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Melting Points.
The temperature at which a solid melts is known as the melting point (MP) of that substance. The melting point is a physical property of a solid and can be used to help identify a substance. In practice, a solid usually melts over a range of temperatures rather than at one specific temperature. For this reason it is more useful to speak of a melting point range. Although the term "melting point" is usually used, what is meant is "melting point range". If the compound melts over a very narrow range, it can usually be assumed that the compound is relatively pure. Conversely, compounds that melt over a wide range are assumed to be relatively impure. Besides melting over a wide range, impure solids also melt at a temperature lower than that for the pure compound. For our purposes a range greater than 2° is considered to be wide. For example, if an unknown solid melts at 102-106° C, the 4° range suggests that the sample is impure. If the unknown is one of four possible compounds which melt at 102, 104, 106, and 108° C, it is most likely that which melts at 108° C. To summarize, an impure solid melts over a wide range and at a temperature lower than that of the pure solid. It should be noted that “insoluble” impurities such as bits of filter paper or dust have no effect on the MP of a substance. To affect the MP the impurity must be soluble in the solid. Several devices are available for measuring melting points. A Mel-Temp device is used in this lab (note that there are a few different models in the lab but that all work the same way). Below is a photo of a Mel-Temp device along with a close-up.
Sample is added to a small glass capillary tube, which is closed at one end. It is important to use as small amount of sample as possible so that sufficient heat is present to melt the sample rapidly. The temperature of the sample is measured with a digital thermometer. The sample is heated slowly as the temperature approaches the MP, while the sample is carefully observed. The temperature at which the first drop of liquid is observed is recorded as the beginning of the melting point range. The temperature at which all solid has melted is recorded as the end of the melting point range. Upon heating, the solid may expand and move slightly in the tube. This movement should not be interpreted as the beginning of the MP range. The melting point (MP) is recorded in the lab notebook as, for example, MP (compound A) 102.5-104.0° C. To determine the MP of an unknown solid, to save time, an approximate MP is first determined by heating the sample fairly rapidly. Once the approximate MP is known, a more careful determination is made on a fresh sample. Note that once a sample has been melted, it may have decomposed slightly. Contamination with decomposition product will change the MP of the sample, so a fresh sample must always be used for each determination. The effect of impurities on the MP can actually be used to help identify a compound. For example if an unknown solid is known to be one of two possible known compounds, both having the same MP, the unknown can be mixed with one of the known compounds and a MP taken of the mixture. If the MP range is lowered and widened, it means that the two are different compounds. If the MP stays the same it means that the two compounds are likely identical. This technique is known as a mixed melting point determination. To summarize, melting points can provide information about the identity and the purity of a solid sample.
(1.) Knowns. Determine the MPs of naphthalene, urea, and sulfanilamide. Note that the MPs of samples used in the lab may differ slightly from those given in a handbook. Therefore, use the MP values given below. Samples will be dispensed in labeled crystal dishes on the side bench. Do not move the dishes. Fill the capillaries right there. Determine the MPs of these known compounds in order of increasing MP. This will obviate the need to allow the apparatus to cool between determinations. The values that you find should agree well with those listed and the range should be narrow. If you find a wide range (> about 2°) or a value different from that expected (within ± 2° - remember that the thermometers used here are good only to about ± 2°C), do a second determination on a fresh sample. It is important to use a small sample and to raise the temperature very slowly (about 1°C/minute) when you are near the MP of the sample. If a MP must be redetermined always use a fresh sample - once a sample has melted, you must assume that it may have decomposed, and therefore become contaminated with impurities, causing the MP to become depressed and the range widened. Never dispose of used capillaries or any glass in the regular trash. The person who empties the trash could be injured by broken glass. Place the used capillaries in the dishes provided. ALWAYS turn the device and digital thermometer off when finished. H 2 N O NH 2 H 2 N SO 2 NH 2 Naphthalene MP 79 - 80 ° Urea MP 132 - 134 ° Sulfanilamide MP 165 - 167 ° (2.) Determine the MP of an unknown compound. Your TA will assign an unknown to you. To save time, when you determine the MP of an unknown sample, first find an approximate MP by raising the temperature much more rapidly than you normally would to get the approximate MP, and then redo it using a fresh sample, raising the temperature at about 1°C/minute when you get near the MP. Using the MP, identify the compound. (List of possible compounds posted at end of this handout.) BEFORE LEAVING THE LAB: TURN THE VOLTAGE ON THE MEL-TEMP TO ZERO, TURN THE SWITCH TO OFF, AND TURN THE DIGITAL THERMOMETER OFF. Clean up your work area, lock up and ask your TA for her or his signature. WASTE DISPOSAL : Place used MP capillary tubes in the evaporating dishes on the side benches. Never place any glass into the trash. The custodian could become injured with broken glass. SAFETY : The heating device can become very HOT. Burned tissue caused by hot surfaces or flames should be immediately placed under cold tap water and as soon as possible into ice/water. This will minimize the pain and tissue damage. Keep all lab chemicals off of your skin.
Postlab Questions 1.) A solid sample has a MP of 13 3 - 13 7°C. What can one conclude about the sample? 2.) For problem 1.), if the sample is one of four possible compounds the melting points of which are 133°, 135°, 137°, and 139°C, which is it most likely to be? Why? 3.) Two test tubes contain compounds having the same MP. Using MPs, how could you determine whether the two test tubes contain the same or different compounds? 4.) In a recrystallization (a technique that you will encounter later in the semester), a solid is dissolved in a solvent and later the solvent is removed. If a MP of the sample is taken while the sample is still moist with solvent, what effect would that have on the MP of the sample? 5.) What two pieces of information can a MP determination provide? POSSIBLE UNKNOWN COMPOUNDS COMPOUND MP (oC) 4 - Methylphenol 35 Benzophenone 48 - 50 Maleic anhydride 54 - 56 4 - Bromophenol 64 - 66 4 - Aminobenzaldehyde 71 Naphthalene 79 - 80 3,4-Diaminotoluene 89 - 90 Acenaphthene 94 - 96 Isobutyranilide 106 - 107 Acetanilide 113 - 115 Benzoic acid 122 - 123 Urea 132 - 134 d,l-Glyceraldehyde 145 Adipic acid 152 - 154 Sulfanilamide 165 - 167 2 - Aminophenol 174 4 - Toluic acid 180 - 182 Succinic acid 187 - 189 The melting points listed here vary slightly from those found in reference texts. These are closer to what you will observe because our compounds are not ultra-pure.
