Discussion of filtrate and precipitate is very important in precipitation reactions, recrystallization, washing after extraction, separation of inorganic ions, and purification in organic synthesis.
When filtration is performed, if it is not correctly determined whether the desired product is in the precipitate on the filter paper or dissolved in the filtrate, the desired product may be discarded by mistake.
In particular, the appearance of a precipitate does not necessarily mean that the precipitate is the desired product, and the desired product may remain in the filtrate.
In this discussion, it is not sufficient simply to write that “a precipitate formed” or “the filtrate was discarded.”
It is necessary to explain in which phase the desired product is likely to exist based on the solubility of the desired product, reaction equation, solubility product, pH, temperature, type of solvent, complex formation, salt formation, recrystallization conditions, and washing conditions.
In addition, by examining the filtrate or precipitate using confirmation reactions, TLC, pH measurement, ion confirmation, melting point, IR, NMR, or other methods, evidence for the judgment can be presented.
This article clearly explains, as examples of discussions that can be used in laboratory reports on filtrate and precipitate, the meaning of filtration, the difference between filtrate and precipitate, methods for determining the location of the desired product, solubility, precipitation, solubility product, recrystallization, washing, pH, complex formation, coprecipitation, loss of the desired product, confirmation methods, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of filtration operations and precipitation separation performed in basic chemistry experiments, analytical chemistry experiments, inorganic chemistry experiments, and organic chemistry experiments at universities and similar institutions.
Whether the filtrate or precipitate should actually be retained, and which should be washed, dried, or analyzed, must always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Are Filtrate and Precipitate?
- Main Items to Include in the Results
- When the Desired Product Is in the Precipitate
- When the Desired Product Is in the Filtrate
- Determination Based on Solubility
- Determination Based on a Precipitation Reaction
- Determination Based on the Solubility Product
- Filtrate and Precipitate in Recrystallization
- Determination in Hot Filtration
- Filtrate and Precipitate During Washing
- Movement of the Desired Product Depending on pH
- Determination Based on Complex Formation
- Incorrect Judgment Caused by Coprecipitation
- When the Precipitate Is Too Fine
- Reasons the Desired Product Remains in the Filtrate
- Reasons Impurities Remain in the Precipitate
- Importance of Checking Before Discarding the Filtrate
- Determination by Confirmation Reactions
- Determination by TLC
- Determination by Melting Point and Spectroscopy
- Failure Caused by Misjudging the Filtrate and Precipitate
- Causes of Error Related to Filtrate and Precipitate
- When the Results Can Be Considered Good
- Example Discussions When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing Filtrate and Precipitate
- Summary
What Are Filtrate and Precipitate?
When filtration is performed, the mixture is separated into a solid that remains on the filter paper or filter and a liquid that passes through the filter paper.
The solid remaining on the filter paper is called the precipitate or residue, while the liquid that passes through the filter paper is called the filtrate.
The precipitate contains substances that are poorly soluble in water or solvent, while the filtrate may contain dissolved components or fine particles.
Whether the desired product is in the precipitate or the filtrate depends on the purpose of the experiment and the chemical properties of the desired product.
In experiments where a sparingly soluble salt is formed by a precipitation reaction, the precipitate is often the desired product.
On the other hand, when unwanted impurities are removed as a precipitate, the desired product remains in the filtrate.
Therefore, it is important not to assume that “precipitate = desired product.”
Example Discussion:
Filtration separated the mixture into a precipitate remaining as a solid and a filtrate passing through the filter paper.
Whether the desired product is present in the precipitate or filtrate is determined by the solubility of the desired product and the properties of the substances formed in the reaction.
Therefore, after filtration, it is necessary to determine whether the precipitate or filtrate should be recovered based on the reaction equation and solubility.
Main Items to Include in the Results
To discuss the filtrate and precipitate, it is necessary to organize the condition of the sample before and after filtration.
Recording the color, amount, and particle size of the precipitate, the color and transparency of the filtrate, pH, results of confirmation reactions, TLC or spectroscopic results, and condition of the washing solution makes it easier to explain where the desired product is present.
