There are many instances, however, where other solvents should be used. The use of aqueous alcohol is sometimes preceded by that of alcohol of greater strength or practically free of water. For the extraction of soluble carbohydrates (sugars) cold or tepid water is employed, the temperature of which is not allowed to rise high enough to act upon starch granules. For the solution of the starch itself an acid solvent is used at a boiling temperature, whereby the starch molecules undergo hydrolysis and form dextrin or soluble sugars (maltose, dextrose). By this process also the carbohydrates, whose molecules contain five, or some multiple thereof, atoms of carbon form soluble sugars of which xylose and arabinose are types. The solvent action of acids followed by treatment with dilute alkalies at a boiling temperature, completes practically the solution of all the carbohydrate bodies, save cellulose and nearly related compounds. The starch carbohydrates are further dissolved by the action of certain ferments such as diastase.
Dilute solutions of mineral salts exert a specific solvent action on certain nitrogenous compounds and serve to help separate the albuminoid bodies into definite groups.
Under the proper headings the uses of these principal solvents will be described, but a complete discussion of their action, especially on samples of a vegetable origin, should be looked for in works on plant analysis.[20]
The application of acids and alkalies for the extraction of carbohydrates, insoluble in water and alcohol, will be described, in the paragraphs devoted to the analysis of fodders and cereals. The extraction of these matters, made soluble by ferments, will be discussed in the pages devoted to starch and artificial digestion. It is thus seen that the general preliminary treatment of a sample preparatory to specific methods of examination is confined to drying, extraction with ether and alcohol, and incineration.
43. Recovery of the Solvent.—In using such solvents as ether, chloroform, and others of high value, it is desirable often to recover the solvent. Various forms of apparatus are employed for this purpose, arranged in such a way as both to secure the solvent and to leave the residue in an accessible condition, or in a form suited to weighing in quantitive work. When the extractions are made according to the improved method of Knorr, the flask containing the extract may be at once connected with the apparatus shown in [figure 25].[21] A represents the flask containing the solvent to be recovered, W the steam-bath, B the condenser sealed by mercury, M and R the flask receiving the products of condensation. It will be found economical to save ether, alcohol, and chloroform even when only a few cubic centimeters remain after the extraction is complete. In the [figure] the neck of the flask A is represented as narrower than it really is. The open end of the connecting tube, which is sealed on A by mercury, should be the same size as the tube connecting with the condenser in the extraction apparatus.
Figure 25.—Knorr’s Apparatus for
Receiving Solvents.
Figure 26. Apparatus for Recovering Solvents
from Open Dishes.