Chemistry 化学

Recrystallization of Benzoic Acid Discussion Examples | Relationship Between Improved Purity and Reduced Yield

Recrystallization of benzoic acid is a representative purification experiment commonly performed in organic chemistry laboratories.
In recrystallization, differences in solubility between the target compound, benzoic acid, and impurities are used to obtain crystals of higher purity.
In reports, it is important to discuss why purity improved through recrystallization and why the yield tends to decrease at the same time.

In a discussion of benzoic acid recrystallization, it is not sufficient simply to write that “white crystals were obtained” or “the melting point approached the literature value.”
It is necessary to explain why the crystals precipitated, why impurities were removed, why the recovered amount decreased, and how solvent volume, cooling rate, filtration, washing, and drying affected the results.

This article clearly explains how to interpret the results of benzoic acid recrystallization experiments, the relationship between improved purity and reduced yield, how to discuss melting point, sources of error, points for improvement, and discussion examples that can be used in reports.

Note:
This article is a reference intended to assist with discussions of results obtained in chemistry experiments at universities and similar institutions.
For the actual heating, handling of solvents, hot filtration, cooling, suction filtration, washing, drying, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.

What Is a Benzoic Acid Recrystallization Experiment?

A benzoic acid recrystallization experiment is a purification experiment in which crude benzoic acid is dissolved in an appropriate solvent by heating and then precipitated again as crystals by cooling.
The purity of the obtained benzoic acid crystals increases because impurities either remain dissolved in the mother liquor or are removed by hot filtration.

The key point in recrystallization is to use a solvent in which benzoic acid dissolves well at high temperature but poorly at low temperature.
This property makes it possible to dissolve benzoic acid in the solution during heating and precipitate it as crystals during cooling.

Example Discussion:
In this experiment, recrystallization was performed using the difference in the solubility of benzoic acid with temperature.
Benzoic acid dissolves readily in a hot solvent, but its solubility decreases upon cooling, causing the solution to become supersaturated and crystals to precipitate.
Through this operation, some impurities remained in the mother liquor, and the purity of the benzoic acid is considered to have improved.

Main Items to Include in the Results

In a benzoic acid recrystallization experiment, organize the mass before and after recrystallization, yield, appearance of the crystals, melting point, melting-point range, condition of the mother liquor, and similar observations.
In particular, whether the purity increased through recrystallization can be discussed by comparing the melting point with the literature value and examining changes in the melting-point range.

Main Items to Include in the Results

  • Mass of crude benzoic acid before recrystallization
  • Type and amount of solvent used
  • Whether complete dissolution occurred during heating
  • Whether hot filtration was performed
  • Appearance of crystal precipitation during cooling
  • Color and shape of the obtained crystals
  • Mass of benzoic acid crystals after drying
  • Recovery rate or yield
  • Melting point before and after recrystallization
  • Comparison with the literature value
  • Possibility that benzoic acid remained in the mother liquor
  • Losses during filtration, washing, and drying

Example of How to Write the Results:
Crude benzoic acid was dissolved in a heated solvent, and white crystals precipitated upon cooling after hot filtration.
The obtained crystals were collected by suction filtration, washed, and dried, giving a recovered amount of 0.82 g.
The melting point after recrystallization approached the literature value compared with before recrystallization, and the melting-point range also became narrower.
From this, the purity of benzoic acid is considered to have improved through recrystallization.

Why Purity Improves Through Recrystallization

Purity improves through recrystallization because the difference in solubility between the target compound and impurities is used.
After benzoic acid is dissolved in a hot solvent, cooling causes the benzoic acid to precipitate as crystals.
Meanwhile, many soluble impurities tend to remain in the mother liquor.

In addition, if insoluble impurities are present, they can be removed by filtration while the benzoic acid remains dissolved in the hot solution.
In this way, recrystallization improves purity through two mechanisms: “removing insoluble impurities by filtration” and “leaving soluble impurities in the mother liquor.”

