Partition equilibrium is an experiment that deals with how a substance is distributed between two mutually immiscible phases, such as an aqueous layer and an organic layer.
In extraction operations in organic chemistry experiments and equilibrium-constant measurements in physical chemistry experiments, the partition coefficient is used to discuss how much of the target substance moves into each layer.
In a report on partition equilibrium, it is not sufficient simply to write that “the substance was extracted into the organic layer” or “the partition coefficient was determined.”
It is necessary to explain why a larger partition coefficient leads to higher extraction efficiency, why recovery differs between a single extraction and multiple extractions, why some of the target substance remains in the aqueous layer, and how the phase-volume ratio, pH, and emulsification affect the results.
This article clearly explains the basics of partition equilibrium, how to determine the partition coefficient, its relationship with extraction efficiency, differences between single and multiple extractions, 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 and physical chemistry experiments at universities and similar institutions.
For the actual extraction solvents, separatory funnels, titration, absorbance measurements, pH adjustment, waste-liquid disposal, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is Partition Equilibrium?
- Main Items to Include in the Results
- Reference Experimental Values for Partition Equilibrium and Analysis Examples of Partition Coefficient and Extraction Efficiency
- Reference Experimental Conditions
- Basic Equation for the Partition Coefficient
- Example Measurement of Concentrations in the Aqueous and Organic Phases
- Example Calculation of the Partition Coefficient
- Example Calculation of Extraction Percentage
- Relationship Between Partition Coefficient and Extraction Percentage
- Comparison Between Single and Multiple Extractions
- Example Calculation for Multiple Extractions
- Effect of Organic-Solvent Volume
- Change in Partitioning With pH
- Example of Back Extraction
- Reference Example of the Salting-Out Effect
- Effects of Emulsification and Poor Phase Separation
- Example of Shaking Time and Attainment of Equilibrium
- Sources of Error in Extraction Operations
- Example of How to Write the Results
- Points for Connecting the Results to the Discussion
- Example Discussion
- Summary
- What Is the Partition Coefficient?
- Relationship Between Partition Coefficient and Extraction Efficiency
- Concept of Extraction Percentage
- Why the Target Substance Remains in the Aqueous Layer After a Single Extraction
- Why Multiple Extractions Are Advantageous
- Effect of the Volume Ratio of the Organic and Aqueous Layers
- Effect of pH on Partition Equilibrium
- Relationship Between Partition Coefficient and Acid-Base Extraction
- Effect of Emulsification on Partition Equilibrium
- Effect of Salting Out on Extraction Efficiency
- Methods for Measuring the Partition Coefficient
- Discussion When the Partition Coefficient Is Determined by Titration
- Discussion When the Partition Coefficient Is Determined From Absorbance
- Discussion When Equilibrium Has Not Been Reached
- Error Caused by Mistaking the Layers
- Causes of Low Recovery
- Discussion When the Recovery Is Too High
- Causes of Differences Between the Partition Coefficient and Literature Values
- Effect of Temperature on the Partition Coefficient
- When the Results Can Be Considered Good
- Example Discussion 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 Partition Equilibrium
- Summary
What Is Partition Equilibrium?
Partition equilibrium is a state in which a solute is distributed between two mutually immiscible liquid phases and ultimately exists at a constant concentration ratio.
A representative example is the distribution of a target substance between an aqueous layer and an organic layer according to how readily it dissolves in each phase.
After sufficient shaking and standing, the solute is distributed between the two phases and reaches equilibrium.
In partition equilibrium, the target substance does not move completely into only one layer.
Even a substance that readily dissolves in the organic layer may remain to some extent in the aqueous layer.
Therefore, in extraction operations, it is important to consider the partition coefficient, phase volumes, and number of extractions.
Example Discussion:
In this experiment, the target substance was distributed between the aqueous and organic layers, and the partition coefficient was determined from the concentration ratio after equilibrium had been reached.
The target substance was distributed mainly into the organic layer, but some is considered to have remained in the aqueous layer.
Therefore, in extraction operations, the target substance does not move completely into the organic layer but exists in both phases at a certain ratio according to partition equilibrium.
Main Items to Include in the Results
In the results of partition equilibrium experiments, organize the volumes of the aqueous and organic layers, initial concentration, concentration in each layer after equilibrium, partition coefficient, extraction percentage, and recovery.
If concentrations were determined by titration or absorbance measurement, clearly describe the measurement method and calculation process as well.
Main Items to Include in the Results
- Types of aqueous and organic layers used
- Volume of each phase
- Initial concentration or initial amount of solute
- Aqueous-layer concentration after equilibrium
- Organic-layer concentration after equilibrium
- Partition coefficient
- Extraction percentage or recovery
- Number of extractions
- pH conditions
- State of emulsification and phase separation
- Measured values such as titration volume or absorbance
- Comparison with theoretical or literature values
- Error rate
Example of How to Write the Results:
After shaking the aqueous and organic layers, the mixture was allowed to stand for phase separation, and the concentration of the target substance in each layer was determined.
The concentration in the organic layer after equilibrium was higher than that in the aqueous layer, confirming that the target substance was more readily distributed into the organic layer.
The partition coefficient was determined from the concentration ratio and found to be K = ○○.
Reference Experimental Values for Partition Equilibrium and Analysis Examples of Partition Coefficient and Extraction Efficiency
Here, reference experimental values are organized for partition equilibrium experiments that investigate how a substance is distributed between an aqueous phase and an organic phase.
The partition coefficient, extraction percentage, differences between single and multiple extractions, solvent volume, pH, and the effect of the ionization state are presented in a form that is easy to discuss in reports.
In partition equilibrium, when a solute reaches equilibrium between two mutually immiscible solvents, it exists in each phase at a certain ratio.
