. For your lab report, calculate the percent recovery of the pure crystals that you obtain. Comment on the purity of your crystals and on the percent recovery.