Part 49

In the one case three times the normal weight of milk is 61.53 and in the other twice the normal weight, 65.48 grams.

=244. Error due to Volume of Precipitate.=—Vieth states that the volume allowed for the precipitated solids in the original process, viz., two and four-tenths cubic centimeters, is not sufficiently large. In such cases it is quite difficult to decide on any arbitrary correction based on the supposed quantities of fat and albuminoids present. A better method than to try to compensate for any arbitrary volume is to remove entirely the disturbing cause or eliminate it by indirect means. To wash the precipitate free of sugar without increasing the bulk of the filtrate unduly would be extremely difficult and tend, moreover, to bring some of the precipitated matters again into solution. It is better, therefore, to eliminate the error by double dilution and polarization (=86=). The principle of this method is based on the fact, that, within limits not sensibly affecting the gyrodynat by reason of different densities, the polarizations of two solutions of the same substance are inversely proportional to their volumes.

For convenience, it is recommended that the volumes of the samples in each instance be 100 and 200 cubic centimeters, respectively, in which case the true reading is obtained by the simple formula given in the latter part of =86=.

In this laboratory the double dilution method of determining the volume of the precipitate is conducted as follows:

In each of two flasks marked at 100 and 200 cubic centimeters, respectively, are placed 65.52 grams of milk, four cubic centimeters of mercuric nitrate added, the volume completed to the mark and the contents of the flask well shaken.

After filtering, the polarization is made in a 400 millimeter tube by means of the triple shadow polariscope described in =75=. From the reading thus obtained the volume of the precipitate and the degree of correction to be applied are calculated as in the subjoined example. The flasks should be filled at near the temperature at which the polarizations are made and the observation room must be kept at practically a constant temperature of 20° to avoid the complications which would be produced by changes in the gyrodynat of lactose and the value of the quartz plates and wedges of the apparatus by marked variations in temperature.

Example.—Weight of milk used in each case 65.52 grams.

Polarimetric reading from the 100 cubic centimeter flask, 20°.84 ” ” ” ” 200 ” ” ” 10°.15 Then 10.15 × 2 = 20.30 20.84 - 20.30 = 0.54 0.54 × 2 = 1.08 20.84 - 1.08 = 19.76 19.76 ÷ 4 = 4.94,

which is the corrected reading showing the percentage of lactose in the sample used.

The volume of the precipitate is calculated as follows:

20.84 ÷ 4 = 5.21, the apparent percentage of lactose present.

Then 5.21: 4.94 = 100: x.

Whence x = 94.82. From this number it is seen that the true volume of the milk solution polarized is 94.82 instead of 100 cubic centimeters, whence the volume occupied by the precipitate is 100 - 94.82 = 5.18 cubic centimeters. So little time is required to conduct the analysis by the double dilution method as to render it preferable in all cases where incontestable data are desired. Where arbitrary corrections are made the volume allowed for the precipitate may vary from two and a half cubic centimeters in milks poor in fat, to six for those with a high cream content.

For milks of average composition sufficient accuracy is secured by making an arbitrary correction of five cubic centimeters for the volume of the precipitate.

SEPARATION OF SUGARS BY CHEMICAL AND CHEMICAL-OPTICAL METHODS.

=245. Conditions of Separation.=—In the foregoing paragraphs the optical methods for determining certain sugars have been described. Many cases arise, however, in which these processes are inapplicable or insufficient. In these instances, the analyst, as a rule, will be able to solve the problem presented by the purely chemical methods which have been previously described, or by a combination of the chemical and optical processes. Not only have the different sugars distinctive relations to polarized light, but also they are oxidized by varying quantities of metallic salts and these differences are sufficiently pronounced to secure in nearly every instance, no matter how complex, data of a high degree of accuracy.

The carbohydrates of chief importance, from an agricultural point of view, are starch and sucrose; while the alternation products of chief importance, derived therefrom by chemical and biological means, are dextrin, maltose, dextrose and invert sugar.

=246. Sucrose, Levulose, and Dextrose.=—The purely chemical methods of separating these three sugars have been investigated by Wiechmann. They are based on the data obtained by determining the percentage of reducing sugars, both before and after the inversion of the sucrose, and before and after the removal of the levulose. For the destruction of the levulose, the method of Sieben is employed, and attention is called to the fact that the complete removal of the levulose by this process is difficult of accomplishment, and is probably attended with alterations of the other sugars present.

=247. Sieben’s Method of Determining Levulose.=—The decomposing action of hot hydrochloric acid on levulose, and its comparative inaction on dextrose are the basis of Sieben’s process. The hydrochloric acid employed should contain about 220 grams of the pure gas per liter, that is, be of twenty-two per cent strength, corresponding to 1.108 specific gravity. If the substance acted on be invert sugar, its solution should be approximately of two and a half per cent strength. To 100 cubic centimeters of such a solution, sixty of the hydrochloric acid are added, and the mixture immersed in boiling water for three hours.

After quickly cooling, the acid is neutralized with sodium hydrate of thirty-six times normal strength. Ten cubic centimeters of the hydrate solution will thus neutralize the sixty of hydrochloric acid which have been used to destroy the levulose. The work of Wiechmann discloses the fact, easily prevised, that the method used for destroying levulose is not always effective and that action of the reagent is not exclusively confined to the levogyrate constituent of the mixture. Nevertheless, data of reasonable accuracy may be secured by this process, which is best carried out as described by Wiechmann. In this connection the possibility of the polymerization of the dextrose molecules, when heated with hydrochloric acid, must not be overlooked.

=248. The Analytical Process.=—The total quantity of invert sugar in a given solution is determined by the methods already given (=136, 141=.)

After this has been accomplished, the levulose is destroyed as described above, and the dextrose determined by any approved method (=136, 140=). In the presence of sucrose, the sum of the reducing sugars is first determined as in =136, 142=. After the inversion of the sucrose, the invert sugar is again determined, and the increased quantity found, calculated to sucrose. The levulose is then destroyed by hydrochloric acid, and the dextrose determined as described above. The quantity of sucrose may also be determined by an optical method (=91, 92, 94=.).

=249. Calculation of Results.=—If we represent by a the weight of metallic copper reduced by the invert sugar present in a solution containing sucrose, and by b that obtained after the inversion of the sucrose, the quantity of copper corresponding to the sucrose is b - a = c. After the destruction of the levulose, the copper reduced by the residual dextrose may be represented by d. The weight of copper equivalent to the levulose is, therefore, b - d = e. From the tables already given, the corresponding quantities of the sugars equivalent to c, d, and e are directly taken. Example:

{ 163.8 milligrams invert sugar. a = 300 milligrams = { 156.5 ” dextrose. { 185.63 ” levulose.

b = 500 ”

d = 275 ” = 142.8 ” dextrose.

c = 200 ” = 106.3 ” invert sugar.

e = 225 ” = 133.89 ” levulose.