By expressing this ratio as the partition coefficient, it is possible to discuss the efficiency of extraction operations and the conditions under which the target substance can be transferred more readily into the organic phase.
Reference Experimental Conditions
| Item | Details |
|---|---|
| Target substances | Iodine, organic acids, phenols, colored substances, etc. |
| Aqueous phase | Water, buffer solution, acidic aqueous solution, basic aqueous solution |
| Organic phase | Hexane, ethyl acetate, dichloromethane, etc. |
| Measurement methods | Absorbance measurement, titration, mass measurement, comparison of color intensity, etc. |
| Evaluation items | Partition coefficient, extraction percentage, residual percentage, effects of multiple extraction, pH, emulsification, and poor phase separation |
| Basic procedure | Shake the aqueous and organic phases, allow them to stand until separated into two layers, and then measure the concentration in each layer |
Basic Equation for the Partition Coefficient
The partition coefficient K can be expressed as the ratio of the solute concentration in the organic phase to that in the aqueous phase at equilibrium.
K = Corg / Caq
Here, Corg is the solute concentration in the organic phase and Caq is the solute concentration in the aqueous phase.
The larger K is, the more readily the solute is considered to transfer into the organic phase.
| Partition Coefficient K | Meaning | Ease of Extraction |
|---|---|---|
| K < 1 | More remains in the aqueous phase | Difficult to extract into the organic phase |
| K = 1 | Distributed to both phases to a similar extent | Moderate |
| K > 1 | More moves into the organic phase | Easy to extract |
| K is very large | Almost all moves into the organic phase | Easy to extract even with a small amount of organic solvent |
Example Measurement of Concentrations in the Aqueous and Organic Phases
The following is a reference example in which a solute was shaken with 50.0 mL of aqueous phase and 50.0 mL of organic phase, and the concentrations after equilibrium were measured.
| Sample | Aqueous-Phase Concentration | Organic-Phase Concentration | Partition Coefficient K | How to Interpret the Result |
|---|---|---|---|---|
| Substance A | 0.20 mmol/L | 1.00 mmol/L | 5.0 | Readily transfers to the organic phase |
| Substance B | 0.50 mmol/L | 0.50 mmol/L | 1.0 | Distributed to both phases to a similar extent |
| Substance C | 0.80 mmol/L | 0.20 mmol/L | 0.25 | Tends to remain in the aqueous phase |
For Substance A, K = 1.00 / 0.20 = 5.0.
From this value, Substance A can be judged to be more readily distributed into the organic phase than into the aqueous phase.
Example Calculation of the Partition Coefficient
If the concentration in the organic phase after equilibrium is 1.20 mmol/L and the concentration in the aqueous phase is 0.30 mmol/L,
K = Corg / Caq = 1.20 / 0.30 = 4.0
Therefore, the partition coefficient of this solute is 4.0.
Because the concentration in the organic phase is four times that in the aqueous phase, the substance is considered to transfer relatively readily into the organic phase.
Example Calculation of Extraction Percentage
Extraction percentage can be expressed as the proportion of the solute initially present in the aqueous phase that moves into the organic phase.
Extraction percentage (%) = Amount transferred to the organic phase ÷ Initial amount × 100
If 0.100 mmol of solute was initially present in 50.0 mL of aqueous phase and 0.080 mmol moved into the organic phase after extraction,
Extraction percentage = 0.080 / 0.100 × 100 = 80%
Under these conditions, 80% of the solute moved into the organic phase in a single extraction.
Relationship Between Partition Coefficient and Extraction Percentage
If the volume of the aqueous phase is Vaq and the volume of the organic phase is Vorg, the proportion remaining in the aqueous phase after one extraction can be expressed as follows.
Fraction remaining in aqueous phase = Vaq / (K Vorg + Vaq)
The extraction percentage is obtained by subtracting the percentage remaining in the aqueous phase from 100%.
| K | Aqueous-Phase Volume | Organic-Phase Volume | Percentage Remaining in Aqueous Phase | Extraction Percentage |
|---|---|---|---|---|
| 1 | 50 mL | 50 mL | 50.0% | 50.0% |
| 2 | 50 mL | 50 mL | 33.3% | 66.7% |
| 5 | 50 mL | 50 mL | 16.7% | 83.3% |
| 10 | 50 mL | 50 mL | 9.1% | 90.9% |
The larger the partition coefficient, the greater the proportion that moves into the organic phase and the higher the extraction percentage.
Comparison Between Single and Multiple Extractions
When the same total amount of organic solvent is used, extraction efficiency may be higher if the solvent is divided into several smaller portions rather than used all at once.
Here, K = 5, the aqueous phase is 50 mL, and the total volume of organic solvent is 50 mL.
| Extraction Method | Organic-Solvent Volume | Percentage Remaining in Aqueous Phase | Extraction Percentage | How to Interpret the Result |
|---|---|---|---|---|
| Single extraction | 50 mL × 1 | 16.7% | 83.3% | Standard |
| Two extractions | 25 mL × 2 | 8.2% | 91.8% | Efficiency increases |
| Five extractions | 10 mL × 5 | 3.1% | 96.9% | Very high efficiency |
In multiple extractions, fresh organic phase is used each time, so the solute remaining in the aqueous phase is further transferred into the organic phase.
Therefore, even when the same total amount of organic solvent is used, dividing the extraction into multiple portions allows more of the target substance to be recovered.
Example Calculation for Multiple Extractions
If K = 5, the aqueous phase is 50 mL, and the organic phase is 25 mL for one extraction, the fraction remaining in the aqueous phase is as follows.
Fraction remaining in aqueous phase = 50 / (5 × 25 + 50) = 50 / 175 = 0.286
After the first extraction, 28.6% remains in the aqueous phase.
