Determination of Melting Points

The melting point of a substance is the temperature at which the solid phase converts to the liquid phase under 1 atmosphere of pressure. The melting point is one of a number of physical properties of a substance that is useful for characterizing (describing) and identifying the substance. To measure the melting point of a substance, it is necessary somehow to gradually heat a small sample of the substance while monitoring its temperature with a thermometer. The temperature at which liquid is first seen is the lower end of the melting point range. The temperature at which the last solid disappears is the upper end of the melting point range. A pure substance normally has a melting point range no larger than 1-1.5 ^oC.

Although many substances melt cleanly and can be melted, crystallized, and remelted repeatedly without chemical decomposition, others chemically decompose before they melt, forming substances of lower molecular weight. The temperature of decomposition is just as useful as the melting point in physically characterizing a substance. Decomposition is usually signalled by a color change; for example, white substances invariably start to turn brown near the decomposition temperature. The temperature at which the color change is first observed signals that the substance is approaching the decomposition temperature. At somewhat higher temperature, liquid may form. At this temperature or at an even somewhat higher temperature, gas bubbles may be seen if gaseous decomposition products are formed. All of these temperatures aid in characterizing a substance, so all should be noted and reported.

General Melting Point Procedures

Preparing a melting point capillary. A glass capillary tube is normally used to contain the sample for a melting point determination. Therefore the tube must have one open end into which the sample can be loaded, and one sealed end so that the capillary will retain the solid sample. Glass capillary tubes for holding the solid sample are commercially available. Some companies provide capillaries with one end already sealed; others provide tubes with both ends open. If you are working with the second type, it is necessary that you seal one end before loading the sample. Light a Bunsen burner, and adjust for a medium-hot flame. Hold one end of the capillary in the edge of the flame, rotating it so that the end does not sag. You will see the glass melt and the end close. When the end has closed, withdraw the capillary from the flame and allow it to cool. Prepare a total of 5 capillaries to start.

Loading the melting point capillary. Place a VERY SMALL quantity of the solid of interest on a watch glass, and use a stirring rod to grind the solid to a powder. Use a spatula to gather the powder into a small pile. Stick the open end of a melting point capillary into the pile to a depth of about 1 mm, then invert the capillary and tap the sealed end on the bench to encourage the solid to drop to the bottom. The height of solid in the capillary should be no more than 1-2 mm. If you have more solid than this in the tube, you should try to shake some out, then reseat the solid at the bottom of the tube.

Determination of the Melting Point Using a Mel-Temp Apparatus. Load your capillary as above and place it into one of the 3 sample wells of the Mel-Temp. Turn the Mel-Temp power switch on and choose a voltage setting, using the following table as a guide. Because most of the amino acids decompose at temperature greater than 200 ^oC, you should choose a setting that allows you to heat rapidly to, say, 190. A voltage setting of 55 would be a reasonable choice. While the sample is heating, watch it through the magnifying window, while frequently checking the temperature reading of the thermometer. When the temperature nears 190, you will need to increase the voltage setting to 60. Observe the sample as the temperature rises. If the temperature reaches the max range for this voltage reading without affecting the sample, increase the setting to 70 and again observe the sample as the temperature rises. Continue with this procedure until you observe the first discoloration of the sample. Note the temperature at which this occurs. Continue observing the sample, noting temperatures at which changes occur. These might include the appearance of some liquid and/or gas bubbles in the capillary. When you have recorded all significant temperatures, turn off the Mel-Temp and remove your capillary. CAREFUL--DO NOT TOUCH THE END THAT WAS HEATED! Let the capillary cool, then discard it in the glass waste receptacle.

Practice with Melting Points. Many covalent organic compounds have relatively clean melting points at low temperatures. Before tackling your amino acid, you should practice the melting point techniques on a couple of compounds of this type. We will use benzophenone (C₁₃H₁₀O, mp = 48-49 ^oC) and benzil (C₁₄H₁₀O₂, mp = 94-95 ^oC) as practice substances. Load a capillary with 1-2 mm depths of each of these solids, and determine their melting points by the Meltemp method.

Melting Points of Amino Acids. Unfortunately, amino acids do not exhibit clean melting points. Instead, they decompose at characteristic temperatures into lower molar mass materials, one of which is probably CO₂. Further, these temperatures are fairly high, usually in the range 200-300 ^oC. You will therefore be looking for 2 or 3 characteristic temperatures for your amino acid: the temperature at which discoloration is first noticed; the temperature at which liquid is first noticed; the temperature at which gas evolution begins (probably CO₂). The characteristic temperatures for alanine, glycine, proline, and threonine are given in the table.

Load a melting point capillary with 1-2 mm of your amino acid, and use the Meltemp approach to determine the characteristic temperatures for your amino acid.