Unknown sample analysis is an experiment in which qualitative and quantitative analyses are combined to estimate the components and concentrations of a sample whose composition is unknown.
In university analytical chemistry experiments, the components contained in an unknown sample may be determined based on the results of precipitation reactions, color reactions, pH measurements, titrations, spectrophotometry, chromatography, and other methods.
In an unknown sample analysis report, it is not sufficient simply to write “what was contained in the sample.”
It is important to discuss which experimental results were used as the basis for that judgment, whether the results of qualitative and quantitative analyses are consistent with each other, and to what extent the conclusion may change because of sources of error.
This article clearly explains how to organize the results of unknown sample analysis, how to write discussions of qualitative and quantitative analyses, how to integrate multiple results, sources of error, 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 handling of unknown samples, reagents, analytical procedures, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is Unknown Sample Analysis?
- Difference Between Qualitative and Quantitative Analysis
- Results to Examine in Unknown Sample Analysis
- How to Write the Discussion for Qualitative Analysis
- How to Write the Discussion for Quantitative Analysis
- How to Relate Qualitative and Quantitative Analysis
- When Multiple Qualitative Reactions Agree
- When Qualitative and Quantitative Analyses Are Inconsistent
- How to Avoid Overstating the Identification of an Unknown Sample
- Common Sources of Error in Unknown Sample Analysis
- Errors Caused by Interfering Components
- Errors Caused by Sample Dilution and Preparation
- Comparison With Standard Substances and Standard Solutions
- Discussion of Unknown Sample Analysis Using a Calibration Curve
- Discussion of Unknown Sample Analysis Using Titration
- Discussion of Unknown Sample Analysis Using Chromatography
- Discussion of Unknown Sample Analysis Using Precipitation Reactions
- Discussion Using pH and Appearance Observations
- A Structure for Integrating Multiple Results
- 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 Unknown Sample Analysis
- Summary
What Is Unknown Sample Analysis?
Unknown sample analysis is an analysis in which the components or concentrations of a sample are not disclosed in advance, and the components present and their amounts are estimated from experimental results.
In some cases, a conclusion is reached from a single experiment, but in many cases, multiple analytical results are combined to reach a conclusion.
For example, a specific ion may be confirmed by a precipitation reaction, and its concentration may then be determined by titration or spectrophotometry.
In this case, precipitation reactions and color reactions are qualitative analyses used to determine “what is contained,” while concentration calculations based on titration or calibration curves are quantitative analyses used to determine “how much is contained.”
In a report, it is important not only to write qualitative and quantitative analyses separately, but also to organize their results in relation to each other.
Writing in a sequence such as “because this reaction was positive, the sample was judged to contain this component” and “quantitative analysis of that component gave a concentration of XX” produces an easy-to-read discussion.
Difference Between Qualitative and Quantitative Analysis
In unknown sample analysis, it is necessary to distinguish the roles of qualitative and quantitative analyses.
Qualitative analysis investigates which components are contained in a sample.
Quantitative analysis determines the amount or concentration of the components present.
| Type of Analysis | Purpose | Example of Result |
|---|---|---|
| Qualitative analysis | Determine what is contained | Chloride ions may be present |
| Quantitative analysis | Determine how much is contained | The chloride-ion concentration was 0.050 mol/L |
In the discussion section of a report, it is natural to narrow down candidate components from the qualitative-analysis results and then show their amounts using quantitative analysis.
However, a positive qualitative-analysis result does not necessarily mean that only that component is present, so the possibility of interfering components and misidentification must also be considered.
Example Discussion:
In qualitative analysis, the components contained in the unknown sample were estimated from the results of precipitation and color reactions.
In quantitative analysis, the concentration of the estimated component was determined using titration volume or absorbance.
Therefore, in unknown sample analysis, it is important to identify candidate components by qualitative analysis and evaluate their amounts by quantitative analysis.
Results to Examine in Unknown Sample Analysis
In unknown sample analysis, it is important not only to organize individual experimental results but also to summarize what each result indicates about the presence or concentration of a component.
When multiple results are available, the discussion should also include whether the results are consistent and which results should be given greater weight.
Main Items to Include in the Results
- Unknown sample number or sample name
- Observations such as appearance, color, and odor
- Basic measurements such as pH
- Results of precipitation reactions
- Results of color reactions
- Chromatographic spots and Rf values
- Titration volume or average titration volume
- Absorbance or concentration determined from a calibration curve
- Estimated components
- Calculated concentration or content
- Sources of error and uncertainty
Example of How to Write the Results:
For Unknown Sample A, a white precipitate formed in the precipitation reaction and the reaction was similar to that of the standard sample.