Example Discussion:
Because the melting point of benzoic acid after recrystallization approached the literature value, the purity is considered to have improved through recrystallization.
This is because benzoic acid crystallized selectively during cooling while many soluble impurities remained in the mother liquor.
In addition, if insoluble impurities were present, they may have been removed by hot filtration.

Why Yield Decreases Through Recrystallization

Although purity improves through recrystallization, the yield tends to decrease.
This is because not all of the benzoic acid precipitates as crystals, and some remains dissolved in the mother liquor.
Benzoic acid is not completely insoluble even at low temperature, so a certain amount remains in the mother liquor after cooling.

In addition, some crystals may be lost during filtration, washing, transfer, and drying.
In other words, during recrystallization, part of the target benzoic acid is also lost in the process of “removing impurities to increase purity,” so the recovery rate tends to decrease.

Example Discussion:
One possible reason the recovery rate of benzoic acid after recrystallization was lower than 100% is that part of the benzoic acid remained dissolved in the mother liquor.
Recrystallization can remove impurities and increase purity, but a certain amount of the target compound also remains dissolved in the solvent.
Therefore, purity improved while the recovered amount of benzoic acid decreased.

Relationship Between Improved Purity and Reduced Yield

In recrystallization, improved purity and reduced yield tend to occur at the same time.
If impurities are thoroughly removed, the target compound is also more easily lost into the mother liquor or washing solution.
Conversely, if washing or recrystallization is insufficient in an attempt to maintain a high yield, impurities are more likely to remain.

Therefore, evaluation of recrystallization requires consideration not only of recovery rate but also of purity based on melting point and crystal appearance.
Rather than aiming only for a high yield, it is important to consider the balance between purity and yield according to the purpose.

Operation / Condition Effect on Purity Effect on Yield
Recrystallization performed thoroughly Tends to increase Tends to decrease
Washing performed thoroughly Tends to increase May decrease
Insufficient washing Tends to decrease May appear high
Mother liquor not removed sufficiently Tends to decrease May appear high

Example Discussion:
Although the melting-point range became narrower after recrystallization, the recovery rate decreased.
This is considered to be because impurities were removed into the mother liquor through recrystallization, improving purity, while part of the benzoic acid also remained dissolved in the mother liquor.
Therefore, in recrystallization, improved purity and reduced yield may have a trade-off relationship.

Discussion of Solvent Selection

Selecting an appropriate solvent is important in recrystallization.
An ideal solvent dissolves benzoic acid well at high temperature but only slightly at low temperature.
Using such a solvent makes it possible to completely dissolve benzoic acid during heating and efficiently precipitate it as crystals during cooling.

If a solvent that dissolves benzoic acid well even at low temperature is used, sufficient crystals do not precipitate even after cooling, and the yield decreases.
Conversely, if benzoic acid does not dissolve well even at high temperature, recrystallization itself will not proceed properly.

Example Discussion:
A recrystallization solvent should dissolve benzoic acid well at high temperature and poorly at low temperature.
Because crystals precipitated upon cooling in this experiment, the solubility of benzoic acid in the solvent is considered to have decreased as the temperature fell, producing a supersaturated state.
On the other hand, if the recovered amount of crystals was small, some benzoic acid may have remained dissolved in the mother liquor even at low temperature.

When Too Much Solvent Is Used

If too much solvent is used in recrystallization, a larger amount of benzoic acid remains dissolved in the mother liquor even after cooling, reducing the amount that can be recovered as crystals.
As a result, the yield decreases.
In recrystallization, it is important to use an amount of solvent close to the minimum necessary to completely dissolve the target compound.

Example Discussion:
One possible reason the recovery rate of benzoic acid was low is that too much solvent was used.
When a large amount of solvent is used, a greater amount of benzoic acid remains dissolved in the mother liquor even after cooling.
Therefore, the amount of benzoic acid precipitating as crystals decreases, possibly reducing the recovery rate.