If a second extraction is performed under the same conditions,
Fraction remaining after two extractions = 0.286 × 0.286 = 0.0818 = 8.18%
Therefore, the extraction percentage is as follows.
Extraction percentage = 100 − 8.18 = 91.8%
This calculation shows that extracting twice with 25 mL portions provides higher extraction efficiency than using 50 mL in a single extraction.
Effect of Organic-Solvent Volume
The larger the volume of the organic phase, the greater the amount of substance that transfers into the organic phase in a single extraction.
However, if too much organic solvent is used, the work required for concentration and recovery may also increase.
| Aqueous-Phase Volume | Organic-Phase Volume | K | Percentage Remaining in Aqueous Phase | Extraction Percentage |
|---|---|---|---|---|
| 50 mL | 10 mL | 5 | 50.0% | 50.0% |
| 50 mL | 25 mL | 5 | 28.6% | 71.4% |
| 50 mL | 50 mL | 5 | 16.7% | 83.3% |
| 50 mL | 100 mL | 5 | 9.1% | 90.9% |
Increasing the volume of the organic phase increases the extraction percentage, but beyond a certain point the benefit of further increases becomes smaller.
Change in Partitioning With pH
For acidic or basic substances, the proportion of molecular and ionic forms changes depending on pH.
In general, the molecular form transfers more readily into the organic phase, whereas the ionic form tends to remain in the aqueous phase.
| Condition | Main Form Present | Organic-Phase Concentration | Aqueous-Phase Concentration | Apparent Partition Coefficient | How to Interpret the Result |
|---|---|---|---|---|---|
| Acidic condition pH 2 | Mainly molecular form | 1.20 mmol/L | 0.30 mmol/L | 4.0 | Readily transfers to the organic phase |
| Neutral condition pH 7 | Partially ionic form | 0.60 mmol/L | 0.90 mmol/L | 0.67 | More likely to remain in the aqueous phase |
| Basic condition pH 10 | Mainly ionic form | 0.05 mmol/L | 1.45 mmol/L | 0.034 | Almost entirely remains in the aqueous phase |
For an organic acid, the molecular form predominates under acidic conditions and the substance is more readily extracted into the organic phase.
Under basic conditions, the ionic form increases and the substance tends to remain in the aqueous phase.
Example of Back Extraction
The operation of returning a substance extracted into the organic phase to an aqueous phase by changing the pH is called back extraction.
After an acidic substance has been extracted into the organic phase, treatment with a basic aqueous solution converts it into the ionic form and makes it easier to transfer back into the aqueous phase.
| Operation | Aqueous-Phase Condition | Main Form Present | Destination | Purpose |
|---|---|---|---|---|
| Extraction | Acidic | Molecular form | Organic phase | Transfer the target substance to the organic phase |
| Washing | Neutral | Removal of impurities | Aqueous phase | Remove water-soluble impurities |
| Back extraction | Basic | Ionic form | Aqueous phase | Return the target substance to the aqueous phase |
| Reacidification | Acidic | Molecular form | Precipitate or organic phase | Recover again |
Because the partition state can be changed by changing pH, this property can be used for the separation of acidic and basic substances.
Reference Example of the Salting-Out Effect
Adding salt to the aqueous phase may reduce the solubility of the solute in the aqueous phase and make it easier for the solute to transfer into the organic phase.
This is called the salting-out effect.
| Aqueous-Phase Condition | Organic-Phase Concentration | Aqueous-Phase Concentration | Partition Coefficient K | How to Interpret the Result |
|---|---|---|---|---|
| No salt | 0.80 mmol/L | 0.40 mmol/L | 2.0 | Standard condition |
| NaCl 0.5 mol/L | 1.05 mmol/L | 0.28 mmol/L | 3.8 | Readily transfers to the organic phase |
| Saturated brine | 1.25 mmol/L | 0.20 mmol/L | 6.3 | Large salting-out effect |
When salting out reduces the solubility of the solute in the aqueous phase, partitioning into the organic phase may be promoted and extraction efficiency may increase.
Effects of Emulsification and Poor Phase Separation
During separation in a separatory funnel, shaking too strongly or the presence of surface-active substances may cause emulsification and make it difficult for the two layers to separate.
| Condition | Observation | Effect on Measured Values | Direction of Discussion |
|---|---|---|---|
| Good phase separation | Aqueous and organic phases are clearly separated | Concentration measurement is stable | Standard |
| Mild emulsification | Interface becomes cloudy and white | Volume of collected phase deviates | Error in extraction percentage |
| Severe emulsification | Difficult to separate into two layers | Aqueous and organic phases contaminate each other | Partition coefficient becomes inaccurate |
| Water droplets in organic phase | Organic phase becomes cloudy | Apparent concentration shifts | Drying or re-separation is necessary |
When emulsification occurs, it becomes unclear which phase is actually being measured, causing large errors in the calculation of the partition coefficient and extraction percentage.
Example of Shaking Time and Attainment of Equilibrium
In partition equilibrium experiments, the phases must be mixed sufficiently and then separated after equilibrium has been reached.
If the shaking time is too short, equilibrium may not have been reached.
| Shaking Time | Organic-Phase Concentration | Aqueous-Phase Concentration | Apparent K | Judgment |
|---|---|---|---|---|
| 10 s | 0.45 mmol/L | 0.75 mmol/L | 0.60 | Insufficient mixing |
| 30 s | 0.80 mmol/L | 0.42 mmol/L | 1.90 | Still insufficient |
| 1 min | 0.98 mmol/L | 0.25 mmol/L | 3.9 | Close to equilibrium |
| 3 min | 1.00 mmol/L | 0.20 mmol/L | 5.0 | Equilibrium reached |
| 5 min | 1.01 mmol/L | 0.20 mmol/L | 5.1 | Almost no further change |
If increasing the shaking time no longer causes significant changes in the measured values, it becomes easier to judge that partition equilibrium has been reached.
