For volumetric work, the solution of copper and the method of manipulation described in paragraph [117] are most conveniently used.
209. Preservation of Sugar Juices for Analysis.—Lead subacetate not only clarifies the juices of canes and thus permits of their more exact analytical examination, but also exercises preservative effects which enable it to be used as a preserving agent and thus greatly diminish the amount of work necessary in the technical control of a sugar factory. Instead, therefore, of the analyst being compelled to make an examination of every sample of the juice, aliquot portions representing the different quantities can be preserved and one analysis made for all. This method has been thoroughly investigated by Edson, who also finds that the errors, which may be introduced by the use of the lead subacetate in the analytical work, may be entirely avoided by using the normal lead acetate.[177]
In the use of the normal lead acetate, much less acetic acid is required in the polariscopic work than when the subacetate is used. The normal lead acetate is not so good a clarifying agent as the subacetate, but its efficiency in this respect is increased by the addition of a little acetic acid. In its use, it is not necessary to remove the lead, even for the determination of the reducing bodies.
For further details in regard to the technical determination of reducing bodies, special works may be consulted.[178]
210. Direct Determination of Sugars in Canes.—The methods, which have just been described, of securing the juices of cane by pressure and of determining the sugars therein, do not give the actual percentage of sugar in the cane. An approximate result may be secured by assuming that the cane is composed of ninety parts of juice and ten parts of cellular tissues and other insoluble matters. This assumption is approximately true in most cases, but there are often conditions arising which render the data calculated on the above assumption misleading. In any particular case in order to be certain that the correct percentage of sugar is secured it will be necessary to determine the fiber in the cane. This is an analytical process of considerable labor and especially so on account of the difficulty of securing samples which represent the average composition of the cane. The fibrous structure of the canes, the hardness of their external covering and the toughness of their nodes or joints render the sampling extremely difficult. Moreover, the content of sugar varies in different parts of the cane. The parts nearest the ground are, as a rule, richer than the upper joints and this is especially true of sugar cane. In order, therefore, to get a fair sample, even of a single cane, all parts of it must be considered. Several methods of the direct determination of sugar in canes have been proposed and will be described below.
Figure 65. Machine for Cutting Canes.
211. Methods of Cutting or Shredding the Cane for Analytical Purposes.—A simple method of cutting canes into small pieces which will permit of an even sampling is very much to be desired. The cutting apparatus shown in [Fig. 65] has been long in use in this laboratory. The canes by it are cut into thin slices, but the cutting edge of the knife being perpendicular to the length of the cane renders the use of the instrument somewhat laborious and unsatisfactory. A considerable time is required to cut a single cane and the slices which are formed should be received in a vessel which will protect them as much as possible from evaporation during the process of the work. Instead of the apparatus above a small cane cutting machine arranged with four knives on a revolving disk maybe used. The apparatus is shown in [Fig. 66]. The cane is fed against the knives through the hole shown in the open front of the apparatus and the knives thus strike the cane obliquely.[179] The knives can be set in the revolving disk at any desired position so as to cut the canes into chips as fine as may be desired. The cossettes furnished by this method may be sampled directly for the extraction of the sugar. In the case of the cossettes from both instruments described above a finer subdivision may be secured by passing them through a sausage cutter.
Figure 66. Cane Cutting Mill.