In chemistry experiments, volumetric measuring instruments such as volumetric flasks, volumetric pipettes, and burettes are frequently used. These instruments are used for solution preparation, measuring fixed amounts, titration, and other procedures, and they have a major influence on the accuracy of experimental results.
This article explains the differences in the roles of volumetric flasks, volumetric pipettes, and burettes, common causes of error in volume measurement, their effects on measured values, and discussion examples that can be used in reports. Use it as a reference when writing reports on acid–base titration, complexometric titration, preparation of standard solutions, concentration calculations, and similar experiments.
Note: This article is a reference for discussing results obtained in chemistry experiments at universities and similar institutions. For actual use of the instruments and safety precautions, always follow your university’s laboratory manual and the instructions of your instructor or teaching assistant (TA).
- Why Volume Measurement Is Important in Chemistry Experiments
- Differences Between a Volumetric Flask, Volumetric Pipette, and Burette
- Reference Experimental Values for Errors in Volumetric Glassware and Examples of Volume Measurement and Error Evaluation
- Reference Experimental Conditions
- Differences in Uses and Accuracy of Volumetric Instruments
- Reference Examples of Allowable Errors for Volumetric Instruments
- Example of Confirming Volume from the Mass of Water
- Examples of Repeated Measurements with Different Instruments
- Example of Meniscus Reading Error
- Example of Burette Reading Error
- Example of Volume Error in Concentration Preparation
- Example of Error Propagation in Dilution
- Comparison of Concentration Errors Caused by Instrument Selection
- Example of Volume Changes with Temperature
- Examples of Errors Due to Bubbles and Poor Drainage
- Example of Overall Error in Standard Solution Preparation
- Example of Writing the Results
- Points to Connect to the Discussion
- Example Discussion
- Summary
- Meniscus Reading Error
- Causes of Error in a Volumetric Flask
- Causes of Error in a Volumetric Pipette
- Causes of Error in a Burette
- Effect of Volume Measurement Errors on Concentration Calculations
- Discussion Example for Volume Measurement in Acid–Base Titration
- Discussion Example for Standard Solution Preparation
- Difference Between a Superficial Discussion and a Good Discussion
- Example Expressions That Can Be Used in Reports
- Points to Check When Discussing Volume Measurement
- Summary
Why Volume Measurement Is Important in Chemistry Experiments
In chemistry experiments, volume values are used when determining the concentration of a solution or the amount of a substance. Therefore, errors in volume measurement affect concentration calculations, titration results, yield, calculations of reaction amounts, and other results.
For example, if water is added beyond the calibration mark of a volumetric flask when preparing a standard solution, the actual concentration becomes lower than intended. In addition, if a sample solution is not correctly measured with a volumetric pipette, the concentration determined by titration will also be affected.
Errors in volume measurement may appear small, but because they are directly related to concentration calculations, they are an important point in the discussion section of a report.
Differences Between a Volumetric Flask, Volumetric Pipette, and Burette
Volumetric measuring instruments may look similar, but they have different purposes. When writing a discussion in a report, confirming which instrument was used for which operation makes it easier to identify possible sources of error.
| Instrument | Main Use | Role in the Experiment |
|---|---|---|
| Volumetric flask | Prepare a solution of a fixed volume | Adjust standard solutions or diluted solutions to an accurate final volume |
| Volumetric pipette | Measure a fixed volume of solution | Transfer a fixed amount of sample solution or standard solution |
| Burette | Measure the volume of solution delivered | Determine the volume of standard solution consumed in a titration |
It is easy to understand the difference by thinking of a volumetric flask as “an instrument for adjusting the final volume,” a volumetric pipette as “an instrument for measuring a fixed amount,” and a burette as “an instrument for reading the amount added.”
Reference Experimental Values for Errors in Volumetric Glassware and Examples of Volume Measurement and Error Evaluation
This section organizes reference experimental values for discussing allowable error, reading error, differences in instrument precision, the effects of temperature, repeated measurements, and error propagation in volume measurements using volumetric flasks, volumetric pipettes, burettes, graduated cylinders, and similar volumetric instruments. In volumetric analysis and preparation of standard solutions, even small volume errors directly affect concentration calculations, so it is important to understand the characteristics of each instrument and use them appropriately.
