A hydroxide precipitation experiment is an experiment in which a base is added to an aqueous solution containing metal ions to investigate the reaction in which the metal ions precipitate as metal hydroxides.
Many metal ions react with OH- in solution to form poorly soluble hydroxides.
Whether a precipitate forms is strongly affected by the metal-ion concentration, OH- concentration, pH, solubility product, complex formation, temperature, and other factors.
In a discussion of hydroxide precipitation, it is not sufficient simply to write that “a precipitate formed” or “a precipitate formed when the pH was increased.”
It is necessary to explain why precipitation occurs as the pH increases, how precipitation can be explained from the relationship between the solubility product and ionic product, and why aging the precipitate makes the particles larger and easier to filter.
The effects of coprecipitation, adsorption, insufficient washing, redissolution, and amphoteric hydroxides can also be discussed.
This article clearly explains, as examples of discussions that can be used in laboratory reports on hydroxide precipitation experiments, pH, OH- concentration, solubility product, precipitation-start pH, supersaturation, nucleation, particle growth, aging, coprecipitation, amphoteric hydroxides, filtration, washing, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of hydroxide precipitation results obtained in inorganic chemistry experiments, analytical chemistry experiments, and materials chemistry experiments at universities and similar institutions.
For the actual metal ions, base, pH adjustment method, precipitate-separation method, washing conditions, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is Hydroxide Precipitation?
- Main Items to Include in the Results
- Relationship Between pH and OH- Concentration
- What Is the Solubility Product?
- Discussion of Precipitation-Start pH
- Differences in Ease of Precipitation Among Metal Ions
- Supersaturation and Nucleation
- Discussion of Particle Growth
- Effect of Aging
- Discussion of Coprecipitation
- Incorporation of Impurities by Adsorption
- Redissolution of Amphoteric Hydroxides
- Effect of Complex Formation
- Discussion of Precipitate Color
- Discussion of Filterability
- Discussion of the Washing Operation
- Effects of Drying and Heating
- Causes of Error in Hydroxide Precipitation
- 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 Hydroxide Precipitation
- Summary
What Is Hydroxide Precipitation?
Hydroxide precipitation is a phenomenon in which a metal ion Mn+ reacts with hydroxide ions OH- and precipitates as a poorly soluble metal hydroxide M(OH)n.
For example, Fe3+, Al3+, Cu2+, Ni2+, Mg2+, and other metal ions form hydroxide precipitates depending on the conditions.
Whether precipitation occurs is not determined simply by the presence of metal ions and OH-.
A precipitate forms when the ionic product in the solution exceeds the solubility product.
Therefore, changes in metal-ion concentration and pH change whether precipitation occurs and how much precipitate forms.
Example Discussion:
When a base is added to a solution containing metal ions, the OH- concentration increases and the ionic product of the metal hydroxide becomes larger.
When the ionic product exceeds the solubility product, the metal hydroxide can no longer remain fully dissolved in the solution and precipitates.
Therefore, hydroxide precipitation can be explained in terms of pH and the solubility product.
Main Items to Include in the Results
In the results of a hydroxide precipitation experiment, organize the type of metal ion, initial concentration, base used, pH change, precipitation-start pH, precipitate color, amount of precipitate, filterability, aging conditions, washing conditions, and mass after drying.
If multiple metal ions are compared, it is useful to summarize in a table which metal precipitated at which pH.
Main Items to Include in the Results
- Type of metal ion
- Initial concentration of the metal ion
- Type of base used
- Concentration of the base
- Amount of base added
- Change in pH
- Precipitation-start pH
- pH at which precipitation was considered complete
- Color of the precipitate
- Amount of precipitate
- Particle size and appearance of the precipitate
- Aging time
- Ease of filtration
- Washing conditions
- Mass after drying
- Presence or absence of redissolution
- Causes of error and points for improvement
Example of How to Write the Results:
When a base was gradually added to the metal-ion solution, a white or colored precipitate formed as the pH increased.
The amount of precipitate increased rapidly above a certain pH, and after that, further addition of base caused little change in the amount of precipitate.
These results suggest that increasing the OH- concentration caused the ionic product of the metal hydroxide to exceed the solubility product and promoted precipitation.
Relationship Between pH and OH- Concentration
pH is very important in hydroxide precipitation.
As pH increases, the OH- concentration in solution increases.
Because precipitation of metal hydroxides depends on the OH- concentration, increasing the pH makes precipitation more likely.
For example, for an M(OH)2-type precipitate, the ionic product is expressed as [M2+][OH-]2.
