Measuring a Melting Point

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A Mel-Temp™ is commonly used to measure the melting points of compounds. A sample is loaded into a capillary tube and placed into the Mel-Temp™. While the tube is being viewed through an eyepiece, the Mel-Temp™ gradually heats the sample. By carefully observing the temperature range at which the sample turns from a solid into a liquid, the melting point is determined.

First, the sample should be crushed to a fine powder.

Video: Crushing the sample (3.20M ) Text description

Next, the sample is loaded into a capillary tube.

Video: Filling a capillary tube (1.98M ) Text description

Tap the capillary tube on a hard surface until the sample packs into the bottom.

If the solid will not go to the bottom of the capillary tube when the tube is tapped on a hard surface, drop the tube through a long, narrow cylinder.

Video: Dropping down cylinder ( 1.71M )

About 1 to 2 mm of sample should be added to the capillary tube.

Use a new capillary tube for every sample.

Why is this sample not appropriate for use in a melting point experiment? Answer

Place a thermometer with a relatively high temperature range (greater than 200 degrees) into a relatively cool Mel- Temp™. Also place the capillary tube into the machine. Why must the Mel-Temp™ be cool before the experiment?

The Mel-Temp™ can hold up to three samples at once.

Video: More than one sample ( 1.69M )

Turn the heating stage on to a medium setting. This setting should result in a heating rate of about 15 degrees per minute.

Video: What does 15 degrees per minute look like? ( 2.75M )

Carefully observe the sample through the eyepiece. About ten degrees before the temperature approaches the expected melting point, reduce the rate of heating to one degree per minute. Why? Be sure to decrease the rate of heating well before melting actually takes place. Why?

If you are unsure what the melting point of your sample should be, perform an initial melting point experiment in which you melt the sample rapidly to determine the approximate melting point. Then perform another experiment using the technique described above to measure the melting point of the sample more accurately.

Video: What does one degree per minute look like? ( 2.92M )

Instead of recording one temperature as the melting point, measure the melting point range. To measure the range, record the temperature at which the solid first begins to turn to liquid and the temperature at which the solid has melted completely. The melting range may span several degrees.

Video: Recording a melting range ( 8.34 M )

Video ( 8.55M ) What melting range should you record for the sample shown in the video? Answer

Discard used capillary tubes into a broken glass container unless directed otherwise.

The melting range, if measured properly, can give you useful information about a sample.

You carefully measure the melting range of a sample of salicylic acid that you prepared in the lab to be 140.7 to 149.3 °C. The known value of the melting point is 159 °C. What does the melting range you measured tell you about your sample? Answer

What is wrong with this student's work? Answer

The Thomas Hoover Capillary Melting Point Apparatus™ is used in a manner similar to the Mel-Temp™. However, while the Mel-Temp™ uses a heating stage to melt a sample, the Hoover uses an oil bath.

To measure a melting point using a Hoover apparatus, prepare the sample in a capillary tube as described previously. If the sample has not been packed well into the capillary tube, place the tube into the vibrator compartment near the back of the machine and turn the vibrator on briefly. The button must be held continuously for the vibrator to work.

Place the sample into one of the sample compartments. Up to five samples may be placed into the apparatus at once.

Begin stirring the oil bath. The faster the rate of heating, the faster the oil bath should be stirred.

Heat the sample and record the melting point in a similar manner as if using a Mel-Temp™. Remember to heat the sample relatively rapidly at first, but then to slow the rate of heating well before the expected melting point is reached. Also remember to record the melting range, not just a single temperature.

When finished with the Thomas Hoover Melting Point Apparatus™, turn off the stirrer and the power, and discard the capillary tube in a broken glass container.

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