Melting Point
Melting Point Determination
Identity and Purity of Solid Organic Compounds
Objectives
• 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.
Introduction
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
Identity and Purity of Solid Organic Compounds
Objectives
• 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.
Introduction
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