When Too Little Solvent Is Used

If too little solvent is used, benzoic acid may not dissolve completely even when heated.
If undissolved benzoic acid or impurities remain and the mixture is cooled, the purification effect may decrease or impurities may become mixed into the crystals.

In addition, if the solution is too concentrated, crystallization may occur rapidly, producing small crystals or crystals containing impurities.

Example Discussion:
If too little solvent was used, benzoic acid may not have dissolved completely and undissolved material may have remained.
If cooling is performed while undissolved material remains, impurities cannot be sufficiently separated and the purity after recrystallization may not improve adequately.
In addition, if the concentration is too high, crystals may precipitate rapidly and may more readily incorporate impurities.

Discussion of Hot Filtration

Hot filtration is an operation used to remove insoluble impurities while benzoic acid remains dissolved in the hot solvent.
This operation makes it possible to remove insoluble impurities before crystallization.
However, if the solution cools during filtration, benzoic acid may crystallize on the filter paper or inside the funnel, reducing the recovered amount.

Example Discussion:
Insoluble impurities are considered to have been removed by hot filtration.
However, if the temperature of the solution decreased during filtration, benzoic acid may have precipitated on the filter paper or inside the funnel and become difficult to recover.
Therefore, during hot filtration, handling the solution while keeping it as hot as possible is important for preventing a decrease in yield.

Cooling Rate and Crystal Size

Cooling rate affects the size and purity of benzoic acid crystals.
Rapid cooling causes the degree of supersaturation to increase quickly, making it easier for many nuclei to form at once.
As a result, many small, fine crystals form and may more readily incorporate impurities or mother liquor.

In contrast, slow cooling suppresses nucleation and allows a smaller number of crystals to grow over time.
Therefore, relatively large and highly pure crystals may be obtained more easily.

Example Discussion:
One possible reason the obtained benzoic acid crystals were fine is that cooling was rapid.
Rapid cooling caused the degree of supersaturation to rise quickly and many nuclei to form at once, making it difficult for each crystal to grow large.
In addition, rapid crystallization can cause impurities to be incorporated, so purity may also have been affected.

Discussion When Cooling Slowly

When cooling is slow, the increase in supersaturation is gradual and formation of crystal nuclei is suppressed.
As a result, a small number of nuclei grow over time, making it easier to obtain relatively large crystals.
Large crystals are easier to handle during filtration and washing and may retain less mother liquor.

Example Discussion:
One possible reason relatively large benzoic acid crystals were obtained is that cooling proceeded slowly and the number of crystal nuclei formed was limited.
When the number of nuclei is small, benzoic acid in the solution is incorporated into a limited number of nuclei, allowing the crystals to grow larger.
Therefore, slow cooling is considered to have been effective in improving crystal size and purity.

Discussion When the Crystals Are Fine

If benzoic acid crystals are fine, they may be more easily lost through the filter paper or adhere to the filter paper or apparatus during filtration.
In addition, fine crystals have a larger surface area and may more readily adsorb mother liquor and impurities, causing reduced purity.

Example Discussion:
Because the crystals were fine, part of the benzoic acid may have been lost during suction filtration or washing.
Fine crystals can pass through the filter paper or adhere more easily to the apparatus, reducing the recovery rate.
In addition, because they have a large surface area, they tend to retain mother liquor, which may also cause impurity contamination and a broader melting-point range.

Losses During Filtration and Transfer

Benzoic acid crystals after recrystallization are recovered by suction filtration or gravity filtration.
During this process, if crystals remain on the filter paper, adhere to the apparatus, or are spilled during transfer, the recovered amount decreases.
These are representative causes of reduced yield in recrystallization experiments.

Example Discussion:
One possible reason the recovery rate decreased is that some benzoic acid crystals were lost during filtration or transfer.
If crystals remained attached to the filter paper, beaker, glass rod, or other apparatus and were not recovered, the mass measured after drying would become smaller.
Therefore, mechanical loss during recovery operations is also one cause of reduced yield.