Sources of Error in Extraction Operations
| Source of Error | Effect | Trend in the Result | Improvement |
|---|---|---|---|
| Insufficient shaking | Equilibrium is not reached | Apparent K becomes smaller | Mix sufficiently |
| Emulsification | Insufficient phase separation | Concentration measurement becomes unstable | Allow to stand; consider centrifugation or salting out |
| Error in collecting phases | Aqueous and organic phases contaminate each other | K and extraction percentage deviate | Avoid collecting near the interface |
| Error in reading solvent volume | Affects amount-of-substance calculations | Extraction percentage deviates | Use graduated cylinders and pipettes correctly |
| Evaporation of organic solvent | Organic-phase volume decreases | Concentration appears high | Keep covered and work quickly |
| Insufficient pH adjustment | Ionization state changes | Apparent K changes | Measure and record pH |
Example of How to Write the Results
A partition experiment for Substance A was performed using 50.0 mL of aqueous phase and 50.0 mL of organic phase.
After equilibrium, the concentration in the aqueous phase was 0.20 mmol/L and that in the organic phase was 1.00 mmol/L.
The partition coefficient was determined as K = Corg / Caq = 1.00 / 0.20 = 5.0.
From this result, Substance A can be judged to be more readily distributed into the organic phase than into the aqueous phase.
When K = 5 and the aqueous and organic phases were each 50 mL, the fraction remaining in the aqueous phase after one extraction was 50 / (5 × 50 + 50) = 0.167, and the extraction percentage was 83.3%.
In contrast, when the same total volume of 50 mL organic solvent was divided into two 25 mL extractions, the residual percentage was 8.18% and the extraction percentage was 91.8%.
Therefore, even with the same amount of organic solvent, multiple extraction allows the target substance to be recovered more efficiently.
In the experiment in which pH was changed, the apparent partition coefficient was large under acidic conditions and small under basic conditions.
This is because under acidic conditions the substance was more likely to exist in the molecular form and therefore transferred more readily into the organic phase.
In contrast, under basic conditions the ionic form increased and the substance is considered to have remained more readily in the aqueous phase.
Points for Connecting the Results to the Discussion
In a discussion of partition equilibrium, it is important to relate not only the partition coefficient as a concentration ratio but also extraction percentage, solvent volume, multiple extraction, pH, and the state of phase separation.
- Can you calculate the partition coefficient K from the concentrations in the organic and aqueous phases?
- Can you explain that the larger K is, the more readily the substance transfers into the organic phase?
- Can you distinguish between partition coefficient and extraction percentage?
- Can you explain why multiple extraction is more efficient than single extraction?
- Can you explain the effect of organic-phase volume on extraction percentage?
- For acidic and basic substances, can you discuss how the proportion of molecular and ionic forms changes with pH?
- Can you explain that the ionic form tends to remain in the aqueous phase while the molecular form tends to transfer into the organic phase?
- Can you explain that the salting-out effect may promote extraction into the organic phase?
- Can you discuss how emulsification and mistakes in phase collection affect the partition coefficient and extraction percentage?
- Can you explain that measurements taken before equilibrium is reached do not provide the correct apparent partition coefficient?
Example Discussion
In this experiment, Substance A was distributed between the aqueous and organic phases, and the partition coefficient was determined from the concentration in each phase after equilibrium.
Because the concentration in the aqueous phase was 0.20 mmol/L and that in the organic phase was 1.00 mmol/L, the partition coefficient was K = 5.0.
Because this value is greater than 1, Substance A is considered to exist more readily in the organic phase than in the aqueous phase.
Regarding extraction efficiency, the larger K is, the greater the proportion that transfers into the organic phase.
When K = 5 and the aqueous and organic phases have equal volumes, approximately 83% is calculated to transfer into the organic phase in a single extraction.
However, even when the same total amount of organic solvent is used, dividing the solvent into multiple extractions leaves a smaller proportion in the aqueous phase than using it all at once.
This is because in each extraction, fresh organic phase again takes up solute remaining in the aqueous phase.
Regarding the effect of pH, the apparent partition coefficient was large under acidic conditions and small under basic conditions.
For an acidic substance, the nonionized molecular form is more abundant under acidic conditions and is more soluble in the organic phase.
In contrast, under basic conditions the ionic form increases, becomes hydrated, and tends to remain in the aqueous phase.
Therefore, in the extraction of acidic and basic substances, the partition state can be controlled by pH adjustment.
Possible sources of error include insufficient shaking, phase separation before equilibrium was reached, emulsification, errors in collecting the phases, evaporation of the organic solvent, and errors in reading solvent volumes.
If the shaking time is too short, sufficient partition equilibrium is not reached, so the concentration in the organic phase remains low and the apparent K may become smaller.
In addition, if aqueous and organic phases are collected while still mixed because of emulsification, the concentration in each phase becomes inaccurate and evaluation of extraction percentage is also affected.
Under conditions where salting out was performed, the partition coefficient increased.
This is considered to have occurred because addition of salt reduced the solubility of the solute in the aqueous phase and made it easier to transfer into the organic phase.
Thus, partition equilibrium is affected not only by the hydrophobicity of a substance but also by pH, salt concentration, solvent volume, and mixing conditions.
Summary
In partition equilibrium, the partition coefficient can be determined from the ratio of the concentration in the organic phase to that in the aqueous phase after equilibrium.
The larger the partition coefficient, the more readily the substance transfers into the organic phase and the higher the extraction percentage becomes.
However, extraction efficiency is affected not only by the partition coefficient but also by the volume ratio of the aqueous and organic phases and the number of extractions.
This reference example covered the partition coefficient, extraction percentage, single and multiple extractions, organic-solvent volume, pH-dependent changes in partitioning, back extraction, salting-out effect, emulsification, attainment of equilibrium, and sources of error.
