Melting Point Measurement Discussion Examples | Causes of a Broad Melting Point Range and the Effects of Impurities

What Is Melting Point Measurement?

The melting point is the temperature at which a solid changes into a liquid. Crystalline substances with high purity melt within a relatively narrow temperature range. Therefore, measuring the melting point provides a clue for determining whether a sample is the desired substance and whether it contains impurities.

In experiments, it is common to record both the temperature at which melting begins and the temperature at which the sample has completely melted. For example, a melting point is expressed as a range, such as “132–134°C,” rather than as a single temperature. This range is called the melting point range.

Melting point measurement results are also frequently used in reports on recrystallization and organic synthesis. Because purity cannot be determined from yield alone, comparison with the literature melting point is important.

Results to Examine in Melting Point Measurement

For melting point measurement results, organize the temperature at which the sample began to melt, the temperature at which it completely melted, the difference from the literature value, and the width of the melting point range. If possible, comparing melting points before and after recrystallization makes it easier to discuss improvements in purity due to purification.

Main Items to Include in the Results

• Temperature at which melting began
• Temperature at which melting was complete
• Melting point range
• Comparison with the literature value
• Difference in melting point before and after recrystallization
• Color and crystalline condition of the sample
• Heating rate during measurement
• Variation in measured values

Reference Experimental Values for Melting Point Measurement and Examples of Purity Evaluation and Error Analysis

This section organizes reference experimental values for discussing sample purity and identification through melting point measurement. The melting point ranges of pure substances and samples containing impurities, differences from literature values, mixed melting point tests, heating rates, sample amounts, packing methods in capillary tubes, reproducibility of measurements, and error factors are summarized in a form that can be easily used in reports.

The melting point is the temperature at which a solid sample changes from a solid to a liquid. Pure substances have narrow melting point ranges and melt at temperatures close to their literature values. In contrast, samples containing impurities may show lower melting points or broader temperature ranges from the beginning to the completion of melting. Therefore, melting point measurement is frequently used for identifying organic compounds and evaluating their purity.

Reference Experimental Conditions

How to Record a Melting Point

A melting point is normally recorded as a range from the “temperature at which melting begins” to the “temperature at which melting is complete.” For example, if melting begins at 123.0°C and the sample becomes completely liquid at 124.2°C, the melting point is recorded as 123.0–124.2°C.

Example Comparison of a Pure Substance and Samples Containing Impurities

The following is a reference example showing changes in melting point range caused by differences in purity for the same substance.

When impurities are present, the melting point generally becomes lower than the literature value, and the melting point range becomes broader. Therefore, not only the “temperature” of the melting point but also the “width of the range” is important in evaluating purity.

Approximate Evaluation of Purity from the Melting Point Range

Even if the melting point range is narrow, if it differs greatly from the literature value, it is necessary to consider the possibility that the sample is a different substance or that the thermometer is inaccurate.

Example of Calculating Differences from the Literature Value

If the literature melting point is 122–123°C and the measured melting point is 121.8–123.1°C, the measured value can be judged to agree well with the literature value. On the other hand, if the measured melting point is 116.5–121.0°C, it is lower than the literature value and the melting point range is also broader, suggesting an effect of impurities.

The difference from the literature value should not be evaluated only by comparing average values; both the beginning and the end of melting should be considered.

Example of Changes in Melting Point Before and After Recrystallization

The following is a reference example showing how the melting point changes before and after purification by recrystallization.

When impurities are removed by recrystallization, the melting point approaches the literature value and the melting point range becomes narrower. This provides evidence that purification improved the purity of the sample.

Reference Example of a Mixed Melting Point Test

In a mixed melting point test, an unknown sample is mixed with a known standard sample and the melting point is measured. If they are the same substance, the melting point does not change greatly, whereas if they are different substances, melting point depression and broadening of the melting point range occur.

Even if two substances have similar melting points when measured separately, if melting point depression occurs in a mixed melting point test, they are more likely to be different substances.

Differences in Melting Point Caused by Heating Rate

If the heating rate is too fast, the actual temperature of the sample may not keep up with the displayed temperature of the apparatus, and the melting point may be observed as being higher.

When comparing with a literature value, slower heating near the melting point makes it easier to obtain an accurate value.

Effects of Sample Amount and Packing Method

If too much sample is placed in the capillary tube or the sample remains coarse, heat may not be transferred uniformly, which can broaden the melting point range.

In melting point measurement, finely dividing the sample and uniformly packing a small amount improves reproducibility.

Effects of Insufficient Drying and Residual Solvent

If solvent or moisture remains in a sample after recrystallization, it acts as an impurity and causes melting point depression and broadening of the melting point range.

If the melting point is lower than the literature value and the range is broad, insufficient drying should be considered in addition to chemical impurities.

Example of Measurement Reproducibility

The following is a reference example in which the same recrystallized sample was measured multiple times.

Because there were no large differences among the three measurements, the reproducibility of the measurement can be considered relatively good. However, even for the same sample, results may shift if the heating rate or the observer’s judgment changes.