Loss Caused by Washing

Washing crystals is necessary to remove mother liquor and soluble impurities.
However, if benzoic acid dissolves in the washing solution, part of it is lost during washing.
The recovery rate tends to decrease when too much washing solution is used, when the washing solution is warm, or when a solvent in which benzoic acid is relatively soluble is used.

Example Discussion:
If part of the benzoic acid dissolved in the washing solution during washing, the recovered amount would decrease.
In particular, when a large amount of washing solution is used or the washing solution is warm, the amount of benzoic acid dissolved may increase.
Therefore, washing should be performed using the minimum amount necessary to remove impurities.

Reduced Purity Caused by Insufficient Washing

If washing is insufficient, mother liquor and soluble impurities remain on the crystal surface.
As a result, the mass after drying becomes larger and the recovery rate may appear high.
However, because impurities are actually present, the melting point may decrease or the melting-point range may broaden.

Example Discussion:
If washing was insufficient, mother liquor and soluble impurities may have remained on the surface of the benzoic acid crystals.
In this case, the mass after drying includes impurities, so the recovery rate may appear high.
On the other hand, because impurities are present, the melting point may decrease and the melting-point range may broaden.

Effects of Insufficient Drying

If benzoic acid crystals are not sufficiently dried, water or solvent remains on the crystal surface and the measured mass becomes larger than the actual mass.
Therefore, the recovery rate may be overestimated.
Residual solvent may also affect melting-point measurement and may cause a lower melting point or a broader melting-point range.

Example Discussion:
If drying was insufficient, water or solvent may have remained in the benzoic acid crystals and caused the measured mass to be too large.
As a result, the recovery rate would be estimated higher than the actual value.
In addition, residual solvent affects the melting behavior of the crystals and may cause a lower melting point or a broader melting-point range.

Discussion of Purity Evaluation Using Melting Point

In benzoic acid recrystallization, purity can be evaluated by melting-point measurement.
Highly pure benzoic acid is expected to have a melting point close to the literature value and a narrow melting-point range.
If impurities are present, the melting point may decrease or the melting-point range may broaden.

Example Discussion:
If the melting point of benzoic acid after recrystallization was close to the literature value and the melting-point range was also narrow, impurities are considered to have been removed through recrystallization and the purity improved.
On the other hand, if the melting point was low or the melting-point range was broad, remaining mother liquor, unremoved impurities, insufficient drying, or similar factors may be responsible.
Therefore, melting point is an important indicator for evaluating the purification effect of recrystallization.

Causes of a Low Melting Point

If the melting point of benzoic acid after recrystallization is lower than the literature value, impurities may be present.
When impurities are incorporated into the crystal lattice, the regularity of the crystal is disturbed and melting may begin at a lower temperature than for pure benzoic acid.
Residual solvent and moisture may also cause melting-point depression.

Example Discussion:
One possible reason the melting point after recrystallization was lower than the literature value is that impurities or residual solvent were present in the crystals.
When impurities are mixed in, the crystal lattice becomes disturbed and melting begins at a lower temperature than for pure benzoic acid.
Therefore, a lower melting point provides a clue that the purity after recrystallization may still have been insufficient.

Causes of a Broad Melting-Point Range

If the melting-point range is broad, the sample may contain multiple components rather than a single component.
A pure substance melts over a relatively narrow temperature range, but when impurities are present, the temperature difference from the beginning of melting to complete melting becomes larger.

Example Discussion:
One possible reason the melting-point range was broad is that impurities remained in the benzoic acid after recrystallization.
When multiple components are present, their melting behaviors overlap, making it difficult to show a sharp melting point like a pure substance.
Therefore, a broadened melting-point range may indicate that purification by recrystallization was insufficient.

Discussion Comparing Melting Points Before and After Recrystallization

If the melting point is measured before and after recrystallization, the purification effect can be discussed more clearly.
If the melting point approaches the literature value and the melting-point range becomes narrower after recrystallization, it becomes easier to conclude that impurities were removed and purity improved.