In a report, rather than simply calculating K, it is useful to discuss why extraction efficiency became higher under particular conditions in relation to solubility, ionization state, and operating conditions.
What Is the Partition Coefficient?
The partition coefficient is a value representing the ratio of the solute concentration in the organic layer to that in the aqueous layer at equilibrium.
If the concentration in the organic layer is Corg and that in the aqueous layer is Caq, the partition coefficient K is expressed as follows.
K = Corg ÷ Caq
The larger K is, the more readily the target substance is distributed into the organic layer.
A small K means that the substance tends to remain in the aqueous layer.
However, even if K is large, the concentration in the aqueous layer does not become zero, so complete extraction is not achieved.
Example Discussion:
The partition coefficient K is the ratio of the concentration in the organic layer to that in the aqueous layer at equilibrium.
Because the K determined in this experiment was greater than 1, the target substance is considered to have been distributed more readily into the organic layer than into the aqueous layer.
However, because K is finite, a certain amount of the target substance remains in the aqueous layer and the extraction percentage does not reach 100%.
Relationship Between Partition Coefficient and Extraction Efficiency
Extraction efficiency expresses how much of the total target substance can be transferred into the desired layer.
The larger the partition coefficient, the more readily the target substance transfers into the organic layer, so extraction efficiency increases.
However, extraction efficiency is affected not only by the partition coefficient but also by the volume ratio of the organic and aqueous layers.
For example, even if the partition coefficient is the same, increasing the volume of the organic layer tends to increase the amount of target substance transferred into it.
Conversely, when the volume of the organic layer is small, the amount that can be recovered may be limited even if the concentration ratio is the same.
Example Discussion:
The larger the partition coefficient, the more readily the target substance transfers into the organic layer, so extraction efficiency increases.
However, the actual amount extracted depends not only on concentration but also on the volumes of the organic and aqueous layers.
Therefore, even with the same partition coefficient, increasing the volume of the organic layer may increase the amount of target substance recovered.
Concept of Extraction Percentage
Extraction percentage represents the proportion of the target substance initially present that is transferred into the organic layer by extraction.
It is determined by dividing the amount of substance transferred into the organic layer by the initial amount of substance.
Extraction percentage (%) = Amount transferred to organic layer ÷ Initial amount × 100
Even when the partition coefficient is large, the extraction percentage does not reach 100% because some of the target substance remains in the aqueous layer after equilibrium.
If the extraction percentage is low, residual target substance in the aqueous layer, emulsification, poor phase separation, insufficient extraction frequency, and similar factors can be discussed as possible causes.
Example Discussion:
The extraction percentage did not reach 100%.
This is considered to be because part of the target substance remained in the aqueous layer because of partition equilibrium.
As long as the partition coefficient is finite, the target substance cannot be transferred completely into the organic layer in a single extraction, so there is a limit to the extraction percentage.
Why the Target Substance Remains in the Aqueous Layer After a Single Extraction
In a single extraction, the aqueous and organic layers are brought into contact once and allowed to reach partition equilibrium.
At that time, the target substance distributes between the two phases according to the partition coefficient, so some remains in the aqueous layer.
In particular, compounds with non-negligible water solubility or compounds containing polar functional groups may show substantial retention in the aqueous layer.
Therefore, the recovery may be low in a single extraction.
To improve this problem, multiple extractions may be performed.
Example Discussion:
One possible reason the recovery was low in the single extraction is that some of the target substance remained in the aqueous layer.
In partition equilibrium, the target substance exists at a certain ratio in both the organic and aqueous layers.
Therefore, even if partitioning into the organic layer is favorable, a single extraction cannot completely remove the target substance from the aqueous layer.
Why Multiple Extractions Are Advantageous
When the same total amount of organic solvent is used, extraction efficiency may be higher if the solvent is divided into several smaller portions rather than used all at once.
This is because each extraction brings fresh organic solvent into contact with the aqueous layer, allowing the target substance remaining in the aqueous layer to be distributed again into the organic layer.
For example, target substance remaining in the aqueous layer after the first extraction can transfer further into the organic layer during the second and third extractions.
Therefore, in multiple extractions, the amount of target substance remaining in the aqueous layer decreases stepwise and the recovery improves.
Example Discussion:
In multiple extractions, the target substance remaining in the aqueous layer after each extraction is redistributed into fresh organic solvent.
Therefore, even when the same total amount of organic solvent is used, dividing it into multiple extractions can reduce the amount of target substance remaining in the aqueous layer compared with a single extraction.
Thus, multiple extraction is an effective method for increasing extraction efficiency.
Effect of the Volume Ratio of the Organic and Aqueous Layers
The partition coefficient represents a concentration ratio, but the actual amount of substance recovered also depends on the volume of each phase.
If the volume of the organic layer is large, the amount of target substance contained in it increases even if the concentration is the same.
Conversely, if the organic-layer volume is small, the amount recovered may be small even if the partition coefficient is large.
In a report, it is useful to discuss extraction efficiency by considering not only the partition coefficient but also the volume ratio.
Example Discussion:
Extraction efficiency is affected not only by the partition coefficient but also by the volume ratio of the organic and aqueous layers.
The larger the volume of the organic layer, the greater the amount of target substance that can be contained in the organic layer.
Therefore, even with the same partition coefficient, the recovery is considered to vary depending on the amount of organic solvent used.
Effect of pH on Partition Equilibrium
For acidic or basic organic compounds, the partition coefficient changes greatly depending on pH.
Even if a compound dissolves readily in the organic layer in its neutral molecular form, ionization makes it more soluble in the aqueous layer.
Therefore, extraction efficiency into the organic layer changes greatly depending on pH conditions.