Some volumetric instruments are intended to measure a specific volume accurately, while others are intended only to measure an approximate volume. For example, volumetric pipettes and volumetric flasks are suitable for quantitative operations that require high accuracy. In contrast, graduated cylinders and beakers are convenient but tend to have larger errors when used for precise concentration calculations.
Reference Experimental Conditions
| Item | Details |
|---|---|
| Instruments examined | Volumetric flask, volumetric pipette, burette, graduated cylinder, Komagome pipette, beaker, etc. |
| Evaluation methods | Measurement of water mass, density conversion, repeated measurements, comparison with allowable error |
| Measurement conditions | Room temperature of 20–25°C, volume confirmation using water |
| Evaluation items | Mean volume, standard deviation, relative error, allowable error, reading error, appropriateness of instrument selection |
| Main error factors | Meniscus reading, bubbles, drainage, temperature, instrument contamination, drying condition, parallax, instrument accuracy class, etc. |
Differences in Uses and Accuracy of Volumetric Instruments
| Instrument | Main Use | Approximate Accuracy | Use in Quantitative Experiments |
|---|---|---|---|
| Volumetric flask | Accurately prepare a solution of a fixed volume | High | Suitable for preparing standard and diluted solutions |
| Volumetric pipette | Accurately measure a fixed volume | High | Suitable for sample collection |
| Burette | Measure titration volume | High | Suitable for titration |
| Graduated cylinder | Measure an approximate volume | Moderate | May be unsuitable for precise quantitative measurements |
| Beaker | Mixing and temporary holding | Low | Generally not used for volume measurement |
| Komagome pipette | Transfer a small amount of liquid | Low | Unsuitable for quantitative sampling |
In quantitative experiments, the reliability of the results varies greatly depending on which instrument is used. As a general rule, volumes used in concentration calculations should be measured with volumetric instruments such as volumetric pipettes, volumetric flasks, and burettes.
Reference Examples of Allowable Errors for Volumetric Instruments
Volumetric instruments have allowable errors depending on their standards and grades. The table below shows reference values for learning purposes.
| Instrument | Nominal Volume | Example of Allowable Error | Relative Error | Direction of Discussion |
|---|---|---|---|---|
| Volumetric pipette | 10.00 mL | ±0.02 mL | ±0.20% | Relatively precise |
| Volumetric pipette | 25.00 mL | ±0.03 mL | ±0.12% | Suitable for sample collection |
| Volumetric flask | 100.00 mL | ±0.10 mL | ±0.10% | Suitable for standard solution preparation |
| Volumetric flask | 250.00 mL | ±0.15 mL | ±0.06% | Small relative error |
| Burette | 50.00 mL | ±0.05 mL | ±0.10% | Suitable for measuring titration volume |
| Graduated cylinder | 100 mL | ±0.5 mL | ±0.5% | Less accurate than volumetric instruments |
| Beaker graduations | 100 mL | Approx. ±5 mL | ±5% | Unsuitable for quantitative measurements |
Even when measuring the same 100 mL, the magnitude of error differs greatly between a volumetric flask and a beaker. For accurate concentration preparation, a volumetric flask rather than the graduations on a beaker must be used.
Example of Confirming Volume from the Mass of Water
The following example calculates the actual volume from the mass of water collected using a volumetric pipette, assuming the density of water to be 0.9970 g/mL.
Volume V = Mass of water ÷ Density of water
| Measurement | Mass of Water | Density | Calculated Volume | Difference from Nominal Volume |
|---|---|---|---|---|
| 1st | 9.970 g | 0.9970 g/mL | 10.00 mL | 0.00 mL |
| 2nd | 9.960 g | 0.9970 g/mL | 9.99 mL | −0.01 mL |
| 3rd | 9.980 g | 0.9970 g/mL | 10.01 mL | +0.01 mL |
| Average | 9.970 g | 0.9970 g/mL | 10.00 mL | 0.00 mL |
If the measured volume of a 10.00 mL volumetric pipette is 9.99–10.01 mL, the reproducibility can be considered good and within the allowable error.
