DETERMINATION OF MELTING POINTS

DETERMINATION OF MELTING POINTS

Theory and Experiments 

This is a very common and important operation for it affords a means of judging the purity of crystalline organic substances and of identifying them. As carried out by the organic chemist the determination is very simple and rapid, and requires but a minute quantity of the sample. It is inferior only in accuracy to the method used in physical chemistry for determining the freezing point of a liquid (the temperature at which the solid and liquid phases are in equilibrium). The method will be best understood by considering the apparatus which is to be used (Fig. 5). A very small amount of the sample is packed into the sealed end of a capillary tube which is then attached to a thermometer

centered in a heating bath. The flask is heated slowly and regularly and one can observe easily

the temperature at which the sample melts. In the type of vessel shown the convection currents rise

around the outer walls of the bulb and then descend and converge at the center. The mechanical

stirring required in other types of apparatus is unnecessary.

It may be noted that this melting point bath is only one of a great host of contrivances designed for the same purpose. There are other types relying on the convection principle for the steady uniform heating of the thermometer bulb and the small tube containing the sample. There are baths which can

be stirred by hand, baths with a rotary stirrer propelled by an air blast, by suction, or by electricity, and baths with an up-and-down stirrer actuated by a discarded windshield wiper. The bulb flask shown in the drawing has many advantages other than its simplicity and low cost, and it is recommended both for beginners and for research workers. When the thermometer bulb is properly centered and when the heating is done carefully entirely reliable results may be obtained. A more elaborate form is shown on page 326, and the beginning student will do well to adopt one feature of this set-up, namely, the test tube suspended on a wire to receive used capillaries. Another piece of equipment required in this and in all other work involving the manipulation of solid organic substances is a good spatula. Porcelain and horn spatulas are almost useless in the organic laboratory. For the experiments described in this manual a small spatula made of sterling silver or of nickel and

having the design shown in Fig. 6 will be found most useful. Whether silver alloy or nickel is used, the spatula should never be heated in a flame for this will ruin it. It is bad practice to use a spatula as a stirring rod. The metal can be kept spotlessly clean by polishing it with steel wool, and when the spatula is properly cared for it will last almost indefinitely.

Experiment: The long-necked, round-bottomed melting point flask is supported by means of a clamp at the very top of the neck on an asbestos-centered wire gauze at a height convenient for heating. A calibrated thermometer is fitted through a cork in the manner shown in the drawing, a section of the cork being cut away so that the thermometer scale may be visible. The bulb of the thermometer should be adjusted carefully to the exact center of the outer flask. If possible choose a thermometer

with a very short bulb. The bulb of the flask is three-quarters filled with concentrated sulfuric acid. This serves as a convenient heating liquid for temperatures up to 280°.' If the acid darkens after long use or from contamination, add a small crystal of sodium nitrate or a drop of concentrated nitric acid; the bath will clear after the next good heating. The capillary melting point tubes are best made by drawing out 12 mm. tubing, and the student should strive by practice to acquire some skill in the at first difficult feat of producing 10-15 good tubes in a single drawing. The large tube is rotated in the hottest part of the Bunsen flame until it is very soft and begins to sag. It should not be drawn at all during the heating. Remove it from the flame, hesitate for a second or two, and then draw it out steadily but not too rapidly. To cut the long tube into sections bring a sharp file which rests on your forefinger underneath the tube, rest your thumb over the place to be cut, and gently draw in the file so as to produce a slight scratch. On applying gentle pressure with the thumb the tube will now break clean.2 The tube is sealed by touching the end to the very edge of a small flame. The proper size of the tube is indicated in the sketch, Fig. 5. A number of these tubes will be required in the course of the year. The capillary tube is filled by crushing a very small pile of crystals on a filter paper or a piece of clay plate with the silver spatula and pushing the powder with the open end of the tube against the spatula. The sample may be shaken down to the bottom by gentle stroking with a file, or it may be tamped down by dropping it through a long vertical tube onto a hard surface. The column of solid should be not more than 2-3 mm. in length and it should be tightly packed. If the sample is in the form of a compact cake it is often convenient to fill the capillary by cutting out a tiny cylinder with the open end of the tube and then shaking it down.

The melting point tube is held to the thermometer simply by capillary attraction. To attach it the thermometer is partially withdrawn and held in an oblique position with the bulb resting on the back lip of the flask. This steadies it and prevents spilling of acid; never withdraw it completely. The capillary is touched to the drop on the thermometer and this liquid is painted onto the side of the thermometer. The capillary will now stick nicely to this wetted spot and can be slid into the proper position, that is, with the substance just opposite the enter of the thermometer bulb. The best way of inserting the thermometer without knocking off the capillary is to keep it always parallel to and touching the back wall of the  flask. Make sure that the bulb is submerged and properly centered and that the column of substance is in the proper position.

In heating the bath it is admissible to apply a rather strong flame until the temperature rises to about 200 of the melting point. If the approximate melting temperature is not known, make first a rapid, approximate determination and then a more slow and careful one. In order to obtain good results it is

essential that the heating during the last 15-200 rise be both slow and regular (2° per minute). Observe carefully both the sample and the temperature and record the temperatures of initial and of final melting. The flask (Pyrex!) may be detached and cooled for a second determination as follows: with an air blast, if the temperature is above ioo°; under the tap, if it is no hotter than 100°.

The melting point apparatus will be used throughout the year. After completing the following experiments, remove the thermometer, save the cork, stopper the flask containing the acid with a cork (loosely, until it is cold), and put it away in an upright position (best in a beaker). Be very careful to see that the acid cannot spill or leak, for it will destroy any wood with which it comes in contact. To discard the acid.after each period of use would be wasteful.

from : Fieser 1941