For example, carboxylic acids become carboxylate salts under basic conditions and transfer more readily into the aqueous layer.
Amines become ammonium salts under acidic conditions and also transfer more readily into the aqueous layer.
This property may be used in acid-base extraction.
Example Discussion:
If the target substance contains acidic or basic functional groups, its ionization state changes with pH and greatly affects partition equilibrium.
When the target substance becomes ionized, its solubility in the aqueous layer increases and extraction efficiency into the organic layer decreases.
Therefore, to extract the target substance efficiently into the organic layer, it is necessary to adjust the pH to conditions under which the target substance is likely to exist as a neutral molecule.
Relationship Between Partition Coefficient and Acid-Base Extraction
In acid-base extraction, the partition coefficient is effectively changed by changing the pH, allowing the target substance to move into either the aqueous or organic layer.
Neutral organic compounds tend to remain in the organic layer, whereas ionized acidic and basic compounds tend to transfer into the aqueous layer.
The observed partitioning in this case results from the combination of chemical equilibrium and partition equilibrium rather than simple solubility alone.
Therefore, if pH adjustment is insufficient, the target substance may not move into the expected layer and recovery may decrease.
Example Discussion:
In acid-base extraction, separation is performed using partition equilibrium by changing the ionization state of the target substance.
When an acidic compound is ionized under basic conditions, it moves more readily into the aqueous layer, while a basic compound moves more readily into the aqueous layer when ionized under acidic conditions.
If pH adjustment is insufficient, part of the target substance may remain as a neutral molecule in the organic layer and extraction efficiency may decrease.
Effect of Emulsification on Partition Equilibrium
Emulsification is a state in which the aqueous and organic layers are finely mixed and the boundary between the layers becomes unclear.
When emulsification occurs, the target substance may be dispersed among the organic layer, aqueous layer, and emulsion layer, making accurate separation difficult.
As a result, errors occur in recovery and concentration measurements.
Emulsification is more likely to occur when the mixture is shaken too strongly, when surface-active impurities are present, or when fine solids are present.
If the emulsion layer is lost, the target substance may also be lost, causing a decrease in recovery.
Example Discussion:
Emulsification occurred during extraction, making the boundary between the organic and aqueous layers unclear.
Because the target substance may be dispersed in the emulsion layer, failure to recover this layer completely as part of the organic layer may result in loss of some target substance.
As a result, the actual extraction percentage may decrease and errors may also occur in the calculation of the partition coefficient.
Effect of Salting Out on Extraction Efficiency
Salting out is an operation in which salt is added to the aqueous layer to reduce the solubility of an organic compound in water.
When a large amount of salt is present in the aqueous layer, the organic compound may become less soluble in the aqueous phase and more likely to transfer into the organic layer.
Therefore, salting out is used to reduce the amount of target substance remaining in the aqueous layer and increase extraction efficiency.
Salting out may also help improve emulsification.
However, the effect differs depending on the type of compound and solvent conditions.
Example Discussion:
Salting out is considered to have reduced the solubility of the target substance in the aqueous layer and made it more readily distributed into the organic layer.
As a result, the amount of target substance remaining in the aqueous layer may decrease and extraction efficiency may improve.
Salting out may also improve emulsification and make phase separation clearer.
Methods for Measuring the Partition Coefficient
The partition coefficient can be calculated by determining the concentrations in the aqueous and organic layers after equilibrium.
Concentrations may be determined by titration, absorbance measurement, gravimetric measurement, chromatography, or other methods.
In student experiments, the amount of substance remaining in the aqueous layer may be determined by titration and the amount in the organic layer calculated by difference.
Regardless of the method used, it is important that equilibrium has been reached, the layers have been separated accurately, and the concentration measurements are correct.
Example Discussion:
The partition coefficient was calculated from the ratio of the concentrations in the aqueous and organic layers after equilibrium.
When the concentration in the aqueous layer is determined by titration, errors in reading the titration volume and determining the endpoint affect the partition coefficient.
In addition, when the concentration in the organic layer is determined by subtraction, error in the aqueous-layer concentration is also reflected in the organic-layer concentration, so the error propagates through the entire calculation.
Discussion When the Partition Coefficient Is Determined by Titration
The substance remaining in the aqueous layer may be quantified by titration, and the amount transferred into the organic layer determined from the difference from the initial amount.
In this method, endpoint determination, concentration of the standard solution, and burette readings are important.
If titrant is added too far beyond the endpoint, the concentration in the aqueous layer may be overestimated.
Example Discussion:
Because the concentration of the target substance in the aqueous layer was determined by titration, error in endpoint determination may have affected the partition coefficient.
If too much titrant is added beyond the endpoint, the amount of target substance in the aqueous layer is overestimated.
As a result, the amount transferred into the organic layer is calculated as smaller and the partition coefficient may be underestimated.
Discussion When the Partition Coefficient Is Determined From Absorbance
If the target substance is colored or can be made to develop color, the concentration in each layer may be determined from absorbance.
When absorbance is used, the linearity of the calibration curve, blank correction, measurement wavelength, contamination of the cell, and contamination between phases become sources of error.
In particular, water droplets mixing into the organic layer or organic solvent mixing into the aqueous layer may affect absorbance measurements.
Example Discussion:
When concentration was determined from absorbance, errors in the calibration curve affected the calculation of the partition coefficient.
If the concentrations of the standard solutions were prepared incorrectly or blank correction was insufficient, the concentration determined from absorbance would deviate from the actual value.
Therefore, the partition coefficient, which is the ratio of the concentrations in the organic and aqueous layers, may be overestimated or underestimated.
Discussion When Equilibrium Has Not Been Reached
To determine the partition coefficient correctly, the solute must be sufficiently distributed between the aqueous and organic layers and equilibrium must be reached.
