44 SUBSTANCES SOLUBLE IN AWOHOL. 



Lowenthal,' 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, 

 hoVever, 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. Lowenthal and others 

 have found the following to be the most advantageous method of 

 procediu-e. The liquid under examination is mixed with ameasured 

 quantity of solution of indigo-cartnine, the value of which, in terms 

 of permanganate, has been previously determined. The perman- 

 ganate solution is then run in till the blue colovu* 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 Giinther ascertained that 

 16 parts of oxygen from the permanganate oxidized 32 "5 parts 

 of gallo-tannio acid, 3S0 of sumach-tanni« acid, 250 (5-54) of 

 catechu-tannic acid, 24-0 (5*5|2) 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 aeid in oak-barks with per- 

 manganate by taking advantage of the power possessed by aninial 

 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 assimiption being made that 

 the substances which acted upon permanganate in the liquid after 

 treatment with animal charcoal were foreign bodies. Lowenthal(8ee 

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

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

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



^ Joiim. {. pr. Chem. Ixxii. 150. 



* Owing to a mistake in the calqulations, the figures here given for catechu- 

 tannic acid and catechuic acid are much too high. The correct nurahers are 

 placed Jn brackets after them. Lehmatan, in checking the.experitnent6 (Vergl. 

 TJntera. einiger Catechu- Und GambierTProben. DisB, 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). 



