Gravimetric analysis is a quantitative analytical method in which the target component is isolated in the form of a precipitate or another solid, and the amount of the component is determined from its mass.
In university analytical chemistry experiments, examples include precipitating sulfate ions as barium sulfate and precipitating metal ions as sparingly soluble salts.
In a gravimetric analysis report, it is important not only to calculate the amount of the target component from the precipitate mass, but also to discuss the completeness of precipitation, filtration, washing, drying, ignition, constant mass, and sources of error caused by coprecipitation and adsorption.
In particular, the discussion becomes deeper when it explains whether the measured mass was too large or too small and which operation may have caused the error.
This article clearly explains how to interpret gravimetric analysis results, how to quantify a target component from precipitate mass, sources of error, points for improvement, and discussion examples that can be used in reports.
Note:
This article is a reference intended to assist with discussions of results obtained in chemistry experiments at universities and similar institutions.
For the actual experimental procedures, precipitation conditions, filtration, washing, drying, and ignition conditions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is Gravimetric Analysis?
- Results to Examine in Gravimetric Analysis
- How to Quantify From Precipitate Mass
- Errors When Precipitation Is Incomplete
- Errors Caused by Dissolution of the Precipitate
- Loss of Precipitate During Filtration
- Errors Caused by Insufficient Washing
- Errors Caused by Excessive Washing
- Errors Caused by Insufficient Drying
- Mass Changes Caused by Ignition
- What Is Constant Mass?
- Errors Caused by Coprecipitation
- Errors Caused by Adsorption
- Precipitate Particle Size and Digestion
- Errors in Measuring the Mass of the Crucible or Filter Paper
- Why a Desiccator Is Used
- When the Precipitate Mass Is Larger Than the Theoretical Value
- When the Precipitate Mass Is Smaller Than the Theoretical Value
- Discussion When the Sample Was Weighed Multiple Times
- When the Result Can Be Considered Good
- Example Discussion When the Analysis 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 Gravimetric Analysis
- Summary
What Is Gravimetric Analysis?
Gravimetric analysis is an analytical method in which the target component is isolated as a substance of constant composition and its amount is determined by measuring its mass.
Unlike titration, which measures volume, gravimetric analysis determines the amount of the target component from the mass of the final precipitate or residue obtained.
In a typical gravimetric analysis, the target ion is reacted with a precipitating reagent to form a sparingly soluble precipitate.
The precipitate is filtered, washed, dried or ignited, and then weighed.
If the chemical formula of the precipitate is known, the amount or content of the target component can be calculated from the mass of the precipitate.
Gravimetric analysis requires time to perform, but if the precipitate forms completely and is obtained in a pure form with constant composition, highly accurate analysis is possible.
On the other hand, loss of precipitate, contamination by impurities, and insufficient drying can greatly affect the results.
Results to Examine in Gravimetric Analysis
In gravimetric analysis results, organize the sample amount, precipitate mass, mass after drying or ignition, confirmation of constant mass, and the calculated amount or content of the target component.
The color and shape of the precipitate, the condition of the filtrate, and changes after washing also provide material for discussion.
Main Items to Include in the Results
- Mass or volume of the sample
- Type of precipitating reagent
- Type of precipitate formed
- Filtration method
- Whether washing was performed
- Mass after drying or ignition
- Whether constant mass was reached
- Mass of the precipitate
- Amount of substance of the target component
- Content of the target component
- Comparison with theoretical or labeled values
Example of How to Write the Results:
A precipitating reagent was added to the sample solution to form the target component as a sparingly soluble precipitate.
The obtained precipitate was filtered and washed, and its mass was measured after drying.
The precipitate mass was determined by subtracting the mass of the empty crucible from the mass of the crucible containing the dried precipitate.
The amount of the target component in the sample was calculated using this precipitate mass.
How to Quantify From Precipitate Mass
In gravimetric analysis, the amount of the target component is determined from the mass of the precipitate.
To do this, the quantitative relationship between the chemical formula of the precipitate and the target component is used.
Amount of precipitate = Mass of precipitate ÷ Molar mass of precipitate
If the target component is contained in the precipitate in a 1:1 ratio, the amount of the target component can be determined from the amount of the precipitate.
The mass of the target component can then be calculated by multiplying by the molar mass of the target component.
Mass of target component = Amount of target component × Molar mass of target component
When determining the content in the sample, divide the mass of the target component by the mass of the sample and express the result as a percentage.