If the shaking time is too short or mixing is insufficient, equilibrium may not yet have been reached and the concentration ratio may not reflect the true partition coefficient.
If partition equilibrium has not been reached, target substance that should move into the organic layer remains in the aqueous layer and the partition coefficient and extraction percentage may be underestimated.
Example Discussion:
One possible reason the obtained partition coefficient was smaller than the literature value is that sufficient partition equilibrium had not been reached.
If the shaking time was too short, the target substance would not have fully transferred into the organic layer and the concentration in the aqueous layer would remain high.
As a result, the ratio of the concentration in the organic layer to that in the aqueous layer becomes smaller, and the partition coefficient is considered to have been underestimated.
Error Caused by Mistaking the Layers
Depending on the extraction solvent, the organic layer may be the upper or lower layer.
If the organic and aqueous layers are mistaken for one another, the layer containing the target substance may be discarded accidentally or the concentration calculations may be reversed.
This causes large errors in the partition coefficient and extraction percentage.
To identify the layers, check the density of the solvent used, the movement observed after adding a small amount of water, and the instructions in the laboratory manual.
Example Discussion:
One possible source of error in the partition coefficient is insufficient identification of the organic and aqueous layers.
Depending on the density of the organic solvent, the organic layer may be either the upper or lower layer.
If the layers are mistaken, the concentrations of the two layers are handled in reverse, causing large errors in the partition coefficient and extraction percentage.
Causes of Low Recovery
Causes of low recovery in extraction using partition equilibrium include residual target substance in the aqueous layer, emulsification, poor phase separation, insufficient number of extractions, inappropriate pH conditions, transfer losses, and inappropriate solvent selection.
When the partition coefficient is small, the target substance is difficult to transfer into the organic layer and recovery in a single extraction tends to be low.
| Cause | What Happens | Effect on Recovery |
|---|---|---|
| Small partition coefficient | Tends to remain in the aqueous layer | Extraction percentage decreases |
| Too few extractions | Large amount remains in the aqueous layer | Recovered amount decreases |
| Emulsification | Poor phase separation | Target substance is easily lost |
| Inappropriate pH | Target substance becomes ionized | Difficult to transfer into the organic layer |
| Transfer loss | Substance adheres to apparatus | Actual yield decreases |
Example Discussion:
One possible reason the recovery was low is that part of the target substance remained in the aqueous layer.
Because the partition coefficient is finite, a certain amount of the target substance remains in the aqueous layer even after extraction.
Furthermore, if emulsification or poor phase separation occurred, the organic layer containing the target substance may not have been completely recovered and the recovery may have decreased.
Discussion When the Recovery Is Too High
If the recovery is too high, components other than the target substance may be present in the recovered material.
Residual solvent, water, impurities, inorganic salts, unreacted substances, and contamination of the aqueous layer caused by emulsification may cause the recovered amount to be overestimated.
In partition equilibrium experiments, contamination between phases and insufficient drying can lead to overestimation of recovery.
Example Discussion:
One possible reason the recovery appeared too high is that water or water-soluble impurities were mixed into the organic layer.
If the aqueous layer mixes into the organic layer because of emulsification or poor phase separation, the recovered mass includes components other than the target substance.
As a result, the apparent recovery becomes high, but the purity of the product may be low.
Causes of Differences Between the Partition Coefficient and Literature Values
Possible reasons the experimentally determined partition coefficient differs from the literature value include differences in temperature, concentration-measurement errors, insufficient attainment of equilibrium, poor phase separation, emulsification, differences in pH, solvent purity, and mutual solubility of the phases.
Because the partition coefficient depends on conditions, it is necessary to check whether the temperature, solvent, concentration range, and pH are the same when comparing with literature values.
Example Discussion:
Differences in measurement temperature and pH conditions may explain why the obtained partition coefficient differed from the literature value.
The partition coefficient is based on the solubility of the solute and changes depending on temperature and ionization state.
In addition, if phase separation was insufficient and one phase contaminated the other, errors would occur in the concentration measurements of each layer and the partition coefficient may have deviated from the literature value.
Effect of Temperature on the Partition Coefficient
The partition coefficient may change with temperature.
Because the solubility of a solute in the aqueous and organic layers is temperature-dependent, changing the measurement temperature may also change the concentration ratio.
When comparing with literature values, it is important to confirm whether the temperature conditions are the same.
Example Discussion:
One possible reason the partition coefficient differed from the literature value is a difference in measurement temperature.
The solubility of a solute in each phase depends on temperature, so changing the temperature can alter the partition ratio between the organic and aqueous layers.
Therefore, when comparing partition coefficients, the measurement temperature must be kept constant and matched to the conditions used for the literature value.
When the Results Can Be Considered Good
Partition equilibrium results can be considered good when phase separation is clear, emulsification is minimal, concentration measurements in each layer are consistent, and partition coefficients obtained from repeated measurements are close to one another.
In addition, if the results do not greatly contradict literature values or theoretical trends and an increase in extraction percentage with multiple extractions is confirmed, the results can also be considered reasonable in terms of understanding partition equilibrium.
Example Discussion:
Phase separation after extraction was clear and almost no emulsification was observed.
The partition coefficients determined from repeated measurements were close to one another, and the result that the target substance was readily distributed into the organic layer was also consistent with the extraction percentage.
From these observations, partition equilibrium is considered to have been established relatively well in this experiment and the partition coefficient to have been determined reasonably.
Example Discussion When the Experiment Did Not Go Well
If a partition equilibrium experiment does not go well, possible causes can be considered from results such as variation in the partition coefficient, large differences from literature values, low extraction percentage, severe emulsification, unclear phase separation, or deviation in pH conditions.
It is easier to discuss the results by separately considering the partition coefficient, number of extractions, phase-volume ratio, concentration measurement, emulsification, and mistakes in identifying the layers.
