(b) a-DEXTROSE (ANHYDROUS)

Class 6: Rhombic bisphenoidal.

Ratio of axes: a: b: c=0.704:1:0.355 [15].

Plane of optic axis: a {100}.

Refractive indices: a=1.530, 8=1.550, y=1.560.

TABLE 72.—Angular values between faces of anhydrous dextrose

[1] W. G. Hankel, Ann. Physik 49, 495 (1840).

[2] W. H. Miller, Trans. Cambridge Phil. Soc. 7, 209-216 (1842).

[3] E. Wolff, J. prakt. Chem. 28, 129 (1843).

[4] C. F. Rammelsberg, Handb. Kryst. Chem. (1855) and later editions. [5] F. Rinne, Rischbiets Dissertation (Göttingen, 1885), p. 514. Z. ver. deut. Zucker-Ind. 38, 972 (1888).

[6] F. P. Phelps, Proceedings of the Fourth Congress of the International Society of Sugar Cane Technologists Bul. 104, San Juan, P. R. (1932). [7] G. Vavrinecz, Z. Zuckerind. cechoslovak. Rep. 51, 39 (1926); Z. Magyar Chemiai Folyoirat 31, 29 (1925).

[8] F. Becke, Tschermak's mineralog. petrog. Mitt., p. 261 (1877).

[9] H. E. Merwin, Int. Crit. Tables 7, 30 (1930).

[10] L. Longchambon, Bul. soc. Française minealogie 45, 242 (1922).

[11] K. Stammer, Z. Ver. deut. Zucker-Ind. 31, 794 (1881).

[12] W. Schaaf, Z. Ver. deut. Zucker-Ind. 33, 699 (1883).

[13] J. Bock, Z. Ver. deut. Zucker-Ind. 38, 965 (1888).

[14] L. Wulf, Z. Ver. deut. Zucker-Ind. 37, 917 (1887); 38, 226.

[15] F. Becke, Tschermak's mineralog. petrog. Mitt. 2, 184 (1880); 10, 464, 495 (1889). Absts. Z. Krist. 5, 283; 20, 298 (1888).

[16] M. J. Schuster, Tschermak's mineralog. petrog. Mitt. 9, 216 (1888).

XXXIII. MELTING POINTS

1. GENERAL

The melting point is a valuable aid in the identification of purified sugars and their derivatives and also in the determination of the purity of these compounds. At any given pressure the solid and liquid phases of a substance are at equilibrium at a definite temperature. The presence of a small trace of impurity generally alters the melting point. Some substances, however, exist in more than one crystal

form and thereby have their melting points affected by the method of heating, thus making the melting point a less definite indication of purity. A number of different methods have been devised for making the melting point determination. Various types of melting point tubes are shown in figure 127.

2. CAPILLARY-TUBE METHODS

The simplest apparatus, illustrated by No. 1, consists of a roundbottomed tube of Pyrex or other suitable heat-resistant glass approximately 100 mm long, 30 mm inside diameter, and with walls not more than 1.5 mm thick at any point. The stirring device consists of a glass rod bent to form a ring at the bottom. The sample under test is in a capillary tube about 60 mm long, 0.8 to 1.2 mm inside diameter, with walls from 0.2 to 0.3 mm thick, and is closed at one end. In making the determination, place the finely powdered substance in the capillary tube by pushing the open end into the powder, and pack it

down by moderate tapping on a solid surface. The sample should form a column about 3 mm in length when packed down. Attach the capillary to the thermometer by wetting them with the liquid of the bath or attach them by means of a piece of fine platinum wire. Adjust the position of the capillary so that the substance is centrally located by the side of the thermometer bulb. As a heating bath, fill the large tube with a suitable liquid such as sulfuric acid to a depth which will permit the top of the thermometer bulb to be immersed 20 to 30 mm below the surface of the liquid. Stir the bath constantly while heating slowly over a Bunsen flame or on an electric heater. Retard the rate of heating as the suspected melting point is approached, until finally the heating is so regulated that the rise in temperature is about 0.5 degree per minute. The temperature at which the substance liquifies is taken as the melting point.