Examples of Repeated Measurements with Different Instruments
The following is a comparison in which approximately 10 mL of water was measured with several instruments and the volume was calculated from the mass.
| Instrument | Nominal Volume | Mean Measured Volume | Standard Deviation | Relative Standard Deviation | Direction of Discussion |
|---|---|---|---|---|---|
| Volumetric pipette | 10.00 mL | 10.00 mL | 0.01 mL | 0.10% | High reproducibility |
| Graduated pipette | 10.00 mL | 9.98 mL | 0.03 mL | 0.30% | Depends on reading |
| Graduated cylinder | 10.0 mL | 10.1 mL | 0.15 mL | 1.5% | Large variation |
| Komagome pipette | Approx. 10 mL | 9.6 mL | 0.35 mL | 3.6% | Unsuitable for quantitative measurement |
| Beaker graduations | Approx. 10 mL | 10.8 mL | 0.80 mL | 7.4% | Only an approximate guide |
The mean value obtained with a volumetric pipette is close to the nominal volume, and the variation is small, so it is suitable for quantitative operations. In contrast, beaker graduations and Komagome pipettes show large variations and are unsuitable for accurate concentration calculations.
Example of Meniscus Reading Error
The liquid surface becomes curved due to interactions with the wall of the container. For aqueous solutions, the bottom of the meniscus is normally aligned with the graduation when taking a reading.
| Reading Condition | Actual Volume | Read Volume | Error | Direction of Discussion |
|---|---|---|---|---|
| Eye level aligned horizontally | 25.00 mL | 25.00 mL | 0.00 mL | Appropriate |
| Viewed from above | 25.00 mL | 24.90 mL | −0.10 mL | Read too low due to parallax |
| Viewed from below | 25.00 mL | 25.10 mL | +0.10 mL | Read too high due to parallax |
| Upper edge of the meniscus read | 25.00 mL | 25.15 mL | +0.15 mL | Incorrect reading reference |
When reading a meniscus, it is important to keep the eye level horizontal with the graduation and to use the same reading reference consistently.
Example of Burette Reading Error
With a burette, the titration volume is determined from the difference between the readings before and after titration. Because reading errors are included in both the initial and final readings, these errors are propagated into the difference in volume.
| Measurement | Initial Reading | Final Reading | Titration Volume | Direction of Discussion |
|---|---|---|---|---|
| Appropriate reading | 0.12 mL | 18.76 mL | 18.64 mL | Reference |
| Initial reading read 0.05 mL too low | 0.07 mL | 18.76 mL | 18.69 mL | Titration volume becomes larger |
| Final reading read 0.05 mL too high | 0.12 mL | 18.81 mL | 18.69 mL | Titration volume becomes larger |
| Both readings shifted in opposite directions | 0.07 mL | 18.81 mL | 18.74 mL | Errors accumulate |
Because the titration volume is the difference between the initial and final readings, both readings must be taken carefully.
Example of Volume Error in Concentration Preparation
When preparing 100.00 mL of a 0.1000 mol/L standard solution, the volume error of the volumetric flask directly becomes a concentration error.
| Actual Final Volume | Difference from Ideal Volume | Calculated Concentration | Resulting Tendency |
|---|---|---|---|
| 100.00 mL | 0.00 mL | 0.1000 mol/L | Reference |
| 99.90 mL | −0.10 mL | 0.1001 mol/L | Slightly higher |
| 100.10 mL | +0.10 mL | 0.0999 mol/L | Slightly lower |
| 101.00 mL | +1.00 mL | 0.0990 mol/L | Clearly lower |
If too much volume is used, the concentration becomes lower, while if the volume is too small, the concentration becomes higher. Errors in the concentration of a standard solution affect subsequent titration and analytical results as a whole.
Example of Error Propagation in Dilution
The following is an example of a tenfold dilution in which 10.00 mL of a stock solution is taken with a volumetric pipette and diluted to 100.00 mL in a volumetric flask.
| Operation | Instrument Used | Volume | Example of Allowable Error | Relative Error |
|---|---|---|---|---|
| Sampling of stock solution | 10.00 mL volumetric pipette | 10.00 mL | ±0.02 mL | ±0.20% |
| Making up to volume | 100.00 mL volumetric flask | 100.00 mL | ±0.10 mL | ±0.10% |
| Dilution factor | 10-fold dilution | 100.00/10.00 | Combined error | Approx. ±0.22% |
Because the dilution factor is determined by the ratio of the sampled volume to the final volume, errors in both volumes affect the concentration after dilution.