Change Discussion
Melting point approached the literature value Purity may have improved
Melting-point range became narrower Impurities may have decreased
Melting point remained low Impurities or solvent may remain
Melting-point range remained broad Purification may have been insufficient

Example Discussion:
Compared with before recrystallization, the melting point of benzoic acid after recrystallization approached the literature value and the melting-point range also became narrower.
This suggests that impurities were removed by recrystallization and the purity of the benzoic acid improved.
On the other hand, because the recovery rate decreased, part of the benzoic acid is considered to have been lost into the mother liquor or washing solution during purification.

Discussion Based on Crystal Appearance

Benzoic acid after recrystallization is often obtained as white crystals.
However, the fact that crystals appear white alone does not prove high purity.
Observe the color, shape, size, luster, moisture, and whether the sample is powdery, and evaluate these together with the melting point.

Example Discussion:
Because white crystals were obtained after recrystallization, some colored impurities may have been removed.
However, even if the crystals are white, colorless impurities or residual solvent may still be present.
Therefore, purity should not be judged from crystal appearance alone but should be evaluated together with the melting point and melting-point range.

Discussion When Activated Carbon Is Used

Activated carbon may be used to remove colored impurities.
Activated carbon adsorbs colored components and similar substances and can reduce the color of the solution.
However, if too much activated carbon is used, the target compound, benzoic acid, may also be adsorbed and the yield may decrease.

Example Discussion:
The use of activated carbon is considered to have adsorbed colored impurities and made the crystals after recrystallization appear closer to white.
On the other hand, if an excessive amount of activated carbon was added, some benzoic acid may also have been adsorbed.
Therefore, while activated carbon is effective for improving purity, excessive use may cause a decrease in recovery.

Example Discussion When Recrystallization Went Well

When recrystallization goes well, the crystals are white, the melting point is close to the literature value, and the melting-point range becomes narrower.
On the other hand, the recovery rate is often lower than before recrystallization.
In such a case, it is useful to discuss improved purity and reduced yield together.

Example Discussion:
After recrystallization, white benzoic acid crystals were obtained, and the melting point showed a value close to the literature value.
In addition, the melting-point range became narrower than before recrystallization, so impurities are considered to have been removed and the purity improved.
On the other hand, the recovery rate was lower than 100%.
This is considered to have been caused by some benzoic acid remaining dissolved in the mother liquor and by loss of crystals during filtration and washing.

Example Discussion When Recrystallization Did Not Go Well

When recrystallization does not go well, possible causes are considered from results such as no crystal formation, excessively low yield, deviation of the melting point from the literature value, a broad melting-point range, fine crystals, or wet crystals.
It is easier to organize the discussion by separately considering solvent volume, cooling rate, filtration, washing, drying, and residual impurities.

Example Discussion:
One possible reason the melting-point range remained broad after recrystallization is that impurities were not sufficiently removed.
If too little solvent was used and impurities crystallized together with the benzoic acid, or if rapid cooling caused mother liquor to become trapped in the crystals, the purity would be difficult to improve sufficiently.
In addition, residual solvent caused by insufficient drying may also lead to melting-point depression and a broader melting-point range.

How to Write Points for Improvement

In a discussion of benzoic acid recrystallization, including points for improvement in addition to sources of error makes the report easier to organize.
Improvements are easier to organize when divided into those for increasing purity and those for increasing yield.

Improvements for Increasing Purity

  • Select an appropriate recrystallization solvent
  • Remove insoluble impurities by hot filtration
  • Cool the solution slowly rather than rapidly
  • Use activated carbon to remove colored impurities when necessary
  • Remove the mother liquor sufficiently
  • Wash the crystals appropriately
  • Dry the crystals sufficiently before measuring the melting point

Improvements for Increasing Yield

  • Do not use too much solvent
  • Cool sufficiently to promote crystallization
  • Take care not to let the solution cool excessively during filtration and cause crystals to precipitate inside the apparatus
  • Recover as much of the crystals remaining on the filter paper and apparatus as possible
  • Use the minimum necessary amount of washing solution
  • Avoid rapid cooling so that the crystals do not become excessively fine

Example of How to Write Points for Improvement:
To increase purity, it is important to use a solvent in which benzoic acid dissolves well at high temperature and poorly at low temperature and to cool the solution slowly to suppress incorporation of impurities.
On the other hand, to increase yield, the amount of solvent should not be excessive and sufficient cooling should be performed so that as much benzoic acid as possible precipitates as crystals.
In addition, careful operation during filtration and washing can reduce the decrease in recovery caused by crystal loss.

