TLDR;
This video provides a comprehensive guide to determining the melting point of a substance, an important criterion for identifying and assessing the purity of a compound. It covers various apparatus used for melting point measurement, from simple setups to more complex hot stage microscopes. The video also demonstrates the preparation of samples, the process of making both rough and accurate melting point measurements, and the technique of mixed melting point determination for compound identification.
- Different types of melting point apparatus are shown, ranging from simple to complex.
- The process of preparing a sample for melting point determination is demonstrated.
- Both rough and accurate melting point measurement techniques are explained.
- The mixed melting point determination method for compound identification is illustrated.
Introduction to Melting Point Measurement [0:06]
The melting point of a substance is a key indicator of its identity and purity. The equipment used to measure melting points varies in complexity. Simple setups involve suspending a melting point tube containing the sample in liquid paraffin inside a boiling tube, heated by a micro burner. More complex apparatus includes electrically heated aluminium blocks and hot stage microscopes, where the sample is heated on a stage and viewed through a microscope as it melts.
Common Melting Point Apparatus [1:19]
The most commonly used apparatus features an electrically heated aluminium block with a thermometer and space for up to three melting point tubes. Heating is controlled electrically via a rheostat, often activated by a knob that also illuminates a digital display of the temperature. Rapid heating can be achieved with a rapid heat switch, and the display can be held at its current value using a hold display switch. Some models also include a boost heater for rapid heating, controlled by a switch.
Sample Preparation for Melting Point Determination [2:39]
To prepare a sample, it needs to be finely ground using a spatula on a clock glass or, more effectively, in a pestle and mortar. The sample tube must be sealed at one end using a Bunsen flame. The tube is held in the edge of the flame and rotated gently until a blob of glass seals the end. It's important not to overheat the tube. Once cooled, the sample is inserted by pressing the open end of the tube into the ground sample and tapping the tube firmly on the bench to move the sample to the bottom. Alternatively, the serrated edge of tweezers or the milled edge of a coin can be used.
Rough Melting Point Measurement [5:49]
A rough melting point measurement is made using the boost heater. The rheostat heater and light are switched on, followed by the boost heater. This provides a quick, initial estimate of the melting point range.
Accurate Melting Point Measurement [6:37]
For an accurate measurement, the heater and lamp are switched on, and the boost heater is used until the temperature is about 15°C below the rough melting point. The boost heater is then switched off, and the rheostat is adjusted to achieve a temperature rise of approximately 2 to 3°C per minute. The temperatures at which the sample begins to soften, the bulk of the sample melts, and melting is complete should be recorded. For a pure compound, this range should be only 0.5 to 1°C.
Mixed Melting Point Determination [8:04]
Mixed melting point determination is used for compound identification. If two compounds are identical, they will melt separately and as a mixture at the same temperature. Dissimilar compounds may have the same melting point, but a mixture will melt at a lower temperature. Three melting point tubes are needed: two with separate samples of the two components and one with a finely ground mixture of the two. The mixture is prepared by grinding the two components together in a mortar. The tubes are then inserted into the melting point apparatus, and the melting points are observed.