In addition, in spectrophotometry, the absorbance of Unknown Sample A was within the range of the calibration curve, allowing its concentration to be determined from the calibration curve.
From these results, Unknown Sample A is highly likely to contain the target component, and its concentration was determined to be XX.
How to Write the Discussion for Qualitative Analysis
In the discussion of qualitative analysis, explain what the observed reaction indicates.
For example, if a precipitate formed, explain which ion may have reacted with which reagent.
In the case of a color reaction, consider which component the color change indicates.
The important point is not to end with statements such as “a precipitate formed” or “the color changed.”
Discuss which component the observation supports and whether other components could produce a similar reaction.
Example Discussion:
When a reagent was added to the unknown sample, a white precipitate formed.
This reaction is considered to have occurred because the target ion in the sample reacted with an ion in the reagent to form a sparingly soluble salt.
Therefore, this result suggests that the target ion may be present in the unknown sample.
However, because other ions may form similar precipitates, the result must be judged together with other confirmation reactions.
How to Write the Discussion for Quantitative Analysis
In the discussion of quantitative analysis, explain which measured values were used to calculate the concentration or content.
For titration, explain the titration volume and stoichiometric relationship in the reaction equation; for spectrophotometry, explain the calibration-curve equation; and for gravimetric analysis, explain the relationship between precipitate mass and the chemical formula.
In addition to the calculated result, discuss whether the value is reasonable.
If comparison with a theoretical value, labeled value, standard-sample result, or results from other groups is possible, it is useful to consider the reasons for any differences.
Example Discussion:
In quantitative analysis, the concentration of the target component in the unknown sample was determined from the volume of standard solution required for titration.
According to the reaction equation, the target component and titrant react in a fixed molar ratio, so the amount of the target component can be calculated from the amount of titrant.
Because the calculated concentration was close to the expected value, the quantitative result is considered generally reasonable.
On the other hand, possible causes of the difference include endpoint-determination error and error in the concentration of the standard solution.
How to Relate Qualitative and Quantitative Analysis
In unknown sample analysis, it is important not only to write the qualitative and quantitative results separately but also to relate them to each other.
The discussion becomes well organized when written in the sequence of estimating the presence of a component by qualitative analysis and then determining its concentration by quantitative analysis.
Example Discussion:
In qualitative analysis, confirmation reactions performed on the unknown sample produced a reaction characteristic of the target component.
From this, the unknown sample was judged highly likely to contain the target component.
Furthermore, quantitative analysis gave a concentration of XX for the target component.
Therefore, in this experiment, qualitative analysis confirmed the presence of the component and quantitative analysis evaluated its content.
When Multiple Qualitative Reactions Agree
When multiple qualitative reactions indicate the presence of the same component, the reliability of the conclusion increases.
For example, if both a precipitation reaction and a color reaction suggest the same component, the conclusion is more persuasive than one based on a single reaction.
Example Discussion:
For the unknown sample, both the precipitation reaction and color reaction produced results indicating the presence of the target component.
Because multiple different confirmation reactions support the same conclusion, the target component is highly likely to be present in the unknown sample.
However, because confirmation reactions may sometimes be affected by interfering components, the result should be judged together with quantitative analysis or other confirmation methods when necessary.
When Qualitative and Quantitative Analyses Are Inconsistent
In some cases, qualitative analysis is positive but the quantitative value is very small, or qualitative analysis is nearly negative while quantitative analysis gives a high concentration.
In such cases, possible errors in one of the procedures or measurements should be considered.
Considering the sensitivity of the qualitative reaction, interfering components, sample dilution, endpoint determination, calibration-curve range, and blank correction makes the discussion easier to write.
Example Discussion:
Although a reaction indicating the presence of the target component was observed in qualitative analysis, the concentration determined by quantitative analysis was low.
One possible reason is that the target component was present only in a small amount, allowing it to be detected by the qualitative reaction while the quantitative result was more strongly affected by measurement error.
In addition, errors in dilution procedures and variation at the low-concentration end of the calibration curve may also have affected the concentration difference.
How to Avoid Overstating the Identification of an Unknown Sample
In unknown sample analysis, components are estimated from experimental results, but when the evidence is insufficient, it is important not to make an overly definitive statement.