Because the OH- concentration appears as the second power, even a small change in pH can greatly change the ease of precipitation.
Therefore, pH adjustment must be performed carefully.
pOH = 14 – pH
[OH-] = 10-pOH
Example Discussion:
The precipitate formed as the pH increased because the OH- concentration increased.
In metal hydroxide precipitation, the OH- concentration strongly affects the ionic product.
In particular, [OH-]2 is involved for M(OH)2 and [OH-]3 for M(OH)3, so even a small change in pH can produce a large difference in precipitation.
What Is the Solubility Product?
The solubility product is an equilibrium constant representing the product of ion concentrations in a saturated solution of a poorly soluble salt.
For a metal hydroxide M(OH)n, the dissolution equilibrium is expressed as follows.
When the product of the metal-ion concentration and OH- concentration in solution reaches the solubility product, precipitation and dissolution are in equilibrium.
M(OH)n(s) ⇄ Mn+ + nOH-
Ksp = [Mn+][OH-]n
When the ionic product is smaller than Ksp, precipitation is unlikely to occur.
When the ionic product exceeds Ksp, the solution becomes supersaturated and precipitation occurs.
Therefore, whether hydroxide precipitation occurs can be explained by comparing the ionic product with the solubility product.
Example Discussion:
Whether hydroxide precipitation occurs is determined by whether the ionic product of the metal ion and OH- exceeds the solubility product Ksp.
Adding a base increases the OH- concentration and increases the ionic product.
When the ionic product exceeded Ksp, the solution became supersaturated and the metal hydroxide was considered to have precipitated.
Discussion of Precipitation-Start pH
The precipitation-start pH is the pH at which a metal hydroxide begins to appear as a visible precipitate.
If the metal-ion concentration and solubility product are known, the OH- concentration and pH at which precipitation theoretically begins can be estimated.
Hydroxides with smaller solubility products precipitate more readily at lower OH- concentrations, that is, at lower pH.
However, the precipitation-start pH actually observed may differ from the theoretical value.
Possible reasons include supersaturation, delayed nucleation, complex formation, pH measurement error, local increases in pH, and the effects of coexisting ions.
It is important to discuss the difference between the experimental and theoretical values.
Example Discussion:
The precipitation-start pH of a metal hydroxide can be explained from the solubility product and the metal-ion concentration.
Hydroxides with smaller solubility products require lower OH- concentrations for the ionic product to exceed Ksp and therefore precipitate at lower pH.
However, the actual precipitation-start pH may differ from the theoretical value because of delayed nucleation, pH measurement error, or complex formation.
Differences in Ease of Precipitation Among Metal Ions
The solubility products of hydroxides differ greatly depending on the type of metal ion.
Therefore, at the same pH, some metal ions precipitate while others do not.
Metal hydroxides with small solubility products readily precipitate at low pH, whereas hydroxides with larger solubility products require higher pH to precipitate.
This property is also used to separate metal ions.
By increasing the pH stepwise, metal hydroxides that precipitate more readily may sometimes be separated first.
However, when coprecipitation, complex formation, or redissolution of amphoteric hydroxides occurs, the behavior may not be explained by the solubility product alone.
Example Discussion:
The precipitation-start pH differed among metal ions because the solubility products of the corresponding metal hydroxides were different.
Hydroxides with smaller Ksp values precipitate at lower OH- concentrations and therefore at lower pH.
By utilizing this difference, metal ions may be selectively separated by adjusting the pH.
Supersaturation and Nucleation
For precipitation to occur, the solution must become supersaturated.
Supersaturation is a state in which the ion concentration in solution exceeds the amount that can remain dissolved at equilibrium.
However, a visible precipitate does not necessarily form immediately after supersaturation is reached.
Small solid nuclei must first form before precipitation begins.
At high supersaturation, many nuclei form at once and fine precipitate particles are more likely to form.
At low supersaturation, fewer nuclei form and existing particles can grow more readily.
The particle size and filterability of a precipitate are affected by the balance between nucleation and particle growth.
Example Discussion:
Fine precipitate formed when the base was added rapidly because the local OH- concentration became high and the degree of supersaturation increased rapidly.
At high supersaturation, many nuclei form simultaneously and precipitate as fine particles before they can grow sufficiently.
As a result, the precipitate may have become difficult to filter.
Discussion of Particle Growth
After a precipitate forms, ions in solution are incorporated onto the surfaces of existing precipitate particles and the particles grow.
