Part 17

The latter formula is the one easier of application since it is only necessary in applying it to dissolve a given weight of the active body in an appropriate solvent and to complete the volume of the solution exactly to 100 cubic centimeters. It is therefore unnecessary in this case to determine the specific gravity.

=102. Formulas for Calculating Specific Rotatory Power.=—In order to determine the specific rotatory power (gyrodynat) of a given substance it is necessary to know the specific gravity and percentage composition or concentration of its solution, and to examine it with monochromatic polarized light in an instrument by which the angular rotation can be measured. The gyrodynat of any body changes with its degree of concentration, in some cases with the temperature, and always with the color of the light. With the red rays the gyrodynat is least and itprogressively increases as the violet end of the spectrum is approached. In practice the yellow ray of the spectrum has been found most convenient for use, and in the case of sugars the gyrodynat is always expressed either in terms of this ray or if made with color compensating instruments in terms of the sensitive or transition tint. In the one case the symbol used is (a){D} and in the other (a){j}. From this statement it follows that (a){D} is always numerically less than (a){j}. Unless otherwise specified the gyrodynat of a body is to be considered as determined by yellow monochromatic light, and therefore corresponds to a_{D}.

=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.

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. In the table constructed by Schmitt the data obtained are as follows:

In 100 parts by weight Specific Rotation a of solution. gravity Concentration for 100 mm. Sugar p. Water q. at 20° C. d. c = pd. at 20° C. [a]_{D}. 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. 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]{D}20° = 53°. As calculated by Tollens the gyrodynats corresponding to several degrees of concentration are shown in the following table:

p = grams in 100 [a]_{D}20° calculated for grams of solution. 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.

The gyrodynat of levulose is not definitely established. At 14° the number is nearly expressed by [a]_{D}14° = -93°.7.

Invert sugar, which should consist of exactly equal molecules of dextrose and levulose, has a gyrodynat expressed by the formula [a]{D}0° = -27°.9, with a concentration equivalent to 17.21 grams of sugar in 100 cubic centimeters. The gyrodynat decreases with increase of temperature, according to the formula [a]{D}t° = -(27°.9 - 0.32t°). According to this formula the solution is neutral to polarized light at 87°.2, and this corresponds closely to the data of experiment.

Maltose, in a ten per cent solution at 20°, shows a gyrodynat of [a]_{D}20° = 138°.3.

The general formula for other degrees of concentration is [a]{D} = 140°.375 - 0.01837p - 0.095t, in which p represents the number of grams in 100 grams of the solution and t_ the temperature of observation.

In the case of lactose [a]_{D} = 52°.53, and this number does not appear to be greatly influenced by the degree of concentration; but is somewhat diminished by a rising temperature.

The gyrodynat of raffinose in a ten per cent solution is [a]_{D} = 104°.5.

CHEMICAL METHODS OF ESTIMATING SUGARS.

=108. General Principles.=—The methods for the chemical estimation of sugars in common use depend on the reducing actions exerted on certain metallic salts, whereby the metal itself or some oxid thereof, is obtained. The reaction is either volumetric or the resulting oxid or metal may be weighed. The common method is, therefore, resolved into two distinct processes, and each of these is carried out in several ways. Not all sugars have the faculty of exerting a reducing action on highly oxidized metallic salts and the most common of them all, viz., sucrose is practically without action. This sugar, however, by simple hydrolysis, becomes reducing, but the two components into which it is resolved by hydrolytic action do not reduce metallic salts in the same proportion. Moreover, in all cases the reducing power of a sugar solution is largely dependent on its degree of concentration, and this factor must always be taken into consideration. Salts of copper and mercury are most usually selected to measure the reducing power of a sugar and in point of fact copper salts are almost universally used. Copper sulfate and carbonate are the salts usually employed, and of these the sulfate far more frequently, but after conversion into tartrate. Practically, therefore, the study of the reducing action of sugar as an analytical method will be confined almost exclusively to the determination of its action on copper tartrate.

Direct gravimetric methods are also practiced to a limited extent in the determination of sugars as in the use of the formation of sucrates of the alkaline earths and of the combinations which certain sugars form with phenylhydrazin. Within a few years this last named reaction has assumed a marked degree of importance as an analytical method. The most practical treatment of this section, therefore, for the limited space which can be given it, will be the study of the reducing action of sugars, both from a volumetric and gravimetric point of view, followed by a description of the best approved methods of the direct precipitation of sugars by such reagents as barium hydroxid and phenylhydrazin.

VOLUMETRIC METHODS.

=109. Classification.=—Among the volumetric methods will be given those which are in common use or such as have been approved by the practice of analysts. Since the use of mercuric salts is now practiced to a limited extent, only a brief study of that process will be attempted. With the copper methods a somewhat extended description will be given of those depending on the use of copper sulfate, and a briefer account of the copper carbonate process.

In the copper sulfate method two distinct divisions must be noted, viz., first an indirect process depending first upon the reduction of the copper to a suboxid, the subsequent action of this body on iron salts, measured finally by titration with potassium permanganate; and second, a direct process determined either by the disappearance of the blue color from the copper solution, or by the absence of copper from a drop of the solution withdrawn and tested with potassium ferrocyanid. This last mentioned reaction is one which is found in common use. The volumetric methods are not, as a rule, as accurate as the gravimetric, depending on weighing the resultant metal, but they are far more rapid and well suited to technical control determinations.

=110. Reduction of Mercuric Salts.=—The method of determining sugar by its action on mercuric salts, is due to Knapp. The method is based on the observation that dextrose and other allied sugars, will reduce an alkaline solution of mercuric cyanid, and that the mercury will appear in a metallic state.

The mercuric liquor is prepared by adding to a solution of ten grams of mercuric cyanid, 100 cubic centimeters of a solution of caustic soda of 1.145 specific gravity, and making the volume to one liter with water. The solution of sugar to be titrated, should be as nearly as possible of one per cent strength.

To 100 cubic centimeters of the boiling solution, the sugar solution is added in small portions from a burette and in such a way as to keep the whole mass in gentle ebullition.

To determine when all the mercuric salt has been decomposed, a drop of the clear boiling liquid is removed and brought into contact with a drop of stannous chlorid solution on a white surface. A brownish black coloration or precipitate will indicate that the mercury is not all precipitated. Fresh portions of the sugar must then be added, until no further indication of the presence of mercury is noted. The approximate quantity of sugar solution required to precipitate the mercury having thus been determined, the process is repeated by adding rapidly, nearly the quantity of sugar solution required, and then only a few drops at a time, until the reduction is complete.