A sol coagulation experiment is an experiment that investigates how colloidal particles dispersed in a liquid aggregate when an electrolyte is added.
A sol may appear at first glance to be a homogeneous solution, but in reality, colloidal particles larger than molecules or ions are dispersed in the liquid.
The main focus of the experimental discussion is to consider why these particles remain stably dispersed and why this stability is lost when an electrolyte is added.
In a discussion of a sol coagulation experiment, it is not sufficient simply to write that “a precipitate formed” or “the solution became cloudy.”
It is necessary to explain why coagulation becomes more likely as the electrolyte concentration increases, why divalent and trivalent ions cause stronger coagulation than monovalent ions, and how these effects are related to the surface charge of the sol particles and the electrical double layer.
In particular, using the critical coagulation concentration and the Schulze-Hardy rule makes it easier to organize the results chemically.
This article explains, as examples of discussions that can be used in laboratory reports on sol coagulation experiments, sol stability, the mechanism of coagulation, the effect of electrolyte concentration, critical coagulation concentration, the Schulze-Hardy rule, ionic valence, hydrophilic and hydrophobic sols, protective colloids, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of sol coagulation experimental results obtained in basic chemistry experiments, physical chemistry experiments, and colloid chemistry experiments at universities and similar institutions.
For the actual type of sol, electrolyte, concentration, amount added, observation time, pH, temperature, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is a Sol?
- Main Items to Include in the Results
- What Is Coagulation?
- Why Sol Particles Remain Dispersed
- Relationship Between Electrolyte Concentration and Coagulation
- The Electrical Double Layer and Coagulation
- What Is the Critical Coagulation Concentration?
- What Is the Schulze-Hardy Rule?
- Case of a Negatively Charged Sol
- Case of a Positively Charged Sol
- Comparison of Ionic Valence and Critical Coagulation Concentration
- Difference Between Hydrophilic and Hydrophobic Sols
- Effect of Protective Colloids
- Effect of pH
- Methods for Determining Coagulation
- Cases That Deviate from the Schulze-Hardy Rule
- Causes of Error in Sol Coagulation Experiments
- 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 Sol Coagulation Experiments
- Summary
What Is a Sol?
A sol is a state in which colloidal particles are dispersed in a liquid.
Gold sol, sulfur sol, iron hydroxide sol, silica sol, and starch sol are representative examples.
The particles in a sol are larger than molecules or ions in a true solution but smaller than coarse suspended particles, so they tend not to settle readily over a short period.
A sol appears stable because the particles remain dispersed while repelling one another.
In many hydrophobic sols, the particle surfaces carry charges of the same sign, and electrostatic repulsion prevents the particles from approaching one another easily.
Therefore, when considering sol coagulation, it is important to relate particle surface charge to dispersion stability.
Example Discussion:
A sol is a system in which colloidal particles are dispersed in a liquid.
Although the particles are difficult to see with the naked eye, when an electrolyte is added they aggregate and can be observed as turbidity or precipitate.
Therefore, in a sol coagulation experiment, it is important to compare the conditions under which the particles remain stably dispersed with the conditions under which aggregation begins.
Main Items to Include in the Results
In a sol coagulation experiment, organize the type of sol used, the color and transparency of the sol, the type of electrolyte added, electrolyte concentration, amount added, presence or absence of coagulation, time until coagulation begins, amount of precipitate, clarity of the supernatant, and other information.
When comparing multiple electrolytes, organizing ionic valence and ease of coagulation in a table makes the discussion easier.
- Type of sol used
- Color, turbidity, and transparency of the sol
- Type of electrolyte added
- Electrolyte concentration and amount added
- Presence or absence of coagulation
- Changes in precipitation and turbidity
- Time until coagulation
- Critical coagulation concentration
- pH, temperature, and observation time
- Causes of error and points for improvement
Example of How to Write the Results:
When an electrolyte was added to the sol, no major change was observed at low concentrations, but above a certain concentration, turbidity increased and a precipitate formed.
In addition, even at the same amount added, electrolytes containing ions with higher valence caused coagulation with smaller amounts.
From these results, the dispersion stability of the sol was considered to be strongly affected by electrolyte concentration and ionic valence.
What Is Coagulation?
Coagulation is the phenomenon in which colloidal particles dispersed in a sol gather together to form larger aggregates that appear as precipitate or turbidity.