Main Items to Include in the Results
- Condition of the solution or suspension before filtration
- Presence or absence of a precipitate
- Color of the precipitate
- Amount of precipitate
- Particle size of the precipitate
- Color of the filtrate
- Transparency of the filtrate
- pH of the filtrate
- Color and pH of the washing solution
- Mass of the precipitate after drying
- Confirmation reactions in the filtrate
- Confirmation reactions in the precipitate
- Results of TLC, IR, NMR, melting point, and other analyses
- Determination of whether the desired product is in the precipitate or filtrate
- Possibility of loss of the desired product
- Points for improvement
Example of How to Write the Results:
Filtration produced a white precipitate and a colorless, transparent filtrate.
After the precipitate was washed and dried, a confirmation reaction showed a reaction corresponding to the desired component.
On the other hand, a slight reaction of the desired component was also observed in the filtrate, suggesting that most of the desired product was present in the precipitate, but some may have remained dissolved in the filtrate.
When the Desired Product Is in the Precipitate
The desired product is present in the precipitate when it is formed as a solid that is poorly soluble in water or the solvent.
For example, when a sparingly soluble salt is produced in a precipitation reaction, the precipitate itself is the desired product.
In recrystallization as well, the crystals that precipitate upon cooling are the desired product, and the filtrate is treated as the mother liquor.
In this case, the precipitate remaining on the filter paper is collected, washed, dried, weighed, and analyzed.
However, the desired product is not necessarily completely precipitated.
Because the desired product has some solubility, part of it may remain in the filtrate.
When discussing a decrease in yield, dissolution loss into the filtrate should be considered.
Example Discussion:
In this experiment, because the desired product is formed as a sparingly soluble solid, the precipitate remaining on the filter paper is considered to be the desired product.
The desired product can be isolated by collecting, washing, and drying the precipitate.
However, because the desired product is not completely insoluble, some may have remained dissolved in the filtrate and may have caused a decrease in yield.
When the Desired Product Is in the Filtrate
The desired product is present in the filtrate when unwanted solids or impurities are removed as a precipitate.
For example, after a reaction, insoluble by-products, drying agents, activated carbon, or inorganic salts may be removed by filtration while the desired product remains dissolved in the solution.
In this case, it is important to collect the filtrate.
If only the precipitate is collected in an experiment where the desired product is in the filtrate, the desired product will be discarded.
Conversely, in filtration intended to remove a precipitate, the desired product is recovered by concentrating, extracting, or recrystallizing the filtrate.
When the desired product is in the filtrate, the color of the filtrate, TLC results, and residue after concentration are checked.
Example Discussion:
In this operation, the precipitate was formed to remove insoluble impurities or a drying agent, while the desired product was considered to be dissolved in the filtrate.
Therefore, after filtration, it is necessary to collect the filtrate rather than the precipitate and obtain the desired product by concentration or purification.
When the desired product is in the filtrate, mistakenly identifying the precipitate as the desired product can lead to a large loss of yield.
Determination Based on Solubility
The most basic criterion for determining whether the desired product is in the filtrate or precipitate is the solubility of the desired product.
If the desired product is poorly soluble in the solvent used, it tends to precipitate or crystallize, while if it is readily soluble, it tends to remain in the filtrate.
Solubility changes depending on the type of solvent, temperature, pH, salt form, and molecular polarity.
In organic compounds, highly polar substances tend to dissolve more easily in water, while nonpolar substances tend to dissolve more easily in organic solvents.
Acidic and basic compounds may change their ionization state depending on pH and may become more likely to move into the aqueous phase or filtrate.
In inorganic salts, the tendency to precipitate changes depending on the solubility product and complex formation.
Example Discussion:
Whether the desired product exists in the precipitate or filtrate can be judged from the solubility of the desired product.
If the desired product is poorly soluble in the reaction solvent, it tends to precipitate or crystallize, while if it is readily soluble, it tends to remain in the filtrate.
Therefore, it is important to consider the difference in solubility between the desired product and impurities in the solvent used.
Determination Based on a Precipitation Reaction
In a precipitation reaction, ions in solution react to form salts or hydroxides that are poorly soluble in water.
If the desired product is this sparingly soluble precipitate, the desired product is in the precipitate.
