07_Melting Points Manual

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7. Determination of Melting Points

This experiment consists of three parts. In the first part, you will

determine the melting point range of three known compounds. This part ismostly for practice, to make sure you know how to use the Mel-Tempapparatus to obtain melting point ranges that are close to the literature values(that is, the values in this manual) of fairly pure samples.

In the second part, you will determine the melting point range of anunknown compound and tentatively identify the compound.

In the last part, you will confirm the identity of your unknown by doingwhat is known as a mixed melting point determination. Your instructor mayhave you simultaneously determine who else in the lab has your sameunknown.

Since the actual melting point determination procedure is the same for

all of your samples, the procedure will be described at the beginning and notrepeated. Determining the melting point range of a compound is a procedurethat you will perform many times during this course.

PRE-EXPERIMENT ASSIGNMENT

Study this chapter of the manual and the lecture notes on the OrganicChemistry departmental web site. Complete the first seven sections of yournotebook write up. (Instructions on completing these sections may be found in

the laboratory notebook chapter of the lab manual.)

A student who has prepared for the Melting Point experiment should beable to:

1. Define or explain: melting point, melting point range, phase transitions, andeutectics.2. Explain why chemists determine the melting point ranges of samples. (Toidentify unknown compounds, to determine the purity of the samples, and tocharacterize new compounds.)3. Draw the structure given the name, or give the name from the structure, of

any of the compounds used in the day's experiment.4. Explain how soluble and insoluble impurities will alter a melting point

range.5. Explain how a melting point capillary is filled and how an accurate meltingpoint range is obtained.

6. Identify and explain safety considerations for the day's experiment.7. Perform the day's experiment safely and successfully.

Quizzes given after the experiment has been performed may also include:


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Record the temperature when you see the first signs of liquid droplets inthe sample. Do not mistake settling of the sample for melting. Keep observing

the sample and record the temperature at which the entire sample becomesliquid. Report the melting point range of the sample as the first temperature tothe second temperature. Always report a melting point range. Mel-Temps

will not give you a single melting point. The amount of sample that you use and the heating rate through the

melting range are critical. If you use too much sample or heat too rapidly youwill not get accurate results. Be patient and do it right the first time.

Once the sample in the capillary melts, don't use it again. If you have torepeat a measurement for any reason, you must start with a fresh sample in anew capillary.

This procedure requires some idea of the expected melting point of thesample. If this is not given to you in the manual or text, then you must look itup in a handbook or other sources. If the sample is unknown, then you need

to do two runs. First do a rapid measurement (about 15 to 20°C per minute) to

determine an approximate melting point. Then do the normal measurement ona fresh sample using the approximate melting point as your expected meltingpoint.

The Mel-Temp takes a long time to cool. If you are doing more than one

sample, be sure to start with the material with the lowest melting point. Assoon as all of the desired samples have finished melting, turn the Mel-Temp offand allow it to cool.

Up to three samples can be run at the same time. This takes someorganization, but saves a significant amount of time.

Comparison of Reported and Observed Melting Points of Known Materials.

In the first section of the experiment the melting point ranges of threeknown compounds (naphthalene, acetanilide, and urea) will be determined.Run all three at the same time. Read the thermometer to one tenth of onedegree or one decimal place. You should be able to obtain melting point rangesof four degrees in width and within a few (2-3) degrees of the literature valuesgiven above. Common mistakes are using too large a sample, heating toorapidly, and mistaking the onset of melting. Record all melting point rangesdirectly into your lab notebook. As soon as the last compound has melted,

switch off the Mel-Temp and allow to begin to cool. Show data to instructor. Ifthe data is of acceptable quality, an unknown will be given. If not, you may be

asked to repeat the measurements.

Melting Point of an Unknown

In the second section of the experiment, the melting point range of a pure

unknown will be determined and a preliminary identification will be made.Obtain an unknown and record its number in your lab notebook. Fill twocapillary tubes with ~1-2mm of unknown. The following table lists the melting


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points for possible identities for the unknown2. Your unknown will be one ofthese 11 compounds. Note, the Mel-Temps do not allow for a single melting

point to be determined. Only ranges of melting points are possible when usinga Mel-Temp.