As particle growth proceeds, the precipitate particles become larger and easier to filter.
Excessively fine precipitates may pass through filter paper or cause clogging, so particle growth is important for precipitate separation.
Methods for promoting particle growth include adding the base slowly, stirring thoroughly, and warming or allowing the precipitate to stand for some time after precipitation.
These methods keep the degree of supersaturation relatively low, suppress the formation of numerous fine nuclei, and allow existing particles to grow more readily.
Example Discussion:
The precipitate became easier to filter after aging because fine particles were considered to have transformed into larger particles through dissolution and reprecipitation or surface growth.
Larger particles are more readily retained by filter paper and also improve the filtration rate.
Therefore, particle growth after precipitation greatly affects the separation operation.
Effect of Aging
Aging is an operation in which the precipitate is allowed to stand or is heated for some time in the mother liquor after precipitation.
During aging, Ostwald ripening may occur, in which fine particles dissolve and reprecipitate on larger particles, or rearrangement of the particle surfaces may proceed.
As a result, precipitate particles may become larger and crystallinity and filterability may improve.
Some hydroxide precipitates readily become gelatinous or colloidal and are difficult to filter in that state.
Aging may cause the particles to aggregate and grow, making them easier to settle.
However, because pH may change or the precipitate may change during aging, control of the conditions is necessary.
Example Discussion:
The improved filterability after aging was considered to result from fine particles dissolving and reprecipitating as larger particles.
Smaller particles have higher surface energy and tend to transfer to larger particles during aging.
As a result, the precipitate became easier to settle and clogging of the filter paper was reduced.
Discussion of Coprecipitation
Coprecipitation is a phenomenon in which other ions or impurities that should originally remain in solution are incorporated together with the target precipitate.
Hydroxide precipitates often have large surface areas and readily become colloidal, so they may adsorb other ions or incorporate them into the precipitate.
Coprecipitation lowers the purity of the precipitate.
Coprecipitation includes surface adsorption, occlusion, and mixed-crystal formation.
If precipitation occurs rapidly, the particles become fine and have a large surface area, making coprecipitation more likely.
Aging, reprecipitation, and sufficient washing may sometimes reduce the effects of coprecipitation.
Example Discussion:
One possible reason for the low purity of the precipitate is that impurity ions in the solution were incorporated into the precipitate through coprecipitation.
Hydroxide precipitates are fine and have large surface areas, so they readily adsorb other ions.
In particular, rapidly adding the base and producing a fine precipitate makes coprecipitation more likely, so it is important to precipitate slowly and perform aging.
Incorporation of Impurities by Adsorption
Hydroxide precipitates may carry surface charge and readily adsorb ions from the solution.
Gelatinous or colloidal precipitates in particular have large surface areas and readily adsorb impurity ions.
This adsorption affects the mass and composition of the precipitate.
Adsorbed impurities may sometimes be difficult to remove by simple washing.
Changing the type of washing solution, pH, or ionic strength may reduce the amount of adsorbed ions.
However, excessive washing may cause the precipitate to redisperse or dissolve.
Example Discussion:
One possible reason why impurities were present in the precipitate is adsorption of ions onto the precipitate surface.
Fine hydroxide precipitates have large specific surface areas and readily retain ions from the mother liquor on their surfaces.
Therefore, to increase the purity of the precipitate, it is important to grow the particles larger and remove adsorbed ions through appropriate washing.
Redissolution of Amphoteric Hydroxides
Some metal hydroxides, such as Al(OH)3, Zn(OH)2, and Cr(OH)3, are amphoteric hydroxides that dissolve in both acids and bases.
They precipitate under neutral to mildly basic conditions, but under strongly basic conditions they may form complex ions and dissolve again.
Therefore, if an excessive amount of base is added, the precipitate may decrease or disappear.
For example, Al(OH)3 forms [Al(OH)4]- in the presence of excess OH- and dissolves in the solution.
Therefore, when the purpose is to obtain a precipitate, it is important not to raise the pH excessively.
The behavior of amphoteric hydroxides is also important in metal-ion separation and qualitative tests.
Al(OH)3 + OH- → [Al(OH)4]-
Zn(OH)2 + 2OH- → [Zn(OH)4]2-
Example Discussion:
If the amount of precipitate decreased under conditions with excess base, the metal hydroxide formed may have been amphoteric and redissolved by forming a complex ion with excess OH-.
Al(OH)3 and Zn(OH)2 form hydroxo complex ions under strongly basic conditions.
Therefore, when performing precipitation separation, the pH must be appropriately controlled according to the target metal.