For example, when AgCl is formed from Ag+ and Cl-, the precipitate is collected if AgCl is the desired product.
On the other hand, when a precipitation reaction is used to remove interfering ions, the precipitate is an impurity and the desired product remains in the filtrate.
For example, in a separation operation in which one metal ion is precipitated and removed while another ion remains in the filtrate, the filtrate may be the important fraction.
It is necessary to check the reaction equation and the purpose of the experiment.
Ag+ + Cl- → AgCl(s)
Ca2+ + CO32- → CaCO3(s)
Example Discussion:
If the sparingly soluble salt produced in the precipitation reaction is the desired product, the desired product is collected as the precipitate on the filter paper.
However, if the precipitation reaction is being used to remove impurities, the precipitate is the material to be removed and the desired product is present in the filtrate.
Therefore, it is necessary to determine from the reaction equation whether the precipitate is the desired product or an impurity, rather than judging only from the fact that a precipitate formed.
Determination Based on the Solubility Product
In inorganic precipitation, the solubility product Ksp can be used to explain the conditions under which a precipitate forms.
Whether the sparingly soluble salt MaXb precipitates is determined by whether the ionic product in the solution exceeds Ksp.
If the ionic product exceeds Ksp, a precipitate forms, whereas if it is below Ksp, the substance tends to remain dissolved in the solution.
Even when the desired product is a sparingly soluble precipitate, Ksp is not zero, so a small amount remains dissolved in the filtrate.
Therefore, dissolution loss into the filtrate can be discussed as one reason why the yield does not reach 100% even when the precipitate is collected.
Precipitation may also be made more complete by the common-ion effect or pH adjustment.
MaXb(s) ⇄ aMn+ + bXm-
Ksp = [Mn+]a[Xm-]b
Example Discussion:
Even when the desired product is formed as a sparingly soluble precipitate, it is not completely insoluble, so some remains dissolved in the filtrate.
Precipitation can be explained by the ionic product exceeding the solubility product Ksp, but at equilibrium, a small amount of the desired product remains in solution.
Therefore, dissolution loss into the filtrate can be considered one cause of decreased yield.
Filtrate and Precipitate in Recrystallization
In recrystallization, the crude product is dissolved in a warm solvent and the desired product is precipitated as crystals by cooling.
In this case, the crystals remaining on the filter paper are the desired product, and the filtrate is the mother liquor.
The mother liquor contains many impurities, but a certain amount of the desired product also remains dissolved.
One reason for decreased yield in recrystallization is that the desired product remains dissolved in the mother liquor.
If cooling is insufficient, too much solvent is used, or the desired product has high solubility, a large amount of the desired product remains in the filtrate.
Therefore, recrystallization requires discussion of the balance between improved purity and decreased yield.
Example Discussion:
In recrystallization, the crystals precipitated by cooling are the desired product, and the filtrate is treated as the mother liquor.
The mother liquor contains impurities, but some of the desired product also remains according to its solubility.
Therefore, one possible reason for the decrease in yield after recrystallization is that the desired product remained dissolved in the filtrate.
Determination in Hot Filtration
Hot filtration is an operation in which the desired product dissolved in a warm solution is allowed to pass into the filtrate while insoluble impurities are removed on the filter paper.
In hot filtration before recrystallization, the desired product is dissolved in the warm solvent, so it is generally present in the filtrate.
The solids remaining on the filter paper are often activated carbon or insoluble impurities.
However, if the temperature decreases during hot filtration, the desired product may crystallize on the filter paper or inside the funnel.
In that case, part of the desired product is lost to the precipitate side and the yield decreases.
In hot filtration, it is important to warm the filtration apparatus, work quickly, and use an appropriate amount of solvent.
Example Discussion:
In hot filtration, because the desired product is dissolved in the warm solvent, it is basically present in the filtrate.
The solid remaining on the filter paper is considered to be an insoluble impurity.
However, if the solution temperature decreases during filtration, the desired product may precipitate on the filter paper and be lost to the precipitate side, so hot filtration must be performed quickly.
Filtrate and Precipitate During Washing
When a precipitate or crystals on the filter paper are washed, the washing solution flows to the filtrate side.
Because the washing solution contains mother liquor, impurities, excess reagents, salts, and other substances, it is normally treated as a liquid containing impurities.
