Recrystallization: Purification
of
Solids
Introduction * Procedure
(Adapted from Mount Holyoke College's Organic Experiments: Profs. Hamilton & Li, 2007)
Introduction:
(Read Lab Text/Guide pp. 107-111; pp. 40-43)Purification of a solid by recrystallization from a solvent relies on the fact that different substances are soluble to differing extents in a given solvent or solvent mixture. In the simplest case all of the impurities present in a solid sample will be so much more insoluble in the chosen solvent that all that remains in solution is the pure dissolved product (the solute). It has often been said that recrystallization is an art: the master crystallizer knows just when to add a little more solvent, just when to stop heating, and seems to be able to conjure beautiful crystalline samples from nasty materials. This is all true. However, most of us are able to achieve good results by following some simple guidelines.
The process of recrystallization can be broken down into seven discrete steps:
1. Identify a good candidate solvent;
2. Dissolve the sample in the minimum volume of hot solvent;
3. If necessary, filter the hot saturated solution under gravity keeping the filter funnel hot;
4. Cool the filtrate, first in air and then in an ice–water bath;
5. Filter the cold solution under vacuum, if necessary dislodge and transfer remaining crystalline material with a little more cold solvent;
6. Wash the crystals with a small volume of cold solvent;
7. Dry the crystals.
1. Choosing the solvent
The choice of solvent is crucial, but how do we decide which solvent will work best? An essential characteristic of a successful solvent is that the compound be soluble in the hot solvent but insoluble when the solvent is cold. And, the impurities either are insoluble in the solvent at all temperature or remain at least moderately soluble in the cold solvent. Tests can be performed with small amounts of material in test tubes: a few drops of a solvent are added and if the material proves insoluble then the tube is heated to see if the material will dissolve at a higher temperature—if so, then a good solvent for recrystallization of that material may have been identified. If the material appears insoluble at high temperature a little more solvent can be added, drop by drop, to ascertain whether dissolution of the solute will occur in a larger volume of solvent. A solvent should be rejected if the material appears readily soluble in the cold solvent, is not soluble to any appreciable extent in the hot solvent even when the volume of solvent is increased, or requires an impractically large volume in order to fully dissolve the crystals. An appropriate choice of solvent for the crystallization process will pretty much ensure success, all that follows is manipulation.Many single or mixed organic solvents may be used for recrystallizing organic solids. The choice of the solvent system depends on the relative solubility of the solid in the solvent at high and low temperature. Thus, when an ample supply of material is available, the solvent choice is made by performing several small-scale trial recrystallizations. For example: place a small amount of the material in a test tube and add a few drops of 95% ethanol (just enough to cover the crystals). Very little of the material should dissolve. Heat the mixture to boiling using a boiling water bath or preheated sand bath. Do not evaporate all of the solvent, but keep it boiling gently and watch carefully to see if all the material dissolves. Impurities that are inert and insoluble will never dissolve; try to distinguish these impurities visually from the material itself. If undissolved material remains in the boiling mixture after 30 to 60 seconds, add more 95% ethanol dropwise while keeping the mixture boiling. When all the material has dissolved, stop adding solvent. Cool the mixture first with a water bath and then with an ice bath. If necessary, scratch the inside of the cooled tube to induce crystallization. Note whether most, little, or none of the starting material has crystallized. Estimate the volume ratio of ethanol used relative to the amount of starting material (1:1; 2:1; 10:1?). The full sample is then used with the appropriate ratio of solvent.
Example of a two solvent system: Place a small amount of the material in a test tube and add just enough solvent dropwise to dissolve the solid at room temperature. Heat the solution to boiling with a steam bath or preheated sand bath. Slowly add hot water dropwise until the solution turns cloudy. To the turbid boiling solution add just enough solvent (one or two drops) to clarify the solution. Cool the solution first with a water bath and then with an ice bath. Scratch the inside of the tube if necessary to induce crystallization. If the solution oils out~ reheat it until it clears or boils. and once boiling add just enough solvent to effect clearing. Cool the solution again to induce crystallization. Estimate the approximate amounst of solvent and water to be used.