Effect of Complex Formation
When metal ions form complexes with ammonia, cyanide, citric acid, EDTA, or other ligands, the concentration of free metal ions decreases.
As the free metal-ion concentration decreases, the ionic product [Mn+][OH-]n becomes smaller and precipitation becomes less likely.
Therefore, complex formation affects the precipitation-start pH and the amount of precipitate.
For example, Cu2+ and Ni2+ may form complexes with ammonia.
If the complex is stable, hydroxide precipitation may not occur immediately even when OH- is added.
If less precipitate than expected is obtained in a precipitation experiment, the possibility of complex formation should be considered.
Example Discussion:
One possible reason why precipitation did not occur until a higher pH than expected is that the metal ion formed a complex with a ligand in the solution, reducing the concentration of free metal ions.
Because the solubility product depends on the concentration of free Mn+, complex formation decreases the ionic product and makes precipitation less likely.
Therefore, coexisting ligands can have a large effect on hydroxide precipitation.
Discussion of Precipitate Color
Metal hydroxides show different colors depending on the type and oxidation state of the metal ion.
For example, Fe(OH)3 may be reddish brown, Cu(OH)2 blue, Ni(OH)2 green, and Al(OH)3 white.
The color of the precipitate provides a clue for estimating the product.
However, the type of precipitate cannot be conclusively identified from color alone.
Color may change because of oxidation, mixed precipitation, coprecipitation, impurities, or drying.
Color observations should be discussed together with pH, solubility product, confirmation reactions, and analytical results.
| Example Hydroxide | Example Color | Point of Caution |
|---|---|---|
| Fe(OH)3 | Reddish brown | The color may vary depending on oxidation state or hydration state |
| Cu(OH)2 | Blue | It may change to a black oxide through heating or decomposition |
| Ni(OH)2 | Green | The color may change because of impurities or oxidation |
| Al(OH)3 | White | It may redissolve in excess base |
Example Discussion:
The color of the precipitate provides a clue for estimating the type and oxidation state of the metal ion.
However, the color of the precipitate can also change because of hydration state, oxidation, coprecipitation, or impurities, so the product cannot be identified from color alone.
The type of precipitate must be determined together with the pH conditions, solubility product, and confirmation reactions.
Discussion of Filterability
Some hydroxide precipitates readily form fine particles or gelatinous precipitates.
Such precipitates clog the pores of filter paper or pass through the filter paper, so filtration takes a long time.
Filterability is strongly affected by the particle size, aggregation state, and whether aging was performed.
When precipitate particles are large and well aggregated, filtration becomes easier.
In contrast, fine particles formed rapidly under highly supersaturated conditions are difficult to filter.
Aging, warming, slow addition of the base, and sufficient stirring may help improve filterability.
Example Discussion:
One possible reason why filtration of the precipitate took a long time is that the hydroxide precipitate formed was fine or gelatinous.
Fine particles readily clog the filter paper and may also partially pass into the filtrate.
Aging the precipitate and promoting particle growth may improve filterability.
Discussion of the Washing Operation
After the precipitate is filtered, washing is performed to remove ions, salts, and excess base remaining in the mother liquor.
If washing is insufficient, impurities remain on the precipitate surface or between particles, causing the mass after drying to be overestimated or the composition to become inaccurate.
Washing is particularly important when coprecipitation or adsorption occurs.
On the other hand, excessive washing may partially dissolve the precipitate or cause it to redisperse in a colloidal form.
Depending on the type of hydroxide precipitate, washing with pure water may cause peptization and make the precipitate more likely to pass through the filter paper.
The type and amount of washing solution must be selected according to the properties of the precipitate.
Example Discussion:
One possible reason why the mass of the dried precipitate was large is that salts or excess base from the mother liquor remained because of insufficient washing.
On the other hand, excessive washing may partially dissolve the precipitate or cause fine particles to flow out, reducing the yield.
Therefore, appropriate washing conditions must be selected by considering the balance between precipitate purity and loss.
Effects of Drying and Heating
Hydroxide precipitates may contain large amounts of water, and drying changes their mass and appearance.
If drying is insufficient, adhering water or adsorbed water remains and the precipitate mass may be overestimated.
In addition, some hydroxides may dehydrate and transform into oxides when heated.
For example, some metal hydroxides transform from M(OH)n to MOx when heated.
In such cases, it is necessary to clarify whether the mass after drying is being treated as the hydroxide or as the oxide.
In experiments involving strong heating, changes in the chemical species must be considered.