When the desired product is in the precipitate, the ideal washing condition leaves as much of the desired product as possible in the precipitate while washing away only the impurities.
However, if the desired product dissolves in the washing solution, the washing solution also contains the desired product.
The larger the amount of washing solution and the greater the number of washes, the greater the dissolution loss of the desired product.
It is important to consider whether the desired product is dissolved in the washing solution before discarding it.
Example Discussion:
When the precipitate was washed, impurities in the mother liquor were considered to have been removed into the filtrate together with the washing solution.
On the other hand, if the desired product is slightly soluble in the washing solution, some of it is also contained in the washing solution and becomes a cause of decreased yield.
Therefore, during washing, conditions must be selected that transfer impurities to the filtrate while leaving the desired product in the precipitate.
Movement of the Desired Product Depending on pH
Acidic or basic compounds change their ionization state depending on pH.
Ionized compounds become more soluble in water and tend to move into the filtrate or aqueous phase.
On the other hand, in the neutral molecular form, they may become more soluble in organic solvents or more likely to precipitate.
For example, carboxylic acids become carboxylate salts under basic conditions and become more soluble in water.
Amines become ammonium salts under acidic conditions and become more soluble in water.
Because pH adjustment can precipitate the desired product or move it into the filtrate, pH is an important criterion for determining its location.
RCOOH + OH- → RCOO- + H2O
RNH2 + H+ → RNH3+
Example Discussion:
If the desired product has acidic or basic properties, whether it remains in the filtrate or precipitates depends on pH.
In the ionized salt form, it is more soluble in water and tends to exist in the filtrate.
On the other hand, when it returns to the neutral molecular form, its solubility may decrease and it may precipitate, so pH must be considered when determining where the desired product is present.
Determination Based on Complex Formation
Metal ions may form complexes with ammonia, chloride ions, EDTA, cyanide ions, and other substances.
When a complex is formed, the metal ion is stabilized in solution and may become less likely to precipitate.
Therefore, a metal component that would normally be expected to precipitate may remain in the filtrate.
Conversely, complex formation may be used to keep the desired product in the filtrate while precipitating only impurities.
In experiments involving complex formation, judgment cannot be made from the solubility product alone.
The presence of ligands, pH, and stability of the complex must also be considered.
Example Discussion:
When a metal ion forms a complex, the concentration of free metal ions decreases and precipitation becomes less likely.
Therefore, the target metal may remain in the filtrate rather than in the precipitate.
When considering whether a precipitate will form, changes in solubility caused by complex formation must be considered in addition to the solubility product.
Incorrect Judgment Caused by Coprecipitation
Coprecipitation is a phenomenon in which a component that should normally remain in the filtrate is incorporated together with another precipitate.
It occurs when ions or molecules adsorb onto the surface of the precipitate or become incorporated into the crystal interior.
When coprecipitation occurs, even if the desired product should be in the filtrate, some may also be contained in the precipitate.
Coprecipitation is more likely when the precipitate particles are fine, precipitation occurs rapidly, the concentration is high, stirring is insufficient, or washing is insufficient.
In analytical chemistry, coprecipitation is an important cause of error because it lowers the purity of the precipitate and reduces the amount of the desired component remaining in the filtrate.
Example Discussion:
The desired component should originally have remained in the filtrate, but some may have been incorporated into the precipitate through adsorption onto the precipitate surface or coprecipitation.
When precipitate particles are fine and have a large surface area, they readily adsorb ions or molecules from the solution.
Therefore, the amount of the desired product in the filtrate may have decreased, causing errors in yield or quantitative results.
When the Precipitate Is Too Fine
If the precipitate particles are extremely fine, they may pass through the pores of the filter paper and flow into the filtrate.
In this case, even if the desired product should be in the precipitate, the filtrate may become cloudy or contain the desired product.
As a result, the amount that can be recovered as precipitate decreases and the yield decreases.
Fine precipitates tend to form when precipitation occurs rapidly, supersaturation is high, stirring is insufficient, or aging is insufficient.
Possible improvements include aging the precipitate to increase particle size, using appropriate filter paper, or using centrifugation.
