In these cases, the recrystallization is allowed to proceed very slowly, over the course of weeks to months, to allow the crystal lattice to form without the inclusion of any impurities.
Special glassware is needed to allow the solvent to evaporate as slowly as possible during this time, or to allow the solvent to very slowly mix with another solvent in which the compound is insoluble called antisolvent addition.
The pharmaceutical industry also makes heavy use of recrystallization, since it is a means of purification more easily scaled up than column chromatography. These different crystal forms might have different biological properties or be absorbed into the body at different rates. A more common use of recrystallization is in making rock candy. Rock candy is made by dissolving sugar in hot water to the point of saturation. Wooden sticks are placed into the solution and the solution is allowed to cool and evaporate slowly.
After several days, large crystals of sugar have grown all over the wooden sticks. Mayo, D. Armarego, W. Ray, P. Google Patents: Hightower, T.
Journal of Chemical Education 83 11 , Rohani, S. Organic Chemistry. Purifying Compounds by Recrystallization. To learn more about our GDPR policies click here. If you want more info regarding data storage, please contact gdpr jove. Your access has now expired. Provide feedback to your librarian. If you have any questions, please do not hesitate to reach out to our customer success team.
Login processing Previous Video Next Video. Overview Source: Laboratory of Dr. Jimmy Franco - Merrimack College Recrystallization is a technique used to purify solid compounds. Perform all steps in a fume hood to prevent exposure to solvent fumes. Selecting a Solvent Place 50 mg of the sample N-bromosuccinimide in an Erlenmeyer flask. If the sample dissolves completely, the solubility in the cold solvent is too high to be a good recrystallization solvent.
If the sample does not dissolve in the cold solvent, heat the test tube until the solvent boils. If the sample has not completely dissolved at this point, add more boiling solvent drop-wise, until all of the solid dissolves. If it takes more than 3 mL to dissolve the sample in the hot solvent, the solubility in this solvent is probably too low to make it a good recrystallization solvent.
If the first choice of solvent is not a good recrystallization solvent, try others. If a single solvent that works cannot be found, try a two solvent system. If you cannot find a suitable single solvent system, then a solvent pair may be necessary.
When identifying a solvent pair, there are several key considerations 1 The first solvent should readily dissolve the solid. As a general rule "likes dissolve likes" meaning that polar compounds tend to be soluble in polar solvents and non-polar compounds are often more soluble non-polar compounds.
Also make sure the boiling point of the solvent is lower than the melting point of the compound, so the compound forms as solid crystals rather than as an insoluble oil. Confirm that the impurities are either insoluble in the hot solvent so they can be hot-filtered out, once the compound is dissolved or soluble in the cold solvent so they stay dissolved during the entire process.
This is a better choice than a beaker, since the sloping sides help trap solvent vapors and slow the rate of evaporation. Place the solvent water in a separate Erlenmeyer flask, and add boiling chips or a stir bar to keep it boiling smoothly. Heat it to boiling on a hotplate. Add hot solvent to a flask at room temperature containing the compound in small portions, swirling after each addition, until the compound is completely dissolved.
During the dissolution process, keep the solution hot at all times by resting it on the hotplate, too. Do not add more hot solvent than necessary - just enough to dissolve the sample. If a portion of the solid does not seem to dissolve, even after more hot solvent has been added, it is likely due to the presence of very insoluble impurities.
If this happens, stop adding solvent and do a hot filtration before proceeding. To perform a hot filtration, fold a piece of filter paper into a fluted cone shape and place it into a glass stemless funnel. Qualitatively, a solution is considered unsaturated if the maximum amount of dissolved solute has not yet been reached. When the maximum possible solute has dissolved, the solution is saturated.
A supersaturated solution contains more dissolved solute than the maximum possible amount under typical conditions. Recrystallization takes advantage of the differences in solubility between the desired product and the contaminants at high temperatures.
The first step of recrystallization is to dissolve the product mixture in a minimal volume of heated solvent that still results in a saturated — but not supersaturated — solution. Then, the solution is cooled to room temperature, decreasing the solubility of both the desired compound and the impurity. As the solution cools, crystallization of the pure component begins, while the still soluble impurities do not. This occurs when the component of interest is in a significantly higher concentration than the impurity.
First, in the nucleation phase, the solvent initiates the random agglomeration of the solute molecules, forming the first crystal called a seed or nucleus. Next, in the particle growth or crystallization phase, more molecules are added to the seed, forming a crystal. The crystal contains the pure compound, while the impurity remains in the solvent.