Content (%) = Mass of target component ÷ Mass of sample × 100
Example Discussion:
In gravimetric analysis, the amount of the target component is determined from the mass of the obtained precipitate, assuming that the precipitate has a constant chemical composition.
The amount of precipitate can be determined by dividing its mass by its molar mass, and the amount of the target component can then be calculated from the stoichiometric coefficients in the reaction equation.
Therefore, it is important for accurate quantification that the precipitate be pure and completely dried.
Errors When Precipitation Is Incomplete
In gravimetric analysis, ideally all of the target component should be recovered as a precipitate.
However, if precipitation is incomplete and part of the target component remains in solution, the amount of precipitate obtained becomes smaller than it should be.
As a result, the amount or content of the target component is underestimated.
Possible causes of incomplete precipitation include insufficient precipitating reagent, inappropriate pH conditions, insufficient reaction time, inappropriate temperature conditions, and insufficient stirring.
Example Discussion:
If precipitation was incomplete, part of the target component would remain in the filtrate, so the amount of precipitate recovered would be smaller than the actual amount.
As a result, the amount of the target component calculated from the precipitate mass may be underestimated.
Insufficient precipitating reagent, inappropriate pH conditions, insufficient reaction time, or inadequate stirring may have caused incomplete precipitation.
Errors Caused by Dissolution of the Precipitate
If the precipitate formed is not completely insoluble, part of it may remain dissolved in the solution.
In addition, if the type or amount of washing solution is inappropriate, part of the precipitate may dissolve during washing.
In this case, the mass of precipitate recovered becomes smaller.
Example Discussion:
If part of the precipitate dissolved in the filtrate or washing solution, the final mass of precipitate recovered would decrease.
In particular, if too much washing solution was used or washing was performed under conditions where the precipitate was slightly soluble, some of the target component may have been lost.
As a result, the amount of the target component determined from the precipitate mass is considered to have been smaller than the actual value.
Loss of Precipitate During Filtration
If part of the precipitate remains on the filter paper or apparatus during filtration, or passes into the filtrate, the recovered amount decreases.
In particular, when the precipitate particles are fine, they may pass through the filter paper or be washed away during washing.
Loss of precipitate reduces the measured precipitate mass and therefore causes the amount of the target component to be underestimated.
Example Discussion:
If part of the precipitate passed through the filter paper during filtration or remained attached to the apparatus without being recovered, the measured precipitate mass would be smaller than the actual value.
Therefore, the amount of the target component calculated from the precipitate mass may be underestimated.
In particular, fine precipitate particles are easily lost during filtration and washing, so care is required.
Errors Caused by Insufficient Washing
Mother liquor and soluble impurities may adhere to the precipitate.
If the precipitate is dried and weighed without sufficient washing, components other than the precipitate are also included in the measured mass.
Therefore, the precipitate mass becomes larger than the actual value and the amount of the target component may be overestimated.
Example Discussion:
If washing was insufficient, mother liquor and soluble impurities may have remained on the precipitate surface and been dried together with the precipitate.
In this case, the measured mass includes components other than the target precipitate, so the precipitate mass becomes larger than the actual value.
As a result, the amount or content of the target component may be overestimated.
Errors Caused by Excessive Washing
Washing is necessary to remove impurities, but excessive washing may dissolve part of the precipitate or wash away fine precipitate particles.
In that case, the final precipitate mass becomes smaller.
Example Discussion:
If washing was excessive, part of the precipitate may have dissolved in the washing solution or fine precipitate particles may have flowed out with the washing solution.
As a result, the mass of precipitate recovered would decrease and the amount of the target component would be underestimated.
Therefore, washing should be performed only to the minimum extent necessary to remove impurities.
Errors Caused by Insufficient Drying
If the precipitate is not sufficiently dried, it is weighed while still containing water or solvent.
Therefore, the measured mass becomes larger than the true mass of the target precipitate.
Insufficient drying is a very important source of error in gravimetric analysis.
Example Discussion:
If the precipitate was insufficiently dried, water or washing solution would remain in the precipitate during weighing.
In this case, the measured precipitate mass becomes larger than the true mass of the target precipitate.
As a result, the amount of the target component determined from the precipitate mass may be overestimated.
Mass Changes Caused by Ignition
In gravimetric analysis, the precipitate may not only be dried but also ignited to convert it into a compound of constant composition.
If ignition is insufficient, water or volatile components may remain and the mass may become too large.
Conversely, excessive ignition may cause the precipitate to decompose or scatter, resulting in a smaller mass.