Example: M(OH)2 → MO + H2O
Example Discussion:
One possible reason for error in the mass after drying is that adsorbed water or adhering water remained in the precipitate.
In addition, if the heating temperature was high, the hydroxide may have dehydrated and changed into an oxide.
Therefore, when evaluating precipitate mass, it is necessary to clarify the drying conditions and the final chemical form.
Causes of Error in Hydroxide Precipitation
Causes of error in hydroxide precipitation include pH measurement errors, errors in the amount of base added, local increases in pH, insufficient stirring, insufficient aging, loss during filtration, insufficient washing, dissolution during washing, insufficient drying, coprecipitation, complex formation, and redissolution of amphoteric hydroxides.
Because precipitation reactions are sensitive to pH, even small differences in operation affect the results.
In particular, adding the base all at once creates locally high pH and rapidly forms fine precipitates.
This may increase coprecipitation and adsorption and also worsen filterability.
In addition, insufficient calibration of the pH meter and pH measurement in a suspension containing precipitate can also cause errors.
Example Discussion:
Possible causes of deviation of the amount of precipitate from the theoretical value include pH adjustment error, loss during filtration, insufficient washing, and coprecipitation.
If the base is added rapidly, the local OH- concentration becomes high and fine precipitates form quickly, making them more likely to incorporate impurities.
In addition, insufficient washing leaves salts from the mother liquor and may cause the mass after drying to be overestimated.
When the Results Can Be Considered Good
A hydroxide precipitation experiment can be considered to have produced good results when a precipitate forms as the pH increases and the precipitation-start pH generally agrees with the trend predicted from the solubility product.
In addition, if aging causes the precipitate particles to grow and filterability improves, particle growth and Ostwald ripening can be explained.
If multiple metal ions are compared and hydroxides with smaller solubility products tend to precipitate at lower pH, the relationship between pH and Ksp can be considered to have been confirmed.
The color of the precipitate and the presence or absence of redissolution also provide information for considering the properties of the metal hydroxide.
Example Discussion:
In this experiment, metal hydroxide precipitate formed as the pH increased, and the amount of precipitate increased above a certain pH.
This was considered to result from the increase in OH- concentration causing the ionic product to exceed Ksp.
In addition, because filterability improved after aging, the fine particles were considered to have grown and changed into larger particles.
Example Discussions When the Experiment Did Not Go Well
When a hydroxide precipitation experiment does not go well, possible causes should be considered from results such as a small amount of precipitate, a large deviation in precipitation-start pH, inability to filter the precipitate, cloudy filtrate, excessively large mass after drying, or redissolution of the precipitate.
Organizing the causes according to pH, solubility product, complex formation, coprecipitation, aging, filtration, washing, and drying makes the discussion easier.
Example Discussion:
One possible reason why less precipitate than expected was obtained is that the pH was not sufficiently high and the ionic product of the metal hydroxide did not sufficiently exceed Ksp.
In addition, if the metal ion formed a complex, the concentration of free metal ions decreased and precipitation became less likely.
Furthermore, fine precipitate may have flowed out during filtration or washing and reduced the yield.
Another Example Discussion:
One possible reason why the filtrate was cloudy is that the precipitate particles were extremely fine and passed through the filter paper.
Rapid addition of the base may have increased supersaturation and caused the formation of many fine nuclei.
In this case, slowly adding the base and aging the precipitate may promote particle growth and improve filterability.
Another Example Discussion:
If the amount of precipitate decreased after excess base was added, the hydroxide formed may have been amphoteric and redissolved by forming a complex ion with excess OH-.
This behavior is observed for Al(OH)3 and Zn(OH)2.
Therefore, to obtain the precipitate completely, excess base must be avoided and the pH must be maintained within an appropriate range.
How to Write Points for Improvement
In a discussion of hydroxide precipitation, 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 pH adjustment, precipitation, aging, filtration, washing, drying, and analytical evaluation.
Improvements to pH Adjustment
- Calibrate the pH meter
- Add the base in small portions
- Stir sufficiently during addition
- Avoid locally high pH
- Record the precipitation-start pH in detail
- Avoid excess base for amphoteric hydroxides
Improvements to Precipitation and Aging
- Do not increase the degree of supersaturation rapidly
- Age the precipitate for a fixed period
- Promote particle growth by warming when necessary
- Suppress the formation of fine precipitates
- Precipitate slowly to reduce coprecipitation
- Check for complex formation
Improvements to Filtration, Washing, and Drying
- Select appropriate filter paper or filters
- Check for cloudiness in the filtrate
- Keep the amount of washing solution constant
- Sufficiently remove salts from the mother liquor
- Avoid loss of precipitate during washing
- Standardize the drying conditions
- Clarify the final chemical form
Example of How to Write Points for Improvement:
To improve the reproducibility of hydroxide precipitation, it is necessary to add the base in small portions while stirring sufficiently and accurately control the pH.