In the dispersed state, aggregation is suppressed by electrostatic repulsion between particles and by hydration.
However, when the repulsive force is weakened by the addition of an electrolyte or another factor, the particles can approach one another and aggregate.
When coagulation occurs, the particle size increases, so light scattering becomes stronger and the solution may appear cloudy.
As the aggregates become larger, they settle under gravity and form a precipitate.
In other words, coagulation occurs as a result of the loss of dispersion stability of the sol.
Example Discussion:
The sol became cloudy and a precipitate formed after the electrolyte was added because the dispersed colloidal particles coagulated.
The electrolyte weakened the repulsion caused by the charges on the particle surfaces, allowing the particles to approach one another more easily.
As a result, the particles gathered into larger aggregates and were considered to have settled.
Why Sol Particles Remain Dispersed
The main reason sol particles can remain dispersed is that the particle surfaces carry charges of the same sign.
Particles with charges of the same sign electrostatically repel one another and therefore have difficulty approaching each other.
As long as this repulsive force is greater than the attractive force between the particles, the sol maintains a stable dispersed state.
In hydrophilic sols, water molecules may bind to the particle surface and form a hydration layer, which also contributes to dispersion stability.
In contrast, hydrophobic sols are weakly stabilized by hydration and are stabilized mainly by repulsion due to surface charge.
Therefore, hydrophobic sols tend to be more strongly affected by electrolytes and coagulate more readily.
Example Discussion:
Sol particles maintain a dispersed state because their surfaces carry charges of the same sign and they electrostatically repel one another.
Before the electrolyte was added, this repulsive force prevented the particles from approaching each other.
Therefore, sol stability is closely related to particle surface charge.
Relationship Between Electrolyte Concentration and Coagulation
When the electrolyte concentration is low, the surface charge of the particles is sufficiently maintained and repulsion between particles remains, so coagulation is unlikely to occur.
However, as the electrolyte concentration increases, ions in the solution shield the particle surface charges and compress the electrical double layer around the particles.
As a result, electrostatic repulsion between particles becomes weaker and coagulation becomes more likely.
In experiments, little change may be observed when only a small amount of electrolyte is added, whereas turbidity or precipitation may suddenly occur once a certain concentration is exceeded.
This is because the limit at which dispersion stability can be maintained has been exceeded.
By examining this boundary, the critical coagulation concentration can be considered.
Example Discussion:
Under conditions with low electrolyte concentration, electrostatic repulsion between the sol particles remained, so almost no coagulation occurred.
On the other hand, as the electrolyte concentration increased, the particle surface charges were shielded and the electrical double layer was compressed.
As a result, repulsion between particles weakened and coagulation was considered to have occurred above a certain concentration.
The Electrical Double Layer and Coagulation
Colloidal particle surfaces carry charges, and ions with the opposite sign gather around them.
The layer formed by the particle surface charge and the distribution of surrounding ions is called the electrical double layer.
When the electrical double layer is thick, strong repulsion acts when particles approach one another, making aggregation difficult.
When an electrolyte is added, the ion concentration in the solution increases and the electrical double layer is compressed.
As the electrical double layer becomes thinner, the repulsive force between particles becomes weaker when they approach each other.
Therefore, attractive forces between particles become dominant and coagulation occurs.
Example Discussion:
The addition of the electrolyte was considered to have compressed the electrical double layer formed around the colloidal particles.
When the electrical double layer becomes thinner, electrostatic repulsion between particles weakens and the particles can approach one another more easily.
As a result, aggregation progressed because of attractive forces between the particles, and coagulation was observed.
What Is the Critical Coagulation Concentration?
The critical coagulation concentration is the minimum electrolyte concentration required to coagulate a fixed amount of sol within a fixed period.
In English, it is called the Critical Coagulation Concentration and may be abbreviated as CCC.
An electrolyte with a lower critical coagulation concentration can cause coagulation with a smaller amount and is therefore judged to have greater coagulating power.
In an experiment, the electrolyte concentration is varied stepwise to determine the concentration above which clear turbidity or precipitation occurs.
However, because the judgment of coagulation may vary depending on the observation time and visual criteria, it is important to keep the conditions consistent in comparative experiments.
An electrolyte with greater coagulating power has a lower critical coagulation concentration.
Example Discussion:
The critical coagulation concentration is the minimum electrolyte concentration required to coagulate a sol.
