Fig. 21.
The course of analysis is as follows:—
Preparation of the infusions.—Extracts are dissolved in hot water, and if necessary, filtered. Barks and other solid materials are treated in Prof. von Schroeder's extraction-apparatus ([Fig. 20]) (which seems very well adapted for its purpose). This consists of a perfectly cylindrical vessel of cast-tin, about 12·5 c.m. deep and 7 c.m. diameter. A strainer covered with fine muslin fits it like a piston.[Q] The powdered material is placed in the cylinder, and stirred up with 200 c.c. of cold water. At the end of an hour, the piston is inserted and pressed down gently, the clear liquor is poured off, and the process is 4 times repeated with hot water, at intervals of 1/2 hour, placing the cylinder in a water-bath. The liquid is made up to 1 litre, and, if necessary, filtered (Report, [p. 66]). The quantity of material used should be such as to give an infusion of which 10 c.c. do not reduce more than 8 c.c. permanganate. If it is desired to determine separately the "easily soluble tannin" (viz. that extracted by cold water), Real's Press ([Fig. 21]) is employed, which consists of a cylinder a, through which water may be forced by the pressure of a column of liquid. The small sieve d, covered with a disc of linen, is placed in a, next the tanning material previously thoroughly moistened with water, and the tap is closed. The press is then filled with water and left 15 hours under a pressure of 11/2 metres. The tap is then opened and 1 litre is allowed to run through in the course of about 2 hours, and mixed by shaking. The material is finally exhausted like a new material in von Schroeder's apparatus to extract the difficultly soluble tannin.
[Q] Both this apparatus, and the Real's press, may be obtained from C. Focke, Zinngiesser, Grosse Kirchgasse 3, Dresden.
The titration is carried out as before described; in each infusion separately to determine the "not-tannin" 50 c.c. are treated with 3 grm. hide-powder, and 10 c.c. are titrated.
It may be well in conclusion for the writer to state for the information of the non-chemical reader, that though for purposes of comparison of the results of different chemists, it is most desirable to have a standard method of the highest possible perfection; any of the accepted modifications of the Löwenthal method will give excellent practical results in careful hands.