Expressions such as “is highly likely to be present” or “the results suggest that the sample is XX” are more scientific than saying that a component “can be definitively confirmed.”
| Expression to Avoid | Improved Example |
|---|---|
| The unknown sample is XX. | Based on the experimental results, the unknown sample is considered highly likely to be XX. |
| XX is definitely present. | The confirmation-reaction results suggest that XX is present. |
| This reaction alone determined the result. | This reaction supports the presence of XX, but it must be judged together with other confirmation results. |
Example Discussion:
In the qualitative analysis of this experiment, a reaction indicating the presence of the target component was observed in the unknown sample.
In addition, quantitative analysis also gave a concentration corresponding to that component.
From these results, the unknown sample is highly likely to contain the target component.
However, because confirmation reactions and quantitative measurements may be affected by errors and interference, additional analysis is required for complete identification.
Common Sources of Error in Unknown Sample Analysis
Errors may occur in both qualitative and quantitative analyses of unknown samples.
In qualitative analysis, misidentification, interfering components, and inappropriate reaction conditions are common problems, while in quantitative analysis, measured values, standard solutions, calibration curves, and endpoint determination may cause errors.
| Source of Error | What Happens | Effect on the Result |
|---|---|---|
| Presence of interfering components | Similar reactions or absorption occurs | Qualitative and quantitative results deviate |
| Sample dilution error | Concentration differs from the calculated value | Directly affects the quantitative result |
| Inappropriate reaction conditions | Confirmation reaction or precipitation is incomplete | Presence or absence of the component may be misidentified |
| Error in standard-solution concentration | Titration calculation is incorrect | Concentration is overestimated or underestimated |
| Outside the calibration-curve range | Concentration is estimated by extrapolation | Reliability of the quantitative result decreases |
| Improper blank correction | Effects other than the target component remain | Absorbance or concentration deviates |
Errors Caused by Interfering Components
Unknown samples may contain components other than the target component.
If an interfering component produces a reaction similar to that of the target component, absorbs at the same wavelength, or consumes the titrant, errors may occur in the results.
Example Discussion:
If interfering components were present in the unknown sample, they may have affected the confirmation reaction and quantitative-analysis results.
For example, if a component produces a precipitate similar to that of the target component, a positive qualitative result may actually reflect reactions of multiple components.
In titration or spectrophotometry, interfering components may consume titrant or absorb at the same wavelength, causing the quantitative value to be overestimated.
Errors Caused by Sample Dilution and Preparation
When an unknown sample is diluted for measurement, errors in the dilution procedure directly affect the quantitative result.
Possible causes include errors in reading the calibration marks of a volumetric pipette or volumetric flask, insufficient mixing, and sample mix-ups.
Example Discussion:
One possible reason the concentration obtained by quantitative analysis differed from the expected value is an error in dilution of the unknown sample.
If the volume collected with the pipette or the final volume adjusted with the volumetric flask was inaccurate, the actual dilution factor would differ from the calculated value.
As a result, an error would occur when calculating the original sample concentration from the measured value.
Comparison With Standard Substances and Standard Solutions
In unknown sample analysis, comparison with standard substances or standard solutions improves the reliability of the judgment.
In qualitative analysis, the reaction or Rf value can be compared with that of a standard substance.
In quantitative analysis, concentration is determined using a calibration curve prepared from standard solutions or a standardized titrant.
Example Discussion:
The reaction of the unknown sample was similar to that of the standard substance, and both showed the same color change.
In chromatography, the Rf value of the unknown-sample spot was also close to that of the standard substance.
From these results, the unknown sample is highly likely to contain the same component as the standard substance.
However, because complete identification cannot be made from Rf values or color reactions alone, the result must be judged together with other analytical results.
Discussion of Unknown Sample Analysis Using a Calibration Curve
When an unknown sample is analyzed by spectrophotometry or a similar method, the linearity of the calibration curve and whether the absorbance of the unknown sample lies within its range should be checked.
If an unknown concentration is determined outside the calibration-curve range, extrapolation is required and the reliability of the result decreases.
Example Discussion:
The absorbance of the unknown sample was within the range of the calibration curve prepared from the standard solutions.
Therefore, it is considered appropriate to determine the concentration of the unknown sample using the calibration-curve equation.
On the other hand, because some variation was observed among the measurement points at the low-concentration end of the calibration curve, the effects of blank correction and instrument-reading error may become larger for low-concentration samples.
Discussion of Unknown Sample Analysis Using Titration
When an unknown sample is analyzed by titration, discuss endpoint determination, standard-solution concentration, coefficients in the reaction equation, and variation in titration volume.
It is easier to organize the discussion by first confirming the presence of the target component by qualitative analysis and then determining its concentration by titration.