In this experiment, an electrolyte that caused coagulation at a lower concentration was considered to have shielded the particle surface charge more effectively.
Therefore, that electrolyte can be judged to have greater coagulating power.
What Is the Schulze-Hardy Rule?
The Schulze-Hardy rule is an empirical rule stating that, in the coagulation of hydrophobic colloids, the greater the valence of the ion having a charge opposite to that of the colloidal particles, the markedly greater its coagulating power.
For negatively charged sols, the valence of the cation is important, whereas for positively charged sols, the valence of the anion is important.
For example, in a negatively charged sol, Ca2+ causes stronger coagulation than Na+, and Al3+ causes stronger coagulation than Ca2+.
This is because multivalent ions can more strongly neutralize the charge on the particle surface and more effectively compress the electrical double layer.
Example Discussion:
The fact that electrolytes containing ions with higher valence caused coagulation with smaller amounts is consistent with the Schulze-Hardy rule.
Multivalent ions with a charge opposite to that of the colloidal particles strongly neutralize the particle surface charge and strongly compress the electrical double layer.
Therefore, divalent ions have greater coagulating power than monovalent ions, and trivalent ions have greater coagulating power than divalent ions.
Case of a Negatively Charged Sol
In a negatively charged sol, cations are strongly involved in coagulation.
Cations neutralize the negative charge on the particle surface and weaken repulsion between particles.
Therefore, cations with higher valence tend to cause coagulation at lower concentrations.
For example, when NaCl, CaCl2, and AlCl3 are compared, the coagulating power differs because of the different valences of Na+, Ca2+, and Al3+.
In general, Al3+ has the greatest coagulating power, followed by Ca2+, while Na+ has the smallest.
Example Discussion:
In a negatively charged sol, cations that neutralize the negative charge on the particle surface are strongly involved in coagulation.
Because Ca2+ has a greater charge-neutralizing effect than Na+, and Al3+ has a greater effect than Ca2+, coagulation occurs more readily at lower concentrations.
Therefore, the coagulating power tends to follow the order Al3+ > Ca2+ > Na+.
Case of a Positively Charged Sol
In a positively charged sol, anions are strongly involved in coagulation.
Anions neutralize the positive charge on the particle surface and weaken repulsion between particles.
Therefore, anions with higher valence have greater coagulating power.
For example, in positively charged iron hydroxide sol and similar systems, SO42- may show a stronger coagulating effect than Cl-, and PO43- may show a stronger effect than SO42-.
Which ion should be considered depends on the sign of the charge on the sol particles.
Example Discussion:
In a positively charged sol, anions that neutralize the positive charge on the particle surface are strongly involved in coagulation.
Multivalent anions such as SO42- and PO43- can neutralize the surface charge more strongly than Cl-.
Therefore, in a positively charged sol, the coagulating power increases as the valence of the anion increases.
Comparison of Ionic Valence and Critical Coagulation Concentration
According to the Schulze-Hardy rule, the greater the valence of the oppositely charged ion, the greater the coagulating power and the lower the critical coagulation concentration.
In other words, trivalent ions may cause coagulation at far lower concentrations than monovalent ions.
This difference shows that not only electrolyte concentration but also ionic charge has a major effect on dispersion stability.
| Valence of Oppositely Charged Ion | Coagulating Power | Critical Coagulation Concentration |
|---|---|---|
| Monovalent ion | Small | Large |
| Divalent ion | Large | Small |
| Trivalent ion | Very large | Very small |
Example Discussion:
In the experiment, electrolytes containing divalent ions caused coagulation with smaller amounts than those containing monovalent ions, and electrolytes containing trivalent ions caused coagulation with even smaller amounts.
This is because ions with higher valence more efficiently neutralize the charge on colloidal particle surfaces and strongly compress the electrical double layer.
Therefore, the critical coagulation concentration was considered to decrease as ionic valence increased.
Difference Between Hydrophilic and Hydrophobic Sols
A hydrophilic sol is a sol in which particles or polymers with a high affinity for water are dispersed, and a hydration layer forms on the particle surface.
Because this hydration layer prevents particles from approaching one another, coagulation may be difficult even when an electrolyte is added.
Gelatin sol, starch sol, and protein solutions are examples.
A hydrophobic sol is a sol in which particles with little affinity for water are dispersed and is stabilized mainly by surface charge.
