Recrystallization is a basic operation commonly used to purify solid organic and inorganic compounds. By dissolving a crude product in an appropriate solvent and precipitating crystals through cooling, impurities can be removed and the purity can be increased.
On the other hand, during recrystallization, part of the desired product may remain in the solvent or may be lost during filtration or transfer, resulting in a low yield. This article explains how to interpret “yield,” “purity,” and “melting point” in recrystallization experiment reports, as well as discussion examples, causes of error, and improvements.
Note: This article is a reference for discussing results obtained in chemistry experiments at universities and similar institutions. For actual experimental procedures and safety precautions, always follow your university’s laboratory manual and the instructions of your instructor or teaching assistant (TA).
- What Is Recrystallization?
- Results to Examine in a Recrystallization Experiment
- How to Think About Yield and Recovery in Recrystallization
- Main Causes of Low Yield in Recrystallization
- Discussion When Too Much Solvent Was Used
- Discussion When Cooling Was Insufficient
- Discussion of Losses During Washing
- Discussion of Losses During Filtration and Transfer
- Why Purity Increases Through Recrystallization
- How to Evaluate Purity from the Melting Point
- Discussion When the Melting Point Is Lower Than the Literature Value
- Discussion When the Melting Point Range Is Broad
- Relationship Between Yield and Purity
- What Can Be Learned from the Appearance of Crystals After Recrystallization?
- Causes When Recrystallization Did Not Work Well
- Discussion When the Recovery Is Too High
- How to Write Improvements for Recrystallization
- Difference Between a Superficial Discussion and a Good Discussion
- Example Expressions That Can Be Used in Reports
- Points to Check When Discussing Recrystallization
- Summary
What Is Recrystallization?
Recrystallization is a purification method in which a solid sample is dissolved in a solvent and the desired substance is recovered again as crystals by using differences in solubility caused by changes in temperature. In general, a solvent is selected in which the desired substance is highly soluble at high temperatures but poorly soluble at low temperatures.
After dissolving the sample at a high temperature, cooling causes the desired substance to precipitate as crystals. At this time, some impurities remain in the solution, so the obtained crystals are expected to have a higher purity than the crude product.
In a recrystallization report, not only the amount of crystals obtained but also the color, shape, melting point, melting point range, and yield are used to discuss whether the purification was successful.
Results to Examine in a Recrystallization Experiment
For the results of a recrystallization experiment, organize the mass of the recovered crystals, yield, melting point, and appearance of the crystals. By combining this information, it is possible to consider whether the purity improved through recrystallization and how much of the desired substance was recovered.
Main Items to Include in the Results
- Mass of the crude product before recrystallization
- Mass of crystals obtained after recrystallization
- Yield or recovery after recrystallization
- Color of the crystals
- Shape of the crystals
- Melting point
- Melting point range
- Comparison with the literature value
- Condition of the filtrate or mother liquor
Example of Writing the Results:
White needle-like crystals were obtained by recrystallization. The mass of the crude product before recrystallization was 1.50 g, and the mass of the crystals obtained after recrystallization was 1.05 g. The recovery was 70.0%. The melting point of the obtained crystals was 132–134°C, which was slightly lower than the literature value of 135–136°C, and the melting point range was also broader.
How to Think About Yield and Recovery in Recrystallization
In recrystallization, the amount of crystals recovered after recrystallization relative to the crude product before purification may be expressed as the recovery. When considering the entire synthesis reaction, it may be called the yield, while when considering only the recrystallization operation, it may be called the recovery.
Recovery (%) = Mass of crystals obtained after recrystallization ÷ Mass of crude product before recrystallization × 100
Calculation Example:
Crude product before recrystallization: 1.50 g
Crystals after recrystallization: 1.05 g
Recovery = 1.05 ÷ 1.50 × 100 = 70.0%
In recrystallization, it is not unusual for the yield or recovery to be less than 100%. This is because part of the desired substance remains dissolved in the solvent. Crystals may also be lost during filtration, washing, drying, and transfer.