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of benzoic acid recrystallization, simply writing that “it became cleaner” or “the yield decreased” results in a superficial discussion.
Relating the results to differences in solubility, loss into the mother liquor, melting point, filtration, washing, and drying produces a more persuasive discussion.

Superficial Discussion Good Discussion
Purity increased through recrystallization. Because the melting point approached the literature value and the melting-point range became narrower after recrystallization, impurities are considered to have been removed into the mother liquor and the purity of benzoic acid improved.
The yield decreased. Possible reasons the yield decreased include some benzoic acid remaining dissolved in the mother liquor and loss of crystals during filtration, washing, and transfer.
The crystals were fine. Because rapid cooling caused many nuclei to form at once, the benzoic acid crystals are considered to have become fine. Fine crystals tend to retain mother liquor and may lead to losses during filtration and reduced purity.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of a benzoic acid recrystallization experiment.
Adjust the necessary parts according to your own experimental results.

  • Recrystallization is a purification operation that uses differences in solubility between benzoic acid and impurities.
  • Cooling reduced the solubility of benzoic acid, causing the solution to become supersaturated and crystals to precipitate.
  • Because the melting point approached the literature value after recrystallization, the purity is considered to have improved.
  • The narrower melting-point range provides a clue that the amount of impurities decreased.
  • One possible reason the recovery rate decreased is that benzoic acid remained dissolved in the mother liquor.
  • If too much solvent is used, benzoic acid tends to remain in the mother liquor even after cooling and the yield decreases.
  • Rapid cooling may have produced fine crystals that more readily incorporated impurities or mother liquor.
  • If washing is insufficient, mother liquor and soluble impurities remain and may cause a lower melting point and a broader melting-point range.
  • If water or solvent remains because of insufficient drying, the recovery rate may be overestimated.
  • In recrystallization, improved purity and reduced yield may have a trade-off relationship.

Points to Check When Discussing Benzoic Acid Recrystallization

Checking the following points before writing the report makes the discussion easier to write.

  • Have you compared the mass before and after recrystallization?
  • Have you calculated the recovery rate or yield?
  • Have you compared the melting point before and after recrystallization?
  • Have you explained why the melting point approached the literature value?
  • Have you related the narrower melting-point range to increased purity?
  • Have you considered benzoic acid remaining in the mother liquor?
  • Have you considered whether too much or too little solvent was used?
  • Have you written about the effect of cooling rate on crystal size and purity?
  • Have you considered losses and errors caused by filtration, washing, and drying?
  • If activated carbon was used, have you explained both its effect and the associated loss?
  • Have you evaluated purity and yield separately?
  • Do the points for improvement correspond to the sources of error?

Summary

In a benzoic acid recrystallization experiment, differences in solubility with temperature are used to recover crude benzoic acid as crystals of higher purity.
If the melting point approaches the literature value and the melting-point range becomes narrower after recrystallization, impurities are considered to have been removed and the purity improved.

On the other hand, because some benzoic acid remains dissolved in the mother liquor during recrystallization, the recovery rate tends to decrease.
In addition, losses during filtration, washing, and transfer, dissolution into the washing solution, and precipitation during hot filtration can also cause a decrease in yield.

In a report, do not simply write separately that “purity increased” and “yield decreased,” but explain that improved purity and reduced yield tend to occur simultaneously in recrystallization.
A persuasive benzoic acid recrystallization report can be produced by relating the melting point, melting-point range, crystal appearance, solvent volume, cooling rate, filtration, washing, and drying.

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