Therefore, when the surface charge is shielded by an electrolyte, coagulation occurs readily.
The Schulze-Hardy rule applies particularly well to hydrophobic sols.
| Type | Main Stabilizing Factor | Coagulation by Electrolytes |
|---|---|---|
| Hydrophilic sol | Hydration layer and polymer chains | Relatively difficult to occur |
| Hydrophobic sol | Surface charge | Occurs readily |
Example Discussion:
In a hydrophobic sol, dispersion stability is maintained mainly by the charge on the particle surface, so coagulation readily occurs when an electrolyte is added.
On the other hand, in a hydrophilic sol, a hydration layer protects the particle surface, so coagulation may remain difficult even if the surface charge is partially shielded.
Therefore, even when the same electrolyte is added, the ease of coagulation differs between hydrophilic and hydrophobic sols.
Effect of Protective Colloids
A protective colloid is a hydrophilic polymer colloid that makes coagulation less likely when added to a hydrophobic sol.
When polymers such as gelatin or starch adsorb onto the surfaces of hydrophobic colloidal particles, a hydration layer and steric barriers are formed.
Therefore, even when an electrolyte is added, the particles have difficulty approaching one another directly.
If coagulation is delayed or the amount of precipitate is reduced in a sample containing a protective colloid, a protective effect can be considered to have occurred.
This effect is also related to dispersion stabilization in foods, pharmaceuticals, paints, inks, and other products.
Example Discussion:
Coagulation became less likely under conditions where a protective colloid was added because a hydrophilic polymer was considered to have covered the surfaces of the hydrophobic sol particles.
The hydration layer and steric hindrance formed by the polymer made it difficult for the particles to approach one another.
As a result, aggregation was suppressed even after the electrolyte was added, and the dispersion stability of the sol was considered to have increased.
Effect of pH
The surface charge of sol particles may change depending on pH.
If acidic or basic groups are present on the particle surface, the magnitude and sign of the surface charge change depending on the concentrations of H+ and OH-.
Therefore, even when the same electrolyte is added, the ease of coagulation may differ depending on pH.
At a pH where the surface charge becomes small, repulsion between particles weakens and coagulation becomes more likely.
In amphoteric colloids such as proteins, the overall charge may become small near the isoelectric point and precipitation may occur more readily.
Example Discussion:
Changes in pH alter the charge state of the sol particle surface and therefore affect the ease of coagulation.
Under conditions where the surface charge becomes small, electrostatic repulsion between particles weakens and aggregation may occur more readily even without adding an electrolyte.
Therefore, in coagulation experiments, not only the electrolyte concentration but also the pH conditions must be kept constant.
Methods for Determining Coagulation
Coagulation is determined by observing increases in turbidity, formation of precipitate, clarification of the supernatant, presence or absence of sedimentation, and other changes.
However, the onset of coagulation may be difficult to judge visually.
Therefore, it is important to standardize the observation time, background, lighting conditions, and criteria for the amount of precipitate.
Even for the same sample, only turbidity may be visible immediately after addition, while clear precipitation may appear after some time has passed.
When comparing the time until coagulation, the elapsed time after electrolyte addition must be accurately standardized.
For quantitative evaluation, absorbance or transmittance may also be measured.
Example Discussion:
Coagulation was judged from the presence or absence of turbidity and precipitate, but visual judgment includes subjective error.
Particularly in the early stages of coagulation, changes in turbidity may be small and difficult to judge.
Therefore, observation time and background conditions should be standardized, and if possible, evaluation by absorbance or transmittance is desirable.
Cases That Deviate from the Schulze-Hardy Rule
Experimental results may not completely agree with the Schulze-Hardy rule.
Possible causes include surface properties of the sol particles, specific adsorption of ions, pH changes, hydrolysis of electrolytes, errors in concentration preparation, differences in observation time, and contamination with protective colloids.
Actual colloidal systems may not be determined simply by ionic valence alone.
For example, trivalent ions show strong coagulating power, but they may hydrolyze in solution or change the pH.
In addition, if ions specifically adsorb onto the particle surface, effects greater than simple charge shielding may appear.
Deviations from the rule provide material for discussing the experimental conditions and properties of the chemical species.
Example Discussion:
One possible reason why the experimental results deviated slightly from the Schulze-Hardy rule is that factors other than ionic valence had an effect.