Example Discussion:
If ignition was insufficient, water or volatile components may have remained in the precipitate, causing the measured mass to be larger than the actual value.
On the other hand, if ignition was excessive, part of the precipitate may have decomposed or scattered, resulting in a smaller mass.
Therefore, in gravimetric analysis, it is important to heat the precipitate at the temperature and for the time specified in the laboratory manual and weigh it as a precipitate of constant composition.
What Is Constant Mass?
Constant mass refers to a state in which the mass changes very little even when drying or ignition and weighing are repeated.
In gravimetric analysis, constant mass is confirmed to ensure that the precipitate has been sufficiently dried or ignited and has reached a stable mass.
If the sample is weighed before reaching constant mass, water or undecomposed components may remain, preventing accurate mass measurement.
Example Discussion:
In gravimetric analysis, drying or ignition and weighing must be repeated until the mass becomes constant, confirming that constant mass has been reached.
If constant mass has not been reached, water or volatile components may remain in the precipitate and the measured mass may be larger than the actual value.
Therefore, confirmation of constant mass is essential for accurate quantification.
Errors Caused by Coprecipitation
Coprecipitation is a phenomenon in which impurities that should not normally precipitate are incorporated together with the target precipitate as it forms.
When coprecipitation occurs, components other than the target component are included in the precipitate mass, so the measured mass becomes larger.
Coprecipitation is more likely to occur when precipitation occurs too rapidly, when many impurities are present in the solution, or when the precipitate particles are fine.
Example Discussion:
If coprecipitation occurred, impurities that should not normally precipitate would be incorporated into the target precipitate, making the precipitate mass larger than the actual value.
As a result, the amount of the target component calculated from the precipitate mass may be overestimated.
The effect of coprecipitation is considered more likely when the precipitating reagent is added rapidly or when fine precipitate particles are formed.
Errors Caused by Adsorption
Ions and impurities in solution may adsorb onto the surface of the precipitate.
The finer the precipitate particles, the larger their surface area and the more susceptible they are to adsorption.
If adsorbed impurities are not removed by washing, the precipitate mass becomes larger.
Example Discussion:
If ions or impurities in solution were adsorbed onto the precipitate surface, the measured precipitate mass would include components other than the target precipitate.
In particular, fine precipitate particles have a large surface area and are more strongly affected by adsorption.
Therefore, adsorption may increase the precipitate mass and cause the amount of the target component to be overestimated.
Precipitate Particle Size and Digestion
In gravimetric analysis, it is desirable to obtain large crystalline precipitates that are easy to filter and contain few impurities.
If the precipitate is too fine, it may pass through the filter paper and readily adsorb impurities on its surface.
The operation of allowing the precipitate to stand or keeping it warm for some time to promote particle growth is sometimes called digestion.
Digestion makes it easier to obtain precipitates that are easier to filter and have higher purity.
Example Discussion:
When precipitate particles are fine, they pass through the filter paper more easily during filtration and readily adsorb impurities because of their large surface area.
Therefore, in gravimetric analysis, digestion of the precipitate to increase particle size is effective.
If digestion produces a precipitate that is easier to filter and contains fewer impurities, the accuracy of quantification from precipitate mass is considered to improve.
Errors in Measuring the Mass of the Crucible or Filter Paper
In gravimetric analysis, the mass of the crucible or filter paper is measured accurately, and the precipitate mass is determined from the difference between that mass and the mass containing the precipitate.
If the mass of the empty crucible is not measured accurately or if weighing is performed before cooling, errors may occur.
If a hot crucible is weighed, convection and similar effects may prevent accurate mass measurement.
In addition, if the crucible absorbs moisture from the air while cooling, its mass may increase.
Example Discussion:
If the crucible was weighed before it had cooled sufficiently, convection of the surrounding air may have prevented accurate measurement of its mass.
In addition, if moisture was absorbed during cooling, the measured mass may have become larger than the actual value.
Therefore, it is important to cool the crucible and precipitate in a desiccator and weigh them after they have approached room temperature.
Why a Desiccator Is Used
A desiccator is used to cool and store samples under dry conditions.
If a dried or ignited precipitate is left in air for a long time, it may absorb moisture and increase in mass.
Therefore, in gravimetric analysis, a heated sample may be cooled in a desiccator before weighing.
This helps prevent moisture absorption and makes it easier to measure a stable mass.
Example Discussion:
If a dried or ignited precipitate is left in air, it may absorb moisture and increase in mass.