Rapid precipitation tends to cause fine particles and coprecipitation, so it is important not to increase the degree of supersaturation rapidly.
In addition, aging the precipitate after formation and promoting particle growth may improve both filterability and precipitate purity.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of hydroxide precipitation, simply writing that “a precipitate formed when the pH was increased” or “aging made filtration easier” results in a superficial discussion.
A good discussion relates OH- concentration, solubility product, ionic product, supersaturation, particle growth, coprecipitation, and amphoteric hydroxides.
| Superficial Discussion | Good Discussion |
|---|---|
| A precipitate formed when the pH was increased. | The increase in pH increased the OH- concentration, causing the ionic product of the metal hydroxide to exceed the solubility product Ksp and producing a precipitate. |
| The ease of precipitation differed among metals. | Because the solubility product differs among metal hydroxides, the precipitation-start pH differed even at the same metal-ion concentration. |
| Aging made filtration easier. | During aging, fine particles dissolved and reprecipitated as larger particles, making them easier to retain on the filter paper and improving filterability. |
| The precipitate became contaminated. | Impurity ions from the mother liquor may have adsorbed onto the hydroxide-precipitate surface or been incorporated through coprecipitation, lowering the purity of the precipitate. |
| The precipitate dissolved when too much base was added. | Amphoteric hydroxides such as Al(OH)3 and Zn(OH)2 can form hydroxo complex ions in the presence of excess OH- and redissolve. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of hydroxide precipitation experiments.
Adjust the necessary parts according to your own experimental results.
- Metal hydroxide precipitation occurs when the ionic product of the metal ion and OH- exceeds the solubility product.
- As pH increases, the OH- concentration increases and hydroxide precipitation becomes more likely.
- Hydroxides with smaller solubility products tend to precipitate at lower pH.
- The precipitation-start pH changes depending on the metal-ion concentration and Ksp.
- Rapid addition of base causes local supersaturation and makes fine precipitates more likely to form.
- Aging may cause precipitate particles to grow and improve filterability.
- Hydroxide precipitates have large surface areas and readily adsorb impurity ions.
- Coprecipitation lowers the purity of the precipitate.
- Amphoteric hydroxides may form complex ions in the presence of excess OH- and redissolve.
- Insufficient washing and insufficient drying cause errors in precipitate mass.
Points to Check When Discussing Hydroxide Precipitation
Checking the following points before writing the report makes the discussion easier to write.
- Is the precipitation reaction equation written?
- Is the relationship between pH and OH- concentration explained?
- Is the solubility product Ksp used in the discussion?
- Is the ionic product compared with Ksp?
- Are differences in precipitation-start pH explained?
- Is the ease of precipitation among metal ions compared?
- Are supersaturation and nucleation considered?
- Is particle growth through aging explained?
- Are impurity incorporation through coprecipitation and adsorption considered?
- Is redissolution of amphoteric hydroxides considered?
- Are causes of error in filtration, washing, and drying described?
- Do the points for improvement correspond to the causes of error?
Summary
Hydroxide precipitation is a phenomenon in which metal ions react with OH- to form poorly soluble metal hydroxides.
Whether precipitation occurs is determined by the metal-ion concentration, OH- concentration, pH, and solubility product.
As pH increases, the OH- concentration increases and precipitation occurs when the ionic product exceeds Ksp.
The properties of the precipitate change depending on the degree of supersaturation, nucleation, particle growth, and aging conditions.
Rapid addition of base tends to produce fine precipitates and may cause coprecipitation and difficulty in filtration.
Aging may promote particle growth and improve filterability and purity.
In addition, for amphoteric hydroxides such as Al(OH)3 and Zn(OH)2, attention must be paid to redissolution caused by excess base.
In a report, rather than simply writing that “a precipitate formed,” organize and discuss pH, OH- concentration, solubility product, precipitation-start pH, supersaturation, nucleation, aging, coprecipitation, adsorption, amphoteric hydroxides, filtration, washing, drying, causes of error, and points for improvement.
Hydroxide precipitation experiments are important experiments for understanding precipitation equilibrium and separation operations.