2. Dissolving the sample
An Erlenmeyer (conical) flask should be used of such a size that it will only be filled to around half–way when all the solvent has been added. The solid sample is introduced, together with around 75% of the amount of solvent thought to be required—it is always advisable to err on the side of using less solvent at this stage. The flask is heated on a hotplate until dissolution of the solute is complete, additional solvent can be added to the hot solution as necessary to ensure complete dissolution. A boiling (wooden) stick should be added to provide a point for bubbles to form and facilitate an even boiling process. By a process of gradual addition of solvent to the flask until all the material has dissolved you will ensure that your hot solution is saturated and will deposit crystalline material once it is cooled. If some insoluble impurities are left suspended in solution, you must be prepared to judge when all of the desired material has dissolved and only the unwanted impurities are left as a suspension, which must then be filtered hot if present.
3. Hot gravity filtration (If necessary)
Once you are satisfied that all of the desired material has dissolved and only unwanted impurities are left as a suspension prepare to filter the mixture. Heat a clean empty erlenmeyer on a hot plate and heat a funnel either with a heat gun or in an oven. Use hot solvent and run it through fluted filter paper in the funnel. Discard the solvent, then filter the hot recrystallization mixture.
4. Cooling
Cooling the filtered solution will allow crystals to form. The rate of cooling has a role to play in determining the size of the crystals: fast cooling will tend to generate more crystals of relatively small dimensions, slow cooling might allow larger crystals to form. Usually the best compromise of speed, convenience, and crystal quality, is simply to let the solution cool to room temperature on the bench. To ensure maximum recovery of material the solution should be cooled in an ice–water bath after the solution has been allowed to cool to room temperature.
5. Cold vacuum filtration
When the crystallization process is judged to be complete the crystals need to be collected by suction filtration. Both the funnel and suction flask should be chosen so that neither will become more than half full during the filtration process. It is preferable that all of the crystalline material is transferred to the funnel as a suspension in the crystallization solvent, however it is sometimes hard to get all of the crystals moving freely by swirling the flask and occasionally it will be necessary to add more ice–cold solvent in order to transfer the last of the crystalline material. It may also be necessary to dislodge crystalline material from the sides of the flask with a spatula prior to filtration.
6. Washing the crystals
Once the suction filtration process is complete the collected crystals should be washed with a little more ice–cold solvent to remove final soluble impurities which would otherwise be left on the surface of the crystals. The solvent used for this final washing should be as cold as possible to minimize losses from the crystals redissolving: recall that your precious crystals are soluble in warm solvent so you’ll lose material if you wash with solvent that is not cold.7. Drying the crystals
Once the crystals have been collected on the suction funnel they can usually be satisfactorily dried by continuing to draw air over them for a few minutes. The almost dry crystals should then be spread on a filter paper to allow the last traces of volatile solvent to evaporate.
After completing the pre-lab and having Dr. R. initial your notebook, obtain an unknown sample. Record its identification number in your notebook. Retain the vial that it came in.1) Determine a suitable solvent to use for its recrystallization. 2) Perform the recrystallization. 3) Determine the sample’s identity from its melting point. All of the unknowns will crystallize from one, or more, or mixtures of the following solvents: water, methanol, or ethanol. Refer to the information above on selecting a good solvent. Use a small beaker of water on your hotplate to warm the test tubes in which you perform your solubility experiments.
Your unknown will be one of the following:
Acetaminophen 169-172°C
Acetylsalicylic Acid 134–136°C
Benzoic acid 122–123°C
Cholesterol 147–149°C
Salicylamide 140–144°C
Salicylic Acid 158–160°C
(Budget 1.0 lab periods)
Identify a good recrystallization solvent, then weigh and record the mass of your entire sample, which will be used for the recrystallization. Retain the vial. Then proceed to recrystallize your unknown. Weigh your dried product and record its mass.(Budget 0.5 lab periods)
Determine the unknown's melting point. Confirm the unknown's identity by doing a mixed melting point with an authentic sample which is available.
Dispose of solvent waste from vacuum filtrations in the proper waste container. Bottle the recrystallized solid sample in the vial that it came in, relabel the vial, and turn in.Calculate the percent recovery of material from its dry weight.