Cooling in a desiccator reduces the influence of moisture in the air and makes it easier to measure a stable mass.
Therefore, the use of a desiccator is important for improving the accuracy of gravimetric analysis.
When the Precipitate Mass Is Larger Than the Theoretical Value
If the precipitate mass is larger than the theoretical value, components other than the precipitate may have been measured together with it.
Typical causes include insufficient drying, insufficient washing, coprecipitation, adsorption, and reabsorption of moisture.
| Cause | Effect on Mass | Possible Result |
|---|---|---|
| Insufficient drying | Water remains | Mass becomes larger |
| Insufficient washing | Soluble impurities remain | Mass becomes larger |
| Coprecipitation | Impurities are incorporated | Mass becomes larger |
| Adsorption | Impurities remain on the surface | Mass becomes larger |
| Reabsorption of moisture | Moisture is absorbed from the air | Mass becomes larger |
Example Discussion:
Possible reasons the obtained precipitate mass was larger than the theoretical value include insufficient drying and insufficient washing.
If water or soluble impurities derived from the mother liquor remained in the precipitate, the measured mass would include mass other than that of the target precipitate.
As a result, the amount of the target component calculated from the precipitate mass may have been overestimated.
When the Precipitate Mass Is Smaller Than the Theoretical Value
If the precipitate mass is smaller than the theoretical value, part of the target component may have been lost.
Possible causes include incomplete precipitation, loss during filtration, dissolution during washing, adhesion to apparatus or spillage during transfer, and scattering during ignition.
| Cause | Effect on Mass | Possible Result |
|---|---|---|
| Incomplete precipitation | Part remains in solution | Mass becomes smaller |
| Loss during filtration | Precipitate is not recovered | Mass becomes smaller |
| Dissolution during washing | Precipitate dissolves in washing solution | Mass becomes smaller |
| Loss during transfer | Precipitate adheres to apparatus or is spilled | Mass becomes smaller |
| Scattering during ignition | Part of the precipitate is lost | Mass becomes smaller |
Example Discussion:
One possible reason the obtained precipitate mass was smaller than the theoretical value is that part of the precipitate was lost during filtration or transfer.
In addition, if precipitation was incomplete or part of the precipitate dissolved during washing, the amount of precipitate recovered would decrease.
As a result, the amount of the target component determined from the precipitate mass may have become smaller than the actual value.
Discussion When the Sample Was Weighed Multiple Times
When the sample is weighed multiple times after drying or ignition, changes in mass can be used to confirm the drying condition and whether constant mass has been reached.
If the mass changes very little, it is easier to judge that constant mass has been reached.
Conversely, if the mass decreases with each weighing, water or volatile components may still have remained.
Example Discussion:
The mass after drying was measured multiple times, and the differences among the measurements were small, giving nearly constant values.
From this, the precipitate is considered to have generally reached constant mass and to have been sufficiently dried.
On the other hand, if the mass decreased with each weighing, the drying may have been insufficient and water may have remained.
When the Result Can Be Considered Good
A good result in gravimetric analysis is indicated when the precipitate forms sufficiently, filtration and washing are performed properly, drying or ignition is appropriate, and constant mass is reached.
In addition, if the result is close to the theoretical or known value and there is no obvious impurity contamination or loss of precipitate, the result can be considered relatively reliable.
Example Discussion:
The precipitate formed sufficiently, and no noticeable turbidity was observed in the filtrate after filtration.
In addition, the mass after drying became nearly constant, indicating that constant mass had been reached.
From this, the recovery and drying of the precipitate are considered to have been performed generally appropriately, and the amount of the target component determined from the precipitate mass is considered relatively reliable.
Example Discussion When the Analysis Did Not Go Well
When gravimetric analysis does not go well, possible causes can be considered from observations such as a small amount of precipitate, cloudy filtrate, unstable mass after drying, or a large deviation from the theoretical value.
It is easier to write the discussion by separately considering precipitation, filtration, washing, drying, ignition, and weighing.
Example Discussion:
One possible reason the obtained precipitate mass was smaller than the theoretical value is that part of the precipitate was lost during filtration.
If the filtrate appeared cloudy, fine precipitate particles may have passed through the filter paper.
In addition, if part of the precipitate dissolved during washing, the final precipitate mass would also become smaller.
Because of these factors, the amount of the target component may have been underestimated.
How to Write Points for Improvement
In a discussion of gravimetric analysis, including points for improvement as well as sources of error makes the report easier to organize.
