227 
ON THE ESTIMATION OF TANNIC ACID. 
Mimotannic Acid. 
showing error. 
Tormentil. 
5-7 . . 
19-8 . 
Larch Bark .... 
(3-2) 
\ 39 ) • ' 
4-7 . 
Rhatany. 
0*5 . . 
8-4 . 
Willow Bark .... 
30 . . 
6-1 . 
Catechu (Gambir) . . 
7-0 . . 
31-1 . 
Eucalyptus Gum . . 
• • # • • 
40-7 . 
Hemlock Extract . . 
• •» • • 
29-0 . 
Kino. 
• • • • * 
40 0 . 
Such, then, is the process introduced by Dr. Fleck ; as I have shown, it is 
not capable of very general application, but it will estimate gallotannic acid 
with considerable accuracy, if the method of direct weighing be adopted. 
When I first essayed to analyse by it, I thought it would prove eminently 
successful, but experience has shown, that though it possesses certain advan¬ 
tages, inasmuch as it combines accuracy, without that tedious watching, 
which is so essential to the success of the gelatine process, yet it can never 
supersede the latter method as long as mimotannic acid continues to be used 
in tanning. I have myself devised one or two processes for tannin estima¬ 
tion, but at present with only negative results. Among the most promising 
of them, was one in which the tannin was precipitated by a volumetric solu¬ 
tion of cinchonine, and the end of the operation ascertained by means of the 
iodohydrarg} rate of potassium, which would, of course, give.a powerful re¬ 
action the instant that the slightest excess of cinchonine was introduced into the 
solution. This fails from the fact that the tannate of cinchonine is not abso¬ 
lutely insoluble, so that as soon as a few cubic centimetres of cinchonine solu¬ 
tion have been added, a small quantity of cinchonine passes into solution, and 
the iodohydrargyrate commences to react. Another was based upon the fact, 
that a solution of tannin will reduce ferric salts to ferrous salts, the quantity of 
iron reduced was then estimated with potassic dichromate; but since the re¬ 
action does not always proceed uniformly, and since gallic acid possesses the 
same reducing power, this property of tannic acid unfortunately cannot 
be utilized. M. Wagner, a few years since, noticed in the ‘ Annales de 
Chimie,’ a method which he thought w T ould prove of great assistance in the 
gelatine process, viz. by adding to one litre of the ordinary gelatine solution, 
T gramme of hydrochlorate of rosaniline. This body has a great affinity for 
tannin, and is, equally with gelatine, thrown out of solution by it. As soon 
as an excess of gelatine was run in, the powerful colour of the rosaniline salt 
was intended to evidence such excess by its presence in the filtrate ; but, 
upon trial, I found that the affinity of magenta for the organic tannate of ge¬ 
latine, is greater than for tannic acid itself, and that the precipitate w ill 
continue to extract the colour from the solution, even after gelatine has been 
for a long time in excess. 
The Gelatine Process .—The following hints derived from considerable 
practice, may perhaps prove of service to some one who has occasion to make 
use of this method of analysis. 
1 . It is not absolutely necessary in every instance to employ a solution of 
alum in conjunction with the gelatine, as many substances will give a per¬ 
fectly clear filtrate, even to the close of the analysis, and the object in using 
alum is only to ensure a clear filtrate. It is impossible to say w 7 hat material 
will give a clear filtrate, or w'hat will not; it is apparently due to some cause 
entirely beyond control, as the same substances will, with one solution of ge¬ 
latine, filter perfectly bright, while with another, in every respect similar, it 
will run through cloudy ; temperature also appears to have some effect. 
When it is found requisite to use alum, it must be w r ith a very sparing hand, 
