34 
THE TANNINS. 
ammoniacal potassium ferricyanide, which gave a red 
color turning brown as long as any uncombined tannin 
was present. As stated by its author this estimation in¬ 
cluded gallic acid, so would not be available for all cases. 
Dragendorff 1 recommended that precipitations with 
lead acetate should be carried out in moderately con¬ 
centrated solutions, avoiding an excess of the lead salt 
and not continuing the washing too long. 
R. Jackson suggested the use of lead carbonate with 
which to agitate the tannin solution. The specific 
gravity before and after the treatment giving the loss 
of tannin, a difference of .0038 indicated one per cent, 
of tannin. 
For the estimation of tannin in plant analysis Drag¬ 
endorff recommended estimating the tannin soluble in 
absolute alcohol, after replacing the solvent by water, 
by precipitating with lead acetate, rapidly washing, 
drying, and weighing the precipitate, which was then 
burned and the residue of lead oxide subtracted from 
the original weight. This operation was then repeated 
with copper acetate. When the amount of organic 
matter precipitated by each was equal, the substance 
was assumed to be tannin; but, in case the lead salt 
precipitated more than the copper salt, the organic 
matter obtained by the latter was taken as representing 
the amount of tannin, while the excess obtained by lead 
acetate over that by copper acetate was reckoned as 
other organic acids and coloring-matter. 
Copper acetate appears to have been first suggested 
for the quantitative determination of tannin by Sakur. 
1 Plant Analysis, English edition, p. 41. 
