44 SUBSTANCES SOLUBLE IN ALCOHOL. 



LöAventhal,^ and others, have shown that the tannin contained in 

 many vegetable substances may be estimated with sufficient 

 accui'acy for technical purposes by titrating with solution of per- 

 manganate of i^otassium. 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 (abov\t 

 1 in 400), and the oxidation incomplete. Löwenthal and others 

 have found the following to be the most advantageous method of 

 procedure. The liqidd 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 till the blue colour changes to green. 

 The value of the pure tannin in terms of the reagent must have 

 been previously determined by experiments Avith 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- 

 anained 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 assumption being made that 

 the substances which acted upon permanganate in the liquid after 

 treatment with animal charcoal were foreign bodies. Lüwenthal (see 

 below) titrates a part of the tannin solution direct, another part 

 after precipitation with solution of gelatine (XII.). From the differ- 

 ence in the quantity of permanganate used the tannin is calculated. 



^ Joum. 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. , 



3 Zeitschr. f. anal. Chem. x. 1 (1871). 



