290. Melting Point.—The temperature at which fats become sensibly liquid is a physical characteristic of some importance. Unfortunately, the line of demarcation between the solid and liquid states of this class of bodies is not very clear. Few of them pass per saltum from one state to the other. In most cases there is a gradual transition, which, between its initial and final points, may show a difference of several degrees in temperature. It has been noted, further, that fats recently melted behave differently from those which have been solid for several hours. For this reason it is advisable, in preparing glycerids for the determination of their melting point, to fuse them the day before the examination is to be made. The temperature at which a glycerid passes from a liquid to a solid state is usually higher than that at which it resumes its solid form. If, however, the change of temperature could be made with extreme slowness, exposing the sample for many hours at near its critical temperature, these differences would be much less marked.
Many methods have been devised for determining the melting point of fats, and none has been found that is satisfactory in every respect. In some cases the moment at which fluidity occurs is assumed to be that one when the small sample loses its opalescence and becomes clear. In other cases the moment of fluidity is determined by the change of shape of the sample or by observing the common phenomena presented by a liquid body. In still other cases, the point at which the sample becomes fluid is determined by the automatic completion of an electric circuit, which is indicated by the ringing of a bell. This latter process has been found very misleading in our experience. Only a few of the proposed methods seem to demand attention here, and some of those, depending on the visible liquefaction of a small quantity of the fat or based on the physical property, possessed by all liquids when removed from external stress, of assuming a spheroidal state will be described. Other methods which may demand attention in any particular case may be found in the works cited.[239]
291. Determination in a Capillary Tube.—A capillary tube is dipped into the melted fat and when filled one end of the tube is sealed in the lamp and it is then put aside in a cool place for twenty-four hours. At the end of this time the tube is tied to the bulb of a delicate thermometer the length used or filled with fat being of the same length as the thermometer bulb. The thermometer and attached fat are placed in water, oil, or other transparent media, and gently warmed until the capillary column of fat becomes transparent. At this moment the thermometric reading is made and entered as the melting point of the fat. In comparative determinations the same length of time should be observed in heating, otherwise discordant results will be obtained. As in all other methods, the resulting members are comparative and not absolute points of fusion, and the data secured by two observers on the same sample may not agree, if different methods of preparing the fat and different rates of fusion have been employed.
Fig. 83.—
Melting
Point
Tubes.
Several modifications of the method just described are practiced, and perhaps with advantage in some cases. In one of these a small particle of the fat is solidified in a bulb blown on a small tube, as indicated in [Fig. 83], tube a. The tube, in an upright position, is heated in a convenient bath until the particle of fat just begins to run assuming soon the position shown in tube b. This temperature is determined by a thermometer, whose bulb is kept in contact with the part of the observation tube containing the fat particle. The rise of temperature is continued until the fat collected at the bottom of the bulb is entirely transparent. This is called the point of complete fusion.[240]
Pohl covers the bulb of a thermometer with a thin film of fat, and the instrument is then fixed in a test tube, in such a way as not to touch the bottom, and the film of fat warmed by the air-bath until it fuses and collects in a droplet at the end of the thermometer bulb.[241]
Carr has modified this process by inserting the thermometer in a round flask in such a way that the bulb of the thermometer is as nearly as possible in the center. By this device the heating through the intervening air is more regular and more readily controlled.[242]
A particle of fat placed on the surface of clean mercury will melt when the mercury is raised to the proper temperature. Where larger quantities of the fat are employed, a small shot or pellet of mercury may be placed upon the surface and the whole warmed until the metal sinks. Of the above noted methods, the analyst will find some form of capillary tube or the use of a film of the fat on the bulb of a thermometer the most satisfactory.[243]
Hehner and Angell have modified the sinking point method by increasing the size of the sinker without a corresponding increase in weight. This is accomplished by blowing a small pear-shaped float, nearly one centimeter in diameter and about two long. The stem of the pear is drawn out and broken off, and while the bulb is still warm, the open end of the stem is held in mercury, and a small quantity of this substance, sufficient in amount to cause the float to sink slowly through a melted fat, is introduced into the bulb of the apparatus and the stem sealed. The whole bulb should displace about one cubic centimeter of liquid and weigh, after filling with mercury, about three and four-tenths grams. In conducting the experiment about thirty grams of the dry melted fat are placed in a large test tube and cooled by immersing the tube in water at a temperature of 15°. The tube containing the solidified fat is placed in a bath of cold water and the sinker is placed in the center of the surface of the fat. The bath is slowly heated until the float disappears. The temperature of the bath is read just as the bulb part of the float disappears. The method is recommended especially by the authors for butter fat investigations.[244]