Compound Literature Melting Point4-nitrotoluene 54.5°C

Naphthalene 80.5°C

glutaric acid 99°C

2-methylbenzoic acid 107°C

Acetanilide 114.3°C

benzoic acid 122.1°C

Urea 135°C

salicylic acid 158°C

Sulfanilamide 165°C

succinic acid 188°C

3,5-dinitrobenzoic acid 205°C

Do not wait for the Mel-Temp to cool to room temperature before starting

this determination. First perform a rapid and crude melting point analysis toassess the approximate melting point range of the unknown, then repeat usinga slower temperature ramp to obtain a more accurate and precise value. Insertone capillary with the unknown into the Mel-temp and adjust the voltagecontrol so a heating rate of ~15oC per minute is obtained. As soon as thesample begins to melt, note the temperature and shut off the Mel-Temp. Allowthe Mel-Temp to cool to at least 10oC below the temperature at which the

sample just began to melt. Remove first sample from Mel-temp and insert thesecond capillary containing the unknown. Adjust the voltage control so aheating rate of ~1oC per minute is obtained. Record the melting point range.Record all melting point ranges directly into your lab notebook, not on a scrap of

paper or datasheet. Compare your observed melting point to those in the table.From this melting point tentatively identify your unknown. Remember that anysoluble impurities will lower the melting point. Therefore, if you did theprocedure correctly, you are more likely to be under the actual melting pointthan over it.

Mixed Melting Point Determination

In the third section of the experiment mixed-melting points will beobtained and positive identification will be made.

Compare the melting point range of your unknown to the possible unknowncompounds. Select the two compounds closest in temperature to yourunknown. (When selecting, remember that it is easier to obtain an artificially


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low experimental melting point range rather than an artificially high meltingpoint range.)Obtain a small sample of each of the two closest compounds.

On two separate pieces of weigh paper or watch glasses, mix an equalamount of your unknown with each of the two other compounds. Grind each

mixture together so that they are thoroughly mixed. These can be mixed using

a spatula against a watch glass or by rolling the materials together in a piece ofweigh paper. Load a separate capillary with each mixture.

Find a student in the class who you believe has the same unknown as you. Write their name and their unknown number in your notebook. Obtain asmall amount of their unknown on a piece of weigh paper. Add a similar sizeamount of your unknown to the same piece of weigh paper. Mix well. Fill acapillary.

Place all three capillaries into the Mel-temp and simultaneously observe

and record the melting points of each. Be sure to record the location of each

mixture in the Mel-temp in your notebook.If the melting point range of the mixture differs substantially from that

of the pure compounds or if the range is too wide, then the two compounds aredifferent materials. If the melting point behavior of the mixture is the same or

very similar to that of the pure compound, you have confirmed the identity of your unknown.

Record all melting point ranges directly into your lab notebook.

Clean-up

Used capillaries and broken ones should be put into the broken glass

container. Excess chemicals from this experiment should be placed in thenon-halogenated organic solid waste bottle in the hood. Do not put weightpaper or capillaries into chemical waste container. Wash all used glassware,replace in drawer.

POST EXPERIMENT ASSIGNMENT

Complete the datasheet and turn in before leaving class. Turn in thewhite notebook pages from your lab notebook. Staple multiple sheets together.

Tear off rough edges.Prepare for the melting point portion of the next quiz. Remember that

you may be asked questions on anything you should have learned fromassigned readings, pre-experiment lecture, or doing the experiment.

References

1 Mel-Temp image, Scott Smith College,http://www.scottsmithonline.com/interests/medicalschool/chemistry/104a/fi

http://www.scottsmithonline.com/interests/medicalschool/chemistry/104a/file_storage/CourseWebSite/www.vanderbilt.edu/AnS/Chemistry/general_chem/104atest/measurement_of_melting_point.htmhttp://www.scottsmithonline.com/interests/medicalschool/chemistry/104a/file_storage/CourseWebSite/www.vanderbilt.edu/AnS/Chemistry/general_chem/104atest/measurement_of_melting_point.htm


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le_storage/CourseWebSite/ww.vanderbilt.edu/AnS/Chemistry/general_chem/104atest/measurement_of_melting_point.htm (December 10, 2010)2CRC Handbook of Chemist ry and Physics, 65

t h ed, CRC press, 1984, p c-6 5-c-

575

Revised August 26 , 20 15 S.L. Weaver
http://www.scottsmithonline.com/interests/medicalschool/chemistry/104a/file_storage/CourseWebSite/www.vanderbilt.edu/AnS/Chemistry/general_chem/104atest/measurement_of_melting_point.htmhttp://www.scottsmithonline.com/interests/medicalschool/chemistry/104a/file_storage/CourseWebSite/www.vanderbilt.edu/AnS/Chemistry/general_chem/104atest/measurement_of_melting_point.htmhttp://www.scottsmithonline.com/interests/medicalschool/chemistry/104a/file_storage/CourseWebSite/www.vanderbilt.edu/AnS/Chemistry/general_chem/104atest/measurement_of_melting_point.htmhttp://www.scottsmithonline.com/interests/medicalschool/chemistry/104a/file_storage/CourseWebSite/www.vanderbilt.edu/AnS/Chemistry/general_chem/104atest/measurement_of_melting_point.htmhttp://www.scottsmithonline.com/interests/medicalschool/chemistry/104a/file_storage/CourseWebSite/www.vanderbilt.edu/AnS/Chemistry/general_chem/104atest/measurement_of_melting_point.htm

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07_Melting Points Manual