Example Discussion:
In this experiment, variation was observed in the partition coefficient.
Possible causes include failure to reach sufficient partition equilibrium after shaking, unclear phase separation caused by emulsification, and errors in the concentration measurements of each layer.
In addition, residual target substance in the aqueous layer or contamination of the organic layer with the aqueous layer may also have affected the extraction percentage and partition coefficient.
How to Write Points for Improvement
In a discussion of partition equilibrium, including points for improvement as well as sources of error makes the report easier to organize.
Improvements are easier to write when divided into attainment of equilibrium, phase separation, concentration measurement, and improvement of extraction efficiency.
Improvements to Attainment of Equilibrium and Extraction Operations
- Allow sufficient shaking time
- Allow sufficient standing time after shaking
- Avoid shaking so strongly that emulsification occurs
- Separate the phases only after the boundary becomes clear
- Perform multiple extractions
Improvements to Concentration Measurement
- Determine the titration endpoint carefully
- Read the burette accurately
- Perform blank correction in absorbance measurements
- Measure within the linear range of the calibration curve
- Collect samples without contaminating one phase with the other
Improvements for Increasing Extraction Efficiency
- Select an extraction solvent suitable for the target substance
- Consider the volume ratio of the organic and aqueous layers
- Adjust pH so that the target substance is more likely to be neutral
- Use salting out when necessary
- Prevent emulsification and accurately recover the layer containing the target substance
Example of How to Write Points for Improvement:
To determine the partition coefficient more accurately, the aqueous and organic layers must be brought into sufficient contact and separated only after partition equilibrium has been reached.
In addition, to prevent emulsification, excessive shaking should be avoided and the mixture should be allowed to stand sufficiently before phase separation.
To improve extraction efficiency, multiple extraction, salting out, and appropriate pH adjustment are effective for reducing the amount of target substance remaining in the aqueous layer.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of partition equilibrium, simply writing that “the target substance moved into the organic layer” or “the recovery was low” results in a superficial discussion.
Relating the partition coefficient, extraction efficiency, volume ratio, number of extractions, pH, and emulsification produces a more persuasive discussion.
| Superficial Discussion | Good Discussion |
|---|---|
| The target substance moved into the organic layer. | Because the partition coefficient was greater than 1, the target substance is considered to have been distributed more readily into the organic layer than into the aqueous layer. However, because K is finite, a certain amount of the target substance remains in the aqueous layer. |
| Multiple extraction is better. | In multiple extractions, the target substance remaining in the aqueous layer after each extraction is redistributed into fresh organic solvent, so the amount remaining in the aqueous layer can be reduced and extraction efficiency increased compared with a single extraction. |
| The recovery was low. | Possible reasons for the low recovery include residual target substance in the aqueous layer, incomplete recovery of the organic layer because of emulsification, and ionization of the target substance under the pH conditions, causing it to be distributed into the aqueous layer. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of partition equilibrium experiments.
Adjust the necessary parts according to your own experimental results.
- In partition equilibrium, the solute reaches a constant concentration ratio between the aqueous and organic layers.
- The partition coefficient is determined by dividing the concentration in the organic layer by the concentration in the aqueous layer.
- The larger K is, the more readily the target substance is distributed into the organic layer.
- Because the partition coefficient is finite, the target substance cannot be completely recovered in a single extraction.
- In multiple extractions, the target substance remaining in the aqueous layer can be transferred stepwise into the organic layer.
- Extraction efficiency is affected not only by the partition coefficient but also by the volume ratio of the organic and aqueous layers.
- When the target substance becomes ionized, it becomes more soluble in the aqueous layer and extraction efficiency into the organic layer decreases.
- Emulsification makes phase separation unclear and may cause errors in the amount of target substance recovered.
- Salting out may reduce the solubility of the target substance in the aqueous layer and improve extraction efficiency.
- Because the partition coefficient depends on temperature and solvent conditions, these conditions must be matched when comparing with literature values.
Points to Check When Discussing Partition Equilibrium
Checking the following points before writing the report makes the discussion easier to write.
- Have you written the definition of the partition coefficient correctly?
- Have you distinguished between the concentrations in the organic and aqueous layers?
- Have you considered not only concentration but also the volume of each phase?
- Have you calculated the extraction percentage or recovery?
- Have you explained the difference between single and multiple extractions?
- Have you discussed residual target substance in the aqueous layer?
- Have you considered the effect of ionization caused by pH?
- Have you considered emulsification and poor phase separation as sources of error?
- Have you considered errors in titration and absorbance measurements?
- Have you considered insufficient attainment of equilibrium?
- Have you considered differences in temperature and solvent conditions?
- Do the points for improvement correspond to the sources of error?
Summary
Partition equilibrium describes how a target substance is distributed between the aqueous and organic layers.
The partition coefficient K is determined by dividing the concentration in the organic layer by that in the aqueous layer.
The larger K is, the more readily the target substance transfers into the organic layer, but because K is finite, a certain amount of target substance remains in the aqueous layer.
Extraction efficiency is affected not only by the partition coefficient but also by the volume ratio of the organic and aqueous layers, the number of extractions, pH, salting out, emulsification, and the state of phase separation.
Even when the same amount of organic solvent is used, dividing the solvent into multiple extractions may reduce the amount of target substance remaining in the aqueous layer and increase extraction efficiency.
In a report, do not simply write that “the substance was extracted into the organic layer.”
Explain the partition coefficient, extraction percentage, residual amount in the aqueous layer, phase-volume ratio, multiple extraction, pH, and emulsification in relation to one another.
If the partition coefficient differs from the literature value, discussing insufficient attainment of equilibrium, concentration-measurement errors, temperature differences, and poor phase separation can produce a more persuasive report on partition equilibrium.
