Leather Industries Laboratory Book of Analytical and Experimental Methods (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ... of mixture will mutually compensate each other; and this method would have been universally adopted, but for the impossibility of obtaining really pure tannin. Von Schroeder showed that the principal impurity in the purest commercial tannins is gallic acid, which is oxidised similarly to tannin in presence of indigo, but consumes slightly more permanganate, and therefore influences but little the amount of permanganate consumed by such a tannin. According to his experiments, 1 grm. of the purest commercial tannins was equal in reducing power, on the average, to 1-05 grm. of theoretically pure, and he therefore proposed to multiply the amount actually weighed out by this figure. Procter suggested the employment of pure crystallised gallic acid as a standard, which, perhaps, hardly received the attention it deserved, since it is oxidised in the same way as tannin in presence of indigo, and is easily obtained in a pure Loc. cit. See Note, p. 225. state. 1 grm. of crystallised gallic acid is equivalent to about 125 grm. of "pure tannin," as calculated by von Schroeder's data, but further experiments are required to fix the quantity with complete accuracy. The values obtained by this method are only correct for gallotannic acid, and little is known of the proportionate consumption of permanganate by other tannins. For a long time, a statement of Oser's that oak-bark tannin consumed 1 5 times as much permanganate as gallotannic acid was accepted as a basis of calculation, but after the introduction of the hide-powder gravimetric method it was found that the relation between the two varied irregularly, even for the same kind of material, and if it is desirable to calculate even approximate percentages in a series of determinations of any other ta...