Concept of Eutectic Melting and Melting Point Depression

In a solid containing impurities, part of the sample may begin to melt at a lower temperature than the pure substance. This can be explained by impurities disrupting the crystal lattice and reducing the stability of the solid.

Melting point depression and broadening of the melting point range are typical clues indicating the presence of impurities.

Example of Observation of a Sample Accompanied by Decomposition

Some compounds undergo decomposition or discoloration before clearly melting.

When decomposition occurs, it is useful to distinguish it from a normal melting point range and record it as, for example, “melted near ○°C with decomposition.”

Main Error Factors

Example of Writing the Results

When the melting point of the sample after recrystallization was measured, it was 121.8–123.1°C. Compared with the literature value of 122–123°C, the measured value was close to the literature value, and the melting point range was also relatively narrow at 1.3°C. This suggests that the sample after recrystallization had relatively high purity.

In contrast, the melting point of the crude product was 116.5–121.0°C, which was lower than the literature value, and the melting point range was broad at 4.5°C. When impurities are present, the crystal lattice is disrupted and the sample begins to melt at a lower temperature than the pure substance, causing melting point depression and broadening of the melting point range. Therefore, the crude product was highly likely to contain impurities such as unreacted materials, by-products, and residual solvent.

After recrystallization, the melting point approached the literature value and the melting point range became narrower. This is considered to be because recrystallization removed impurities and improved the purity of the crystals. However, if the melting point does not completely agree with the literature value, factors such as temperature calibration of the apparatus, heating rate, drying condition of the sample, and timing of observation may also have had an effect.

Points to Connect to the Discussion

In a discussion of melting point measurement, it is important not only to consider whether the measured value is close to the literature value but also to relate the width of the melting point range, changes before and after recrystallization, mixed melting point tests, and measurement conditions.

• Were the temperature at which melting began and the temperature at which melting ended recorded separately?
• Were differences from the literature value compared at both the lower and upper limits?
• Can you explain the meaning of a narrow or broad melting point range?
• Can you explain that impurities lower the melting point and broaden the melting point range?
• Can you discuss improvements in purity due to recrystallization from the increase in melting point and narrowing of the range?
• Can you explain the concept of identification by a mixed melting point test?
• Can you discuss the possibility that a fast heating rate causes the melting point to appear higher?
• Can you explain that sample amount, particle size, and packing in the capillary tube affect the melting point range?
• Can you explain insufficient drying, residual solvent, apparatus calibration, and observation delay as error factors?

Example Discussion

Summary

In melting point measurement, the purity and identity of a sample can be discussed by checking whether the measured melting point is close to the literature value and whether the melting point range is narrow. Samples with high purity have narrow melting point ranges close to the literature value, whereas samples containing impurities have lower melting points and broader ranges.

This reference example covered comparison between pure substances and samples containing impurities, differences from literature values, changes in melting point before and after recrystallization, mixed melting point tests, heating rate, sample amount and packing, insufficient drying, reproducibility, samples accompanied by decomposition, and error factors. In a report, it is useful to discuss not only the numerical melting point but also the width of the melting point range and the measurement conditions.

What Is the Melting Point Range?

The melting point range is the range from the temperature at which the sample begins to melt to the temperature at which it becomes completely liquid. For example, if the sample begins to melt at 132°C and completely melts at 134°C, the melting point range is 132–134°C.

For a high-purity sample, the melting point range is narrow. In contrast, a sample containing impurities may show a lower melting point or a broader melting point range.

Effects of Impurities on Melting Point

One of the most important points to discuss in melting point measurement is the effect of impurities. In general, samples containing impurities may show a lower melting point and a broader melting point range.

In a pure substance, the crystal lattice is relatively uniform, so the substance melts near a specific temperature. However, when impurities are mixed in, the crystal lattice is disrupted, and melting may begin at a lower temperature than in the pure substance. In addition, because the entire sample is not uniform, the temperature range from the beginning to the completion of melting becomes broader.

Causes of a Melting Point Lower Than the Literature Value

The most typical cause of a measured melting point lower than the literature value is impurity contamination. However, insufficient drying, measurement conditions, and the condition of the sample may also be involved.

Causes of a Broad Melting Point Range

A broad melting point range indicates that the entire sample did not melt at the same time. Impurity contamination is a typical cause, but measurement procedures can also broaden the melting point range.

Impurities Are Present

In samples containing impurities, different portions may begin to melt at different temperatures, resulting in a broad melting point range. This is likely to occur when recrystallization is insufficient or mother liquor remains.

Too Much Sample

In melting point measurement, if too much sample is used, heat does not transfer uniformly throughout the sample. As a result, the lower and upper parts may melt at different times, making the melting point range appear broader.

Heating Rate Is Too Fast

If the heating rate is too fast, a difference is more likely to occur between the displayed temperature and the actual temperature of the sample. It also becomes difficult to determine the beginning and end of melting, which can broaden the melting point range or make the melting point appear higher.

Insufficient Drying of the Sample

If solvent or moisture remains in the sample, it may affect the melting point measurement. Residual solvent acts as an impurity and may cause melting point depression and broadening of the melting point range.

Can the Melting Point Be Higher Than the Literature Value?