If the filtrate is cloudy, the possibility that the desired product has flowed through should be considered.
Example Discussion:
If the filtrate was cloudy, fine precipitate particles may have passed through the filter paper and flowed into the filtrate.
In an experiment where the desired product is in the precipitate, this loss reduces the amount recovered and lowers the yield.
Effective improvements include aging the precipitate to increase particle size, using finer filter paper, or performing centrifugation.
Reasons the Desired Product Remains in the Filtrate
Even in experiments where the desired product is obtained as a precipitate or crystals, some may remain in the filtrate.
Possible reasons include the desired product having a certain solubility, insufficient cooling, insufficient precipitating agent, inappropriate pH, dissolution through complex formation, or fine particles passing through the filter.
If the desired product remains in the filtrate, the amount recovered in the precipitate decreases and the yield becomes lower.
Additional precipitate may form if the filtrate is further cooled, the pH is adjusted, or more precipitating agent is added.
It is important to confirm whether the desired product remains before discarding the filtrate.
Example Discussion:
Even if the desired product is expected to be obtained as a precipitate, some may remain in the filtrate.
This may be caused by the slight solubility of the desired product or insufficient precipitation conditions.
If the desired product remains in the filtrate, the actual yield becomes smaller than the theoretical yield, so this can be considered one cause of decreased yield.
Reasons Impurities Remain in the Precipitate
Even when the precipitate is the desired product, it is not necessarily pure.
Mother liquor adheres to the surface of the precipitate, and unreacted materials, by-products, salts, acids, bases, and other substances may remain.
In addition, impurities may be incorporated into the precipitate through coprecipitation or adsorption.
If impurities remain in the precipitate, the mass after drying becomes larger and the yield may appear high.
However, the purity decreases.
Washing, reprecipitation, aging, recrystallization, or other methods may reduce the impurities.
Even when the precipitate is judged to be the desired product, evaluation of purity is necessary.
Example Discussion:
Even when the desired product was recovered as a precipitate, impurities derived from the mother liquor may have adhered to the precipitate surface.
In addition, components that should normally remain in the filtrate may have been incorporated into the precipitate through coprecipitation or adsorption.
Therefore, even if the precipitate mass is large, this does not necessarily mean that the purity is high, and evaluation by washing and confirmation analysis is necessary.
Importance of Checking Before Discarding the Filtrate
In chemistry experiments, it is important to check whether the filtrate contains the desired product before discarding it.
In particular, it is easy to mistake the location of the desired product during recrystallization, precipitation reactions, filtration after extraction, removal of drying agents, and activated-carbon treatment.
If the filtrate is discarded in an operation where the desired product is in the filtrate, the entire experiment will fail.
Even when the desired product is in the precipitate, additional recovery may be possible if some of the desired product remains in the filtrate.
It can be checked by cooling the filtrate, concentrating it, changing the pH, or performing a confirmation reaction.
In small-scale synthesis, even a small amount of desired product in the filtrate can greatly affect the yield.
Example Discussion:
Because the desired product may remain dissolved in the filtrate, it is important to confirm this before discarding the filtrate.
Particularly in recrystallization and precipitation reactions, the desired product may not completely precipitate and some may remain in the mother liquor.
By cooling or concentrating the filtrate and checking whether additional crystals or precipitate form, it may be possible to reduce loss of the desired product.
Determination by Confirmation Reactions
Confirmation reactions are useful for determining whether the desired product is in the filtrate or precipitate.
In experiments involving inorganic ions, reactions such as precipitation, color development, or complex formation for specific ions can be used to confirm whether the desired component is present in the filtrate or precipitate.
In some cases, the precipitate is dissolved before the confirmation reaction is performed.
For example, AgCl precipitation with Ag+ may be used to confirm Cl-, and red coloration with thiocyanate ions may be used to confirm Fe3+.
If the confirmation reaction is positive, the desired component may be present in that phase.
However, the effects of interfering ions and coexisting substances must also be considered.
Example Discussion:
If a confirmation reaction performed on the filtrate showed a reaction corresponding to the target ion, some of the desired product was considered to have remained in the filtrate.
This indicates that precipitation was incomplete or that the desired product remained dissolved.
