Plant analysis : qualitative and quantitative / by G. Dragendorff ; translated from the German by Henry G. Greenish.

  • Dragendorff, G. (Georg), 1836-1898.
Date:
1884
Catalogue details

Licence: Public Domain Mark

Credit: Plant analysis : qualitative and quantitative / by G. Dragendorff ; translated from the German by Henry G. Greenish. Source: Wellcome Collection.

    64/304 (page 44)
    LöAventhal/ and others, have shown that the tannin contained in many vegetable substances may be estimated with sufficient accuracy for technical purposes by titrating with solution of per- manganate of potassium. In dealing with vegetable infusions, however, almost all authors agree that if a satisfactory result is to be obtained the solution to be titrated must be very dilute (about 1 in 400), and the oxidation incomplete. Löwenthal and others have found the following to be the most advantageous method of procedure. The liquid under examination is mixed with a measured quantity of solution of indigo-carmine, the value of which, in terms of permanganate, has been previously determined. The perman- ganate solution is then run in tili the blue colour changes to green. The value of the pure tannin in terms of the reagent must have been previously determined by experiments with weighed quanti- ties of the same. By such experiments Günther ascertained that 16 parts of oxygen from the permanganate oxidized 32*5 parts of gallo-tannic acid, 33’0 of sumach-tannic acid, 25-0 (5‘54) of catechu-tannic acid, 24'0 (5’32) of catechuic acid,^ 28'0 of kino- tannic acid, 34 to 37 of rhatania-tannic acid, 35 of tormentilla- tannic acid, 34 of cafFeo-tannic acid, and 32 of oak-bark-tannic acid. Neugebauer ^ estimated the tannic acid in oak-barks with per- manganate by taking advantage of the power possessed by animal charcoal of absorbing tannic acid, and thus removing it completely from its aqueous solution. He divided the infusion to be ex- amined in two equal parts. The one was titrated direct with permanganate, the other after the absorption of the tannic acid by animal charcoal. The amount of tannin present was then calculated from the difference, the assumi)tion being made that the substances which acted upon permanganate in the liquid after treatment with animal charcoal were foreign bodies. Löwenthal (see below) titrates a part of the tannin solution direct, another part after precipitation with solution of gelatine(XIL). From the differ- ence in the quantity of permanganate used the tannin is calculated. 1 Journ. f. pr. Chem. Ixxxi. 150. ® Owing to a mistake in the calculations, the figures here given for catechu- tannic acid and catechuic acid are much too high. The correct numbers are placed in brackets after them. Lehmann, in checking the experiments (Vergl. Unters, einiger Catechu- und Gambier-Proben. Diss. Dorpat, 1880), found that 16*0 parts of oxygen were equivalent to 5‘14 parts of catechu-tannic acid and 4’84 catechin. 2 Zeitschr. f. anal. Chem. x. 1 (1871).