When impurities are present, the melting point often becomes lower, but the measured value may also be higher than the literature value. In such cases, the cause is often related more to the measurement procedure or apparatus conditions than to the purity of the sample itself.

For example, if the heating rate is too fast, observation may be delayed even though the sample has actually begun to melt, resulting in a higher recorded melting point. Calibration of the thermometer or apparatus, sample packing, and timing of observation can also have an effect.

Comparison of Melting Points Before and After Recrystallization

Melting point measurement is also used to confirm the effectiveness of purification by recrystallization. Comparing the melting points of the crude product before recrystallization and the crystals after recrystallization makes it easier to discuss whether purity improved.

Mixed Melting Point Tests and Identification

Melting point measurement is also used to identify substances. A method in which a known substance and an unknown sample are mixed and their melting point is measured is sometimes called a mixed melting point test.

If the unknown sample and known substance are the same substance, mixing them does not greatly decrease the melting point or significantly broaden the melting point range. In contrast, when different substances are mixed, the melting point may decrease and the melting point range may broaden, as if an impurity had been added.

Common Sources of Error in Melting Point Measurement

In melting point measurement, the value may change not only because of sample purity but also because of the measurement procedure. In a report, if the measured value differs from the literature value, organize the possible causes of error.

Relationship Between Yield and Melting Point

In recrystallization and synthesis experiments, yield and melting point may be discussed together. A high yield does not necessarily mean high purity. If a sample is recovered while still containing impurities, its mass may be large, but its melting point may be low and the melting point range may be broad.

When the Results Can Be Considered Good

Melting point measurement results can be considered good when the measured value is close to the literature value and the melting point range is narrow. In addition, when comparing before and after recrystallization, if the melting point after recrystallization approaches the literature value and the melting point range becomes narrower, it becomes easier to conclude that purification was effective.

Discussion Example When the Measurement Did Not Go Well

If the melting point measurement differs greatly from the literature value or the melting point range is broad, consider the possibility that there was a problem with sample purity or the measurement procedure. However, rather than simply writing that “the experiment failed,” it is important to explain the causes specifically.

How to Write Improvements

In a discussion of melting point measurement, including improvements as well as causes makes the report easier to organize. It is important to describe improvements specifically in relation to your own measurement results.

Improvements for Increasing Purity

• Perform recrystallization carefully to remove impurities.
• Thoroughly remove the mother liquor and washing solution.
• Dry the sample thoroughly.
• Purify the sample again if necessary.

Improvements for Increasing Measurement Accuracy

• Do not use an excessive amount of sample.
• Finely divide the sample and pack it uniformly.
• Reduce the heating rate near the melting point.
• Carefully observe the beginning and end of melting.
• Perform multiple measurements to confirm reproducibility.

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of melting point measurement, writing only that “there were impurities because the melting point was low” is somewhat superficial. A more convincing discussion explains why impurities lower the melting point, why the melting point range becomes broader, and whether the measurement procedure may also have affected the result.

Example Expressions That Can Be Used in Reports

The following expressions can be used when writing melting point measurement results and discussions. Adjust the necessary parts according to your own experimental results.

• The measured melting point was lower than the literature value, suggesting that the sample may have contained impurities.
• Because the melting point range was broad, the purity of the sample is considered to have been insufficient.
• Disruption of the crystal lattice by impurities may have caused melting point depression and broadening of the melting point range.
• If drying was insufficient and residual solvent remained, the melting point may have been measured as being lower.
• If the sample amount was too large, heat may not have transferred uniformly throughout the sample, causing the melting point range to broaden.
• Because the heating rate was too fast, errors may have occurred in determining the beginning and end of melting.
• Because the melting point approached the literature value after recrystallization, the purity is considered to have improved.
• It is necessary to evaluate the purity of the product using both yield and the melting point and melting point range.

Points to Check When Discussing Melting Point Measurement

Before writing the report, checking the following points can make it easier to write the discussion.

• At what temperature did melting begin?
• At what temperature was melting complete?
• Is the melting point range narrow or broad?
• Is the measured melting point higher or lower than the literature value?
• How did the melting point change before and after recrystallization?
• Did the melting point range become narrower after recrystallization?
• Is it possible that impurities remained in the sample?
• Was the sample sufficiently dried?
• Was the sample amount excessive?
• Was the heating rate appropriate?
• Were the beginning and end of melting observed correctly?
• Can purity be evaluated from both yield and melting point?

Summary

Melting point measurement is an important experiment for confirming the purity and identity of a solid sample. A high-purity substance shows a melting point close to the literature value and a relatively narrow melting point range. In contrast, samples containing impurities may show a lower melting point or a broader melting point range.

Causes of a broad melting point range include impurity contamination, insufficient drying, an excessive sample amount, a fast heating rate, and uneven sample packing. Therefore, when the measured value differs from the literature value, it is necessary to consider not only sample purity but also the effects of the measurement procedure.

In the discussion section of a report, rather than presenting only the melting point value, explain the relationship between comparison with the literature value, the melting point range, the effects of impurities, and changes before and after recrystallization. By considering both yield and melting point, the experimental results can be evaluated from both the amount and purity of the product.