Confirmation reactions are an effective method for determining whether the desired product is present in the precipitate or filtrate.
Determination by TLC
In organic chemistry experiments, TLC can be used to investigate whether the desired product is present in the filtrate or an extract of the precipitate.
If a small amount of filtrate is analyzed by TLC and a spot with the same Rf value as the desired product appears, the desired product may remain in the filtrate.
The precipitate may also be dissolved in a small amount of solvent and analyzed by TLC.
TLC allows comparison of spots from the desired product, impurities, and unreacted materials.
However, substances with the same Rf value are not necessarily identical, so melting point, IR, NMR, or other methods are used together when necessary.
TLC is useful as a simple check before discarding the filtrate.
Example Discussion:
If TLC of the filtrate showed a spot with the same Rf value as the desired product, some of the desired product may have remained in the filtrate.
This indicates that the desired product did not completely crystallize or precipitate and may have caused a decrease in yield.
TLC is a simple method for determining whether the desired product is present in the filtrate or precipitate.
Determination by Melting Point and Spectroscopy
After collecting the precipitate or crystals, melting point, IR, NMR, or other methods can be used to confirm whether they are the desired product.
If the melting point is close to the literature value and the range is narrow, the desired product may have been obtained relatively purely in the precipitate.
If peaks corresponding to the desired product are observed by IR or NMR, this provides evidence that the desired product is present in the precipitate.
On the other hand, if the residue obtained by concentrating the filtrate is analyzed and peaks of the desired product are observed, the desired product also remained in the filtrate.
When the yield is low, examining not only the precipitate but also the concentrated filtrate may reveal the cause of product loss.
Example Discussion:
If the precipitate was dried and its melting point was close to the literature value, the desired product can be judged to be present in the precipitate.
However, if the melting-point range is broad, impurities may be present.
In addition, concentrating the filtrate and measuring IR or NMR can determine whether the desired product remains in the filtrate.
Failure Caused by Misjudging the Filtrate and Precipitate
If the filtrate and precipitate are incorrectly identified, the desired product may be discarded.
For example, in hot filtration, the desired product is in the filtrate, so collecting the insoluble material on the filter paper causes loss of the desired product.
Conversely, in a precipitation reaction, if the desired product is in the precipitate but only the filtrate is collected, the desired product is lost.
To prevent incorrect judgment, it is necessary to understand the purpose of the experimental operation.
Consider whether “this filtration is intended to collect the desired product or to remove impurities.”
It is also important to check instructions in the laboratory manual such as “collect the filtrate,” “wash the precipitate,” or “discard the mother liquor.”
Example Discussion:
Incorrectly determining whether the desired product is in the filtrate or precipitate can cause major product loss.
In hot filtration, the desired product is often dissolved in the filtrate, while the precipitate is often insoluble impurities.
On the other hand, in precipitation reactions, the precipitate is often the desired product, so it is necessary to understand the purpose of the filtration operation and determine which fraction should be recovered.
Causes of Error Related to Filtrate and Precipitate
Causes of error related to filtrate and precipitate include incomplete precipitation, dissolution of the desired product in the filtrate, passage of precipitate particles through the filter paper, coprecipitation, adsorption, dissolution loss during washing, adhesion to the filter paper, remaining mother liquor, insufficient pH adjustment, insufficient cooling, and complex formation.
These factors greatly affect yield and purity.
The filtration operation itself can also cause errors.
Possible operational errors include inappropriate selection of filter paper, excessive suction, excessive washing solution, precipitate remaining during transfer, and spilling part of the filtrate.
It is necessary to consider not only which fraction contains the desired product but also where it may have been lost during transfer.
Example Discussion:
Possible causes of the decreased yield include dissolution of part of the desired product in the filtrate, passage of fine precipitate through the filter paper, and dissolution of the desired product during washing.
In addition, if mother liquor remains on the precipitate surface, impurities become incorporated and reduce purity.
In separation of filtrate and precipitate, solubility, particle size, washing conditions, and filtration method greatly affect the results.
When the Results Can Be Considered Good
Separation of filtrate and precipitate can be considered successful when the desired product is sufficiently recovered in the expected phase and impurities are separated into the other phase.
