The recrystallization process separates impurities from a solid. Some impurities can cause, for example, a solid to have a different color to its pure form or be weaker than it otherwise should. This is achieved by dissolving the solid in a hot solvent and then cooling that solvent to allow the formation of crystals. To perform the recrystallization of a compound, several steps need to be followed; the selection of solvent, dissolution of solid in the hot solvent, addition of activated charcoal followed by a filter, cooling of the solution, and crystal formation.
Figure 1: The process of recrystallization (Image source Labster theory)
When you use a solid sample for recrystallization, students will ask you why we are recrystallizing it as it is already in solid form. So the answer is the purity of the compound; we do it to make it purer. The 2nd thing that may confuse the students is the principle of the recrystallization process. The answer is that the purification process is based on solubility and temperature; if we increase the temperature, the solubility of the desired compound increases and the impurities having low solubilities will be separated from the desired compound.
One of the major hurdles for students in the recrystallization process is the selection of solvent for the procedure because different solvents have different solubilities for various compounds. So selecting an appropriate solvent for any specific compound is very tricky. In the recrystallization process, we mostly use a combination of two solvents, and this may also confuse the students about why we are using a mixture of two solvents and how to make a pair of solvents. These points must be cleared first before going to the practical process.
Theoretical understanding of recrystallization is tricky because it involves phase changes. If the compound is in powder form and after recrystallization, it makes crystals, the students must see the type of crystals the compound form. Students easily understand the recrystallization process practically or by video graphics. If you cannot perform the practical, use video graphics so that students understand it easily.
Before going for the main topic, you must clear some basic terms of the recrystallization process.
A supersaturated solution is a solution in which we increase the solubility of any solution by increasing the temperature and dissolving more substance.
Precipitation is when a dissolved substance is converted to an insoluble substance from a super-saturated solution by reaction or changing the temperature.
Single-solvent recrystallization is the process of dissolving our compound in a single solvent.
Multi-solvent recrystallization is the process of recrystallization in which we dissolve the compound in two solvents. Select one solvent in which the compound dissolves quickly and is insoluble in the 2nd one.
Filtration is the process in which the pure compound is separated from the impurity.
Seeding is the process in which a small amount of pure compound is added to the solution to initiate the crystallization process.
The melting point range is the span of temperature from the point when the solid starts melting to the point at which the entire solid is in a liquid state.
The first step in the recrystallization process involves the dissolution of the compound in the solvent.
The first step is choosing a suitable solvent for the solid to purify. The solvent must dissolve the solid when it's hot, but NOT when it's cold. The solvent must be quickly evaporated so that it will evaporate easily after the filtrations. It should be non-toxic and of low cost.
Heat your solvent to its boiling point and dissolve the solid, resulting in a supersaturated solution. Note that some of the impurities in the solid might not be soluble in the solvent. If you are using multi-solvent recrystallization, make sure that the solid material you are using is not soluble in the second solvent.
This step depends on the type of impurities in your solid. Once the solid is dissolved in your solvent, add a bit of activated charcoal. The activated charcoal will adsorb a lot of the impurities. After the activated charcoal has adsorbed the impurities, filter it out of the solution while it is still VERY HOT; otherwise, you might risk getting crystals in your funnel and filter. Always use a filter the same size as the funnel for effective filtration.
The solution needs to be cooled down in two steps. First, the conical flask needs to be cooled down to room temperature, and once it is at room temperature, it can be placed in an ice bath for further cooling. If the solution is put straight into an ice bath, the fast cooling and formation of crystals could trap some of the impurities again in the crystals, making the purification less efficient.
For filtration, you can use the gravity filtration method or the suction filtration method. In gravity filtration, put the filter paper in a funnel, place it on a conical flask and filter the solution.
Suction filtration (also called vacuum filtration) is a technique used to separate liquids from solids. In this technique, an aspirator sucks out the air in the flask where the Büchner funnel with a filter is placed. This causes a pressure difference, so when the mixture is placed in the funnel, it is forced to go through the filter. After the filtration, the solid stays on the filter while the liquid goes through and accumulates at the Büchner flask.
Suction filtration focuses on the recovery of the solid since the flow of air created by the aspirator will be much drier than with simple gravity filtration. By drying the solid much more, we ensure that most of the weight in the filter is indeed due to the solid and not the solvent.
To do a suction filtration, we need to:
Take filter paper on a glass watch. By doing this before and after the filtration, we will be able to estimate the weight of the solid that was in our mixture
Assemble the suction filtration equipment and activate the pump to make the air start flowing
Place the pre-weighed filter in the Büchner funnel and filter the solution
Once the solution is filtered, leave it a bit longer in the funnel to dry out
Weight your pre-weighed filter with the solid in the scale that you previously tared to obtain the weight of your crystals
At the end of the process, we can estimate the recovery efficiency by simply calculating the percentage of the original solid we have recovered.
Recovery efficiency = (Final weight - Initial weight) / Initial weight
Figure 2. Suction filtration protocol (Image source Labster theory).
Enjoy your crystals
If you perform these steps correctly, you will see crystals forming in your flask. Those crystals will have fewer impurities than the original ones you dissolved.
This technique can be used to determine the purity of a solid. If the melting point range of a pure solid is known, we can estimate the purity of our impure solid as the difference in the melting point compared to the pure solid. An impure solid melts over a wide range of temperatures and at a temperature lower than that of a pure solid. The closer the impure solid's melting point range gets to the pure solid's melting point, the fewer impurities it will contain.
To perform a melting point range in a solid, we should follow the following steps:
Grind your crystals until you have a fine powder
Tap a capillary tube on top of your sample to load it, and then tap it against the workbench to make it go to the other end
Insert the melting capillary tube in the melting point apparatus and turn it on
Determine the temperature at which the solid starts melting
Determine the temperature at which the solid is completely melted
If, at the end of the protocol, the melting point range of the impure solid is below the melting point of the pure solid, it means that we still have impurities in our solid.
Figure 3. Melting Point Range protocol (Image source Labster theory).
With technological advances, it is much easier to explain complex and challenging processes with the help of simulations. Now, you can simulate experiments without the need for any valuable equipment. In this regard, you can take help from Labster’s virtual lab simulations. These simulations engage students through interactive learning scenarios. Students dive into a 3D world, where they visually learn and apply their concepts to solving real-life problems.
In Labster’s interactive Recrystallization: Dissolve your solid and precipitate your crystals Virtual Lab, students will learn why it is important to choose the right solvent and the best ways to recrystallize your solid.
In Labster’s interactive Recrystallization: Filter your crystals and measure the melting point Virtual Lab, students will learn how to use the suction filtration and melting point techniques that will allow them to answer both questions.
In Labster’s interactive Recrystallization: Purify your solid Virtual Lab, students will learn step by step how to purify a solid using the recrystallization technique, and on top of that, you will be able to filtrate your solid and check for its melting point range.
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