Lab Procedure: Recrystallization

Core Concepts

In this tutorial, you will learn about recrystallization analysis and the steps required to perform one. 

Topics Covered in Other Articles 

What is Recrystallization

The definition of recrystallization is a technique for the purification of compounds in which a compound is dissolved in a solvent and slowly cooled to form crystals, which are a purer form of the compound.

Scientists use recrystallization to purify solids, typically products, from different chemical reactions. The process involves dissolving a solid into solution, then allowing the dissolved substance to gradually crystallize. This produces compounds high in purity, a quality which can be seen by the presence of uniform crystals.  

Recrystallization is a challenging procedure to carry out correctly. Researchers must carefully control all influential variables, including temperature and time, to ensure a successful recrystallization of their product.

Recrystallization vs Crystallization

Although the procedures of crystallization and recrystallization display similarities, their respective definitions diverge. Firstly, crystallization is a separation technique. It involves precipitating crystals from solution through transitions in the solute’s solubility conditions. The resulting crystals can be readily distinguished and subsequently filtered out from the solution. Solubility rules will tell you if a precipitate will form.

Recrystallization, on the other hand, requires the purification of crystals already obtained via the crystallization method (hence the “re” prefix, which reinforces this sense of repetition). Scientists turn to this second process because crystals generated through crystallization often contain impurities. Recrystallizing the compounds facilitates a more effective removal of these impurities. This process can even be repeated multiple times, increasing the purity of the desired compound during each trial. 

Learn amazing facts about recrystallization analysis, including its components, steps and applications in the chemistry lab.

Precipitates vs Crystals

We must now discuss another relevant discrepancy, the difference between precipitate formation and crystal formation. A precipitate is a solid mixture of compounds that originates from an oversaturated solution, falling out of it. However, precipitated substances may not necessarily be pure and can contain up to several compounds.

In contrast, only one compound composes any given crystal. This results in a highly organized, structurally uniform product; in contrast, precipitates hold no apparent structure. Therefore, while precipitates occur relatively readily, procuring crystals is a far more difficult process. 

The mechanism of generating crystals involves heating a solution to a high temperature in order to saturate it. This target high temperature often falls near the solvent’s boiling point. Then, you remove the heat source and allow the solution to drop back to room temperature.

As the temperature decreases, so does the solubility of the compound in solution. Soon, the solution becomes supersaturated, prompting crystal formation. As discussed, these crystals then precipitate out of the solution at the cooler recrystallization temperature.

Beautiful Recrystallization of Lead Iodide

Here is the complete procedure for seeing one of the most beautiful recrystallizations on the planet!

The Purification Process 

As described, the recrystallization process further purifies the compounds of the crystallization process. The mechanisms explaining that include the simultaneous dissolving of both the compound and its impurities in the solvent. During this process, either the desired compound or the impurities can exit the solution and leave the other behind.

Scientists filter these solutions to remove the insoluble impurities contained within. They can then evaporate the solvent, isolating the solid compound.

What is Seeding?

To expedite crystallization and recrystallization processes, scientists can employ a technique called seeding. Seeding involves first dipping a small crystal, or seed, of solute into the saturated solution. Following this step, larger crystals can form and subsequently grow on this seed crystal.

This practice boosts the efficiency of recrystallization because it eliminates any dependency on random molecular collisions. The seed crystal prompts more molecular interaction and facilitates crystal formation, speeding the overall process.

Types of Recrystallization 

1. Single Solvent Recrystallization

This type of recrystallization is the most basic and, as a result, the most frequently employed. The process involves dissolving a compound, “A,” and impurity, “B,” in a heated solvent system. As this solution cools back to room temperature, the solubility of the compounds in the solution drops, prompting the desired compound to recrystallize from the mixture.

2. Multi-Solvent Recrystallization

This method of recrystallization closely resembles the first in terms of the procedure involved. However, multi-solvent recrystallization requires two, and sometimes more, solvents. The compound, “A,” and impurity, “B,” dissolve in the first solvent. Then, the addition of a second solvent prompts the insolubility and precipitation of either A or B. The other component of the mixture remains in solution.

Pros of Using a Solvent Pair

Scientists typically employ multiple solvents when no singular solvent meets all the criteria for that given recrystallization procedure. Each solvent pair contains a “soluble solvent,” one in which the compound is soluble, and an “insoluble solvent,” one in which the compound is insoluble. The soluble solvent and insoluble solvent must be miscible, so that the proportions used do not limit their solubility in solution.

To work with these two solvents, you must first dissolve your compound in the hot soluble solvent. Next, you add hot insoluble solvent dropwise to this mixture until it becomes cloudy. You then add a small amount of hot solvent to clarify the mixture, then allow it to cool and crystallize, as you would during a single-solvent recrystallization.

How to Perform a Simple Recrystallization Procedure 

Below is a stepwise guide for completing a basic recrystallization procedure.

1. First, weigh the impure solvent and record that value. Then add the impure compound to a solvent system.

2. Heat this solvent system to your target temperature, or its boiling point. Be sure to raise the temperature in gradual increments to ensure that the rest of the process proceeds smoothly.

3. Let the solution stand without disturbance. Allow the temperature to slowly drop, reaching room temperature. If you do not maintain a gradual temperature decrease, a precipitate may form instead of the desired crystals. Additionally, do not place the flask on a hard surface, which will cause “shock-cooling” of your solution. Instead, either place the flask in an insulated jar or clamp it to a steady, secure device. Recrystallization is a gradual process; do not become impatient as you wait and employ appropriate safety precautions.

4. To further lower the temperature, place the solution in an ice-water bath. This facilitates the formation of crystals by speeding the recrystallization reaction. Shortly after completing this step, you should observe many crystals.

5. Filter the crystals via vacuum filtration.

6. Air dry the crystals. Then weigh this recovered compound.

7. Calculate your percent recovery of desired compound. To accomplish this, divide the weight after purification, obtained in step six, by weight before purification, obtained in step one–then multiply this value by 100%. If your percent recovery falls within the range of 70-100%, you can view your procedure and the amount of recovered product as “very good.”

8. Compliment your percent recovery quantification with a visual inspection. If the crystals appear uniform in shape and size, as well as display shiny surfaces that catch the light, you have successfully completed your procedure. You may wish to further distinguish the purity of your compound, which you can do by conducting a melting point analysis and comparing the obtained temperature to literature values.

Limitations of Recrystallization

Although quite useful, recrystallization possesses some limitations and ramifications. Firstly, the compound in question must occupy a solid state at standard conditions. This means that substances including oils, greases, and waxes cannot be crystalized or recrystallized under standard conditions.

The purity of your crude material also comes into play. The success of recrystallization processes depends directly on the solubilities of each respective component of your starting substance. But this substance must contain approximately 80% of your desired compound; in other words, it should be mostly pure. Otherwise, the recrystallization will not proceed smoothly.

Lastly, for each recrystallization step, some of the desired compound will remain in solution even after cooling it. Because of this, each recrystallization step will decrease the overall yield of the desired compound. For small scale applications, like laboratory testing, this does not have a huge effect. However, for large scale applications like pharmaceutical production, even a small percentage loss represents a large mass of compound.

These limitations all must be considered when deciding to use recrystallization for a given process.

Applications of Recrystallization To Today’s World 

Recrystallization has applications that extend into the industrial, medical, and pharmaceutical industries. Techniques such as texture control, drug development, and treatment purification all involve the procedure.

But the pharmaceutical industry actually makes the most use of recrystallization procedures. Purification and separation processes are key to the isolation of different active ingredients. These steps, in turn, inform the synthesis of many different drugs and medications.