Example Discussion:
Because qualitative analysis suggested that the target component was present in the unknown sample, its concentration was determined by titration.
The titration volumes were similar over multiple measurements, suggesting relatively high reproducibility in endpoint determination.
However, if titrant was added beyond the endpoint, the titration volume would become larger than the true value and the calculated concentration could be overestimated.
Discussion of Unknown Sample Analysis Using Chromatography
When chromatography is used, components are estimated from the number of spots, Rf values, and comparison with standard substances.
However, because complete identification cannot be made from similar Rf values alone, the result must be judged together with other analytical results.
Example Discussion:
Multiple spots were observed for the unknown sample, suggesting that the sample contained multiple components.
One of the spots showed an Rf value close to that of the standard substance, suggesting that the unknown sample may contain the same component as the standard substance.
However, because Rf values depend on development conditions and different substances may show similar values, complete identification requires judgment together with other confirmation results.
Discussion of Unknown Sample Analysis Using Precipitation Reactions
Precipitation reactions use the formation of sparingly soluble precipitates when specific ions react with reagents.
The color, presence or absence, and solubility of the precipitate are compared with standard samples or known reactions.
Example Discussion:
When a reagent was added to the unknown sample, a white precipitate formed.
This precipitate is considered to have formed because the target ion reacted with an ion in the reagent to produce a sparingly soluble salt.
Therefore, this result suggests that the target ion may be present in the unknown sample.
However, because other ions may also form similar precipitates, the result must be judged together with the color and solubility of the precipitate and other confirmation reactions.
Discussion Using pH and Appearance Observations
Appearance observations such as pH, color, odor, and transparency of an unknown sample also provide clues for estimating its components.
However, because components cannot be determined from appearance alone, these observations must be considered together with confirmation reactions and quantitative analysis.
Example Discussion:
The pH of the unknown sample was acidic, suggesting that it may contain an acidic component or an acidic salt.
However, the specific component cannot be identified from pH alone.
Therefore, the result must be judged together with precipitation reactions, color reactions, and titration results.
A Structure for Integrating Multiple Results
A discussion of unknown sample analysis is easier to organize when written in the following sequence.
Observation Result → Meaning of the Reaction → Estimation of the Component → Quantitative Result → Sources of Error → Conclusion
Example of How to Summarize:
In Unknown Sample A, a precipitate characteristic of the target component was observed in the confirmation reaction.
In addition, comparison with the standard substance showed that the reaction behavior was consistent.
From this, Unknown Sample A is highly likely to contain the target component.
Furthermore, quantitative analysis gave a concentration of XX for the target component.
However, the precipitation reaction may have been affected by interfering components, while the quantitative analysis may have included errors in the standard-solution concentration and endpoint determination.
Therefore, Unknown Sample A is estimated to contain the target component at a concentration of approximately XX.
When the Result Can Be Considered Good
A good result in unknown sample analysis is indicated when multiple pieces of qualitative evidence agree and the quantitative value is not inconsistent with the theoretical value or range of the standard sample.
In addition, if the measurements show high reproducibility and major sources of error are small, the conclusion can be considered more reliable.
Example Discussion:
In the unknown sample, multiple qualitative reactions indicated the presence of the same component, and the concentration obtained by quantitative analysis was also within the expected range.
In addition, no large variation was observed among repeated measurements.
From these results, the conclusion that the target component is present in the unknown sample is considered relatively reliable.
Example Discussion When the Analysis Did Not Go Well
When unknown sample analysis does not go well, possible causes can be considered from results such as unclear qualitative reactions, quantitative values that differ greatly from expectations, contradictory results, or poor reproducibility.
It is easier to write the discussion by separately considering reaction conditions, interfering components, dilution procedures, calibration curves, and endpoint determination.
Example Discussion:
In qualitative analysis, the reaction indicating the presence of the target component was weak, and the concentration obtained by quantitative analysis also differed greatly from the expected value.
One possible reason is that the concentration of the target component in the unknown sample was low, making the change in the confirmation reaction unclear.
In addition, if interfering components were present, they may have affected both the confirmation reaction and quantitative analysis, causing errors in component estimation and concentration calculation.
How to Write Points for Improvement
In a discussion of unknown sample analysis, including points for improvement as well as sources of error makes the report easier to organize.
Improvements are easier to organize when written separately for qualitative and quantitative analysis.