Comparison of Concentration Errors Caused by Instrument Selection
Even for the same tenfold dilution, the reliability of the concentration changes depending on which instruments are used.
| Sampling of Stock Solution | Final Volume Adjustment | Expected Relative Error | Suitability for Quantitative Analysis | Direction of Discussion |
|---|---|---|---|---|
| Volumetric pipette | Volumetric flask | Approx. 0.2% | High | Standard precise dilution |
| Graduated cylinder | Volumetric flask | Approx. 1% | Moderate | Large error in sampled volume |
| Komagome pipette | Graduated cylinder | Several percent | Low | Unsuitable for quantitative analysis |
| Beaker graduations | Beaker graduations | 5% or more | Very low | Not used for concentration calculations |
In quantitative experiments, it is important to choose instruments with the accuracy appropriate for the purpose rather than simply choosing the most convenient instrument.
Example of Volume Changes with Temperature
Volumetric instruments are usually calibrated to give the correct volume at a specific temperature. If the temperature differs greatly, slight differences in volume may occur due to expansion of the water and glass.
| Temperature | Approximate Density of Water | Volume of 100.00 g of Water | Direction of Discussion |
|---|---|---|---|
| 15°C | Approx. 0.9991 g/mL | 100.09 mL | Density is slightly higher at lower temperatures |
| 20°C | Approx. 0.9982 g/mL | 100.18 mL | Close to the reference temperature of volumetric instruments |
| 25°C | Approx. 0.9970 g/mL | 100.30 mL | Water density becomes slightly lower |
| 30°C | Approx. 0.9957 g/mL | 100.43 mL | Volume becomes larger at higher temperatures |
In ordinary student experiments, the effect of temperature may be small, but recording the temperature conditions makes it easier to discuss precise volume confirmation or standardization.
Examples of Errors Due to Bubbles and Poor Drainage
| Condition | What Happens | Effect on Measured Value | Direction of Discussion |
|---|---|---|---|
| Bubble in the pipette tip | Actual amount of liquid drawn up is smaller | Sampled volume becomes smaller | Affects concentration calculations |
| Bubble in the burette tip | Bubble is expelled during titration | Apparent titration volume becomes larger | Bubble should be checked before reaching the endpoint |
| Droplets remain on the pipette wall | Delivered volume becomes smaller | Sampled volume becomes smaller | Handling after natural drainage should be consistent |
| Instrument contaminated with oil | Liquid film does not spread uniformly | Poor drainage | Instrument cleaning is necessary |
Bubbles and poor drainage not only cause volume deviations but also reduce reproducibility.
Example of Overall Error in Standard Solution Preparation
When a solid reagent is weighed and a standard solution is prepared by making it up to volume in a volumetric flask, both mass error and volume error affect the concentration.
| Factor | Target Value | Example of Error | Effect on Concentration |
|---|---|---|---|
| Reagent mass | 1.0000 g | ±0.0005 g | ±0.05% |
| Final volume | 100.00 mL | ±0.10 mL | ±0.10% |
| Incomplete dissolution | Complete dissolution | Small amount remains | Lowers concentration |
| Alignment with calibration mark | Aligned with mark | +0.05–0.10 mL | Lowers concentration |
| Insufficient mixing | Uniform solution | Concentration variation | Value changes depending on sampling position |
Because a standard solution serves as the reference for subsequent analyses, mass measurement, complete dissolution, adjustment to volume, and mixing must all be performed carefully.
Example of Writing the Results
Water was collected using a 10.00 mL volumetric pipette, and the actual volume was calculated from its mass. When calculated using a water density of 0.9970 g/mL, the three measured volumes were 9.99–10.01 mL, with an average of 10.00 mL. The standard deviation was also small, and almost no difference from the nominal volume was observed, so the reproducibility of volume measurement with this volumetric pipette is considered good.
