103. Variations in Specific Rotatory Power.—The gyrodynat of any optically active body varies with the nature of the solvent, the strength of the solution, and the temperature.[66]
Since water is the only solvent of importance in determining the gyrodynat of sugars it will not be necessary here to discuss the influence of the nature of the solvent. In respect of the strength of the solution it has been established that in the case of cane sugar the gyrodynat decreases while with dextrose it increases with the degree of concentration. The influence of temperature on the gyrodynat of common sugars is not of great importance save in the case of levulose, where it is the most important factor, the gyrodynat rapidly increasing as the temperature falls. It is of course understood that the above remarks do not apply to the increase or decrease in the volume of a solution at changed temperatures. This influence of temperature is universally proportional to the change of volume in all cases, and this volumetric change is completely eliminated when the polarizations are made at the temperatures at which the solutions are completed to standard volumes.
104. Gyrodynatic Data for Common Sugars.—In the case of cane sugar the gyrodynat for twenty-five grams of sugar in 100 grams of solution at 20° is [a]D = 66°.37. This is about the degree of concentration of the solutions employed in the shadow lamplight polariscopes. For seventeen grams of sugar in 100 grams of solution the number is [a]D = 66°.49. This is approximately the degree of concentration for the laurent instrument.
For any degree of concentration according to Tollens the gyrodynat may be computed by the following formula: [a]D = 66°.386 + 0.015035p - 0.0003986p², in which p is the number of grams of sugar in 100 grams of the solution.[67] In the table constructed by Schmitt the data obtained are as follows:
| In 100 parts by weight of solution. | Specific gravity | Concentration | Rotation a for 100 mm. | [a]D. | |
|---|---|---|---|---|---|
| Sugar p. | Water q. | at 20° C.d. | c = pd. | at 20° C. | |
| 64.9775 | 35.0225 | 1.31650 | 85.5432 | 56°.134 | 65°.620 |
| 54.9643 | 45.0357 | 1.25732 | 69.1076 | 45°.533 | 65°.919 |
| 39.9777 | 60.0223 | 1.17664 | 47.0392 | 31°.174 | 66°.272 |
| 25.0019 | 74.9981 | 1.10367 | 27.5938 | 18°.335 | 66°.441 |
| 16.9926 | 83.0074 | 1.06777 | 18.1442 | 12°.064 | 66°.488 |
| 9.9997 | 90.0003 | 1.03820 | 10.3817 | 6°.912 | 66°.574 |
| 4.9975 | 95.0025 | 1.01787 | 5.0868 | 3°.388 | 66°.609 |
| 1.9986 | 98.0014 | 1.00607 | 2.0107 | 1°.343 | 66°.802 |
105. Bi-Rotation.—Some sugars in fresh solution show a gyrodynat much higher than the normal, sometimes lower. The former phenomenon is called bi- the latter semi-rotation. Dextrose shows birotation in a marked degree, also maltose and lactose. After standing for a few hours, or immediately on boiling, solutions of these sugars assume their normal state of rotation. The addition of a small quantity of ammonia also causes the birotation to disappear.[68] This phenomenon is doubtless due to a certain molecular taxis, which remains after solution is apparently complete. The groups of molecules thus held in place have a certain rotatory power of their own and this is superadded to that of the normal solution. After a time, under the stress of the action of the solvent, these groups are broken up and the solution then assumes its normal condition.
106. Gyrodynat of Dextrose.—The gyrodynat of dextrose, as has already been mentioned, increases with the degree of concentration, thus showing a property directly opposite that of sucrose.
The general formula for the anhydrous sugar is [a]D = 52.°718 + 0.017087p + 0.0004271p². In this formula p represents the grams of dextrose in 100 grams of the solution. In a ten per cent solution the gyrodynat of dextrose is therefore nearly exactly [a]D20° = 53°. As calculated by Tollens the gyrodynats corresponding to several degrees of concentration are shown in the following table:
| p = grams in 100 grams of solution. | [a]D20° calculated for anhydrous dextrose. | |
|---|---|---|
| 7.6819 | 52°.89 | |
| 9.2994 | 52°.94 | |
| 9.3712 | 52°.94 | |
| 10.0614 | 52°.96 | |
| 10.6279 | 52°.98 | |
| 12.9508 | 53°.05 | |
| 18.6211 | 53°.25 | |
| 31.6139 | 53°.83 | |
| 40.7432 | 54°.34 | |
| 43.9883 | 54°.54 | |
| 53.0231 | 55°.17 | |
| 82.6111 | 57°.80 |
107. Gyrodynats of Other Sugars.—Of the other sugars it will be sufficient to mention only levulose, maltose, lactose, and raffinose. For complete tables of gyrodynatic powers the standard books on carbohydrates may be consulted.[69]