Section III.—Preparation of Infusion.
1. Strength of Solution.—The tannin solution employed shall contain from 0·35 to 0·45 grms. per 100 c.c. of tanning matters absorbed by hide. (Paris 1900.)
2. Solution of Liquid Extracts.—A sufficient quantity shall be weighed into a covered basin or beaker, from which it shall be washed into a liter flask with boiling water and well shaken, and the flask shall be filled to the mark with boiling water. The neck being covered with a small beaker, the flask shall be placed under a cold water tap or otherwise rapidly cooled to a temperature between 15° and 20° C., and made up accurately to the mark, after which it shall be thoroughly mixed, and the filtration at once proceeded with.
Note.—Tannin infusions may be kept from fermenting by the addition of 3 to 5 drops of essential oil of mustard per liter. (F. Kathreiner.)
3. Filtration.—The filtration of the solution for analysis may take place through any paper which may be considered most suitable for the particular case, and with or without the use of kaolin, absorption of tanning matter, if any, being corrected for by an amount determined by a similar filtration of a clear solution. Perfectly clear solutions need not be filtered.
To determine the correction, about 500 c.c. of the tanning solution of the strength prescribed for analysis is obtained perfectly clear, preferably by the method of filtration which is to be corrected for. After thorough mixing, 50 c.c. is evaporated to determine “total soluble No. 1.” A portion of the remainder is then filtered in the manner for which correction is to be made, and 50 c.c. of the filtrate is evaporated for “total soluble No. 2.” Deducting No. 2 from No. 1 the difference is the correction required, which must be added to the total soluble found by analysis. It is generally advisable, both in analysis and in the second filtration for correction, to filter first 150 c.c. (which in analysis may be used for the determination of non-tannins), and then to employ the next 50 c.c. for evaporation, keeping the filter full during the operation; but whatever procedure is adopted must be rigidly adhered to in all analyses to which the correction is applied. Where kaolin is employed, a constant weighed quantity (1 or 2 grm.) must be used, which is first washed with 75 c.c. of the liquor by decantation, and then washed on to the filter with a further quantity of liquor, of which 200 c.c. is filtered as above.[191]
[191] It is obvious that in the first instance it will be necessary to determine the correction for each particular material employed, but it will soon be found that the correction is practically constant for large groups of tanning materials, so long as the same method of filtration is rigidly adhered to.
4. Solid Extracts.—Solid extracts shall be dissolved by stirring in a beaker with boiling water, the undissolved portions being allowed to settle, and treated with further quantities of boiling water, and the solution poured into a liter flask. After the whole of the soluble matter is dissolved, the solution is treated similarly to that of a liquid extract.
5. Extraction of Solid Materials.—Such quantities shall be weighed as will give an infusion of the strength already prescribed. (Preparation of Infusion, Resolution 1.) Not less than 500 c.c. of the infusion shall be extracted at a temperature not exceeding 50° C., after which the temperature shall be gradually raised to 100° C.,[192] and the extraction continued till the percolate is free from tannin and the whole made up to one liter, the weaker portions of the solution being first concentrated if necessary by evaporation in a flask, in the neck of which a funnel is placed.
[192] In substances which, like canaigre, contain a large quantity of starch, the extraction should be completed at a temperature of 50° C.—H. R. P.