In contrast, when a graduated cylinder or beaker graduations were used, the deviation of the average volume and the variation in the measured values became larger. This is because the graduation intervals are coarse, errors in reading the meniscus are larger, and the instruments themselves are not designed for precise volume adjustment. Therefore, volumetric flasks and volumetric pipettes rather than graduated cylinders or beaker graduations should be used for standard solution preparation and sampling for titration.
With a burette, the titration volume is determined from the difference between the initial and final readings, so reading errors in both values affect the titration volume. If the eye level is not horizontal, parallax may cause the graduation to be read too high or too low. As a result, the titration volume may become larger or smaller than the actual value, causing an error in concentration calculations.
Points to Connect to the Discussion
When discussing errors in volumetric instruments, it is important not simply to write that “there was an error,” but to explain specifically which operation involving which instrument affected the result.
- Can you distinguish whether the instrument used was intended for precise measurement or approximate volume measurement?
- Can you compare the allowable error with the measured error?
- Can you explain how to read the meniscus and the effect of parallax?
- Can you explain that both the initial and final readings of a burette contain errors?
- Can you explain why volumetric pipettes and volumetric flasks are suitable for quantitative experiments?
- Can you explain why graduated cylinders and beaker graduations are unsuitable for precise concentration preparation?
- Can you explain that in dilution, errors in both the sampled volume and the final volume affect the concentration?
- Can you discuss bubbles, poor drainage, contamination of instruments, temperature, and insufficient mixing as error factors?
- Can you explain that a volume error in a standard solution affects subsequent titration results as a whole?
Example Discussion
In this experiment, the volume of water was measured using several volumetric instruments, and the error and reproducibility of each instrument were compared. With a 10.00 mL volumetric pipette, the volume calculated from the mass was 9.99–10.01 mL, which was very close to the nominal volume. This is considered to be because a volumetric pipette is a volumetric instrument designed to accurately measure a fixed volume, has few graduation points to read, and provides high reproducibility.
In contrast, graduated cylinders and beaker graduations showed larger deviations in average volume and greater variation in measured values. Graduated cylinders have relatively coarse graduation intervals and are easily affected by meniscus reading and parallax. Beaker graduations are intended only as approximate guides and are unsuitable for accurate volume measurements. Therefore, volumes used for concentration calculations in quantitative analysis must be measured using volumetric pipettes, volumetric flasks, burettes, or similar instruments.
In titration using a burette, the titration volume is determined from the difference between the initial and final readings. Therefore, reading errors are included in both the initial and final readings. If the eye level is not horizontal with the graduation, parallax causes the graduation to be read too high or too low, resulting in an error in the titration volume. Because an error in the titration volume directly affects the calculated concentration of the unknown sample, burette readings must be taken carefully.
In dilution, errors are included in both the volume used to sample the stock solution and the final volume used for dilution. For example, when 10.00 mL is taken with a volumetric pipette and diluted to 100.00 mL, both the error in the sampled volume and the error of the volumetric flask affect the dilution factor. Therefore, in standard solution preparation and calibration curve preparation, it is important to perform volume measurements at each stage using precise instruments.
Other possible error factors include bubbles in pipette or burette tips, poor drainage, instrument contamination, temperature differences, and insufficient mixing of solutions. If bubbles remain, the amount of liquid actually transferred changes, while poor drainage reduces the delivered volume. In addition, if a solution is not sufficiently mixed during preparation of a standard solution, the concentration may differ depending on the position from which the solution is sampled. Therefore, correct use of volumetric instruments is a basic operation that supports the reliability of analytical results as a whole.
Summary
Errors in volumetric instruments directly affect concentration calculations, titration volumes, dilution factors, and standard solution preparation. In precise quantitative experiments, volumetric instruments such as volumetric flasks, volumetric pipettes, and burettes must be used appropriately, with attention to the meniscus, bubbles, drainage, temperature, and mixing conditions.
This reference example covered the uses and accuracy of different instruments, allowable errors, confirmation of volume from the mass of water, repeated measurements, meniscus readings, burette errors, concentration preparation, dilution procedures, temperature effects, bubbles and drainage, and overall errors in standard solution preparation. In a report, it is useful to discuss specifically which instrument was used for which purpose and how errors in that instrument affected the result.
Meniscus Reading Error
One of the most basic causes of error in volume measurement is an error in reading the meniscus. The meniscus is the curved liquid surface seen inside glassware due to surface tension.