It is important to write improvements in correspondence with the actual sources of error considered.
Methods for Improving Precipitation and Recovery
- Add an appropriate amount of precipitating reagent
- Maintain appropriate pH and temperature conditions
- Stir the solution sufficiently
- Digest the precipitate
- Avoid losing the precipitate during filtration
- Check whether the filtrate is cloudy
Methods for Improving Washing, Drying, and Weighing
- Perform washing using a necessary and sufficient amount
- Select a washing solution in which the precipitate is not readily soluble
- Perform sufficient drying or ignition
- Confirm that constant mass has been reached
- Cool in a desiccator before weighing
- Measure the mass of the crucible or filter paper accurately
Example of How to Write Points for Improvement:
To reduce loss of precipitate, care must be taken not to leave precipitate on the apparatus during filtration or transfer, and it should be washed into the collection vessel with a small amount of washing solution when necessary.
In addition, to prevent overestimation of the mass caused by insufficient drying, drying and weighing should be repeated until constant mass is reached.
Furthermore, cooling in a desiccator before weighing can reduce the influence of moisture in the air.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of gravimetric analysis, simply writing that “there was little precipitate” or “an error occurred” results in a superficial discussion.
A persuasive discussion can be produced by explaining which operations make the precipitate mass larger and which make it smaller.
| Superficial Discussion | Good Discussion |
|---|---|
| There was little precipitate. | If precipitation was incomplete, part of the target component would remain in the filtrate, making the recovered precipitate mass smaller. As a result, the amount of the target component calculated from the precipitate mass may be underestimated. |
| The mass became larger. | If drying was insufficient and water remained in the precipitate, the measured mass would include the mass of water in addition to the target precipitate. Therefore, the precipitate mass becomes larger than the actual value and the amount of the target component may be overestimated. |
| The washing was poor. | If washing was insufficient, mother liquor and soluble impurities would remain on the precipitate surface and increase the measured mass. On the other hand, excessive washing may dissolve or wash away part of the precipitate and decrease the measured mass. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of gravimetric analysis.
Adjust the necessary parts according to your own experimental results.
- In gravimetric analysis, the target component is recovered as a precipitate of constant composition, and the amount of the target component is determined from its mass.
- The amount of precipitate was determined by dividing the precipitate mass by its molar mass.
- The amount of the target component in the sample was calculated from the stoichiometric relationship between the precipitate and the target component.
- If precipitation was incomplete, the amount of the target component may be underestimated.
- If part of the precipitate was lost during filtration or transfer, the precipitate mass becomes smaller than the actual value.
- If soluble impurities remained because of insufficient washing, the precipitate mass becomes larger than the actual value.
- If water remained because of insufficient drying, the measured mass may become too large.
- If impurities were incorporated through coprecipitation or adsorption, the amount of the target component may be overestimated.
- If constant mass has not been reached, water or volatile components may remain in the precipitate.
- For more accurate measurement, precipitation, filtration, washing, drying, and weighing must all be performed carefully.
Points to Check When Discussing Gravimetric Analysis
Checking the following points before writing the report makes the discussion easier to write.
- Have you confirmed the chemical formula of the precipitate?
- Have you correctly calculated the amount of the target component from the precipitate mass?
- Was precipitation complete?
- Was the filtrate free of turbidity?
- Was any precipitate lost during filtration?
- Have you considered insufficient washing and excessive washing?
- Was drying or ignition sufficient?
- Was constant mass reached?
- Have you considered the effects of coprecipitation and adsorption?
- Was the mass of the crucible or filter paper measured accurately?
- Was the sample cooled in a desiccator before weighing?
- Have you explained whether each error makes the measured mass larger or smaller?
Summary
Gravimetric analysis is a quantitative analytical method in which the target component is isolated as a precipitate or another substance of constant composition, and the amount of the target component is determined from its mass.
Because the calculation is based on precipitate mass, it is important that the precipitate be pure, completely formed, and sufficiently dried or ignited.
Possible sources of error include incomplete precipitation, dissolution of the precipitate, loss during filtration, insufficient washing, insufficient drying, coprecipitation, adsorption, and moisture absorption during weighing.
These errors may make the precipitate mass either larger or smaller.
In a report, do not simply write that “the precipitate was weighed.”
Explain specifically how each operation affects the precipitate mass.
Considering separately the causes of a precipitate mass that is larger than the theoretical value and those of a precipitate mass that is smaller than the theoretical value produces a persuasive discussion of gravimetric analysis.
