Using the Thiele Tube

With the Thiele tube (Fig. 39) you use hot oil to transfer heat evenly to your sample in a melting point capillary, just like the metal block of the Mel-Temp apparatus does. You heat the oil in the sidearm and it expands. The hot oil goes up the sidearm, warming your sample and thermometer as it touches them. Now, the oil is cooler and it falls to the bottom of the tube where it is heated again by a burner. This cycle goes on automatically as you do the melting point experiment in the Thiele tube. 

Don t get any water in the tube or when you heat the tube the water can boil and throw hot oil out at you. Let s start from the beginning. 

This is a bit tricky, so don t do it unless your instructor says so. Also, check with your instructor before you put fresh oil in the tube. 

Pour the old oil out into an appropriate container and let the tube drain. 

Use a hydrocarbon solvent (hexane, ligroin, petroleum ether—and no flames ) to dissolve the oil that s left. 

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The Mel-Temp apparatus (Fig. 33) substitutes for the Thiele tube or open beaker and hot oil methods (see Using the Thiele Tube ). Before you use the apparatus, there are a few things you should look for. 

Use an extension clamp to support the Thiele tube on a ring stand. Add mineral oil or silicone oil to the Thiele tube, and fill to a level above the top of the side arm. Use a thermometer clamp to support the thermometer with the attached melting point capillary tube in the oil. The bulb and capillary tube should be immersed in the oil keep the rubber ring and open end of the capillary tube out of the oil (Fig. 15.4). 

Heat the arm of the Thiele tube very slowly with a Bunsen burner flame. Use a small flame and gently move the burner along the arm of the Thiele tube. 

There are two principal types of melting-point apparatus available the Thiele tube and commercially available, electrically heated instruments. The Thiele tube, shown in Figure 9.3, is the simpler device and was once widely used. It is a glass tube designed to contain a heating oil (mineral oil or silicone oil) and a thermometer to which a capillary tube containing the sample is attached. For best results... 

Proper use of the Thiele tube is required to obtain reliable melting points. Secure the capillary tube to the thermometer at the position indicated in Figure 2.17b using either a rubber band or a small segment of rubber tubing. Be sure that the... 

Some kinds of melting-point apparatus, such as the Thiele tube, use mineral or silicone oils as the heat transfer medium.These oils may not be heated safely if they are contaminated with even a few drops of water. Heating these oils above 100 °C may produce splattering of hot oil as the water turns to steam. Fire can also result if spattered oil comes in contact with open flames. Examine your Thiele tube for evidence of water droplets in the oil. If there are any, either change the oil or exchange tubes. Give the contaminated tube to your instructor. 

If you use a Thiele tube, handle it carefully when you are finished, because the tube cools slowly.To avoid burns, take care when removing it from its support. 

The melting point may be determined using either a commercial melting point apparatus or a Thiele tube. 

Notice that if we use Eq. (6.4.11) for A and Eq. (6.4.10) for pi Sg we recover the same Thiele modulus Eq. (6.4.9) for the straight tube. The geometry of the situation for a slab is identical with that for a single pore, and both give the same expression for the effectiveness factor. The assumption that the edges are sealed forces the problem into a one-dimensional form by making it possible to take the concentration constant over any plane parallel to the face. The graph of j versus h is shown as the left-hand curve in Fig. 6.7. 

Because the melting point of one of the compounds is near 200 °C, do not use a liquid-filled apparatus such as a Thiele tube unless the heating medium is silicone oil. A metal block melting-point apparatus is satisfactory for determining the melting points of the semicarbazones in this experiment. 

Nitrostyrene has been obtained in small yields by a number of methods, but the only practical methods of preparation start with benzaldehyde and nitromethane. The condensation was first accomplished by heating nitromethane and benzaldehyde in sealed tubes with anhydrous zinc chloride.1 Good results are obtained by adding small amounts of a primary aliphatic amine to a mixture of nitromethane and benzaldehyde,2 but it takes a number of days for the reaction to go to completion. Undoubtedly the best method is the use of alkali to condense benzaldehyde and nitromethane, as first discovered by Thiele.3... 

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