If multivalent ions hydrolyze and change the pH or specifically adsorb onto the particle surface, the ease of coagulation cannot be explained by valence alone.
In addition, errors in electrolyte concentration, amount added, and observation time were also considered to have affected the deviation in the results.
Causes of Error in Sol Coagulation Experiments
Causes of error in sol coagulation experiments include differences in sol concentration, errors in preparing electrolyte concentrations, differences in amount added, insufficient mixing, differences in observation time, temperature differences, pH changes, dirty containers, sample deterioration, and subjectivity in determining precipitation.
Because coagulation is sensitive to conditions, even slight differences in operation may change the results.
Particularly when determining the critical coagulation concentration, the definition of what is judged to be “coagulated” greatly affects the result.
Visual judgment of the degree of turbidity tends to vary among observers, so if possible, evaluation by absorbance or transmittance after a fixed period is preferable.
Example Discussion:
Possible causes of error in the sol coagulation experiment include errors in preparing the electrolyte concentration, differences in the amount added, insufficient mixing, and differences in observation time.
In addition, if the presence or absence of coagulation is judged visually, subjective differences may affect the evaluation of turbidity and precipitation.
Therefore, to accurately compare critical coagulation concentrations, the amount added, mixing method, observation time, and judgment criteria must be standardized.
When the Results Can Be Considered Good
A sol coagulation experiment can be considered to have produced good results when coagulation becomes more likely as the electrolyte concentration increases and when ions with a higher valence opposite to that of the colloidal particles cause coagulation at lower concentrations.
These results correspond to compression of the electrical double layer by electrolytes and to the Schulze-Hardy rule.
In addition, if coagulation is suppressed under conditions where a protective colloid is added, dispersion stabilization by a hydrophilic polymer can be considered to have occurred.
If the observations are consistent with the concepts of surface charge, the electrical double layer, ionic valence, and protective colloids, the results can be judged reasonable.
Example Discussion:
In this experiment, coagulation of the sol became more likely as the electrolyte concentration increased.
In addition, electrolytes containing ions with higher valence opposite to that of the colloidal particles caused coagulation with smaller amounts.
These results are consistent with the Schulze-Hardy rule, in which electrolytes shield the particle surface charge and multivalent ions more strongly compress the electrical double layer.
Example Discussions When the Experiment Did Not Go Well
When a sol coagulation experiment does not go well, possible causes can be considered from results such as failure of coagulation to occur, an unexpected order of coagulating power, a small amount of precipitate, variation in the critical coagulation concentration, or poor reproducibility under the same conditions.
Organizing the causes according to sol concentration, electrolyte concentration, amount added, pH, temperature, mixing, observation time, and judgment criteria makes the discussion easier.
Example Discussion:
In this experiment, coagulation was not clearly observed even after the electrolyte was added.
One possible reason is that the electrolyte concentration was too low to sufficiently shield the surface charge of the colloidal particles.
In addition, if the sol was hydrophilic and stabilized by a hydration layer, the particles may have remained difficult to approach even after the surface charge was partially neutralized, making coagulation less likely.
Another Example Discussion:
Possible reasons why the order of coagulating power did not agree with the Schulze-Hardy rule include pH changes caused by electrolyte hydrolysis, specific adsorption of ions, and errors in concentration preparation.
In addition, when coagulation is judged visually, variation may occur in the evaluation of turbidity.
Therefore, when comparing coagulation, it is important to standardize the observation time and judgment criteria.
How to Write Points for Improvement
In a discussion of a sol coagulation experiment, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to sample preparation, electrolyte addition, coagulation judgment, and comparison conditions.
Improvements to Sample Preparation
- Keep the sol concentration consistent
- Mix thoroughly before use
- Measure the sample amount accurately
- Keep pH and temperature constant
- Remove contamination from containers
Improvements to Electrolyte Addition
- Prepare the electrolyte concentration accurately
- Keep the amount added constant
- Standardize the mixing method after addition
- Prepare a stepwise concentration series
- Observe at the same time intervals
Improvements to Coagulation Judgment
- Fix the observation time
- Define criteria for turbidity and precipitation
- Observe under the same background and lighting conditions
- Take photographs for comparison
- Quantify using absorbance or transmittance
- Perform multiple measurements to confirm reproducibility
Example of How to Write Points for Improvement:
To accurately compare critical coagulation concentrations, the sol concentration, sample amount, amount of electrolyte added, mixing method, and observation time must be kept constant.
