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.[204] 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.[205] 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.