Melting Point Tube

Fig. 32). Using a fine pipette insert about i cm. length of the liquid into the bottom of the tube. Now place in the tube A a fine inverted melting-point tube B of about i mm. diameter, sealed at the upper end. Fasten the capillary tube to the ther- Fio. 32. mometer by means of a rubber band and place in a melting-point apparatus. Heat slowly until a stream of bubbles rises from the bottom... 

A metal cylinder A contains two small holes. Into one of these the thermometer is placed and into the other (arranged centrally) is placed the melting-point tube. The cylinder is pierced by a hori-2ontaI tunnel so the melting-point tube can be observed during heating. A lens is... 

The heating of the vessels is accomplished by means of a small bath or a micro-Bunsen burner. The vessel can be clamped at such a distance from the burner that the contained liquid boils gently under reflux. Smooth boiling is ensured by the addition of 1-2 minute pieces of unglazed porcelain, or of a short piece of melting-point tubing open at both ends. 

Pyrex glass is preferable, but this requires an oxy-coal gas blowpipe for manipulation. Suitable melting point tubes may be purchased from dealers in scientific apparatus or chemicals. It is, however, excellent practice, and an essential part of his training, for the student to learn to prepare bis own capillary tubes. 

The filled melting point tube is now attached to the lower end of a thermometer in such a way that the substance is at the level of the middle... 

The decomposition temperature is extremely variable and depends upon the rate of heating. The temperatures reported here were obtained by immersing the melting-point tube in a bath preheated to 200°, and then heating rapidly. 

A melting point test has been described for diazo compounds. The first 1 mm of a melting-point tube filled with c. 10 mg of test compound is inserted in a melting-point apparatus heated at 270°C. Once decomposition starts, the tube is removed. The decomposition rapidly propagates through the entire mass for unstable diazo compounds no such propagation is reported for stable versions. 

One milliliter each of the borneol solution and the oxidizing solution are mixed in a test tube and briefly shaken. A TLC slide is spotted with the borneol solution, the camphor solution, and the ether layer of the reaction mixture. Spotting is done by means of a capillary melting point tube used as a dropper and filled with a 5 mm sample. The slide is developed in a wide-mouth jar containing a filter paper liner and a few milliliters of chloroform (Fig. A3.20). After development (the solvent front rises to within 1 cm of the top), the slide is removed, the solvent is allowed to evaporate, and the slide is placed in a covered wide-mouth jar containing a few crystals of iodine. The spots readily become visible and the progress of the reaction can easily be followed. With periodic shaking, the oxidation is complete in about 30 minutes. 

Schmelzpunkt, m. melting point, fusing point, -bestimmung,/. melting-point determination, -bestimmungsrohr, n. (or-rdhre,/.), melting-point tube-... 

The melting point tube is placed in the hath at 85° and heated rapidly. 

Insensitive to friction or impact, but explodes violently on heating in a melting point tube or on analytical combustion, destroying the apparatus in both cases. 

You usually take melting points in thin, closed end tubes called capillary tubes. They are also called melting point tubes or even melting point capillaries. The terms are interchangeable, and I ll use all three. 

Sometimes you may get a supply of tubes that are open on both ends You don t just use these as is. Light a burner, and close off one end, before you start. Otherwise your sample will fall out of the tube (see Closing Off Melting Point Tubes, following). 

If you have melting point tubes that are open at both ends and you try to take a melting point with one, it should come as no surprise when your compound falls out of the tube. You ll have to close off one end, to keep your sample from falling out (Fig. 32). So light a burner and get a stiff small blue flame. SLOWLY touch the end of the tube to the side of the flame, and hold it there. You should get a yellow sodium flame, and the tube will close up. There is no need to rotate the tube. And remember, touch— just touch—the edge of the flame, and hold the tube there. Don t feel you have to push the tube way into the flame. 

Here you use a loaded melting point capillary tube (see Loading the Melting Point Tube ) and attach it directly to the thermometer. The thermometer, unfortunately, has bulges there are some problems, and you may snap the tube while attaching it to the thermometer. 

After this sample has melted, lift the thermometer and attached sample tube carefully (it may be HOT) by the thermometer up at the clamp, until they are just out of the oil. This way the thermometer and sample can cool, and the hot oil can drain off. Wait for the thermometer to cool to about room temperature before you remove it entirely from the tube. Wipe off some of the oil, reload a melting point tube (never remelt melted samples), and try again. And heat at 2°C per minute this time. 

Fig. 90 Making capillary spotters from melting point tubes.

The heating liquid is pure concentrated sulphuric acid, with which the bulb of the flask is three-quarters filled. The substance, in powder form, is introduced into a small, thin-walled capillary tube. Such tubes are made as follows from test tubes (preferably from damaged tubes which must, however, be clean and dry ). The tubes are rotated in the flame of the blow-pipe till soft and then drawn out rapidly already after short practice the student can strike the correct diameter, which should be 1 0-1 -5 mm. internally. Suitable portions of the drawn-out material are cut off with scissors. It is convenient to cut double lengths (about 12 cm.), so that by sealing each length in the middle (micro-burner) two melting-point tubes are obtained ready for use. 

Make the melting-point tubes as described on p. 40 from a clean test tube the internal diameter should be about 4-5 mm. Cut off pieces about 5 cm. long. Close the lower ends by sealing as uniformly as possible, also keeping the walls as thin as possible. By drawing the softened glass away sideways avoid reducing the bore more than a little. 

Melting-point tubes, thin (to be made by the student). 

The melting point is measured in an evacuated melting point tube. 

Thin-walled melting point tube (diameter 0.7 mm or less) (optional)... 

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