Main Causes of Low Yield in Recrystallization
The main causes of low yield or recovery in recrystallization are “the desired substance remaining in the solvent” and “loss of the desired substance during experimental operations.” In a report, select causes that are likely to have occurred in your own experiment and explain their relationship to the results.
| Cause | What Happens | Effect on Results |
|---|---|---|
| Too much solvent was used | A large amount of the desired substance remains in the mother liquor | The amount recovered decreases |
| Cooling was insufficient | Crystals do not precipitate sufficiently | Recovery decreases |
| The desired substance dissolved during washing | Part of the crystals dissolves in the washing solution | The amount recovered decreases |
| Crystals were lost during filtration | Crystals remain on the filter paper or apparatus | Recovery decreases |
| Adhesion or spilling occurred during transfer | Part of the crystals cannot be recovered | The measured mass becomes smaller |
| The crystals were too fine | They are more likely to pass through during filtration | Recovery decreases |
Discussion When Too Much Solvent Was Used
In recrystallization, a solvent is used to dissolve the desired substance at a high temperature. However, if too much solvent is used, the desired substance is more likely to remain dissolved in the solvent even after cooling. As a result, the amount of crystals that precipitate decreases and the recovery becomes lower.
Discussion Example:
One possible reason for the low recovery is that too much solvent was used during recrystallization. When a large amount of solvent is used, part of the desired substance remains dissolved in the mother liquor even after cooling, reducing the amount of crystals that precipitate. As a result, the mass of crystals recovered after recrystallization decreased, and the recovery is considered to have become lower.
Discussion When Cooling Was Insufficient
In recrystallization, the solubility of the desired substance decreases as the temperature decreases, causing crystals to precipitate. If cooling is insufficient, the desired substance remains in the solution and a smaller amount is recovered as crystals.
Discussion Example:
One possible reason why the amount recovered after recrystallization was small is insufficient cooling. The solubility of the desired substance decreases as the temperature decreases, causing it to precipitate as crystals. However, if the cooling time was short or the temperature did not decrease sufficiently, part of the desired substance may have remained in the mother liquor, resulting in a lower recovery.
Discussion of Losses During Washing
Crystals obtained after recrystallization may be washed to remove mother liquor and impurities. However, if too much washing solution is used or if the desired substance is somewhat soluble in the washing solution, part of the crystals will be lost.
Discussion Example:
Dissolution of part of the desired substance in the washing solution during washing of the crystals may also have contributed to the decrease in recovery. Washing is necessary to remove the mother liquor and impurities, but if the desired substance is slightly soluble in the washing solution, the amount recovered decreases as a result of washing. Therefore, an increase in purity and a decrease in recovery may have occurred at the same time.
This discussion is an important point that makes it easy to explain the essential nature of recrystallization. Including the perspective that “an operation that increases purity can simultaneously decrease yield” makes the discussion more in-depth.
Discussion of Losses During Filtration and Transfer
Crystals obtained by recrystallization are recovered by filtration. During this process, some crystals may adhere to the filter paper, funnel, beaker, glass rod, spatula, or other equipment and be lost.
Discussion Example:
During filtration and transfer, some of the crystals may have adhered to the filter paper or equipment and could not be completely recovered. Particularly when the crystals are fine or wet, they are more likely to remain on the filter paper or equipment, reducing the amount of crystals that can be weighed. Therefore, the actual mass of the obtained crystals became smaller, resulting in a lower recovery.
Why Purity Increases Through Recrystallization
Purity increases through recrystallization because the method uses differences in solubility between the desired substance and impurities. The desired substance precipitates as crystals upon cooling, while many impurities remain in the mother liquor.
In addition, impurities that do not readily enter the crystal lattice tend to be excluded during crystal growth. As a result, the obtained crystals are expected to have a higher purity than the crude product.
Discussion Example:
The crystals obtained after recrystallization were white, and the coloration observed in the crude product became weaker, suggesting that some of the colored impurities were removed into the mother liquor. In recrystallization, the desired substance precipitates as crystals while impurities tend to remain in solution, so the purity of the crystals is considered to have improved.
How to Evaluate Purity from the Melting Point
One method for confirming the purity after recrystallization is melting point measurement. In general, a high-purity substance shows a melting point close to the literature value and has a narrow melting point range. In contrast, when impurities are present, the melting point may decrease or the melting point range may broaden.
| Melting Point Result | Possible Interpretation |
|---|---|
| Close to the literature value with a narrow range | Purity is relatively high |
| Lower than the literature value | Impurities may be present |
| Broad melting point range | Impurity contamination or sample nonuniformity may be present |
| Measured values vary | Possible effects of sample amount, heating rate, drying condition, etc. |
Discussion Example:
The melting point of the sample after recrystallization was slightly lower than the literature value, and the melting point range was also broad. In general, samples containing impurities may show a lower melting point and a broader melting point range. Therefore, a small amount of impurities may have remained in the obtained crystals. However, if the melting point range was narrower than that of the crude product, the purity is considered to have improved through recrystallization.