Nucleation proceeds faster than particle growth in a supersaturated solution. With more seeds, each crystal is smaller. Thus, if the solution is saturated, rather than supersaturated, fewer seeds form, resulting in larger crystals. Heating the solution to a higher temperature before cooling to room temperature enables the dissolution of a higher concentration of solute, decreasing supersaturation.
Additionally, rapid cooling results in quick nucleation, forming many small crystals and trapping the impurity inside of the crystals. Slow cooling is preferred to achieve fewer, larger crystals.
Once the solution has cooled to room temperature, and the crystals have formed, the solution is filtered using vacuum filtration. Then, the crystals are allowed to dry. The percent recovery is calculated by dividing the mass of the recovered product by the mass of the crude product. The solute that can no longer be held in solution forms purified crystals of solute, which can later be collected. Recrystallization works only when the proper solvent is used.
The solute must be relatively insoluble in the solvent at room temperature but much more soluble in the solvent at higher temperature. At the same time, impurities that are present must either be soluble in the solvent at room temperature or insoluble in the solvent at a high temperature. For example, if you wanted to purify a sample of Compound X which is contaminated by a small amount of Compound Y, an appropriate solvent would be one in which all of Compound Y dissolved at room temperature because the impurities will stay in solution and pass through filter paper, leaving only pure crystals behind.
Also appropriate would be a solvent in which the impurities are insoluble at a high temperature because they will remain solid in the boiling solvent and can then be filtered out. When dealing with unknowns, you will need to test which solvent will work best for you.
According to the adage "Like dissolves like," a solvent that has a similar polarity to the solute being dissolved will usually dissolve the substance very well. In general, a very polar solute will easily be dissolved in a polar solvent and will be fairly insoluble in a non-polar solvent.
Frequently, having a solvent with slightly different polarity characteristics than the solute is best because if the polarity of the two is too closely matched, the solute will likely be at least partially dissolved at room temperature. There are five major steps in the recrystallization process: dissolving the solute in the solvent, performing a gravity filtration, if necessary, obtaining crystals of the solute, collecting the solute crystals by vacuum filtration, and, finally, drying the resulting crystals.
Figure 1. Hot gravity filtration. Recrystallization is the process of purification that involves dissolution of a solid in a hot solvent, filtration of the heated solution or mixture, crystal formation, and the isolation of the crystalline compound. In order to perform a recrystallization, the solubility of a compound in a hot solvent must be taken advantage of. In other words, a solution at a higher temperature will have more dissolved solids and as it cools, the solute will return back to its solid phase, forming a precipitate.
Impurities in the solid being recrystallized are usually significantly lower in concentration than the concentration of the substance being purified so as the mixture cools, the impurities remain in solution while the highly concentrated product crystallizes. Crystal formation of a solute from a solution is a selective process because only solids moving at the right speed and are under the appropriate conditions of concentration and solvent form almost perfect crystalline materials as only molecules of with the right shape fit into the crystal lattice.
Recrystallization purifies a compound because dissolution of the impure solid in a suitable hot solvent destroys the crystal lattice of the impure compound and the recrystallization from the cold solvent selectively produces a new, more pure crystal lattice. Crystals that form slowly are larger not always and often purer than ones that form quickly because rapid crystal formation traps impurities within the lattice as they are simply surrounded by the crystallizing solute.
The most important aspect of recrystallization is the choice of solvent because the solute should have maximum solubility in the hot solvent and minimum solubility in the cold solvent. This entails that nonionic compounds generally dissolve in water only when they can associate with the water molecules through hydrogen bonding. Hydrocarbons and alkyl halides are virtually insoluble in water whereas carboxylic acids and alcohols are often recrystallize from water solutions.
In a miniscale recrystallization experiment several steps are followed to complete the purification process. One of the key factors of purification through recrystallization is understanding solubility. In order to proceed with a successful recrystallization process, the mixture must have some important solubility properties.
One solvent must be soluble at all temperatures. The next solvent must be soluble at low temperatures. And another solvent must be soluble at high temps, and insoluble at low temps. The blue square is soluble at all temperatures, The orange circle is mostly insoluble at all temperatures, and the green triangle is soluble at high temperatures and insoluble at low temperatures.
Isoelectric Focusing is an electrophoresis protein purification process in which proteins are separated by their isoelectric points. The isoelectric point of a protein, the pI , is the characteristic pH at which a protein carries no net charge.
The net charge of a protein is determined by the acidity or basicity of the side chains that make up the protein.
If a protein has more acidic groups than basic groups then the protein will have a very low pH and be considered acidic.
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