When the desired product is in the precipitate, it is desirable that confirmation reactions, melting point, or spectra of the precipitate agree with the desired product and that little desired product remains in the filtrate.
When the desired product is in the filtrate, it is desirable that the desired product can be confirmed by concentrating and analyzing the filtrate after removing the precipitate and that little desired product is contained in the precipitate.
The success of the separation should be judged not only from yield but also together with purity and confirmation-reaction results.
Example Discussion:
If the desired product was confirmed in the precipitate and almost no reaction of the desired product was observed in the filtrate, the desired product was considered to have been mostly recovered as the precipitate.
In addition, if the melting point or spectrum of the precipitate agreed with that of the desired product, the separation could be judged to have been good.
Therefore, filtration was considered to have appropriately separated the desired product from soluble impurities.
Example Discussions When the Experiment Did Not Go Well
When separation of the filtrate and precipitate does not go well, possible causes should be considered from results such as the desired product remaining in the filtrate, impurities contaminating the precipitate, cloudy filtrate, low yield, low purity, or confirmation reactions differing from expectations.
Organizing the causes according to solubility, pH, temperature, particle size, washing, and filtration method makes the discussion easier.
Example Discussion:
Although the desired product was expected to be recovered as a precipitate, one possible reason for the low yield is that some of the desired product remained dissolved in the filtrate.
If cooling was insufficient or too much solvent was used, the solubility of the desired product may have remained high and the amount precipitated may have been small.
Therefore, additional desired product might have been recovered by further cooling or concentrating the filtrate.
Another Example Discussion:
One possible reason for the cloudy filtrate is that the precipitate particles were fine and passed through the filter paper.
If the desired product is in the precipitate, this loss causes a decrease in yield.
Effective improvements include aging the precipitate to increase particle size, using finer filter paper, or performing centrifugation.
Another Example Discussion:
One possible reason for the low purity of the precipitate is that the mother liquor was not washed away sufficiently and soluble impurities remained on the precipitate surface.
In addition, components that should normally remain in the filtrate may have been incorporated into the precipitate through coprecipitation.
Therefore, it is necessary to review the washing conditions and rate of precipitation formation and suppress incorporation of impurities.
How to Write Points for Improvement
In a discussion of filtrate and precipitate, writing not only which fraction contains the desired product but also how loss of the desired product and contamination with impurities can be reduced makes the report easier to organize.
Points for improvement can be organized according to precipitation conditions, cooling conditions, pH, filtration method, washing, and confirmation methods.
Improvements When Recovering the Desired Product in the Precipitate
- Cool sufficiently to promote precipitation
- Do not use too much solvent
- Adjust the pH appropriately
- Add an appropriate amount of precipitating agent
- Age the precipitate to increase particle size
- Select appropriate filter paper or a filter
- Reduce dissolution loss into the washing solution
- Check whether the desired product remains in the filtrate
Improvements When Recovering the Desired Product in the Filtrate
- Check whether the desired product is adsorbed onto the precipitate
- Wash the precipitate with a small amount of solvent and combine the washings with the filtrate
- Collect the filtrate without spilling it
- Do not allow the temperature to decrease excessively during hot filtration
- Control the solvent amount and temperature so that the desired product does not precipitate
- Examine the filtrate by TLC or a confirmation reaction
- Concentrate the filtrate and recover the desired product
Improvements to Confirmation Methods
- Check small amounts of both the filtrate and precipitate
- Cool the filtrate and check for additional precipitation
- Dissolve the precipitate and perform a confirmation reaction
- Check the desired-product spot by TLC
- Confirm the desired product by melting point, IR, or NMR
- Use pH or ion confirmation reactions
- Confirm the instructions and purpose before discarding the filtrate
Example of How to Write Points for Improvement:
When the desired product is in the precipitate, it is necessary to cool sufficiently and appropriately adjust the amount of solvent and pH to reduce dissolution loss into the filtrate.
In addition, before discarding the filtrate, it is important to perform a confirmation reaction or TLC to check whether the desired product remains.
In operations where the desired product is in the filtrate, it is useful to check whether the desired product is adsorbed onto the precipitate and, if necessary, wash the precipitate with a small amount of solvent and combine the washing with the filtrate.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of filtrate and precipitate, simply writing that “the precipitate was the desired product” or “the filtrate was discarded” results in a superficial discussion.