In addition, because visual judgment of coagulation is susceptible to subjective error, the samples should be observed under the same background and lighting conditions and, if possible, evaluated quantitatively using absorbance or transmittance.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of a sol coagulation experiment, simply writing that “a precipitate formed” or “multivalent ions caused stronger coagulation” results in a superficial discussion.
A good discussion relates surface charge, the electrical double layer, electrolyte concentration, critical coagulation concentration, and the Schulze-Hardy rule.
| Superficial Discussion | Good Discussion |
|---|---|
| A precipitate formed when the electrolyte was added. | The ions in the electrolyte shielded the charge on the colloidal particle surfaces and compressed the electrical double layer, weakening repulsion between particles and causing coagulation. |
| Coagulation occurred more at higher concentrations. | As the electrolyte concentration increased, the number of ions in the solution increased and shielding of the particle surface charge became stronger, making the reduction in repulsive force required for coagulation more likely. |
| The trivalent ion was stronger. | According to the Schulze-Hardy rule, the greater the valence of the ion with a charge opposite to that of the colloidal particles, the greater the coagulating power, and trivalent ions can strongly neutralize the surface charge even in small amounts. |
| The results were slightly different. | In addition to ionic valence, pH changes, specific adsorption, errors in electrolyte concentration, mixing conditions, observation time, and subjectivity in determining coagulation may have affected the results. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of a sol coagulation experiment.
Adjust the necessary parts according to your own experimental results.
- A sol is a system in which colloidal particles are dispersed in a liquid.
- Sol particles maintain a dispersed state through electrostatic repulsion caused by surface charge.
- When an electrolyte is added, the particle surface charge is shielded and coagulation becomes more likely.
- The higher the electrolyte concentration, the more strongly the electrical double layer is compressed and the weaker the repulsion between particles becomes.
- The critical coagulation concentration is the minimum electrolyte concentration required to coagulate a sol.
- An electrolyte with a lower critical coagulation concentration has greater coagulating power.
- According to the Schulze-Hardy rule, the greater the valence of the oppositely charged ion, the greater the coagulating power.
- In a negatively charged sol, the valence of the cation is strongly related to coagulation.
- In a positively charged sol, the valence of the anion is strongly related to coagulation.
- Hydrophilic sols may be more difficult to coagulate than hydrophobic sols because of their hydration layers.
Points to Check When Discussing Sol Coagulation Experiments
Checking the following points before writing the report makes the discussion easier to write.
- Is the definition of a sol explained?
- Is the meaning of coagulation explained?
- Is the reason sol particles remain dispersed described?
- Are particle surface charge and electrostatic repulsion considered?
- Is compression of the electrical double layer by electrolytes explained?
- Are electrolyte concentration and ease of coagulation related?
- Is the critical coagulation concentration explained?
- Is the Schulze-Hardy rule used in the discussion?
- Is attention paid to the valence of the oppositely charged ion?
- Are the differences between hydrophilic and hydrophobic sols considered?
- Are the effects of pH and temperature considered?
- Do the points for improvement correspond to the causes of error?
Summary
A sol coagulation experiment is an experiment that investigates how colloidal particles remain dispersed in a liquid and under what conditions they aggregate.
Sol particles maintain a dispersed state through electrostatic repulsion caused by surface charge and through hydration.
However, when an electrolyte is added, the particle surface charge is shielded and the electrical double layer is compressed, allowing the particles to approach one another more easily and causing coagulation.
The higher the electrolyte concentration, the more readily coagulation occurs.
In addition, according to the Schulze-Hardy rule, the greater the valence of the ion with a charge opposite to that of the colloidal particles, the greater the coagulating power and the lower the critical coagulation concentration.
In negatively charged sols, the valence of the cation is particularly important, while in positively charged sols, the valence of the anion is particularly important.
In a report, rather than simply writing that “a precipitate formed” or “the solution became cloudy,” organize and discuss the dispersion stability of the sol, surface charge, electrical double layer, electrolyte concentration, critical coagulation concentration, the Schulze-Hardy rule, differences between hydrophilic and hydrophobic sols, causes of error, and points for improvement.
Sol coagulation experiments are important experiments for understanding the stability of colloidal particles and the role of ions.