Discussion When the Melting Point Is Lower Than the Literature Value
When the melting point is lower than the literature value, impurity contamination is a common cause. When impurities are present, the crystal lattice is disrupted, and melting may begin at a lower temperature than in the pure substance.
Discussion Example:
One possible reason why the melting point of the obtained crystals was lower than the literature value is impurity contamination. When impurities are present, the crystal lattice is disrupted, and melting may begin at a lower temperature than in the pure substance. Therefore, the lower melting point suggests that a small amount of impurities remained in the sample after recrystallization.
Discussion When the Melting Point Range Is Broad
For a high-purity substance, the temperature range from the beginning to the end of melting is relatively narrow. If the melting point range is broad, possible causes include impurity contamination, insufficient drying of the sample, an excessive sample amount, or a fast heating rate.
Discussion Example:
One possible reason for the broad melting point range is that impurities were present in the sample. When impurities are present, the entire sample does not melt at a uniform temperature, broadening the temperature range from the beginning to the end of melting. In addition, if the sample amount was too large or the heating rate was too fast, it may also have been difficult to read the melting point accurately.
Relationship Between Yield and Purity
In recrystallization, yield and purity do not necessarily become high at the same time. When impurities are thoroughly removed, part of the desired substance may also be lost in the mother liquor or washing solution, reducing the recovery.
Conversely, even if the yield is high, the purity cannot be considered high if impurities have not been sufficiently removed. Therefore, when evaluating recrystallization, it is necessary to consider not only the yield but also the melting point and appearance.
| Result | Possible Condition |
|---|---|
| Low yield but melting point close to literature value | Purity may be high, but there may have been a large loss of the desired substance |
| High yield but low melting point with a broad range | Impurities may be present |
| Both yield and melting point are good | Recrystallization may have been relatively successful |
| Low yield and low melting point | Both loss of the desired substance and remaining impurities are possible |
Discussion Example:
In this experiment, the recovery after recrystallization was low, but the melting point was close to the literature value and the melting point range was relatively narrow. This suggests that impurities were removed to some extent through recrystallization and that the purity improved. On the other hand, part of the desired substance may have remained in the mother liquor or been lost during filtration and washing, resulting in a lower recovery.
What Can Be Learned from the Appearance of Crystals After Recrystallization?
The color and shape of crystals after recrystallization can also provide information for discussion. However, purity cannot be determined from appearance alone. It is important to evaluate the result together with the melting point and yield.
| Observation | Possible Interpretation |
|---|---|
| White or colorless crystals were obtained | Colored impurities may have been removed |
| Color remains in the crystals | Colored impurities may remain |
| Distinct crystal shapes such as needles or plates are observed | Crystallization may have proceeded relatively well |
| Fine, powdery crystals were obtained | Rapid cooling or stirring may have produced fine crystals |
| Wet or clumped crystals | Insufficient drying or residual mother liquor may be present |
Discussion Example:
The crystals after recrystallization were white, and the coloration observed before recrystallization was almost no longer visible. This suggests that some of the colored impurities were removed into the mother liquor. However, purity should be evaluated not only from appearance but also together with the results of melting point measurement.
Causes When Recrystallization Did Not Work Well
When the expected crystals are not obtained during recrystallization, possible causes include solvent selection, solvent amount, cooling conditions, and filtration procedures.
When Almost No Crystals Were Obtained
- Too much solvent was used
- Cooling was insufficient
- The desired substance was too soluble in the solvent
- The amount of the desired substance in the sample was small
When Fine Crystals Were Obtained
- Rapid cooling was used
- Stirring was too vigorous
- There were many impurities
- Many crystal nuclei formed at the same time
When Color Remained in the Crystals
- Colored impurities were not completely removed
- The number of recrystallizations was insufficient
- Mother liquor remained on the crystals
- Washing was insufficient
Discussion Example:
Because coloration remained in the obtained crystals, impurities may not have been completely removed by recrystallization. In particular, if mother liquor remained on the crystal surfaces or washing was insufficient, colored impurities may remain attached to the crystals. Therefore, this may also have contributed to a decrease in the melting point or broadening of the melting point range.
Discussion When the Recovery Is Too High
If the recovery after recrystallization is too high, substances other than the desired product may have been included in the measurement. Typical causes include insufficient drying, residual mother liquor, and impurity contamination.