For many aqueous solutions, the center of the liquid surface curves downward. In this case, the bottom of the meniscus is normally aligned with the graduation or calibration mark. If the eye level is not at the same height as the mark or graduation, the volume may be read as larger or smaller than the actual value.
Discussion Example:
One possible cause of error in the volume measurement is a shift in the position used to read the meniscus. If the eye level is not at the same height as the calibration mark, the liquid surface may be read as higher or lower than it actually is. Therefore, errors may occur in the sampled volume or titration volume, affecting the results of concentration calculations.
Causes of Error in a Volumetric Flask
A volumetric flask is an instrument used to accurately prepare a solution of a fixed volume. It is used for preparing standard solutions and diluted solutions. Errors involving a volumetric flask directly affect the concentration of the prepared solution itself.
When Water Is Added Beyond the Calibration Mark
If water is added beyond the calibration mark of the volumetric flask, the volume of the solution becomes larger than intended. Because the amount of solute does not change, the concentration of the solution becomes lower than intended.
Discussion Example:
If water is added beyond the calibration mark in a volumetric flask, the volume of the prepared solution becomes larger than intended. As a result, the amount of solution relative to the amount of solute increases, and the actual concentration becomes lower than the calculated concentration. If this solution is used as a standard solution, an error may also occur in the sample concentration determined from the titration results.
When the Solute Is Not Completely Dissolved
If the solute is not completely dissolved in the volumetric flask, it will not be uniformly distributed throughout the solution. If part of the solution is sampled in this state, the actual concentration may differ depending on the sampling location.
Discussion Example:
If the solute was not completely dissolved, the concentration throughout the solution would not be uniform. Therefore, the concentration of the sampled solution may have differed from the theoretical concentration, causing variation in the measurement results.
When Mixing Is Insufficient
If the solution is not sufficiently mixed after water is added to the calibration mark, the concentration may not become uniform. Particularly when a concentrated solution is diluted, insufficient mixing can cause variation in concentration.
Discussion Example:
If mixing inside the volumetric flask was insufficient, the concentration throughout the solution may not have been uniform. If part of the solution was then sampled with a volumetric pipette, the concentration of the sampled portion could differ from the average concentration, causing errors in the titration value or absorbance.
Effect of Temperature
A volumetric flask is generally manufactured to contain the correct volume at a specific temperature. If the solution temperature differs greatly from this reference temperature, the volume of the liquid or glass may change, causing a deviation from the accurate volume.
Discussion Example:
If the solution temperature differed greatly from the reference temperature of the instrument, thermal expansion of the solution or glassware may have caused the actual volume to differ from the indicated volume. Therefore, a small error may also have occurred in the concentration of the prepared solution.
Causes of Error in a Volumetric Pipette
A volumetric pipette is an instrument used to accurately measure a fixed volume of solution. It is used to transfer a fixed amount of sample solution or standard solution. Errors involving a volumetric pipette directly affect the amount of substance sampled.
When the Liquid Level Is Not Correctly Aligned with the Calibration Mark
With a volumetric pipette, the liquid level must be accurately aligned with the calibration mark. If the liquid level is above or below the mark, the sampled volume differs from the intended amount.
Discussion Example:
If the liquid level could not be accurately aligned with the calibration mark of the volumetric pipette, the volume of the sampled solution would differ from the specified amount. If a larger amount of sample solution was collected, the amount of substance contained in it would also increase, and the volume of standard solution required for titration would increase. Conversely, if the sampled amount was smaller, the titration volume would also become smaller.
When an Air Bubble Enters the Pipette
If an air bubble enters the pipette, the actual volume of liquid collected may be smaller than it appears. As a result, the amount of sample or standard solution becomes smaller than intended.
Discussion Example:
If an air bubble was present in the volumetric pipette, part of the collected volume would be replaced by air, so the actual amount of liquid transferred would be smaller than the specified amount. As a result, the amount of substance in the sample would be smaller, possibly causing errors in the titration volume or calculated concentration.
When Droplets Remain on the Inner Wall
If droplets remain on the inner wall of the volumetric pipette, the actual volume transferred may be smaller than intended. However, volumetric pipettes are normally designed to account for the amount of liquid remaining inside after delivery, so care must be taken in how they are handled.
