If the pulp be obtained by any other means than that of a fine rasp, the extraction of the sugar by the aqueous alcohol takes a long time, and even a second extraction may be necessary. It is convenient to use as a flask for holding the solvent, one already graduated at 100 or 110 cubic centimeters. A flask especially constructed for this purpose, has a constricted neck on which the graduations are made, and a wide mouth serving to attach it to the extracting apparatus, as shown in [Fig. 68]. When the extract is obtained in this way, it is not necessary to transfer it to a new flask before preparing it for polarization. When the extraction is complete, the source of heat is removed, and when all the alcohol is collected in the flask, the latter is removed from the extraction apparatus, cooled to room temperature, a sufficient quantity of lead subacetate added, the flask well shaken, the volume completed to the mark with water, again well shaken and the contents of the flask thrown upon the filter. It is important to avoid loss of alcohol during filtration. For this purpose it is best to have a folded filter and to cover the funnel immediately after pouring the contents of the flask upon the filter paper, with a second larger funnel. The stem of the funnel carrying the filter paper, should dip well into the flask receiving the filtrate. As in other cases of filtering sugar juices for polarization, the first portions of the filtrate received should be rejected. The percentage of sugar is obtained in the filtrate in the usual way.
Where a weight of pulp equal to the normal factor of the polariscope employed is used, and the extract collected in a 100 cubic centimeter flask, the percentage of sugar is directly obtained by making the reading in a 200 millimeter tube. With other weights of pulp, or other sizes of flask, the length of the observation tube may be changed or the reading obtained corrected by multiplication or division by an appropriate factor. A battery of sickel-soxhlet extractors is shown in [Fig. 69].[183]
Fig. 70.
222. Scheibler’s Extraction Tube.—In order to secure a speedy extraction of large quantities of pulp, Scheibler recommends the use of the extraction tube shown in [Fig. 70].[184] The apparatus is composed of three concentric glass cylinders. The outer and middle cylinders are sealed together at the top, and the inner one is movable and carries a perforated diaphragm below, for filtering purposes. Near the top it is provided with small circular openings, whereby the alcoholic vapors may gain access to the condenser (not shown). The middle cylinder is provided with two series of apertures, through the higher of which the vapor of alcohol passes to the condenser, while the alcohol which has passed through the pulp and collected between the inner and middle cylinders, flows back through the lower into the flask (not shown) containing the boiling alcohol.
The middle cylinder is provided with a curved bottom to prevent the filtering end of the inner tube from resting too tightly against it.
The tube containing the pulp is thus protected from the direct heat of the alcoholic vapors during the progress of extraction by a thin cushion of liquid alcohol.
223. Alcoholic Digestion.—The fourth method of determining sugar in beet pulp, is by means of digestion with hot alcohol. The principle of this method is precisely the same as that which is involved in aqueous diffusion in the cold. The diffusion, however, in the case of the alcohol, is not instantaneous, but is secured by maintaining the mixture of the pulp and alcohol for some time at or near the boiling point. The methods of preparing the pulp, weighing it and introducing it into the digestion flask are precisely those used for aqueous digestion, but in the present case a somewhat coarser pulp may be employed. The method is commonly known as the rapp-degener process.[185]
Any convenient method of heating the alcohol may be used. In this laboratory the flasks are held on a false bottom in a bath composed of two parts of glycerol and one of water. One side of the bath holder is made of glass, as shown in [Fig. 71], in order to keep the flasks in view. In order to avoid the loss of alcohol, the digestion flask should be provided with a reflux condenser, or be attached to an ordinary condenser, which will reduce the vapors of alcohol again to a liquid. Unless the weather be very warm, the reflux condenser may consist of a glass tube of rather wide bore and at least one meter in length, as shown in [Fig. 71]. A slight loss of alcohol during the digestion is of little consequence. A convenient method of procedure is the following.
Double the quantity of the beet pulp required for the ventzke polariscope, viz., 52.096 grams, weighed in a lipped metal dish, is washed, by means of alcohol, into a flask marked at 202.6 cubic centimeters, and the flask filled two-thirds with ninety-five per cent alcohol and well shaken. Afterwards, a proper quantity of lead subacetate is added, and then sufficient alcohol to complete the volume to the mark. The flask is then attached to the condenser, placed in a water-glycerol bath and heated to a temperature of 75° for about forty-five minutes. At the end of this time, the flask is removed from the bath and condenser, cooled quickly with water, alcohol added to the mark and well shaken. The filtration should be accomplished with precautions, to avoid the loss of alcohol mentioned in paragraph [221]. The filtrate is examined in the polariscope in a 200 millimeter tube, and the reading obtained gives directly the percentage of sugar in the sample examined. Half the quantity of pulp mentioned, in a 101.3 cubic centimeter flask, may also be used. A convenient form of arranging a battery of flasks is shown in the accompanying figure.