Discussion Example:
One possible reason for the high recovery is insufficient drying of the crystals. If the crystals were weighed while solvent or mother liquor remained in them, the measured mass would be greater than the actual mass of the desired product. In addition, if impurities were not completely removed, their mass would also be included, so the recovery may have been overestimated.
How to Write Improvements for Recrystallization
In a discussion of recrystallization, including improvements as well as causes makes the report easier to organize. It is important to describe improvements specifically in relation to your own results.
When You Want to Improve Yield
- Use the minimum necessary amount of solvent
- Cool sufficiently to precipitate the crystals
- Do not use too much washing solution
- Avoid losing crystals during filtration and transfer
When You Want to Improve Purity
- Select an appropriate solvent
- Remove insoluble impurities
- Avoid rapid cooling and allow slow crystallization
- Remove the mother liquor thoroughly
- Perform recrystallization again if necessary
Example of Writing Improvements:
To increase the recovery, it is important to use the minimum necessary amount of solvent during recrystallization and to cool the solution sufficiently. In addition, transfer operations should be performed carefully to avoid losing crystals during filtration and washing. On the other hand, to improve purity, it is also necessary to thoroughly remove the mother liquor and impurities, so it is important to consider the balance between recovery and purity.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of recrystallization, writing only that “the yield was low” or “I think there were impurities” makes the content superficial. Relating the results of yield, purity, and melting point makes the discussion more convincing.
| Superficial Discussion | Good Discussion |
|---|---|
| The yield was low because the experiment failed. | Possible reasons for the low recovery include part of the desired substance remaining dissolved in the mother liquor and loss of crystals during filtration and washing. In recrystallization, part of the desired substance is also lost during the process of increasing purity, so the recovery may decrease. |
| The melting point was low, so there were impurities. | Because the melting point of the obtained crystals was lower than the literature value and the melting point range was also broad, a small amount of impurities may have remained in the sample. Impurities can disrupt the crystal lattice and cause a decrease in melting point and broadening of the melting point range. |
| I think the purity increased through recrystallization. | The crystals after recrystallization became closer to white than before recrystallization, and the melting point also approached the literature value. This suggests that some of the colored impurities were removed into the mother liquor and that the purity of the sample improved. |
Example Expressions That Can Be Used in Reports
The following expressions can be used when writing a discussion of recrystallization. Adjust the necessary parts according to your own experimental results.
- Recrystallization is considered to have removed some of the impurities contained in the crude product into the mother liquor.
- One possible reason for the low recovery is that part of the desired substance remained dissolved in the solvent.
- During washing, part of the crystals may have dissolved in the washing solution, reducing the amount recovered.
- During filtration or transfer, some of the crystals may have adhered to equipment or filter paper and been lost.
- Because the melting point was lower than the literature value, a small amount of impurities may have remained.
- The broad melting point range suggests that the purity of the sample was insufficient.
- Because the crystals became white after recrystallization, colored impurities are considered to have been removed.
- Although the yield decreased, the purity is considered to have improved because the melting point approached the literature value.
- It is necessary to judge the success of recrystallization using not only the yield but also the melting point and appearance.
Points to Check When Discussing Recrystallization
Before writing the report, checking the following points can make it easier to write the discussion.
- How did the mass change before and after recrystallization?
- What was the recovery?
- Where is the likely cause of the low recovery?
- Was too much solvent used?
- Was cooling sufficient?
- Were crystals lost during filtration or transfer?
- Could the desired substance have dissolved during washing?
- What were the color and shape of the crystals?
- Was the melting point close to the literature value?
- Was the melting point range narrow or broad?
- Can the relationship between yield and purity be explained?
Summary
Recrystallization is an important purification operation used to increase the purity of solid samples. It uses differences in solubility between the desired substance and impurities and recovers the desired substance as crystals through cooling.
In recrystallization, although purity may improve, part of the desired substance may dissolve in the mother liquor or washing solution or be lost during filtration and transfer, causing the yield or recovery to decrease. Therefore, a low yield alone should not be regarded as a failure; it is important to evaluate the result together with the melting point and appearance of the crystals.
In the discussion section of a report, relating yield, purity, and melting point makes the explanation more convincing. Consider the success of recrystallization based on combinations of results, such as “the yield was low but the melting point was close to the literature value” or “the yield was high but the melting point range was broad.”
