PEOCESSES FOE THE ESTIMATION OF TANNIN. 
519 
consequently to calculate with 636 must involve an error, more or less, in one 
direction ; but by putting aside all calculations depending upon formulae when 
possible, as in this instance, and preparing the volumetric solution as before 
noticed, we have a very excellent method before us, and capable too of easy 
application. 
The operation is conducted in precisely the same manner as with gelatine: a 
certain quantity having been run into the beaker, it is allowed to settle, and a 
drop taken out on to a piece of black, glass; by letting fall carefully another drop 
of emetic solution on to it, it will be immediately seen whether the precipitation 
be complete or no, and so on till the exact point has been obtained. 
To ascertain the approximate amount of antimony solution required by a cer¬ 
tain quantity of material, I have been in the habit of employing a modification 
which will be found to answer very satisfactorily, viz. by taking six beakers, 
and having exhausted, say 60 grms. of astringent substance, I make the decoc¬ 
tion up to 600 c. c. I then measure 100 c. c. of it into each beaker, in order 
that the soluble portions of 10 grms. of substance may be contained in each. 
10 c. c. of antimony solution are then run into the first, 15 into the second, 
20 into the third, and so on; by testing then each beaker as before explained, it 
is very easy to see in which there is an excess of tannin and in which an excess 
of antimony. Thus, if the beaker into which we ran 15 c. c. of antimony, 
should, on testing, indicate the presence of tannic acid, while that which has had 
20 c. o. of antimony shows an excess of the latter, it is obvious that 10 grms. of 
the substance used will require for the precipitation of its tannin a quantity of 
tartar-emetic solution lying between 15 and 20 c. c.; by exhausting some fresh 
material, and operating over again, the exact point of precipitation will be very 
readily obtained. 
The analysis of catechus and kinos, by this method, still presents some diffi¬ 
culty, and to coax the precipitate into subsiding will tax the patience of most 
operators; sometimes the addition of a little alum or a few cubic centimetres of 
alcohol will effect it, and I have found that merely warming the solution gently 
for a few minutes will sometimes cause the precipitate to settle sufficiently, so as 
to enable it to be tested as above; but in all cases it will be found rather more 
tedious than when working upon such bodies as oak bark, or its analogues. 
This process possesses the same advantage as the gelatine, in not precipitating 
gallic acid, and excels it in the fact that the precipitate is denser, and more 
readily subsides. Brande says that “gallic acid and the alkaline gallates throw 
down a precipitate from solutions of emetic tartar, containg about 43 per cent, 
of oxide of antimony.” This, however, as regards gallic acid is not a fact, for 
I have specially examined it on this point, and find that no reaction takes place 
between them, when the acid is in the free state; but to effect the combination 
of it with antimony the freshly precipitated SbgOg must be employed. 
Mittenzweij's process^^ which is the last that I have to notice, is in substance 
a new application of a well-known fact, namely, that an alkaline solution of 
tannin will rapidly absorb oxygen whenever that element is allowed access to 
it; and it is by means of the volume so absorbed, which is of course proportional 
to the quantity of tannin present, that M. Mittenzwey proposes to calculate the 
percentage of the latter. 
The process is conducted in the following manner:—A bottle capable of hold¬ 
ing about a litre and a half is fitted with a good cork, perforated to admit the 
passage of a length of glass tubing, which projects above the cork about 50 
mm., and on entering the bottle is slightly bent towards the side. This is con¬ 
nected to a second similar piece by a moderately long piece of india-rubber tubing 
provided with a pinchcock to close it. Having arranged the bottle as above. 
Journ. fiirPrakt. Chem. no. 2, 1861, p. 81. 