Discussion Example:
When transferring a solution from a volumetric pipette, if an excessive amount of liquid remained on the inner wall or at the tip of the instrument, the actual amount transferred may have been smaller than the specified amount. As a result, the amount of substance in the sample solution may have been underestimated, causing an error in the concentration calculation.
When Rinsing with the Solution Is Insufficient
If water or another solution remains inside the volumetric pipette, the solution being sampled may be diluted or contaminated. As a result, the concentration of the sampled solution changes.
Discussion Example:
If the volumetric pipette was insufficiently rinsed with the solution and water remained inside, the sampled solution would be diluted. Therefore, the actual amount of substance in the transferred solution would be smaller than intended, possibly affecting the titration volume and concentration calculation.
Causes of Error in a Burette
A burette is an instrument used to measure the volume of solution added during titration. In a titration, the amount of standard solution used is determined from the burette readings, so errors involving the burette directly affect the titration results.
Reading Errors in the Initial and Endpoint Values
With a burette, the graduations are read before and after titration, and the titration volume is determined from the difference. If there is an error in reading either the initial or final value, the titration volume will also contain an error.
Discussion Example:
If the eye level was not aligned with the graduations when reading the initial or final burette value, the titration volume may have been overestimated or underestimated. Because the titration volume is used directly in concentration calculations, this reading error is considered to have caused an error in the calculated concentration.
Air Bubble in the Burette Tip
If an air bubble remains in the tip of the burette, the bubble may be expelled during titration, making it appear from the graduations that liquid has been discharged even though part of that volume was not actually added to the sample solution. Therefore, the titration volume may be overestimated.
Discussion Example:
If an air bubble remained in the tip of the burette, the liquid level in the burette would decrease when the bubble was expelled during titration. However, that portion of the apparent volume would not actually have been added to the sample solution, so the titration volume may have been overestimated. As a result, an error is considered to have occurred in the calculated sample concentration.
When Solution Is Added Beyond the Endpoint
In titration, the solution is added carefully near the endpoint while observing the color change. If too much standard solution is added beyond the endpoint, the titration volume becomes larger than the amount actually required.
Discussion Example:
If standard solution was added beyond the endpoint, the recorded titration volume would be larger than the amount required to reach the actual equivalence point. Therefore, the amount of substance in the sample would be overestimated, and the calculated concentration may become higher than the actual value.
Insufficient Rinsing of the Burette with the Solution
If water remains inside the burette, the standard solution added to it may be diluted. In that case, the actual concentration of the standard solution becomes lower than expected.
Discussion Example:
If the burette was insufficiently rinsed with the solution and water remained inside, the standard solution would be diluted and its actual concentration would become lower than the set concentration. When titration is performed with this standard solution, a larger volume would be required to neutralize the same amount of substance. As a result, the titration volume would increase, possibly causing an error in the concentration calculation.
Effect of Volume Measurement Errors on Concentration Calculations
Concentration is calculated by dividing the amount of substance by the volume of the solution. Therefore, whether the volume is overestimated or underestimated changes the calculated concentration.
Concentration = Amount of substance ÷ Volume
For example, even if a solution contains the same amount of substance, if the volume is overestimated, the concentration is calculated as lower. Conversely, if the volume is underestimated, the concentration is calculated as higher.
| Type of Error | Effect on Volume | Effect on Concentration |
|---|---|---|
| Calibration mark exceeded in a volumetric flask | Actual volume becomes larger | Actual concentration becomes lower |
| Too little solution collected with a volumetric pipette | Sampled volume becomes smaller | Amount of substance sampled becomes smaller |
| Air bubble present in the burette tip | Titration volume is overestimated | Calculated concentration is affected |
| Solution added beyond the endpoint | Titration volume becomes larger | Sample concentration may be overestimated |
In a report, the discussion becomes more in-depth if you explain not only the cause of the error but also whether the measured value becomes larger or smaller.
Discussion Example for Volume Measurement in Acid–Base Titration
In acid–base titration, a sample solution is often collected with a volumetric pipette and a standard solution is added from a burette. Therefore, errors in volume measurement with both the volumetric pipette and burette affect the result.
