Section IV.—Determination of Tanning Matters and Non-Tannins; etc.
1. Total Soluble Matter.—100 c.c.[193] of the clear filtered tanning solution, or a smaller quantity if the balance employed is of sufficient delicacy, shall be evaporated in an open weighed basin of platinum, hard glass, porcelain, or nickel, on the water-bath, and the basin shall afterwards be dried till constant in an air-oven, at a temperature of 100° to 105° C., or at a temperature not exceeding 100° C. in vacuo till constant, care being taken that no loss occurs by splintering of the residue. The use of the vacuum-oven for drying the residues is recommended when possible.
[193] 50 c.c. is sufficient, and is the quantity now generally employed.
2. Determination of Non-Tannins.—That the filter method shall remain the official method until the next Conference, but that members be permitted to employ the chromed hide-powder method of the American Association of Official Agricultural Chemists of 1901 (Appendix C) where it is desired, the fact being clearly stated on the report that the A.O.A.C. method has been employed, and not that of the I.A.L.T.C. (Leeds, 1902, see [note], [p. 480].)
That the “bell form”[194] of filter, as described by Professor Procter, shall be employed; not less than 5 grms. of hide-powder be used; the hide-powder should be so packed in the tube that the detannised liquor shall come over at a rate of about one drop in two seconds; and the filtrate be rejected so long as it gives a turbidity with a clear tanning solution. The filtrate may be used for the determination of non-tannin so long as it gives no reaction with salted gelatine solution.[195] The first 30 to 35 c.c. should be thrown away, and the next 50 c.c. of detannised solution, or an aliquot part of it, evaporated in a weighed basin on a water bath, and then dried till constant in an air-oven at a temperature of 100° to 105° C., or, in vacuo, not exceeding 100° C.
[194] It is obvious that the exact form and dimensions of the filter must be adapted to the character of the hide-powder available, as considerable differences exist in the absorptive power of different samples.
[195] 8 to 9 grams of good gelatine are dissolved in 500 c.c. of hot water, 100 grams of salt added, and the whole cooled and filtered.
3. Hide-Powder.—That the hide-powder must be sufficiently absorbent for use in the filter, and that in a blank experiment conducted with distilled water in the same way as an analysis, the residue from the evaporation of 50 c.c. should not exceed 5 milligrams.
The Freiberg Hide-Powder, made by Mehner and Stransky, containing between 10 and 20 per cent. of cellulose (as suggested by Cerych), is recommended by the Conference (Liège, 1901) and is very suitable for the filter method; but the powder, when analysed by the Kjeldahl method and calculated to 18 per cent. of moisture, must not contain less than 11·5 per cent. of nitrogen (Leeds, 1902).
4. Determination of Moisture and “Total Dry Matter.”—That the moisture in the sample be determined by drying a small portion at the temperature adopted in the determination of the “total soluble.” In extracts yielding turbid solutions which can be thoroughly mixed, it is generally preferable after mixing the solution and before filtration, to measure off and evaporate 50 c.c. for the determination of total dry matter (and moisture) in the same manner as the “total soluble.”