Unable to load video. Please check your Internet connection and reload this page. If the problem continues, please let us know and we'll try to help. An unexpected error occurred. Jimmy Franco - Merrimack College. Recrystallization is a technique used to purify solid compounds. During recrystallization, an impure solid compound is dissolved in a hot liquid until the solution is saturated, and then the liquid is allowed to cool.
Ideally, any impurities that are present will remain in the solution and will not be incorporated into the growing crystals Figure 1. The crystals can then be removed from the solution by filtration.
Not all of the compound is recoverable — some will remain in the solution and will be lost. Recrystallization is not generally thought of as a separation technique; rather, it is a purification technique in which a small amount of an impurity is removed from a compound. However, if the solubility properties of two compounds are sufficiently different, recrystallization can be used to separate them, even if they are present in nearly equal amounts. Recrystallization works best when most impurities have already been removed by another method, such as extraction or column chromatography.
Figure 1. The general scheme for recrystallization. A successful recrystallization depends on the proper choice of solvent. The compound must be soluble in the hot solvent and insoluble in the same solvent when it is cold. In choosing a solvent, the bigger the difference between hot solubility and cold solubility, the more product recoverable from recrystallization.
The rate of cooling determines the size and quality of the crystals: rapid cooling favors small crystals, and slow cooling favors the growth of large and generally purer crystals. Although the terms "crystallization" and "recrystallization" are sometimes used interchangeably, they technically refer to different processes.
Crystallization refers to the formation of a new, insoluble product by a chemical reaction; this product then precipitates out of the reaction solution as an amorphous solid containing many trapped impurities.
Recrystallization does not involve a chemical reaction; the crude product is simply dissolved into solution, and then the conditions are changed to allow crystals to re-form. Recrystallization produces a more pure final product. For this reason, experimental procedures that produce a solid product by crystallization normally include a final recrystallization step to give the pure compound. To perform recrystallization, an impure solid compound is mixed with hot solvent to form a saturated solution.
As this solution cools, the solubility of the compound decreases, and pure crystals grow from solution. Recrystallization is often used as a final step after other separation methods such as extraction, or column chromatography. Recrystallization may also be used to separate two compounds with very different solubility properties. This video will illustrate solvent selection for recrystallization, purification of an organic compound from solution, and will introduce a few applications in chemistry.
Crystallization begins with nucleation. Solute molecules come together to form a stable small crystal, which is followed by crystal growth. Nucleation occurs faster on nucleation sites such as seed crystals, scratches, or solid impurities than spontaneously in solution. Agitation may also encourage rapid nucleation. However, rapid growth can lead to incorporation of impurities if not grown in optimal conditions.
The solubility of a compound tends to increase with temperature, and is highly dependent on the choice of solvent. The greater the difference in solubility at high and low temperature, the more likely it is for the solute to come out of the solution as it cools, and form crystals.
The solvent's boiling point must also be below the melting point of the solute to enable crystallization. Rapid cooling of the solution induces the formation of many nucleation sites, thus favors the growth of many small crystals. However, slow cooling induces the formation of fewer nucleation sites, and favors larger and purer crystals.
Thus, slow cooling is preferred. Additionally, a solvent can be selected to minimize impurities. If a solution impurity is more soluble than the solute itself, it can be washed off of the fully formed crystals with cold solvent.
However, if an impurity is less soluble, it will crystalize first, and can then be filtered out of the heated solution, prior to recrystallization of the solute. If no single solvent has the necessary properties, a mixture of solvents can be used. For a solvent pair, the first solvent should readily dissolve the solid. The second solvent must have a lower solubility for the solute and be miscible with the first solvent. Common solvent pairs include ethyl acetate and hexane, toluene and hexane, methanol and dichloromethane, and water and ethanol.
Now that you understand the principles of recrystallization, let's go through a procedure for purification of an organic compound by recrystallization. Add 0. If the compound dissolves completely, the solubility in the cold solvent is too high to be used for recrystallization. Otherwise, heat the mixture in the test tube to boiling.
If the compound does not dissolve completely in the boiling solvent, heat another portion of solvent to boiling. Add the boiling solvent dropwise to the test tube until the solid dissolves completely or until the test tube contains 3 mL of solvent.
If the solid still does not dissolve, then its solubility in this solvent is too low. Confirm that impurities are either insoluble in the hot solvent so they can be filtered out after dissolution or soluble in the cold solvent so they remain in solution after recrystallization is complete.
If a solvent meets all criteria, it is suitable for recrystallization. To start recrystallization, heat the solvent to boiling on a hot plate in an Erlenmeyer flask with a stir bar.
Place the compound to be recrystallized in another Erlenmeyer flask at room temperature. Next, add a small portion of hot solvent to the compound. Swirl the mixture in the flask and then place it on the hot plate as well.
Repeat this process until the sample has completely dissolved or until addition of solvent causes no further dissolution. Filter the solution to remove insoluble impurities.
If crystals form during filtration, dissolve them with drops of hot solvent. Cool the solution on the benchtop. Cover the flask to prevent solvent loss to evaporation and to keep particulates out of the solution.
How can insoluble impurities be removed? Can an insoluble compound form a saturated solution? What are insoluble impurities? Why do we need to remove impurities from compounds before we use them? How do you remove color impurities? Is used to remove impurities from water? How do you remove impurities? What are the methods to remove impurities from water? What are the methods to remove soluble impurities from water?
Which is the best technique for removal of insoluble impurities? Which of these is the best technique for removal of insoluble impurities? Which is not a way to remove insoluble impurities from water? Which of the following is insoluble in water? What are some insoluble substances?
What is a insoluble? What is an insoluble problem? Is salt insoluble in water? Does insoluble mean no reaction? How do you know if a solution is soluble or insoluble?
What is an insoluble solution? Which salts is insoluble in water? What does insoluble in water mean? Why Some salts are insoluble in water? Is baking soda insoluble in water? What makes a salt soluble or insoluble? What are the five factors that affect solubility? Before which tests should acetone not be used to clean the test tubes? Why run solubility tests before running the.
Suppose that the amount of heat removed when How many kilograms of ethyl alcohol would freeze? The exact question is: Benzyl alcohol bp degree C was selected by a student to crystallize fluorenol mp degrees C because the solubility characteristics of this solvent are appropriate.
However, this solvent is not. Match the following terms about analytical precipitations to their definitions: A. Coprecipitation B. Adsorption C. Absorption D. Gathering E. Masking F.
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