In the method of the United States Pharmacopoeia [1] the temperature at which the column of substance in the capillary tube first begins to liquify at any point is defined as the beginning of melting, and the temperature at which the substance becomes liquid through

out is defined as the end of melting. The usual precuations regarding the heating and stirring of the bath are observed. The result thus obtained as the melting interval is adjusted for the calibration correction of the thermometer and the correction for the emergent stem. In this method the following liquids are suggested for use as heating baths: For temperatures up to 200° C, pure concentrated sulfuric acid; for temperatures up to about 350° C, a pure grade of cotton-seed oil (almost colorless). Other, though less desirable, substitutes for sulfuric acid at high temperatures are pure paraffin freshly distilled, and clean, white artificial (cotton-seed) lard. A very desirable bath for high temperature work is prepared by cautiously boiling together, for 5 or 10 minutes under a hood, a mixture of 70 parts of purified concentrated sulfuric acid and 30 parts of potassium sulfate, stirring constantly until the sulfate is completely dissolved.

In place of the simple tube described above, many workers prefer to use the Thiele melting-point tube (No. 2) or the Thiele-Dennis

J V

tube (No. 3). These tubes are so designed that heating causes a circulation of the liquid in the tube, making stirring unnecessary. further modification (figure 128) consists of a tube similar to the above with a side tube sealed in at an angle of about 30° to the axis of the tube, which permits the capillary containing the sample to be inserted or removed without removing the thermometer. The use of the side arm also has the advantage that the capillary containing the sample can be placed in contact with the thermometer bulb and the usual unsatisfactory methods of fastening the capillary to the thermometer are eliminated. The various modifications of the Thiele tube and a discussion of their advantages are given in reference [2].

3. ADDITIONAL METHODS

Another method of determining melting points employs the Maquenne Block. This consists of a block of brass mounted in a frame above a long gas burner. In the top surface of the block are a number

of small cavities. A hole is bored lengthwise of the block, just below its upper surface, to permit the insertion of a thermometer.

To make a melting-point determination, place a small amount of the sample in one of the cavities and cover with a small glass plate. Insert the thermometer so that the bulb is just under the cavity containing the sample. Heat the block slowly until the substance begins to melt, then adjust the thermometer so that the mercury column just projects beyond the end of the block and note the temperature. As the block has approximately a uniform temperature, the stem correction of the emergent mercury column is eliminated.

Dennis and Shelton [3] have devised an apparatus for the accurate and rapid determination of melting points. It consists of an electric heater controlled by a rheostat and mounted on the end of a square pure copper bar. The temperature of this bar can be varied from room temperature to 300° C by means of the rheostat. A voltmeter shows the potential across the heater. The heat applied at one end of the bar causes a temperature gradation along the length of the bar, with a temperature variation of from 10° to 30° C. The temperature of the bar at any point is determined by turning a knob which lowers a constantan element onto the copper bar. This contact forms a thermocouple, the potential of which is read on a potentiometer graduated directly in degrees centigrade. To determine the melting point of a substance, drop a few finely ground particles along the surface of the bar. If the bar has been heated to the proper temperature, a distinct line of demarcation will be noted between the melted and unmelted particles. Move the knob along its slider and lower the constantan element onto the bar exactly at this line. Read the temperature directly by means of the potentiometer. The apparatus is claimed to give values for melting points to an accuracy of about 0.25° C.

4. REFERENCES

[1] U. S. Pharmacopoeia XI, p. 455-457 (Mack Printing Co., Easton, Pa., 1936). [2] A. A. Morton, Laboratory Technique in Organic Chemistry, p. 27-30 (McGraw-Hill Publishing Co., Inc., New York, N. Y., 1938).

[3] L. M. Dennis and R. S. Shelton, J. Am. Chem. Soc. 52 3128 (1930).