Discussion Example:
Possible causes of error in the titration results include deviation in the volume of sample solution collected with the volumetric pipette and reading errors in the titration volume measured with the burette. If the liquid level could not be accurately aligned with the calibration mark of the volumetric pipette, the amount of substance in the sampled solution would differ from the specified amount. In addition, if the eye position was misaligned when reading the burette graduations, the titration volume could have been overestimated or underestimated. These errors are considered to have directly affected the calculated sample concentration.
Discussion Example for Standard Solution Preparation
When preparing a standard solution, a volumetric flask is used to accurately adjust the final volume. If an error occurs at this stage, it affects all subsequent titration and analytical results.
Discussion Example:
If water was added beyond the calibration mark of the volumetric flask during standard solution preparation, the volume of the prepared solution would be larger than intended. As a result, the actual concentration of the standard solution would be lower than the calculated value. If this solution was used for titration, a larger titration volume would be required to react with the same amount of substance, possibly causing an error in the calculated sample concentration.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of volume measurement, simply writing that “there was an error in reading the graduation” is insufficient. A good discussion explains which instrument was involved, what type of reading error occurred, and how the calculated value changed as a result.
| Superficial Discussion | Good Discussion |
|---|---|
| An error occurred when reading the graduation. | If the eye level was not aligned with the graduations when reading the burette, the titration volume may have been read as larger or smaller than the actual value. Because the titration volume is used directly in concentration calculations, an error is considered to have occurred in the calculated sample concentration. |
| Too much water was added to the volumetric flask. | If water was added beyond the calibration mark in the volumetric flask, the volume of the prepared solution would become larger and the actual concentration would become lower than intended. Therefore, if this solution was used as a standard solution, a systematic error may also occur in the titration results. |
| The pipette was used incorrectly. | If an air bubble remained in the volumetric pipette, the actual volume of liquid collected would be smaller than the specified amount. This would reduce the amount of substance sampled and change the amount of standard solution required for titration. |
Example Expressions That Can Be Used in Reports
The following expressions can be used when discussing errors in volume measurement. Adjust the necessary parts according to your own experimental results.
- One possible cause of error in volume measurement is a shift in the position used to read the meniscus.
- If the eye level was not aligned with the calibration mark, the liquid level may have been read as higher or lower than the actual value.
- If water was added beyond the calibration mark in a volumetric flask, the concentration of the prepared solution would become lower than intended.
- If an air bubble remained in the volumetric pipette, the volume of solution collected may have been smaller than the specified amount.
- If an air bubble in the burette tip was expelled during titration, the titration volume may have been overestimated.
- Adding solution beyond the endpoint is considered to have made the titration volume larger than the actual value.
- Errors in volume measurement directly affect concentration calculations and may therefore have contributed to the deviation in the results.
- Because repeated measurements gave similar values, the reproducibility of the volume measurement is considered to have been relatively high.
Points to Check When Discussing Volume Measurement
Before writing the report, checking the following points can make it easier to write the discussion.
- Which instrument was used to measure the volume?
- Which of the volumetric flask, volumetric pipette, or burette had the greatest effect on the result?
- Was there any error in aligning with the calibration mark or reading the graduations?
- Was the meniscus read correctly?
- Were there any air bubbles remaining inside the instrument?
- Was rinsing with the solution sufficient?
- Was the solution sufficiently mixed?
- Was solution added beyond the endpoint?
- Did the error make the measured value larger or smaller?
- How did the error affect the concentration calculation?
Summary
Volumetric flasks, volumetric pipettes, and burettes are volume-measuring instruments frequently used in chemistry experiments. A volumetric flask is used to prepare a solution of a fixed volume, a volumetric pipette is used to measure a fixed amount of solution, and a burette is used to measure the amount of solution added during titration.
Errors that occur in volume measurement include meniscus reading errors, misalignment with the calibration mark, air bubbles, insufficient rinsing with the solution, insufficient mixing, and overshooting the endpoint. These errors directly affect concentration calculations and titration results.
In the discussion section of a report, rather than simply writing that “there was an error in volume measurement,” it is important to explain specifically which operation involving which instrument changed the measured value and in which direction. Correctly discussing errors in volume measurement makes the entire chemistry laboratory report more convincing.
