Identity and Purity of Solid Organic Compounds
• To introduce the technique of melting point determination. • To use the concept of melting points for identification and characterization of organic compounds. • Properly fill and use a capillary melting point tube. • Determine accurate melting point ranges for a wide variety of organic substances.
The melting point of a solid can easily and accurately be determined using only a small amount of material. In combination with other measurements, melting point information can provide rapid confirmation of the identity of unknown substances. The method of capillary melting point determination involves placing a small amount of sample in the bottom of a narrow capillary tube that has been closed at one end. The melting point is then determined using a melting point apparatus (Figure 1) that simultaneously heats both the sample tube and a thermometer. The temperature range over which the substance melts is recorded.
Melting is said to begin when the first indication of liquid is seen. The end of the melting point range is the temperature at which all of the solid material has become a liquid. Some pure materials possess a very narrow melting range, perhaps as little as 0.5-1.0 °C, while more typically a 2-3 °C range will be observed. You will usually see data recorded as, for example, mp 232-234 °C. Though formally denoting the melting range, this piece of data is almost universally referred to as the melting point (mp).
For the most accurate results, the rate of heating should be kept relatively low, especially for low-melting samples, to ensure that the thermometer reading represents as accurately as possible the true temperature experienced by the sample tube (since the transfer of heat within the apparatus is relatively slow). With this fact in mind, it is sensible when recording a melting point of an unknown material to perform a trial run where the temperature range is increased relatively rapidly in order to ascertain a rough melting range. The determination is then repeated by heating rapidly to within around twenty degrees of the expected melting point and then very carefully increasing the temperature the remaining few degrees until the melting point is reached.
Figure 1: Apparatus for manual melting point determination.
"Mel-temp" device (left) and hot oil method (right).
Although a pure solid might be expected to have a single, sharp melting point, most samples are observed to melt over a narrow range of a few degrees Celsius. The observation of a melting range may be a result of inhomogeneities in the macroscopic nature of the solid sample, or may indicate the presence of other substances in the sample (contaminants or by-products of the method used to prepare the materials.
Melting Points as Criteria of Purity. Thermodynamics tells us that the freezing point of a pure material falls as the amount of an impurity is increased. The presence of an impurity in a sample will both lower the observed melting point and cause melting to occur over a broader range of temperature. Generally, a melting temperature range of 0.5-1.0 °C is indicative of a relatively high level of purity. It follows that for a material whose identity is known, an estimate of the degree of purity can be made by comparing melting characteristics with those of a pure sample.
Melting Points as a Means of Identification and Characterization. For pure samples a clear difference in melting points between two materials reveals that they must possess different arrangements of atoms, or configurations. If two materials are found to have the same melting point then they may, but not necessarily, have the same structure. Clearly, the recording of a melting point is a desirable check of purity and identity but must be combined with measurements from other...