Improvements for Qualitative Analysis
- Compare with standard substances under the same conditions
- Perform multiple confirmation reactions
- Observe the color and solubility of precipitates in detail
- Maintain the reaction conditions specified in the laboratory manual
- Consider the possibility of interfering components
- Record observations immediately
Improvements for Quantitative Analysis
- Prepare or standardize the concentration of standard solutions accurately
- Perform dilution procedures accurately
- Perform multiple measurements and use the average value
- Measure within the range of the calibration curve
- Perform blank correction appropriately
- In titration, add the titrant one drop at a time near the endpoint
Example of How to Write Points for Improvement:
To improve the reliability of identifying an unknown sample, multiple confirmation reactions should be performed rather than relying on a single qualitative reaction, and the consistency of the results should be checked.
In quantitative analysis, errors in the concentration of the standard solution and dilution factor directly affect the result, so accurate volume measurement is important.
When a calibration curve is used, appropriately diluting the sample so that the unknown-sample measurement falls within the calibration-curve range can improve the reliability of the concentration estimate.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of unknown sample analysis, simply writing that “XX was present” or “the concentration was determined” results in a superficial discussion.
A persuasive discussion can be produced by explaining which results support which conclusions and how the qualitative and quantitative analyses are connected.
| Superficial Discussion | Good Discussion |
|---|---|
| The unknown sample contained XX. | A change characteristic of XX was observed in the confirmation reaction, and the result was similar to that of the standard substance. Therefore, the unknown sample is highly likely to contain XX. |
| The concentration was determined. | Because qualitative analysis suggested the presence of the target component, its concentration was determined by quantitative analysis. Using the value obtained from the titration volume or calibration curve made it possible to evaluate the concentration of the target component in the unknown sample. |
| The results were different. | Possible causes of the difference between the qualitative and quantitative results include the presence of interfering components, errors in dilution procedures, measurement outside the calibration-curve range, and deviation in endpoint determination. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of unknown sample analysis.
Adjust the necessary parts according to your own experimental results.
- In qualitative analysis, the components in the unknown sample were estimated from the confirmation-reaction results.
- In quantitative analysis, the concentration of the target component was determined from the titration volume or absorbance.
- Because a change characteristic of the target component was observed in the confirmation reaction, the unknown sample is highly likely to contain the target component.
- Because the unknown sample showed a reaction similar to that of the standard substance, the presence of the same component in the unknown sample was suggested.
- Because multiple qualitative reactions supported the same conclusion, the reliability of the component estimation is considered relatively high.
- The qualitative and quantitative results were generally consistent, so the estimation of the unknown sample is considered reasonable.
- A positive reaction was observed in qualitative analysis, but the quantitative value was small, suggesting that the target component may be present only in a small amount.
- If interfering components were present, they may have affected the confirmation reactions and quantitative analysis.
- If the measured value of the unknown sample was outside the calibration-curve range, the reliability of the concentration estimate decreases.
- Complete identification requires judgment based on a combination of multiple analytical results.
Points to Check When Discussing Unknown Sample Analysis
Checking the following points before writing the report makes the discussion easier to write.
- Have you written qualitative and quantitative analyses separately?
- Have you explained which results were used as the basis for estimating the components?
- Have you compared the results with standard substances or standard solutions?
- Are the qualitative and quantitative results consistent?
- If they are inconsistent, have you considered the cause?
- Have you considered the possibility of interfering components?
- Have you handled the dilution factor and coefficients in the reaction equation correctly?
- Have you measured the unknown sample within the calibration-curve range?
- Have you considered endpoint determination and variation in measured values during titration?
- Have you avoided overstating the conclusion and used wording appropriate to the evidence?
- Have you explained how the sources of error affect the conclusion?
- Does the final conclusion reflect both the qualitative and quantitative results?
Summary
In unknown sample analysis, qualitative analysis is used to estimate “what is contained,” while quantitative analysis is used to determine “how much is contained.”
In a report, it is important not merely to list individual results, but to clearly explain which conclusions each result supports.
In qualitative analysis, components are estimated from results such as precipitation reactions, color reactions, pH, and chromatography.
In quantitative analysis, concentrations and contents are determined from results such as titration, spectrophotometry, and gravimetric analysis.
If the results of both types of analysis agree, the reliability of the unknown-sample estimation increases.
However, unknown samples may contain interfering components, and errors may occur in qualitative reactions and quantitative values.
Therefore, in the discussion, avoid making overly definitive statements and use expressions such as “highly likely,” “suggested,” and “estimated” based on the experimental results.
Connecting qualitative and quantitative analyses in the explanation makes it possible to produce a persuasive unknown sample analysis report.
