Melting Point Technique Quiz
1) You isolated the following compounds in lab and observed the melting points in the table below. For each compound, look up the melting point in the Tables in the Handbook (or see the online Chemical Information page) and judge the purity of the compound.
Compound |
Observed MP |
Naphthalene |
79-80° |
Benzophenone |
45-47° |
p-Anisic acid |
178-182° |
Salicylic acid acetate |
135° |
3-Chlorobenzoic acid |
157-158° |
Sulfanilamide |
165-166° |
Ferrocene |
157.5-161.5° |
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Compound |
Observed MP |
Literature MP |
Conclusion |
Naphthalene |
79-80° |
80.5° |
Only slightly impure, since the range is only 1° and it is very close to the literature value. |
Benzophenone |
45-47° |
48.1° |
Somewhat impure, with a mp range of 2° and the mp depressed by 1-3°. It is probably pure enough for most purposes. |
p-Anisic acid |
178-182° |
184° |
Impure. A mp range of 4° indicates that the compound is less pure than the naphthalene and benzophenone examples given above. |
Salicylic acid acetate |
135° |
135° |
Ultra-pure. Good thing, since this is aspirin. |
3-Chlorobenzoic acid |
157-158° |
158° |
Very pure. (Did you have trouble finding this compound in the Handbook? Often compounds are listed under the parent compound, in this case benzoic acid.) |
Sulfanilamide |
165-166° |
165-6° |
Ultra-pure. Note that the literature reports a 1° mp range for this compound. |
Ferrocene |
157.5-161.5° |
172.5° |
Extremely impure, with a 4° mp range over 10° below the literature value. You might even question whether the compound is even ferrocene. |
2) You isolated lidocaine and novocaine in the laboratory. What are the melting points of these compounds? (They are not listed in the Tables in this Handbook. Hint: go to the Chemical Information page.)
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Look up the melting points in a suitable source, such as the CRC, Merck Index, on-line sites, etc. Two values of melting points are listed for each compound below. The first one is from the Cambridge Soft (CS) site, found by going to the Hazard and Physical Data for Compounds page and following the links to sources of information on chemicals used in the lab. The second value was found in a hard copy of the Merck Index (10th Ed.)
- Lidocaine: 66-69°C at CS site; 68-69°C in the Merck Index.
- Novocaine: 155-156°C at CS site; 153-156°C (procaine hydrochloride) in the Merck Index.
3) You think that you have isolated ibuprofen in the lab. Since you don't totally trust your own laboratory techniques, you want to prove to yourself that you have ibuprofen before you ingest it. Using only melting point techniques, explain how you can prove that you actually have ibuprofen. (Assume the stockroom is able to supply you with any compound you need.)
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Using only methods of melting points, you can prove that you have isolated ibuprofen if you can obtain an authentic sample of ibuprofen. Mix equal amounts of the two compounds together, preferably in a mortar and pestle. Then take the melting point of the mixture. If it is 75-77°C, the compound you isolated is ibuprofen. If the melting point is substantially lowered and has a wide melting point range, the compound is not ibuprofen. This is the technique of "mixed melting points". Even if two different compounds have the same melting point, if they are mixed together the melting point of the mixture will be lowered because the compounds are no longer pure.
4) The melting point of a pure compound is known to be 110-111°. Describe the melting behavior expected if this compound is contaminated with 5% of an impurity.
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An impurity consisting of 5% total mass will lower the melting point from that of the pure compound, and it will increase the melting point range. There is no formula to predict how much the melting point will be lowered. However, the greater the impurity, the greater the melting point depression. A value of 103-107° would be consistent with this amount of impurity with the pure melting point of 110-111°; values of 100-105°, 97-100°, 102-110° are also good estimates.
5) You melt and observe decolorization, but miss the melting point. Should you cool the sample and re-melt it, or start over?
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If a compound changes color upon melting, it is likely that it has undergone a chemical reaction, or decomposed. If the sample has decomposed, it is no longer the same compound and will have a different melting point. Therefore, simply cooling and re-melting the sample will not give you the melting point (or in this case the decomposition point) of the compound you originally intended to analyze. To be safe, start with a fresh sample of the compound.
6) You melt a compound and it disappears: what should you do?
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If a compound "disappears" on melting, it is likely that it has sublimed, or gone directly from the solid to the gas phase. The melting point of such a compound needs to be taken in a sealed capillary tube.
7) What setting should you use on the Fisher-Johns and Mel-Temp melting point apparatus to determine the melting points of acetylferrocene and p-anisic acid?
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Compound |
Literature MP |
Fisher-Johns |
Mel-Temp |
Acetylferrocene |
81-83°C |
Start with the Fisher-Johns apparatus set at 100 until it reads about 75°, then set it to 30. |
Start with the Mel-Temp apparatus set at 5 until it reads about 75°, then set it to 2.5. |
p-Anisic acid |
184°C |
Start with the Fisher-Johns apparatus set at 100 until it reads about 179°, then set it to 75. |
Start with the Mel-Temp apparatus set at 5 until it reads about 179°, then set it to 4.5. |
8) You and your lab partner take melting points of the same sample. You observe a melting point of 101-107°C, while your partner observes a value of 110-112°C. Explain how you can get two different values with exactly the same sample.
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In the teaching labs, there are a few reasons which can account for two different people getting a different melting point range for the same sample of the same compound. (Other reasons might be valid too.)
- There is some variability between melting point apparati. The teaching labs have about 40 such instruments, and occasionally some of them are out of calibration. If you ever suspect that a particular device is out of calibration, you should report it to the Coordinator or ask for some standards so that you can check it yourself.
- One of the lab partners may have heated the melting point device too rapidly. To obtain an accurate melting point value, the device must be heated slowly near the melting point in order for the thermometer's sensor to be the same temperature as the heating block. If the heating block is heated too rapidly, the thermometer cannot keep up, and reads a value which is lower than the actual temperature of the heating block and of the compound.
- When observing a melting point, different people can make different judgements as to when a compound has begun to melt. Only one person can look through the lens at a time, so the decision resides solely on that one person's judgement. Sometimes a person will call the melting point as begun when the sample moves a bit, softens, or shrinks, or a droplet outside the bulk of the sample. Some students take a larger mass of crystals when taking a melting point and this can affect the way a compound looks as it melts (smaller amounts give more accurate melting point values). True melting is taken as the first appearance of liquid within the bulk of the sample. Practice taking melting points helps improve your judgement in taking melting points.
9) Which of the melting point capillaries in the picture below is filled to the proper level for taking a melting point on a MelTemp melting point apparatus?
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The melting point capillary should be filled to 2-3 mm, so A or B is the best choice.
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