A good discussion relates the solubility of the desired product, precipitation, pH, temperature, confirmation reactions, and causes of decreased yield.
| Superficial Discussion | Good Discussion |
|---|---|
| I think the precipitate was the desired product because a precipitate formed. | Because the desired product is poorly soluble in the solvent used and is formed as a sparingly soluble solid according to the reaction equation, the precipitate on the filter paper can be judged to be the desired product. |
| The filtrate was discarded. | Even when the desired product is considered to be in the precipitate, some may remain in the filtrate according to its solubility, so residual desired product in the filtrate can cause a decrease in yield. |
| The filtrate was cloudy. | Fine precipitate particles may have passed through the filter paper and caused the desired product to flow into the filtrate, so aging of the precipitate and selection of the filter paper are important. |
| There were impurities in the precipitate. | Residual mother liquor on the precipitate surface or incorporation of impurities by coprecipitation or adsorption may have caused the decrease in precipitate purity. |
| I did not know which side contained it. | By comparing the solubility of the desired product, ionization depending on pH, and the results of TLC or confirmation reactions, it is possible to determine whether the desired product is in the precipitate or filtrate. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of filtrate and precipitate.
Adjust the necessary parts according to your own experimental results.
- Filtration separated the solid components as the precipitate and the soluble components as the filtrate.
- Because the desired product is sparingly soluble, it is considered to be present in the precipitate.
- In an operation where the desired product is dissolved in solution, the filtrate must be collected.
- The fact that a precipitate formed alone does not establish that it is the desired product.
- The phase to be recovered must be determined from the solubility of the desired product and the purpose of the experiment.
- If the desired product remains in the filtrate, the amount recovered as precipitate decreases and the yield falls.
- If the precipitate particles are fine, they may pass through the filter paper and make the filtrate cloudy.
- Because mother liquor and impurities may adhere to the precipitate, washing is necessary.
- The ionization state of the desired product changes depending on pH, changing its distribution between the filtrate and precipitate.
- Checking both the filtrate and precipitate makes it possible to determine the location of the desired product more reliably.
Points to Check When Discussing Filtrate and Precipitate
Checking the following points before writing the report makes the discussion easier to write.
- Is the purpose of filtration explained?
- Is it clearly stated whether the desired product is in the precipitate or filtrate?
- Has the solubility of the desired product been considered?
- Is the reaction equation for the precipitation reaction written?
- Have the effects of the solubility product and pH been considered?
- In recrystallization, has dissolution loss into the mother liquor been considered?
- Is it understood that the desired product is in the filtrate during hot filtration?
- Has loss of the desired product during washing been considered?
- Has contamination with impurities caused by coprecipitation or adsorption been considered?
- Have cloudy filtrate and loss of precipitate particles been discussed?
- Has the judgment been made using confirmation reactions, TLC, or other methods?
- Do the points for improvement correspond to the causes of error?
Summary
Whether the desired product is in the filtrate or precipitate is determined by the purpose of the experiment and the solubility of the desired product.
In precipitation reactions and recrystallization, the desired product often remains on the filter paper as a precipitate or crystals.
On the other hand, in hot filtration and removal of insoluble impurities, the desired product is often in the filtrate and the filtrate must be collected.
Even when the desired product is in the precipitate, some may remain dissolved in the filtrate.
In addition, if the precipitate particles are fine, they may pass through the filter paper and make the filtrate cloudy.
Even when the desired product is in the filtrate, some may be lost to the precipitate through adsorption onto the precipitate surface or coprecipitation.
Therefore, it is important to check both the filtrate and precipitate as necessary.
In a report, rather than simply writing that “a precipitate formed,” organize and discuss the purpose of filtration, solubility of the desired product, precipitation reactions, solubility product, pH, complex formation, recrystallization, hot filtration, washing, coprecipitation, loss into the filtrate, confirmation reactions, TLC, melting point, spectroscopy, causes of error, and points for improvement.
Determining whether the desired product is in the filtrate or precipitate is an important concept for correctly recovering the desired product and improving yield and purity.
